ABSTRACT UNDERSTANDING FIBER-COUPLED ...

ABSTRACT

UNDERSTANDING FIBER-COUPLED DIODE LASER SUPERHEATING IN LASER-ASSISTED MACHINING OF SILICON NITRIDE (Si3N4) SriHarsha Panuganti, M.S. Department of Mechanical Engineering Northern Illinois University, 2009 Thesis Adviser: Dr. Federico Sciammarella

The use of advanced ceramics like silicon nitride (Si3N4) can be found in many high-tech engineering applications ranging from NASA’s Space Shuttle program to more recent areas such as ballistic protection (ceramic armor). In order for advanced ceramics to become more prevalent in industry, it is necessary to develop the adequate manufacturing equipment for processing at a much cheaper price and faster manufacturing time than conventional grinding does. Laser-assisted machining (LAM) of ceramics is one tool that can enable this growth while maintaining reasonable costs for making advanced ceramic parts in small quantities. Mostly CO2 and YAG lasers have been used for processing but now with the development of diode lasers a new tool has been introduced. The diode laser offers more flexibility over others and cheaper equipment costs, smaller footprint and highest wall plug efficiencies. In LAM, the laser does not remove the material but helps the tool in removing the material with much greater ease, thus providing many advantages over conventional machining methods like grinding. In LAM of Si3N4 it is required to melt the glassy phase

present (at the grain boundaries) in the workpiece material around 1000 °C rather than evaporating the bulk of the material which happens above 1600 °C. The current thesis work obtains the temperatures achieved by a Si3N4 rod (1 × 6 inch) during laser-assisted turning at different sets of parameters: rpm, laser spot size, laser power, laser preheat time, and bulk preheat temperature (temperature of preheating the whole workpiece externally without laser). The temperatures obtained are analyzed with the help of a previously carried out numerical modeling in LAM and a mathematical study is done relating these temperatures with the corresponding laser energies absorbed. In the process of the research conducted, the calibration of all the equipment/processes involved in the accurate estimations of temperatures, power outputs, and other parameters – which plays a major role in this study – has been done. The whole thesis work helps develop a complete understanding of various laser and machining parameters in LAM of ceramics, and makes some important conclusions that help in continuing the research for a fully developed LAM of ceramics system.

NORTHERN ILLINOIS UNIVERSITY DE KALB, IL

AUGUST 2009

UNDERSTANDING FIBER-COUPLED DIODE LASER SUPERHEATING IN LASER-ASSISTED MACHINING OF SILICON NITRIDE (Si3N4)

BY SRIHARSHA PANUGANTI ©2009 SriHarsha Panuganti

A THESIS SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE MASTER OF SCIENCE

DEPARTMENT OF MECHANICAL ENGINEERING Thesis Adviser: Federico M. Sciammarella

ACKNOWLEDGEMENTS First I would like to express my sincere indebtedness to Prof. Federico Sciammarella for his advisership and backing since the inception of this project. I am grateful to Dr. Joseph Santner and Prof. Pradip Majumdar for their support as my thesis committee members. I appreciate the financial support that I received throughout the period I was involved in this research, from Dean Pramod Vohra and Dr. Richard Johnson of the ROCK (Rapid Optimization of Commercial Knowledge) program. I also want to thank the Department of Mechanical Engineering at NIU for facilitating this research. I want to thank my fellow student Mr. Michael Matusky for his active involvement in all the experimentation trials. I also want to thank Mr. Jesus Arceo, who assisted me with the documentation of this research data. Special thanks to the technical staff at the Engineering Building, to Reliance Tool & Manufacturing Co., Elgin, to Dr. Stephen Gonczy of Gateway Materials Technology Inc. and to Dr. Frank Pfefferkorn of University of Wisconsin at Madison. Finally I would like to mention the moral support from my parents and family who played an important role in helping me complete this thesis work.

DEDICATION

To PETA

TABLE OF CONTENTS

LIST OF TABLES ………………………………………………………………... vii LIST OF FIGURES ………………………………………………………………. viii Chapter 1. INTRODUCTION ................................................................................................1 1.1 Ceramics .....................................................................................................1 1.2 Machining Methods of Ceramics ...............................................................2 1.3 Advanced Ceramics ....................................................................................3 1.4 Silicon Nitride (Si3N4) ................................................................................4 1.5 Literature Review .......................................................................................8 1.6 Objectives .................................................................................................11 2. LASERS IN MATERIALS PROCESSING.......................................................12 2.1 Introduction ..............................................................................................12 2.2 Classification of Commercial Lasers........................................................13 2.3 High-Power Lasers – CO2 and Nd:YAG ..................................................15 2.4 Diode Lasers .............................................................................................17 2.5 Laser-Assisted Machining ........................................................................18

v

Chapter

Page

3. EXPERIMENTAL .............................................................................................21 3.1 Numerical Model ......................................................................................21 3.2 NIU Laser-Assisted Machining of Ceramics (LAMC) Equipment ..........25 3.3 Measurement of Spot Size as a Function of Focal Distance for the M3 Lab LAMC System.........................................................................................29 3.4 Determination of Emissivity of Si3N4 and Calibration of the IR Camera 34 3.5 Experiments to Determine Approximate Temperature Losses Due to Distance from the Thermal Camera ...............................................................38 3.6 Experiments to Determine Approximate Temperature Losses Due to Angle of View of the Thermal Camera ..........................................................43 4. RESULTS ...........................................................................................................47 4.1 Laser Heating Results ...............................................................................47 4.2 Effect of Bulk Preheating .........................................................................55 5. GRAPHICAL REPRESENTATION OF THE EXPERIMENTAL RESULTS AND ANALYSES .................................................................................................57 5.1 Temperature vs. Time Graphs ..................................................................57 5.2 Effect of the Laser Spot Size at 500 and 250 rpm Turning Speeds at Three Laser Power Outputs ......................................................................................62 5.3 Mathematical Formulation .......................................................................66 5.4 Efficiency vs. Turning Speed (rpm) for Different Laser Power Outputs and Spot Sizes.................................................................................................72

vi

Chapter

Page

6. DISCUSSION AND CONCLUSIONS ..............................................................77 6.1 Discussion.................................................................................................77 6.2 Conclusions ..............................................................................................78 7. RECOMMENDATIONS AND FUTURE WORK ............................................81 7.1 Recommendation(s) of Parameters for LAMC ........................................81 7.2 Future Work..............................................................................................82 REFERENCES .......................................................................................................85

LIST OF TABLES

Table 1.1: Production applications of silicon nitride....................................................7 Table 2.1: Some commercial industrial lasers. ...........................................................14 Table 2.2: Some R&D industrial lasers. .....................................................................15 Table 2.3: Typical diode laser materials.....................................................................18 Table 3.1: Specifications of the LAM system used at the M3 Lab, NIU. ..................28 Table 3.2: Specifications of the LAM system used at Reliance Tool and Manufacturing (RTM), Elgin, IL................................................................................28 Table 3.3: Specifications of instrumentation. .............................................................29 Table 3.4: Thermocouple tests results. .......................................................................37 Table 3.5: Effect of distance on temperature readings of the thermal camera. ..........42 Table 3.6: Temperature compensations made for losses due to different factors. .....46 Table 4.1: Series 1 results. ..........................................................................................49 Table 4.2: Series 2 results. ..........................................................................................49 Table 4.3: Series 3 results. ..........................................................................................50 Table 4.4: Series 3 results: repetition of 189W – 2 mm spot trials. ...........................51 Table 5.1: Second-order curve fit equations (250 W) and first derivatives for Series 2 results. ……………………………………...……...…………...……….... 64

LIST OF FIGURES

Figure 1.1: Si3N4 gas turbine radial rotor from Allied Signal Ceramic Components, Torrance, CA. ...............................................................................................................5 Figure 1.2: Development of silicon nitride alloys. ......................................................5 Figure 1.3: Automotive components (left) and industrial components (right) made from Ceralloy 147-31N (SRB silicon nitride). .............................................................6 Figure 2.1: A schematic of laser-assisted milling. .....................................................20 Figure 2.2: A demonstration of laser-assisted turning at Purdue University, IN. ......20 Figure 3.1: Model parameters considered in laser-assisted turning of a cylindrical workpiece. ..................................................................................................................22 Figure 3.2: Material removal plane at the tool–workpiece interface. .........................22 Figure 3.3: Temperature–time graph showing different temperatures achieved during a LAM simulation for the different parameters shown. .............................................23 Figure 3.4: LAMC (turning) system at the M3 Lab, Engineering Building, NIU. ....26 Figure 3.5: Experimental setup for the laser preheat trials at the M3 Lab, NIU. .......27 Figure 3.6: Brass shim stocks with spots created by the laser. ...................................31 Figure 3.7: The spots created on the soot being measured using an optical comparator. .................................................................................................................32 Figure 3.8: Graph showing laser spot size–focal distance linear curve fits (and equations) for the LAMC system at the M3 Lab, NIU...............................................33 Figure 3.9: Strip heater at 1.21 m (4 ft). .....................................................................39

ix

Page Figure 3.10: Strip heater at 0.91 m (3 ft). ...................................................................39 Figure 3.11: Strip heater at 0.60 m (2 ft). ...................................................................40 Figure 3.12: Strip heater at 0.30 m (1 ft). ...................................................................40 Figure 3.13: Directional emittance of typical classes of materials shown as polar diagram . .....................................................................................................................43 Figure 3.14: Thermal image of the 1×1 inch Si3N4 rod with lines (left) and the temperature profile of Line 2 (right). .........................................................................44 Figure 4.1: Temperature distribution of the Si3N4 rod at 189 W (100rpm, 2mm spot) around 10 sec. .............................................................................................................52 Figure 4.2: Temperature distribution of the groove (brightest) at 189 W (100rpm, 2mm) around 5 min. ...................................................................................................52 Figure 4.3: Temperature axial and circumferential distribution at 170 W, 100rpm, approx. 2 mm (left) and 3 mm (right) laser spot sizes................................................53 Figure 4.4: The thermal profiles of the lines drawn across the turning direction (shown in Figure 4.3) for the laser spot sizes approx. 2 mm and 3 mm respectively. ....................................................................................................................................54 Figure 4.5: Comparison of maximum temperatures achieved by a bulk preheat trial and only laser preheat trial. ........................................................................................56 Figure 5.1: Temperature–time graph for 100 W, 2 mm laser spot size. .....................58 Figure 5.2: Temperature–time graph for 100 W, 3 mm laser spot size. .....................58 Figure 5.3: Temperature–time graph for 150 W, 2 mm laser spot size. .....................59 Figure 5.4: Temperature–time graph for 150 W, 3 mm laser spot size. .....................59 Figure 5.5: Temperature–time graph for 189 W, 2 mm laser spot size. .....................60

x

Page Figure 5.6: Temperature–time graph for 189 W, 3 mm laser spot size. .....................60 Figure 5.7: Temperature–time graph for 250 W (at RTM), 2 mm laser spot size......61 Figure 5.8: 500 rpm turning speed, for three laser power outputs 100, 150 and 189 W ....................................................................................................................................62 Figure 5.9: 250 rpm turning speed, for three laser power outputs 100, 150 and 189 W ....................................................................................................................................63 Figure 5.10: DT/dt versus time for Series 2 results (see Table 4.2) ...........................65 Figure 5.11: Thermal efficiency vs. energy/sq.mm for 100 W, 2 mm laser spot size. ....................................................................................................................................69 Figure 5.12: Thermal efficiency vs. energy/sq.mm for 100 W, 3 mm laser spot size. ....................................................................................................................................69 Figure 5.13: Thermal efficiency vs. energy/sq.mm for 150 W, 2 mm laser spot size. ....................................................................................................................................70 Figure 5.14: Thermal efficiency vs. energy/sq.mm for 150 W, 3 mm laser spot size. ....................................................................................................................................70 Figure 5.15: Thermal efficiency vs. energy/sq.mm for 189 W, 2 mm laser spot size. ....................................................................................................................................71 Figure 5.16: Thermal efficiency vs. energy/sq.mm for 189 W, 3 mm laser spot size. ....................................................................................................................................71 Figure 5.17: Thermal efficiency vs. energy/sq.mm for 250 W, 2 mm laser spot size. ....................................................................................................................................72 Figure 5.18: Thermal efficiency vs. rpm for three power outputs, 2 mm laser spot size, at 1 minute. .........................................................................................................73

xi

Page Figure 5.19: Thermal efficiency vs. rpm for 250 W, 2 mm laser spot size, at 1 minute. ........................................................................................................................73 Figure 5.20: Thermal efficiency vs. rpm for three power outputs, 2 mm laser spot size, at 2 minutes. .......................................................................................................74 Figure 5.21: Thermal efficiency vs. rpm for 250 W, 2 mm laser spot size, at 2 minutes. ......................................................................................................................74 Figure 5.22: Thermal efficiency vs. rpm for three power outputs, 2 mm laser spot size, at 3 minutes. .......................................................................................................75 Figure 5.23: Thermal efficiency vs. rpm for 250 W, 2 mm laser spot size, at 3 minutes. ......................................................................................................................75 Figure 5.24: Thermal efficiency vs. rpm for three power outputs, 2 mm laser spot size, at 4 minutes. .......................................................................................................76 Figure 5.25: Thermal efficiency vs. rpm for 250 W, 2 mm laser spot size, at 4 minutes. ......................................................................................................................76

1. INTRODUCTION 1.1 Ceramics

Ceramics are very hard, inorganic, non-metallic materials, most of them having crystalline structure formed by metallic and non-metallic elements. When we think of ceramics, we think of the traditional ceramics like the ones that make dinnerware, pottery, tiles, etc., to name a few. The first use of ceramics is known to be made in what was formerly known as Czechoslovakia in around 24,000 BC. The ceramics found there were a form of animal and human figurines, slabs, and balls. But the first use of operative pottery vessels, believed to be used for storing grain, is thought to be made in 9,000 BC [1]. In the modern era, the use of ceramics has been absolutely necessary for the development of technology in all fields. Ceramics are used as the necessary sophisticated refractory materials to produce metals, whose importance does not need an explanation. Construction would not have been possible without essential ceramics like bricks, which don’t succumb to termites and heat. In the same way we can also recognize the necessity of ceramics in electronics industries where they are used as very good insulators, semiconductors, superconductors and magnets.

2 1.2 Machining Methods of Ceramics

Grinding and diamond machining are the most widely used techniques for machining ceramics. Even though grinding of ceramics has very low material removal rates when compared to grinding of other materials like metals, it is the best available commercial technique for ceramic machining. While there have been many advancements in the process of grinding of ceramics, there are only very few other techniques available for machining ceramics. Methods like honing and lapping are the same kind of techniques as grinding, where the material is removed through the process of abrasion. The lack of an accurate and economical ceramic machining method to produce complex shapes like borings, grooves and spherical surfaces makes such ceramic parts even more expensive [25]. Ultrasonic machining and electrical discharge machining (EDM) are two examples of non-abrasive ceramic machining methods used to produce intricate shapes, which have several disadvantages. While EDM can be used for only conductive materials, low material removal, high wear on tool molds and inadequate dimensional accuracy are some disadvantages of ultrasonic machining. However there have been advancements like ultrasonic-assisted lapping, laser-assisted grinding, etc., which are still not able to reduce the cost of ceramic parts to the required extent [25]. To realize easy replacements of ceramics for metals, there is an urgent need to find a much cheaper alternative to ceramic grinding process. Laser-assisted

3 machining (LAM) is one developing machining technique that is anticipated to reduce the cost of ceramic parts by at least 50 % [21]. While the previous research conducted on LAM already showed promising results (see Section 1.5), it is also believed that LAM can produce complex-shaped ceramic parts after being commercialized.

1.3 Advanced Ceramics

The terms “advanced ceramics” or “technical ceramics” or “engineering ceramics” refer to the ceramic materials with superior mechanical, thermal, electrical, optical and magnetic properties and oxidation/corrosion resistance. The synthesis of advanced ceramics is said to have started during the second half of the twentieth century [24]. Advanced ceramics are classified as oxides, non-oxides and particulate reinforced materials which are combinations of oxides and non-oxides. Alumina (Al2O3) and zirconia (ZrO2) are examples of oxides and borides, nitrides and carbides are examples of non-oxides [24]. Alumina is used in hip replacements and in water filters [2]. Silicon carbide (SiC) is used in ceramic brake discs in automobiles [27], in blue LEDs, in thin film pyrometry and as mirror material in astronomical telescopes [8]. Boron carbide (B4C) is used in personal and vehicle anti-ballistic armored plating and in neutron absorbers in nuclear reactors [26]. Cubic zirconia is used as diamond substitute in jewelry [12] and stabilized zirconia is used in oxygen sensors [7] and fuel cell

4 membranes. Advanced ceramics can also be used in some applications that help stop global warming. “Greenhouse gas emissions from power stations could be cut to almost zero by controlling the combustion process with tiny tubes made from advanced ceramic materials,” claim engineers [20].

1.4 Silicon Nitride (Si3N4) Silicon nitrides are a range of advanced ceramics which stand for their high strength, toughness, hardness and excellent thermal and chemical stability. When silicon nitride was first discovered in the mid-19th century, it was hard to manufacture it due to the covalent bonded nature. But the necessity to manufacture silicon nitride with ease gave birth to RBSN (reaction-bonded silicon nitride) and HPSN (hot pressed silicon nitride); and later SSN (sintered silicon nitride) and SRBSN (sintered reaction-bonded silicon nitride) were manufactured [10]. The first major application of silicon nitride was cutting tool inserts, which can withstand thermal shock, impact, contact stress, severe erosion and temperatures around 1100 °C [18]. See Figures 1.1 and 1.2. Most of the current research on silicon nitride was developed from the idea in 1980’s that parts for automobile gas turbine and piston engines could be made from silicon nitride; and eventually make automobile engines predominantly with silicon nitride, thus reducing weight and enabling higher temperature operations than conventional engines.

5

Figure 1.1: Si3N4 gas turbine radial rotor from Allied Signal Ceramic Components, Torrance, CA [18].

Figure 1.2: Development of silicon nitride alloys [18].

6 Unfortunately this goal was not achieved owing to the factors like the high cost of fabrication of silicon nitride parts [10]. Therefore, there is a huge need to develop cheaper and faster alternatives to the current conventional process of grinding to make silicon nitride parts. Apart from the applications in metal forming, industrial wear and molten metal handling, silicon nitride has growing applications in the automotive industry as shown in Figure 1.3. Now that the cost of producing silicon nitride has come down, there have been rapidly increasing production applications. Table 1.1 gives some production applications of silicon nitride.

Figure 1.3: Automotive components (left) and industrial components (right) made from Ceralloy 147-31N (SRB silicon nitride) [18].

7 Table 1.1: Production applications of silicon nitride [18].

Applications

Conditions imposed on the silicon nitride

Cutting tool inserts (introduced late 1970s)

High contact stress, severe physical and chemical erosion, temperatures to around 1100 °C

Diesel cam follower rollers (introduced 1990s)

Highly concentrated rolling contact stress

Bearings (introduced late 1980s)

High contact loads that can exceed 2 million psi (~13,800 MPa)

Wear parts for many applications

Depends on application, but typically abrasion, erosion, and some impact over a broad range of temperature

Aircraft engine seal (gas turbine aircraft propulsion engine (introduced mid1990s)

Sliding seal operating at engine shaft speed and elevated temperature

Benefits demonstrated or perceived by customers 75-90% reduction in machining time for cast iron, 5-10 times increase in the metalremoval rate, and increase in the number of parts machined per tool

Increased life at higher injection pressures, reduced frictional losses, improved engine performance 3-10 times life of best metal bearings, 80% lower friction, 80% higher speed, 60% lighter, higher operation temperature, ability to operate with lubrication starvation Dramatic increase in life and decrease in maintenance

Much higher reliability and life than metals

8 1.5 Literature Review

Many lasers are already being used commercially in materials processing industries throughout the world. The processes like laser welding, laser cutting, laser surface heat treatment and laser forming have been successful in the industry whose parametrical studies have been developed and understood to a significant extent. But newer methods like laser-assisted machining of ceramics (LAMC), where laser helps the cutting tool in machining, do not have established laser and machining parameters. Even though a lot of research in the form of numerical modeling has been done in LAMC, little has been done with regard to establishing practical working parameters. Responding to the growing demand of ceramics, it is necessary to understand the processes involved in the LAM of ceramics (i.e., LAMC), which could be a much better and effective alternative to the conventional grinding process. The current research focuses on LAMC (turning), which seems to have more potential in the future than LAM of metals. The advantages of LAMC over conventional diamond grinding are anticipated to be realized through the following factors [6]: •

Faster machining time

•

Better surface quality

•

More complex shapes

9 The following are some publications that will justify the need for LAMC while exposing what needs to be done in the future to develop a commercial LAMC system. Chang Chih-Wei and Chun-Pao Kuo propose that LAM considerably reduces the cutting force by 22% (feed force) and 20% (thrust force) when compared to conventional planing operation. The surface integrity of the workpiece in the case of LAM process was better than that of conventional planing [3]. Budong Yang and Shuting Lei discuss the machinability of Si3N4 under laser- assisted milling by observing tool wear and workpiece edge chipping. It was found that LAM could significantly improve the machinability of Si3N4 by reducing the cutting forces and increasing the tool life [28]. Chang Chih-Wei and Chun-Pao Kuo propose that Nd:YAG LAM of Al2O3 produced a much better surface quality than conventional machining. They used Taguchi method to evaluate the surface quality of the Al2O3 workpieces and identified an optimum machining conditions for LAM [4]. Rozzi and colleagues propose that

the temperature around cutting tool location

must be greater than about 1000 ºC, for successful LAM. They observed brittle fracture of chips at lower temperatures and visco-elastic flow in chips at higher temperatures around 1000 ºC confirming the softening of glassy phase of Si3N4 at higher temperatures [19]. Patten and colleagues modeled orthogonal cutting of silicon nitride with a single-point cutting tool. It was found theoretically that for a small tip radius, high

10 speeds and small values of feeds, ductile machining of silicon nitride was possible, favored by positive or zero-degree rake angles [15]. Konig and Zaboklicki conclude that in LAM, the softening of glassy Si3N4 present at the grain boundaries of the β phase softens, allowing ductile machining of Si3N4 [13]. Purdue University alone published about 25 papers on LAM, which include both experimental and numerical studies [16]. Among those papers, there have been some successful studies on experimental validation of theoretical LAM simulations suggesting that LAM can replace grinding and diamond machining processes to machine advanced ceramics. There have been successful attempts made to prove the feasibility of LAMC as a good low-cost alternative to the current ceramic machining methods. There are studies on surface analysis, temperature analysis, tool wear and life, machining parameters and numerical simulations on LAMC. But lack of establishment of laser parameters like spot size, laser power output, preheat time, etc., compatible with optimum machining parameters is one of the issues that needs to be developed for LAMC . The current thesis work focuses on obtaining and understanding the maximum temperatures achieved during laser preheating of laser-assisted turning of silicon nitride, for different laser parameters and turning speeds. The goal is to help provide workable parameters for a successful LAMC (turning) process.

11 1.6 Objectives

•

Obtain the maximum laser spot temperatures (at center) achieved by a Si3N4 rod (1×6 in) for the different parameters: turning speed, laser spot size, laser power, laser preheat time and bulk preheat temperature (the temperature of heating the entire bulk of the workpiece using an external heating source before laser preheating).

•

Quantify the temperature dependence on the above parameters. Identify any practical critical parameters and trends favoring laser-assisted turning of Si3N4.

•

Establish a mathematical relation between the actual parametrical temperatures obtained and laser energy input on the rod.

•

Suggest any appropriate conditions (parameters, modifications, etc.) for maximizing the thermal efficiency in the process of laser-assisted turning of Si3N4.

2. LASERS IN MATERIALS PROCESSING 2.1 Introduction

Laser (also LASER) is the acronym for “light amplification by stimulated emission of radiation”, which is different from other electro-magnetic radiations as the laser possesses some special characteristics, like high monochromaticity and high coherence. Laser light is generated through a complicated quantum-mechanical process where an electron in a higher energy state of the laser-generating medium is stimulated by an external radiation to emit a photon. The details and intricasies of this mechanism are discussed in many commercial books on the laser light generation. As the current focus is on LAM, the details on how the laser is generated will not be discussed in this thesis work. Some of the various unique characteristics that make laser beams special for material processing are as follows [5]: •

High brightness, which implies low divergence and small, focused spot size.

•

Propagates over large distances through air with minimal attenuation.

•

No interaction with magnetic field.

•

High power focusability to the size of wavelengths.

13 Since the first time lasers were used for material processing, the newer advantages of lasers in materials processing can be attributed to the following items [5]: •

The multitude of reliable lasers, wavelengths, and wide ranges of powers.

•

The laser can work in vacuum and in the presence of shielding gasses.

•

Delivery systems, such as optical wave guides (fiber optics), added tremendous flexibility.

•

Lasers have an edge over conventional material processing techniques, for long-term goals.

•

Lasers are readily coupled with computer-controlled processes, NC systems, and robotics.

2.2 Classification of Commercial Lasers

Of the many types of lasers that are being commercially used in the current materials processing industry, there are many practical applications that incorporate lasers from the UV to IR and from very low power to very high power. Lasers are named with reference to their active media, which can be classified as solid, liquid, or gaseous, based on the physical state. Since power is the most important factor for choosing a laser in the materials processing industry, classifying the industrial lasers based on power would be most appropriate. Table 2.1 provides important information about some most prevalent commercial industrial lasers with their nonabsolute common power ratings [5].

14 Table 2.1: Some commercial industrial lasers. Active Medium

Max. Avg. Power (W)

Operating Modes

Wavelength (micron)

CO2 Nd:YAG

25,000 1,800

CW*, P** CW, P

9.6/10.6 1.06/1.32

Excimer Lasers: F2 ArF KrCl XeCl

3 70 20 200

P P P P

0.157 0.193 0.222 0.308

Solid State Lasers: Cr:sapphire (Ruby) Cr:alexandrite

20 20

P P

0.68-0.95 0.72-0.79

DPSS Lasers: Nd:YAG Nd:YLF

10 10

CW, P CW, P

1.06/1.32 1.06/1.32

1 0.1

CW, P CW, P

0.780-0.865 0.98

10 0.3 0.2 0.1

CW P CW CW

0.52,0.53,…, 0.58 0.337 0.325/0.442 2//3.51//4

Semiconductor Lasers: AlGaAs, Diode InGaAs, Diode Gas Lasers: CO N2 HeCd HeXe CW* = Continuous Wave P** = Pulsed

15 R&D Lasers

Research and development (R&D) lasers are advanced lasers that are being developed in a very few industrial, academic, or government laboratories around the world. In some cases time may be purchased on these lasers for customer experimentation. Table 2.2 gives some examples of R&D lasers, which indicates significant potential for high-power lasers in the future market [5].

Table 2.2: Some R&D industrial lasers.

Active Medium

Max. Avg. Power (W)

Operating modes

Wavelength (micron)

Iodine

30,000

CW, P

1.315

CO

5,000

CW

5.2,5.3,…,5.8

HF

1000

CW

2.6-3

Dye

1000

P

0.500-0.640

Nd:glass/slab

1000

P

1.06

Free Electron

100

P

0.500-10,000

2.3 High-Power Lasers – CO2 and Nd:YAG CO2 and Nd:YAG (neodymium doped yttrium aluminum garnet) lasers stand out for their ability to produce high-power laser beams with very high flexibility in power variation and control. As mentioned in Table 2.1, the average power

16 capabilities of CO2 and Nd:YAG lasers are 25,000 and 1,800 W respectively, with the flexibility of being run in continuous wave (CW) and pulsed modes and with a wide variation of pulse widths and shapes. Because of these capabilities CO2 and Nd:YAG lasers have become the key lasers for high-power applications. The following are some cost issues associated with CO2 and Nd:YAG lasers [5].

CO2 Lasers CO2 lasers offer the highest average power for materials processing. The average cost of CO2 laser runs from about $5000 for a 10 W laser to $1 M for a 25,000 W laser. This averages form $400/W at low power, $200/W at medium power, $70/W at high power, and $40/W at super high power. As the lasers reach small production volumes, a 10 – 20 % reduction in price is reasonable [5].

Nd:YAG

The cost of Nd:YAG lasers can be significantly different between continuous wave (CW) and pulse pumping because of the power supply technology, and between rod and slab lasers because of the resonator technology [5]. CW-pumped rod lasers up to 1800 W

100 – 200 $/W

Pulse-pumped rod lasers up to 600 W

200 $/W

Pulse-pumped slab lasers up to 500 W

300 – 400 $/W

17 2.4 Diode Lasers

The first diode lasers were used in the early 1960s, which required cryogenic working temperatures and allowed only pulsed operation with milliwatt range. But now diode lasers are capable to operate also in CW mode and at room temperatures. Diode lasers are mainly used in telecommunication applications, but for material processing the diodes have been modified to yield high power output [17]. Diode lasers are the most sought developing lasers in the materials processing industry which are considered as the laser of the future due to their size, efficiency and potential for cheap mass production. The life time of the laser LEDs varies from 10-20,000 hours – about 100 times longer than that of an incandescent lamp [22]. The luminous Performance of visible diodes is increasing ten folds per decade and in the decade of 1990-2000 the cost of the diodes dropped by ten times [11]. Diode lasers are extremely interesting because of their ability to convert electrical current directly into laser light leading to high efficiency of about 30-50 %. Table 2.3 gives some typical compounds used for diode lasers [17].

18 Table 2.3: Typical diode laser materials. Material

Wavelength (micron)

In1-xGaxAsyP1-y

1.2 – 1.6

GaAsxP1-x

1.2 – 1.5

InAsxP1-x

1.0 – 3.1

(AlxGa1-x)yIn1-yAs

0.9 – 1.5

AlxGa1-xAs

0.7 – 0.9

GaAs1-xPx

0.6 – 0.9

InxGa1-xAs

0.55 – 3.0

(AlxGa1-x)yIn1-yP

0.55 – 0.8

CdSxSe1-x

0.5 – 0.7

CdxZn1-xSe

0.3 – 0.5

AlGaInN

0.2 – 0.64

The current research utilizes fiber-coupled diode lasers, whose specifications are discussed in Chapter 3, for the potential of the diode lasers in the future as mentioned above. The availability of fiber coupling greatly enhances the flexibility in the way that the laser beam can be transported through fiber optic cable to the laser head with only negligible power losses.

2.5 Laser-Assisted Machining

Laser machining (LM) and Laser-assisted machining (LAM) are the two most popular three-dimensional laser machining methods. While LM is a non-

19 traditional machining approach where the laser acts as the tool to remove workpiece material, LAM utilizes traditional machining methods like milling, planing and turning where the laser assists the cutting tool in machining [6]. Laser-assisted machining of ceramics (LAMC), a special case of LAM, is a practical way of making ceramic parts through machining, instead of forming the material using expensive dies. Usually a grinding process in done in several stages using about seven grinding machines, each costing about a million dollars. LAM can replace multiple grinding machines and assembly lines with one integrated LAM system worth half a million dollars, thus resulting in huge savings. At the same time, with LAM it can be possible to produce parts of intricate shapes in a single cut [21]. LAM is a process specially developed as an economical alternative to traditional methods for machining very hard materials like ceramics. In LAM, the laser softens the workpiece material prior to removal by the tool, thus offering many advantages over conventional machining methods. By heating a local portion of the workpiece with the laser, the machinability of the heated portion is considerably improved, which is then removed with reduced tool forces. Improved life of the tool, closed tolerances, and better workpiece surface integrity are some advantages expected from LAM when compared to conventional machining methods. Figure 2.1 shows that the laser precedes the tool and softens the material prior to being removed by the tool.

20

Figure 2.1: A schematic of laser-assisted milling [23].

Figure 2.2: A demonstration of laser-assisted turning at Purdue University, IN. [21].

3. EXPERIMENTAL 3.1 Numerical Model

An existing finite volume method model was utilized to quantify the interrelationship among the laser parameters and machining parameters (feed, speed and depth of cut) to manage the laser heating. The surface heating isotherms are used to gain confidence in the predictions of the depth of heating below the surface. The modeling was done by Dr. Frank Pfefferkorn, University of Wisconsin at Madison. Figure 3.1 defines the parameters considered in the model, and Figure 3.2 defines the temperature nomenclature in the tool engagement zone. The radial meshing was very fine (about 20 µ) near the surface since steep thermal gradients are expected. Similarly the axial and circumferential grid near the laser impingment was finer (30 µ × 50 µ) than far field as a compromise for accurate answers and reasonable computational time.

22

Figure 3.1: Model parameters considered in laser-assisted turning of a cylindrical workpiece.

Figure 3.2: Material removal plane at the tool–workpiece interface.

23 Figure 3.2 magnifies the material removal plane and shows the locations for the maximum and minimum temperatures in the material removal plane, named TMR, max and TMR, min accordingly. Tmax is the temperature achieved at the center of the radial laser heating on the rod. Figure 3.3 shows TMR, max and TMR, min for the parameters mentioned on the right side of the figure.

Figure 3.3: Temperature–time graph showing different temperatures achieved during a LAM simulation for the different parameters shown.

The solid orange line in Figure 3.3 represents the temperature of interest, 1000 °C, while the red line represents TMR, max.

24 The following are the summary results obtained from the numerical model. The magnitude of parameters chosen in the model were selected after a few initial experimental trials were made to determine a set of parametrical magnitudes yielding maximum temperatures (experimental) of above 900 °C. Effect of laser spot size: •

A plus or minus 0.1 mm change in laser spot size results in a plus or minus 500 °C in Tmax (250 W, 2 mm, 500 rpm).

•

No significant change is resulted in the temperature distribution at the material removal plane Tmr, due to slight changes in the laser spot size (250 W, 2 mm, 500 rpm).

Effect of laser beam position: •

The position of the laser beam with respect to the cutting zone will probably have great influence on the temperature distribution (for a given set of parameters).

Effect of bulk preheat: •

For 2mm laser spot size, 250 W laser power output, and turning speed of 500 rpm without bulk preheat, a Tmax of 2500 °C was achieved after 20 sec. For a similar simulation trial but with a bulk preheat temperature of 100 °C, the same Tmax was achieved 5 sec earlier, which suggests that there is no significant effect of bulk preheating on the maximum temperatures achieved.

25 Effect of turning speed: •

1000 rpm turning speed produced Tmax of 1700 °C for 250 W, 2 mm laser spot and 3 min laser preheat time.

•

500 rpm turning speed produced Tmax predicted 2500 °C for the same parameters.

Effect of coefficient of convection (hc): •

The coefficient of convection (hc) at the workpiece-air interface has no significant effect on the temperature distribution.

3.2 NIU Laser-Assisted Machining of Ceramics (LAMC) Equipment (Figure 3.4)

As mentioned in Section 1.6, the focus of this thesis is on the maximum temperatures achieved by a turning Si3N4 (1×6 in) rod on being heated by the laser, for different parameters. The previous research conducted in LAMC suggests that the temperature of about 1000 °C is required at the machining zone for Si3N4 for a successful LAM [19]. Since the laser parameters and the corresponding turning speeds to achieve the above temperature are not known, a temperature study (without involving machining) of the maximum temperatures achieved during the laser preheating seems important, hence this study. Obtaining the maximum temperatures is practically much easier than obtaining the temperatures at the machining zone, owing to the limited resolution of the IR instruments commercially available today.

26 Therefore, the experiments conducted in this research concentrate on the maximum temperatures.

Figure 3.4: LAMC (turning) system at the M3 Lab, Engineering Building, NIU. The CNC lathe can be seen to the right while the controller is to the left.

All the laser preheat trials mentioned in this work were conducted with the setup shown in Figure 3.5. The strip heater was used only for the bulk preheating trials, where the rod would rotate at 100 rpm above the hot strip heater till the bulk preheating temperature was achieved. The temperature of the strip heater was controlled by the temperature probe shown. The laser would fire on the rotating rod for a fixed period of time (5 minutes in most cases) as programmed for different

27 turning speed and laser parameters. The IR camera would record the temperature near the laser spot as the local temperature is increased from the initial preheat temperature. The recorded footages were analyzed and post-processed to obtain the maximum temperatures of the laser spots on the rod as a function of time. A set of experimental trials (for 250 W laser output power) was conducted at Reliance Tool and Manufacturing Co. (RTM), Elgin, IL, which could not be done at NIU (M3 Lab) due to the equipment limitations on maximum power output and turning speed at NIU (see Tables 3.1 and 3.2). The experimental set up at RTM is similar to that shown in Figure 3.5.

Laser Head

IR Camera 1 × 6 inch Si3N4 Rod 12” Strip Heater

Temperature Probe

Figure 3.5: Experimental setup for the laser preheat trials at the M3 Lab, NIU.

28 Table 3.1: Specifications of the LAM system used at the M3 Lab, NIU. Equipment

CNC lathe (custom built)

Jenoptik® (Germany) Fiber-Coupled Diode Laser JOLD-250-CPXF2P2

Specifications Turning speed 0 – 500 rpm Feed rates up to 10 in/min

0 - 189 W output after losses (250 W max. diode output) 0.976 µ wavelength

Table 3.2: Specifications of the LAM system used at Reliance Tool and Manufacturing (RTM), Elgin, IL. Equipment

Specifications

MAZAK CNC Lathe

Turning speed 0 – 4000 rpm Feed rates upto 0.2 in/rev

QPC Bright Lase® Fiber Coupled Diode Laser

0 - 450 W 0.976 µ wavelength

MA (Germany) 2color Pyrometer.

0 - 1800 °C 2 mm spot size at focus

®

29 Table 3.3: Specifications of instrumentation. Instrument

®

FLIR A-325 Thermal Camera with ExamineIR Software

Power Meter COHERENT ® PM 300F-19

Specifications 0 - 2000 °C Operating Range Resolution: 320×240 pixel

0-450W operating range

3.3 Measurement of Spot Size as a Function of Focal Distance for the M3 Lab LAMC System

It is necessary to know the sensitivity of spot size as a function of focal distance. The knife edge technique and the plastic material burning method are judged inadequate because the percentage error cannot be easily determined. The most accurate way of obtaining the laser spot size–focal distance relationship is to use an appropriate laser profiling system, which was initially unavailable during this study. Therefore, our work group came out with an innovative technique whose accuracy was later determined by comparing our results with the actual laser profile

30 discussed later. The following is the procedure followed to obtain the laser spot size as a function of focal distance. •

Arrange for a metallic wedge of known angle. Cut a piece of brass shim stock in rectangular shape of dimensions equal to those of the wedge’s top face consisting of the hypotenuse.

•

Bond the shim stock on to the top face of the wedge using a bonding cement or super glue.

•

Uniformly cover the shim stock with soot emitted by a cigarette lighter. Caution must be taken not to overheat the shim stock and de-bond the shim stock.

•

Place the wedge under the laser head such that the beam would be focused closer to the lower end of the wedge. Make sure the laser beam translates parallely along the centre of the axis parallel to the base of the wedge.

•

Record the initial focal distance, i.e., the distance between the laser head (collimating lens) and the shim stock, to obtain the first spot size at this initial focal distance.

•

Program the laser to shoot the shim stock for appropriate amount of time (use trial and error) to form a bright spot on the black soot (Figure 3.6). This happens as the laser carries away the soot present in the area where it is being fired, thus exposing bright metallic surface under it. (The time period of shooting the laser must be enough to create a spot, which can be determined using trial and error method.)

31 •

Translate the laser head (or the wedge table) parallel to the base of the wedge, by a fixed increment distance. This changes focal distance to obtain a different spot size.

Figure 3.6: Brass shim stocks with spots created by the laser.

•

Likewise make at least 10 spots at regular intervals for various focal distances. The burned spot sizes are a measure of actual laser spot sizes at those focal distances.

32 •

Repeat the experiment to obtain three sets of spots.

•

Measure the diameter of all the spots to an accuracy of one hundreds of a mm with an optical comparator (see Figure 3.7).

Figure 3.7: The spots created on the soot being measured using an optical comparator.

•

Plot the average of the three spots for each focal distance on a chart of spot size versus focal distance (Figure 3.8).

•

Curve fit the graph linearly to obtain the equation(s) for the spot size (diameter) as a function of the focal distance.

33

Spot Size (mm)

Linear Curve Fits of Spot Size vs. Focal Length 3.4 3.3 3.2 y= 3.1 3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 100.000

-0.194x + 23.074 R2 = 0.9961

105.000

y = 0.1815x - 19.773 R2 = 0.9772

110.000

115.000

120.000

125.000

130.000

Focal Length (mm)

Figure 3.8: Graph showing laser spot size–focal distance linear curve fits (and equations) for the LAMC system at the M3 Lab, NIU.

It was found that the focal length obtained for 2 mm laser spot size through this method was 2 mm more than the actual focal length obtained during laser profiling by a qualified technician. Therefore, necessary corrections were made while setting up the focal distances for the spot sizes 2 mm and 3 mm during the actual laser preheat trials. Note: Since only 2 mm laser spot trials were conducted at RTM, determination of the spot size–focal length curve was not necessary. The focal length of 2 mm laser spot was set as per the manufacturer’s specifications.

34 3.4 Determination of Emissivity of Si3N4 and Calibration of the IR Camera Determination of the emissivity of the object and calibration of the IR instrument must go together as different instruments may require different emissivity set points to record the correct temperature of the object. To measure the absolute surface temperatures attained by laser heating the Si3N4 rod, an IR instrument requires the emissivity of the specific material as a function of temperature. Since the emissivity of an engineering material as a function of temperature may not follow a standard curve, several thermocouple tests are to be conducted over a range of temperatures. Moreover the grade and surface finish of the material can significantly affect the emissivity. Furthermore, different IR instruments may require different values of emissivity set points for the given material. This is because different instruments have different amounts of losses of IR radiation within them, and in the case of twocolor instruments, the band of spectrum a particular instrument utilizes is usually different from any other instrument. And the losses encountered in IR entering an instrument can be attributed to inadequate size of the target, distance, angle of view and internal losses, the effect of which will be discussed in later in this chapter. Hence, thermocouple tests not only reveal the emissivity of the material of interest but also calibrate the specific IR instrument used.

35 Procedure used for thermocouple tests: •

Precautions/warning: o

Use of thermal gloves, tongs and safety glasses is strongly encouraged.

o The temperature drops rapidly once the rod is taken out of the furnace. Hence, the test must be done as quickly as possible. •

Arrange a ceramic plate where the Si3N4 rod rests onto which the IR camera is focused.

•

Heat a 1×1 inch piece of Si3N4 rod uniformly to the lowest temperature around which emissivity is to be obtained, using a programmable furnace.

•

After enough time is allowed (about 15 min) to have the rod reach uniform temperature, open the furnace door and quickly hold the rod with tongs and place it on a thermally insulating plate.

•

While the IR camera is recording the live image of the rod, stick firmly the thermocouple bead onto the centre portion of the rod mechanically using hand. Allow enough time for the thermocouple to reach equilibrium temperature. (For the thermocouple beads of sizes 0.05 mm, the losses due to convection are estimated to be negligible.)

•

Like in case of the camera, the thermocouple live readings should be recorded using a data acquisition system or an alternative method like the one discussed below.

36 •

Emissivity can be determined first by synchronizing the recordings of camera and thermocouple and later adjusting the emissivity set in the camera to match the thermocouple reading.

•

This set point can be considered as the emissivity of the Si3N4 rod for that temperature (read by thermocouple) for the given FLIR A-325 IR camera.

•

Repeat the trials for different increments of temperatures, from lower to higher, to obtain emissivity values at those temperatures.

The above procedure was followed till the thermocouples used started to degrade, which was around 600 °C. Choosing a thermocouple for applications like these is not easy because once we look for fast response, it would mean a sacrifice of upper temperature limits and/or the life of the thermocouple. Also, making a good contact between the rod’s surface and the thermocouple’s bead becomes challenging at higher temperatures. As an alternative to a data acquisition system, the thermocouple was connected to a digital meter display whose real-time readings were manually recorded with a digital camera. Later each of the digital camera’s video files was carefully synchronized (based on the event of initial thermocouple contact) with its corresponding file from the IR camera, to obtain the emissivity. The following are the specifications and results of these tests (Table 3.4). Thermocouple specifications for Round 1 tests: •

Name: Omega CO2-K Style 2, Cement On Type

•

Wire/bead diameter: 0.05 mm

37 •

Rated for: 540 °C max.

•

Response time: 2-5 ms (still air)

Thermocouple specifications for Round 2 tests: •

Name: Omega CHAL-002

•

Wire/bead diameter: 0.05 mm

•

Rated for: 540 °C max. (expected up to 1000 °C)

•

Response time: < 1 s (still air)

Table 3.4: Thermocouple tests results. Thermocouple read °C

ε(emissivity) obtained for FLIR A-325

Time to reach max. temp (sec)

Round 1 Tests 41 0.74 10 70.5 0.65 9 116.6 0.63 20 153.4 0.62 21 118 0.67 9 234.4 0.66 12 264 0.67 8 331 0.67 23 413* 0.69 20 418* 0.66 17 Round 2 Tests 670* 0.64 8 - 11 *Instable readings due to poor thermocouple contact. Maximum recorded reading.

38 3.5 Experiments to Determine Approximate Temperature Losses Due to Distance from the Thermal Camera

The FLIR A-325 thermal camera allows the user to set the approximate distance of the target from the camera lens, in the software. If the correct distance is not set, the temperature readings the instrument reads will have increased error. This was confirmed by observing the variation of the temperature of a target by changing the distance set point. But the effect of the distance itself, in spite of setting the correct distance set point in the software, for this specific instrument was not known. That is, the loss in temperature reading, if any, when a target is observed from a farther distance rather than from a nearer distance, with the correct distance set points in the camera’s software was not known. Knowing this effect at different temperatures is necessary to record the temperature of an object more accurately. Hence, a set of tests was done by recording the temperatures of a strip heater having uniform and almost steady-state temperature. Five temperatures (420, 378, 287, 179 and 79 °C) with four distances (1, 2, 3 and 4 ft) for each of the temperatures were used to study the effect of distance on the temperature readings. Since the strip heater did not have uniform temperature distribution throughout its surface, isothermal pixels of the camera around the maximum temperature reached by the strip heater in each trial were obtained. The temperatures of these pixels were considered as the reference temperature, or the temperature of interest in each trial. A typical result is shown in Figures 3.9 – 3.12 at 420 °C and at

39 the four different distances mentioned. The accompanying isotherms in the images show the number of pixels allotted for the highest temperatures in each case, shown as the red areas. The same kind of images was obtained for the other three temperatures and the same kind of analysis was done, to know the effect of distance on the temperature readings at temperatures 79, 179, 287, 378 °C.

Figure 3.9: Strip heater at 1.21 m (4 ft). Right: isotherm pixels from 412 to 416 °C


40



41 Table 3.5 gives the central, the maximum, and the minimum temperatures of the isothermal pixels around the maximum temperatures achieved by the strip heater, at the four different temperatures and at the four distances. The number of pixels around the maximum temperature achieved by the strip heater, increases as the camera moves closer to the target and as the temperature of the strip heater decreases. Hence, the little temperature variation along the rows of Table 3.5 can be considered more as the effect of pixels, rather than the effect of the distance. Therefore, it can be concluded that distance does not have significant effect (i.e., no losses) on the temperature readings of the FLIR A-325 thermal camera, if the target is big enough to have sufficient number of pixels allocated. However, approximate temperature compensations can made as seen in Table 3.6 at the end of this chapter. For the current application, distance has negligible effect as the target distance has always been within one-quarter meter.

42 Table 3.5: Effect of distance on temperature readings of the thermal camera. 420 °C Distance

Image max. (°C)

1.21 m 0.91 m 0.60 m 0.30 m

420 419.9 421.4 425

1.21 m 0.91 m 0.60 m 0.30 m

379 380 381 386

1.21 m 0.91 m 0.60 m 0.30 m

288 289.4 290.1 292.7

*iso. center (°C)

iso. max. (°C)

409.4 420 405 419.89 412.6 421.45 408.54 425 378 °C 374.3 379 363.93 380.24 363.88 381 357.6 386.91 287 °C 277.46 287.98 277 289.4 285.04 290.06 285.2 292.7 179 °C

412 414.28 414.97 418.14

Pixels Allotted for isotherm 45 43 93 252

372.33 371.51 373.48 379

37 90 169 447

280.36 280.47 281.44 281.53

70 142 306 1419

iso. min.(°C)

Distance

Image max.

Iso. Center

Iso. Max

Iso. Min

1.21 m 0.91 m 0.60 m 0.30 m

175.1 196 177 179

170.29 168.93 170 170.24

175.1 176 177 179

168.43 170.2 172 172

Pixels Allotted for isotherm 120 190 349 1776

75.7 75.6 76 76.8

70.5 71 70 22

294 495 1250 76800

79 °C 1.21 m 75.7 0.91 m 75.6 0.60 m 76.1 0.30 m 76.8 *iso. stands for isothermal

73.3 73.3 72.82 53.87

43 3.6 Experiments to Determine Approximate Temperature Losses Due to Angle of View of the Thermal Camera

Consider Figure 3.13, which shows the influence of the angle on the directional emittance of an object (e) from zero to 90 degrees represented in a polar diagram [14]. A lambertian blackbody has radius of unity represented by ‘1’. Similarly, lambertian grey body is represented by ‘2’, dielectrics like Si3N4 by ‘3’, and metals by ‘4’. According to Figure 3.13, we need not be concerned about the viewing angle for most materials as long as we are within about 50 degrees.

Figure 3.13: Directional emittance of typical classes of materials shown as polar diagram [14].

44 Some simple tests were conducted to validate the temperature changes with the angle of view of the thermal camera. Si3N4 rods similar to the rods used in LAMC trials were heated to a uniform temperature. First, a 1×1 inch silicon nitride solid rod was heated uniformly to the temperature around 600 °C using a furnace. Then the top view of the rod was observed with the thermal camera and a thermal image was taken. Using the post-processing software that came with the camera, the image was analyzed by observing how the thermal profiles of a line drawn across the rod varied. Figure 3.14 shows the thermal image of the Si3N4 rod with two lines drawn across at different areas of the rod, and the thermal profile of the better of the two lines, Line 2, respectively. The profile of “Line 2” indirectly shows how the temperature decreases by moving away from the normal view, which is around the pixel 79 on the abscissa.

Figure 3.14: Thermal image of the 1×1 inch Si3N4 rod with lines (left) and the temperature profile of Line 2 (right).

45 The thermal profile of Line 2 in Figure 3.14 validates to some extent the theory stated by Figure 3.13. As we move away from the center of Line 2, we are actually moving away from the normal view. The temperatures at the ends of Line 2 on the rod represent the temperature readings at a 90 degree angle of view from the normal, but not the temperature of the rod there, because we know for sure that the rod’s entire surface had uniform temperature. It can be noted that the temperature reading drops steeply when the angle of view approaches 90 degrees from the normal. Similar tests were done at 350, 470, and 740 °C to encounter an average loss of temperature reading of about 3.8 % for a viewing angle of 45 degrees. The estimation of the average loss was obtained from pictorial analysis of the temperature–pixel graph obtained in each case. The region of each graph within 45 degrees on either side of the normal was chosen and the drop in temperature value from 0 to 45 degree was computed, which resulted in the above average loss in temperature reading. The extrapolation is based on the trend in how the emissivity changes with temperature. The emissivity results suggest that the change in the emissivity of Si3N4 decreases with increasing temperature. The emissivity drops by 0.6 from 100 ºC to 286 ºC. But the emissivity drop from 336 °C to 501 °C is only by 0.3. Based on this inference, the emissivity for the higher temperatures has been set as shown in Table 3.6. Si3N4 being a dark-colored dielectric, it is anticipated that the emissivity does not drop to very low value (like 0.4) at any temperature.

46 Table 3.6: Temperature compensations made for losses due to different factors. Temp. Reading at Emissivity =0.95 °C

Actual Temp. Estimation (emissivity)

% Increase Emissivity (ε) Compensation

% Increase Angle of View Compensation

100 117(0.74) 17 3.8 286 351(0.68) 22.72 3.8 336 418(0.68) 24.4 3.8 501 639(0.65) 27.5 3.8 537 684(0.65) 27.3 3.8 614 813(0.64) 32.4 3.8 738 1009.3(0.62) 36.76 3.8 854 1192(0.61) 39.6 3.8 1017 1452(0.59) 42.8 3.8 1023 1481(0.60) 44.8 3.8 1090 1578(0.60) 44.8 3.8 Note: Colored figures are guessed extrapolations

Increase Distance Compensation

Net % Increase

1°C/m 5 °C/m 5 °C/m 5 °C/m 5 °C/m

20.8 26.52 28.2 31.3 31.1 36.2 40.56 43.4 46.6 48.6 48.6

4. RESULTS 4.1 Laser Heating Results

The results of the entire laser preheat trials are represented in the form of Tables 4.1 to 4.4 whose cells consist of the maximum temperatures obtained for the different parameters considered. There are three sets of results: Series 1 and 2 correspond to the debut trials done at the M3 Lab and the RTM respectively, which were conducted before any calibrations were made. However, appropriate compensations were made based on the calibration results (see Table 3.6) for any IR losses encountered. Series 3 correspond to the latest laser preheat trials done at the M3 Lab (in May 2009) where the calibration results were used to compensate for the IR losses during the laser preheat trials. Four different power levels (100, 150, 170 and 250 W) were chosen considering the power capabilities of either laser system at the M3 Lab and RTM; and the probable power levels required in the process of LAMC. In the same way, the turning speeds chosen for different sets of laser preheat trials are based on the capabilities of the lathes used in the experiments. Since the main objective of this study is to understand the thermal effects of different relevant parameters in LAMC, any appropriate selection of the sets of parameters could be used for analysis.

48 In the Series 1 and 2 results (Tables 4.1 and 4.2), the values inside the brackets represent the maximum temperature values of the laser spots on the Si3N4 rod at different preheat times, recorded by the IR camera at the emissivity set point 0.95. The values outside the brackets show the corrected values after accounting for the obtained emissivities (at different temperatures) and the angle of view based on the Table 3.6. Some cells in the series results tables are blank since the data for those cells was not obtained owing to some practical difficulties and/or unimportance. The color shaded cells are the ones of interest which have temperatures above 800 °C. It should be noted that in the Series 1 results the spot sizes were known to be greater than 2 mm and 3 mm respectively, although the exact values are not known. For this reason, there was a necessity to conduct similar kind of trials with known spot sizes. Hence, the other two sets of laser preheat trials (Series 2 and Series 3 results) were conducted whose data was primarily used in the analysis and discussion of this study. Series 2 results was obtained from the laser preheat trials conducted at RTM, whose equipment had the capabilities of a maximum of about 450 W power output and about 4000 rpm maximum turning speed. Since the laser system and the lathe at the M3 Lab had a maximum of 189 W laser final power output and 500 rpm maximum turning speed, Series 2 power/turning speed trials were performed at RTM. It can be noted that there is no data for 3 mm spot size as the focal distance to obtain 3 mm spot size was unknown.

49 Table 4.1: Series 1 results. 170 W Max. Temperatures (°C) 20sec

1 min

2 min

(1057) 1604 (627) (666) 859 919 (446) 561

(712) 989

(309) 392 (221) 274

(394) 512 (305) 387

(352) 457 (264) 330

3 min

4 min 5 min

Turning Laser Avg. Speed Intensity Spot (rpm) W/sq.mm Size 100

(755) 1064

(791) 1123

(806) 1144

250

< 54.11

>2 mm

< 24.05

>3 mm

500 (424) 551 (336) 430

(445) 578 (365) 470

(464) 603 (386) 497

100 250

Table 4.2: Series 2 results.

20sec

1 min

(300) 381 (258) 325 (235) 293 (198) 243

(430) 559 (355) 457 (290) 366 (270) 340

250 W Max. Temperatures (°C) (@ RTM) Laser Avg. Turning 2 3 Spot Intensity Speed min min Size (rpm) W/sq.mm (535) (610) 500 700 829 (430) (506) 750 559 664 79.57 2 mm (360) (425) 1500 462 552 (325) (370) 3000 416 477

6 min

12 min

(840) 1201

(880) 1261

50 Table 4.3: Series 3 results. 100 W Max. Temperatures (°C) 20sec

1 min

2 min

3 min

4 min

5 min

695

752

789

853

876

901

334 156

377 201

414 226

432 260

453 277

490 300

420 313 134

460 364 203

528 412 263

540 479 291

559 485 318

595 512 351

Turning Intensity Speed (Avg.) (rpm) W/sq.mm 100 31.8 250

Laser Spot Size 2 mm

500 100 250 500

14.14

3 mm

150 W Max. Temperatures (°C) 20sec

1 min

2 min

3 min

4 min

5 min

900 535 424

973 622 508

1009 687 564

1065 780 605

1133 800 693

1117 881 688

681 447 347

891 600 452

1,034 1,066 1,161 1,189 675 750 800 885 557 622 665 673

Turning Intensity (Avg.) Speed (rpm) W/sq.mm 100 47.7 250 500 100 250 500

Laser Spot Size 2 mm

21.22

3 mm

60.16

2 mm

26.73

3 mm

189 W Max. Temperatures (°C) 939 716 472

669 824 554

573 916 616

976 692

996 722

1061 791

100 250 500

919 565 428

1036 647 514

1135 724 577

1183 791 621

1247 828 680

1291 876 710

100 250 500

51 Series 3 results are more complete when compared to the other series. However the row of 189W-100 rpm-2 mm set of parameters is incomplete, which shows a decreasing trend of temperature with increasing time. Therefore, this particular set of trials was repeated to confirm the trend. The results of the repeated trials are presented in Table 4.4.

Table 4.4: Series 3 results: repetition of 189W – 2 mm spot trials. 189 W Max. Temperatures (°C) 20 sec

1 min

2 min

3 min

4 min

5 min

1473

1116

835

804

924

1015

Turning Avg. Intensity Speed W/sq.mm (rpm) 100 60.16

701

1300 1442 1510 1670 838 938 1063 1091

1733 1167

Laser Spot Size

2 mm

250 500

The first aspect that can be noticed in the repeated trials is the differences in the magnitudes of the temperatures when compared to the previous corresponding trials. This indicates poor repeatability of the laser preheat trials, which was experienced on many occasions. The key factors that seem to influence the repeatability of any laser preheat trials are the environmental factors like humidity, etc., at the time of the trial. The reason for the anomalous decreasing trend is attributed to the inaccurate measurement of temperature by the IR camera due to the

52 barrier of the smoke that was being released from the Si3N4 material. Simultaneously the groove formed upon material removal must have made the laser go out of focus, which favors the decreasing trend. Figures 4.1 and 4.2 are the IR images for the repeated 189 W-100 rpm-2 mm (Table 4.4) set of parameters at the specified times, showing the smoke (Figure 4.1) and the groove formed (Figure 4.2).

Figure 4.1: Temperature distribution of the Si3N4 rod at 189 W (100rpm, 2 mm spot) around 10 sec.

Figure 4.2: Temperature distribution of the groove (brightest) at 189 W (100rpm, 2 mm) around 5 min.

53 Figures 4.3 compares the thermal images of the Si3N4 rod for approximately 2 mm and 3 mm laser spot size trials not shown in any of the series results. The lines shown are drawn to obtain the thermal profiles along the widths (line profiles) of the laser spots across the turning direction. Either line passes through the maximum centre temperature of each laser spot.

Figure 4.3: Temperature axial and circumferential distribution at 170 W, 100rpm, approx. 2 mm (left) and 3 mm (right) laser spot sizes.

54 It can be noticed in Figure 4.4 that the temperature profile in the case of the approximate spot size 2 mm has very sharp temperature gradients within the central two millimeter region, which is undesired in LAM. Steep temperature gradients at high temperatures might induce microthermal stresses in the workpiece, leading to microcracks. Therefore, small laser focal spots should be avoided unless it is necessary.

170W Laser Preheat Temp. Comparison Along Width with Change in Spot Size 1100

2.91, 1034.01

1000

Temperature (Celsius)

900 800

2 mm

700

3 mm

600 500

2.91, 283.24

400 300 200 100 0 0

1

2

3

4

5

6

Approx. Distance (mm)

Figure 4.4: The thermal profiles of the lines drawn across the turning direction (shown in Figure 4.3) for the laser spot sizes approx. 2 mm and 3 mm respectively.

55 4.2 Effect of Bulk Preheating

Bulk preheating is the process of heating the entire workpiece to a desired uniform temperature using an external heating source prior to laser preheating. The idea of bulk preheating the workpiece before laser preheating is to improve the laser absorption, especially for large-mass workpieces. An example of such an advantage may be achieving higher temperatures than in the case without bulk preheating. Bulk preheating (to 100 °C) of the Si3N4 rod was done with the help of the strip heater shown in the Figure 3.6. The rod was made to rotate at 100 rpm turning speed above the hot strip heater around 500 °C, for uniform heating of the rod. The strip heater was turned off once the rod reached the desired temperature of 100 °C, following which a laser preheat trial was made. Unfortunately, bulk preheating did not achieve significantly higher temperatures. Figure 4.5 compares a 170 W-3 mm spot size-100 rpm bulk preheat trial with a similar trial where bulk preheating was not done before the laser was fired onto the rod. Similar trends were obtained for different bulk preheat temperature trials at different laser parameters and turning speeds, which concluded that bulk preheating was not advised in the current LAMC process.

56 170W, 3 mm Spot Size Temp. Distribution Comparison With and Without Bulk Preheating 400


350

3 mm

300

3 mm Bulk

250 200 150 100 50 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Approx. Distance (mm)

Figure 4.5: Comparison of maximum temperatures achieved by a bulk preheat trial and only laser preheat trial.

5. GRAPHICAL REPRESENTATION OF THE EXPERIMENTAL RESULTS AND ANALYSES 5.1 Temperature vs. Time Graphs

The temperature–time graphs help us understand the effect of turning speed, laser spot size, etc. Even though not all graphs are shown in this work, the important ones are discussed. The specifications of the graphs can be found in their titles. Figures 5.1–5.7 show the effect of turning speed for each of the laser power output (W) and spot sizes (mm) considered. The average intensities are shown in the brackets in the header titles.

58 Temp. vs time- Effect of RPM 100W, 2mm [31.8W/sq.mm]

1400


1200

1000

100 rpm 800

250 rpm 600

500 rpm 400

200

0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)

Figure 5.1: Temperature–time graph for 100 W, 2 mm laser spot size.

Temp. vs time- Effect of RPM 100W, 3mm [14.14W/sq.mm]

1400

1200


100 rpm 1000

250 rpm 800

500 rpm 600

400

200

0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)


59



1400 1200

100 rpm 1000

250 rpm 800

500 rpm 600 400 200 0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)


Temp. vs time- Effect of RPM 150W, 3mm [21.22W/sq.mm] 1400

100 rpm


1200

250 rpm 1000

500 rpm

800 600 400 200 0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)


60



1400 1200 1000 800

100 rpm

600

250 rpm

400

500 rpm 200 0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)




1400 1200 1000

100 rpm 800

250 rpm

600

500 rpm

400 200 0 0

30

60

90

120

150 180

210

240

270 300

330

Time (s)


61

Temp. vs time- Effect of RPM 250W, 2mm [31.8W/sq.mm] 1400


1200

1000

500 rpm

800

750 rpm 1500 rpm

600

3000 rpm

400

200

0

0

50

100 150 200 250 300 350 400 450 500 550 600 650 700 750

Time (s)

Figure 5.7: Temperature–time graph for 250 W (at RTM), 2 mm laser spot size.

62 5.2 Effect of the Laser Spot Size at 500 and 250 rpm Turning Speeds at Three Laser Power Outputs (Figures 5.8 and 5.9)

Temp. vs time- Effect of Spot Size 500 rpm ; at 100 W, 150 W & 189 W

1400

100W, 2mm


1200

100W, 3mm 1000

150W, 2mm

800

150W, 3mm

600

189W, 2mm 189W, 3mm

400 200 0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)

Figure 5.8: 500 rpm turning speed, for three laser power outputs 100, 150 and 189 W.

63 Temp. vs time- Effect of Spot Size 250 rpm; 100, 150, 189 W 1400

100W, 2mm


1200

100W, 3mm

1000

150W, 2mm 800

150W, 3mm 600

189W, 2mm 400

189W, 3mm 200

0 0

30

60

90

120

150

180

210

240

270

300

330

Time (s)

Figure 5.9: 250 rpm turning speed, for three laser power outputs 100, 150 and 189 W.

Analyzing a second-order curve fit equations for the temperature–time graphs shown in Figures 5.1–5.9 will give a simple mathematical understanding of the temperature–time trends. Table 5.1 shows the second-order curve fit equations and their first derivatives of the temperature–time trends represented in the Series 3 results. Since the corresponding R2 values shown are greater than 0.95, higher order curve fits were not taken into consideration. Moreover, if the order of an equation is raised, it becomes more difficult to pursue a mathematical analysis.

64

Table 5.1: Second-order curve fit equations (250 W) and first derivatives for Series 2 results (Table 4.2). T = Max. temperature (°C) t = time (s)

R2 for Curve Fit

Turning Speed (rpm)

T = – 0.003t2 + 3.48t + 327.95 DT/dt= – 0.006t + 3.48

0.9963

500

T = – 0.0057t2 + 3.1956t + 270.36 DT/dt= – 0.0114t+3.1956

0.9927

750

T = – 0.0014t2 + 1.8855t + 256.44 DT/dt= – 0.0028t + 1.8855

0.9999

1500

T = – 0.0056t2 + 2.5505t + 198.28 DT/dt= – 0.0112t + 2.5505

0.9950

3000

Avg. Intensity W/sq.mm

Laser Spot Size

79.57

2 mm

Similarly, second-order curve fit equations and their first derivatives were obtained for the results Series 1 and Series 3 results. Mathematically, DT/dt or heating rate at a given laser preheat time ‘t’ gives the differential change in the temperature ‘T’ per unit time at that instant. Since all DT/dt terms have negative coefficients for the independent variable ‘t’, it is evident that the rate at which the temperature increases with time, is falling. The reasons for these trends are expected as the dissipation of heat is anticipated to be increasing with time. Figure 5.10 shows DT/dt values versus time for Series 2 results (Table 4.2). Similar graphs were obtained for Series results 1 and 3. Further analysis of the

65 temperature–time trends through mathematical formulation will be discussed in the following sections.

Differential of Temp. vs time- Effect of RPM 250W, 2mm 4 3.5 3

DT/dt

2.5 2

500 rpm

1.5

750 rpm

1

1500 rpm

0.5

3000 rpm

0 -0.5

0

30

60

90

120

150

180

210

240

270

300

330

-1 -1.5

Time (s)

Figure 5.10: DT/dt versus time for Series 2 results (see Table 4.2).

A negative value of DT/dt means that the temperature drops with time. All the above curves may be good for time values below 300 s, but only positive DT/dt values should be considered.

66 5.3 Mathematical Formulation

The purpose of this mathematical formulation is to relate the maximum temperatures obtained for the laser preheat trials with the corresponding laser input energies. A temperature–energy relationship is not only a necessary tool to understand the laser preheating process in LAMC but also helps predict some thermally efficient laser/machining parameters. The approach of the following derivation is to compute the input laser energy impinging on a unit circular area (sq.mm) in the given real laser preheat time ‘t’. So the first step necessary would be computing the surface velocity ‘V’ of the Si3N4 rod, which is a function of turning speed. Based on the surface velocity, the time taken (te) by a point on the rod to traverse through a distance of laser spot size ‘S’ is calculated. By multiplying ‘te’ with the laser power per square millimeter, it will give us the energy ‘E0’ absorbed by a unit circular square millimeter area while passing through the laser spot for one revolution. Henceforth, the total energy ‘Etotal’ (per square millimeter) absorbed by the rod in the given time ‘t’ is obtained by using the parameter ‘Nt’ as shown in the derivation.

67 Derivation

V = 2.π.r / tr

Nomenclature:

(Units: mm, s, J)

te = S / V = 30S / π.r.R

V = surface velocity

E0 / mm2 = te × LP / mm2

r, D = radius, diameter of the rod

Etotal / mm2 = Total energy input on a unit circular area in real time‘t’

R = rpm S = laser spot size

2

= E0 / mm × Nt

tr = time / revolution = 60 / R te = exposure time / revolution

2

Etotal / mm = 4×LP×t π2S.D 2

Etotal / mm = K × t where K = 4×LP π2S.D

(Eq. 5.1) LP = avg. laser power output

(Eq. 5.2)

E0 = laser energy input / 1 time exposed E0 / mm2 = E0 on a mm2 circular area on the rod. Nt = total no. of times exposed in time‘t’ = t × tr

Notice that ‘Etotal / mm2’ in Eq.5.1 is independent of turning speed, reflecting conservation of energy. Regardless of the turning speed, the input laser energy onto the rod is constant per unit time for a given power output. ‘Etotal / mm2’ is proportional to time, where the constant of proportionality ‘K’ in Eq. 5.2 is a function of laser power, laser spot size and diameter of the workpiece.

68 Therefore, it is understood that to supply a given energy input on the rod in time ‘t’, one or all of the three parameters in ‘K’ can be modified. This is the main advantage and purpose of the derivation, which would lead to the temperature– energy relationship after substituting ‘t’ in terms of energy in the temperature–time equations. To say in other words, the temperature–energy relationships can be obtained by substituting ‘E/K’ for ‘t’ in the second order curve fit equations for the Series 1, 2 and 3 results. Thus, the terms of ‘DT/dE’ can be obtained as the first derivatives of the second-order temperature–energy equations. A DT/dE term is the value of the differential change in temperature per unit energy absorbed per unit square millimeter area on the rod, at a particular energy value. Therefore, DT/dE is what we can call thermal efficiency in terms of laser input energy per unit area. Figures 5.11–5.17 show how thermal efficiency varies with energy input for 2 mm and 3 mm laser spot sizes at different laser power outputs. (All units in mm, s, °C, and J .)

69

DT/dE vs Energy 100W, 2mm

DT/dE

1.4 1.2

100 rpm

1

250 rpm

0.8

500 rpm

0.6 0.4 0.2 0 0

50

100

150

200

250

300

Energy / sq.mm

Figure 5.11: Thermal efficiency vs. energy/sq.mm for 100 W, 2 mm laser spot size.

DT/dE vs Energy 100W, 3mm 1.4 1.2

100 rpm 1

DT/dE

250 rpm 0.8

500 rpm

0.6 0.4 0.2 0 0

20

40

60

80

100

120

140

160

180

Energy/sq.mm


70 DT/dE vs Energy 150W, 2mm 1.8 1.6 1.4

100 rpm

DT/dE

1.2

250 rpm 1

500 rpm 0.8 0.6 0.4 0.2 0 0

50

100

150

200

250

300

350

400

Energy/sq.mm


DT/dE vs Energy 150W, 3mm 4 3.5

DT/dE

3 2.5

100 rpm

2

250 rpm 500 rpm

1.5 1 0.5 0 0

50

100

150

200

250

300

-0.5

Energy/sq.mm


71 DT/dE vs Energy 189W, 2mm 25

20

DT/dE

15

100 rpm 250 rpm

10

500 rpm 5

0 0

50

100

150

200

-5

250

300

350

400

450

500

Energy/sq.mm

-10

Figure 5.15: Thermal efficiency vs. energy/sq.mm for 189 W, 2 mm laser spot size. The anomalous trend for the 100 rpm is due to the material evaporation and inaccurate temperature reading (see Figures 4.1 and 4.2).

DT/dE vs Energy 189W, 3mm 2.5

2

DT/dE

100 rpm 1.5

250 rpm 500 rpm

1

0.5

0 0

50

100

150

200

250

300

350

Energy/sq.mm


72 DT/dE vs Energy 250W, 2mm 4

500 rpm

3.5

750 rpm 3

1500 rpm

2.5

3000 rpm

DT/dE

2 1.5 1 0.5 0 0

100

200

300

400

500

600

700

-0.5 -1 -1.5

Energy/sq.mm


5.4 Efficiency vs. Turning Speed (rpm) for Different Laser Power Outputs and Spot Sizes

So far, the variation of thermal efficiency is seen versus energy per square millimeter, which does not compare efficiency and turning speed exclusively. In order to discuss the effect of turning speed on thermal efficiency, it is required to understand how efficiency changes with turning speed for different laser parameters. Figures 5.18–5.25 are the heating efficiency–rpm graphs for 2 mm and 3 mm laser spot sizes and for different laser power outputs, shown at four different laser preheat times 1, 2, 3 and 4 minutes respectively.

73 DT/dE vs RPM At 1 min 3.5 3

100W, 2mm 150W, 2mm

2.5

DT/dE

100W, 3mm 2

150W, 3mm 189W, 3mm

1.5 1 0.5 0 0

100

200

300

400

500

600

RPM

Figure 5.18: Thermal efficiency vs. rpm for three power outputs, 2 mm laser spot size, at 1 minute.

DT/dE vs RPM 250W, 2mm, At 1 min 3.5 3

DT/dE

2.5 2 1.5 1 0.5 0 0

500

1000

1500

2000

2500

3000

3500

RPM

Figure 5.19: Thermal efficiency vs. rpm for 250 W, 2 mm laser spot size, at 1 minute.

74 DT/dE vs RPM At 2 min 3.5

3 2.5

DT/dE

100W, 2mm 150W, 2mm

2

100W, 3mm 1.5

150W, 3mm 189W, 3mm

1 0.5

0 0

100

200

300

400

500

600

RPM

Figure 5.20: Thermal efficiency vs. rpm for three power outputs, 2 mm laser spot size, at 2 minutes.

DT/dE vs RPM 250W, 2mm, At 2 min 3.5 3

DT/dE

2.5 2 1.5 1 0.5 0 0

500

1000

1500

2000

2500

3000

3500

RPM

Figure 5.21: Thermal efficiency vs. rpm for 250 W, 2 mm laser spot size, at 2 minutes.

75 DT/dE vs RPM At 3 min 3

2.5

2

DT/dE

100W, 2mm 150W, 2mm

1.5

100W, 3mm 150W, 3mm

1

189W, 3mm 0.5

0 0

100

200

300

400

500

600

RPM


DT/dE vs RPM 250W, 2mm, At 3 min 3

2.5

DT/dE

2

1.5

1

0.5

0 0

500

1000

1500

2000

2500

3000

3500

RPM


76 DT/dE vs RPM 2mm, At 4 min 3

2.5

100W, 2mm

DT/dE

2

150W, 2mm 1.5

100W, 3mm 150W, 3mm

1

189W, 3mm

0.5

0 0

100

200

300

400

500

600

RPM


DT/dE vs RPM 250W, 2mm, At 4 min 3 2.5

DT/dE

2 1.5 1 0.5 0 0

500

1000

1500

2000

2500

3000

3500

-0.5

RPM


6. DISCUSSION AND CONCLUSIONS 6.1 Discussion

The major discrepancy that can be noticed when the results from the numerical model and the experimental trials are compared, is the difference between the magnitudes achieved during the laser preheating. For example, the maximum temperature shown in Figure 3.3 is much higher than that of any of the experimental laser preheat trials conducted for the same range of parameters. The primary explanation for such difference is that the model has a much higher resolution (grids) than that of the IR imaging devices used in the experiments. An appropriate modification(s) that can be made to compare experimental results with the model will be discussed in the “Future Work” section of this chapter. Numerical models can be modified by using different mesh sizes and distributions to yield different maximum temperatures for a given set of laser and machining parameters. But it is very difficult to measure the temperatures achieved during experimental laser preheat trials because of the resolution issues with any IR instrument. While pyrometers record weighted average temperatures in their fields of view, IR cameras read the average temperatures recorded by the pixels allocated to a specific area on the target. Hence, for targets having very steep temperature gradients

78 like that of LAMC applications, it is very hard to measure the temperatures accurately with a finer resolution. The temperature–time graphs and graphical analyses in this thesis work give an understanding of the maximum temperature dependence on the laser parameters and turning speed, versus time.

6.2 Conclusions

1. Effect of bulk preheating: Bulk preheating the Si3N4 ceramic workpiece does not have a significant influence on the maximum temperatures achieved in a LAMC laser preheat trial (Figure 4.5). The maximum temperature increases only by almost the initial bulk preheat temperature of the workpiece, when compared to a trial with out bulk preheat. The numerical model also suggests that bulk preheating does not have significant effect on the maximum temperatures achieved (see Section 3.1). 2. Effect of laser spot size: The experimental maximum temperatures measured for 2 mm and 3 mm laser spot size preheat trials suggest that the temperatures are close, unlike suggested by the numerical modeling (see Section 3.1). This trend is favored (or holds better) for extended preheat times, lower power outputs, and higher turning speeds (Figures 5.8 and 5.9).

79 3. Effect of turning speed: An increment in turning speed would lead to a decrement in the maximum temperatures achieved during a laser preheat trial. Among the three turning speeds – 100, 250, and 500 rpm – the temperatures achieved (versus time) for the 250 rpm are closer to those of 500 rpm than they are to those of 100 rpm (Figures 5.1-5.6). Similar effect can be observed for the turning speeds 500, 750, 1500, and 3000 rpm (Figure 5.7). The effect of turning speed reduces at higher turning speeds. 4. Effect of laser power output: Higher laser power outputs yield higher temperatures but the effect of power output diminishes for extended preheat times (due to steady state) and higher turning speeds (higher heat dissipation). o Higher power outputs when combined with lower turning speeds should be avoided as the effect can evaporate the material leading to inaccurate temperature readings. For example, the 189 W output and 100 rpm trial in the Series 3 results (Table 4.4) formed an undesired groove on the workpiece (Figure 4.2). 5. Laser heating efficiency (DT/dE): Efficiencies for 250 and 500 rpm turning speeds are very close (at 100 and 189 W power output, both spot sizes. See Figures 5.11 to 5.16). o This implies that the rate of heat losses (versus time or energy input) for turning speeds 250 and 500 rpm are almost equal. Therefore,

80 either rpm speeds will yield almost the same temperature increment in a given time. o This does not mean that either turning speed yields the same maximum temperature. o As expected, the efficiency for the 250 W trial (Series 2 results) is higher for 500 rpm turning speed than for 750, 1500, 3000 rpm turning speeds, but D2T/ dE2 for the 500 rpm trial is lower than only that of 1500 rpm turning speed (Figure 5.17). This implies that the loss in the efficiency (versus time or energy input) for the 500 rpm trial is lesser than that of 750 and 3000 rpm turning speeds. Hence, there may not be a fixed trend on how the efficiency is affected with increase in turning speed (Figures 5.18 - 5.25). This trend leads to Recommendation 1 in Chapter 7 based on good thermal efficiency for 500 rpm turning speed.

7. RECOMMENDATIONS AND FUTURE WORK 7.1 Recommendation(s) of Parameters for LAMC

Assuming that the laser spot center is very close to the edge of the workpiece and that the laser–tool distance is very small, the following laser parameters and turning speeds are recommended for very slow longitudinal feeds based on the series results and all the graphical analyses. The goal is to reach around 1000 °C workpiece (Si3N4) temperatures close to the machining zone (see material removal plane in Figure 3.2) while achieving faster machining parameters and lower laser power outputs. 1. Recommendation(s) for LAMC (single-beam turning) at RTM for 1×6 inch Si3N4 rod: o Laser output power > 200 W, 2 mm laser spot, turning speed around 500 rpm (Figures 5.18, 5.20, 5.22, 5.24), at least 4 min of laser preheat time. Comment: 750 and 3000 rpm turning speeds may not work as the steadystate temperature is below 1000 °C; 1500 rpm turning speed may work for very extended preheat period.

82 2. Recommendation(s) for LAMC (single-beam turning) at the M3 lab, NIU, for 1×6 inch Si3N4 rod: o Laser output power 189 W, turning speed 250 rpm. The laser preheat time is about 4 min for 2 mm laser spot and about 7 min for 3 mm laser spot size. o For a constant depth of cut and constant faster longitudinal feeds, a second laser beam positioned at a different position than the first beam is necessary for continuous cuts.

7.2 Future Work

1. The current work focused on the analysis of temperature–time relationship and relates the maximum temperatures to the energy input versus time. But in the future, a complete mathematical formulation can be developed by adding appropriate energy dissipation term Dp (see Eq. 5.1) as shown in Eq. 5.3. By doing so, the maximum temperatures for a given set of laser/machining parameters can be predicted.

Etotal / mm2 = 4×LP×t - Dp π2SD

Eq. (5.3)

The dissipation term may strongly depend on the forced convection due to the turning speed (rpm) of the workpiece and the thermal mass of the workpiece itself. One of the approaches to find the effect of convection is to find the rate of

83 decrease of temperature immediately after the laser beam turns off in a preheat trial. The cooling effect can be studied for various laser preheat temperatures and turning speeds. 2. The mathematical formulation discussed in this work can be extended to different diameter workpieces. For example, if the dissipation term in Eq. 5.3 is ignored, it can be noticed that doubling the diameter (D) of the workpiece would require doubling the laser power (LP) to have the same energy input, for a given set of other parameters. Hence, the correctness of such theoretical inferences can be checked through practical studies. 3. Estimation of Tmr through Tmax : In the experimental LAMC, the temperature at the machining zone (Tmr) where the tool makes contact with the workpiece needs to be maintained around 1000 °C for Si3N4. Hence Tmr can be estimated using the maximum laser spot temperature by studying the axial and circumferential temperature distribution as shown in Figures 4.3 and 4.4. For the depth of cut (DOC) determination, the depth of 1000 °C isotherm is necessary. 4. Multi-beam LAMC temperature study: Similar temperature–time studies and graphical analyses can be done with a two-beam LAMC system, where suggestions from a numerical model seem more important due to the addition of an extra parameter – positioning distance/angle of the two beams relative to each other and the cutting tool. 5. Since there is limitation on the resolution sensitivity of any IR camera in reading the maximum temperature of a laser spot on workpiece, the numerical modeling

84 can be modified to fit the resolution of the IR camera used. Such a modification helps to directly compare the modified model with the experimental results.

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