This will be followed by a brief presentation of. ArcHLM, an arc welding based HRM process under development at IIT Bombay. 2. Hybrid Rapid Manufacturing.
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Hybrid Rapid Manufacturing of Metallic Objects K.P. Karunakaran1, S. Suryakumar1 and Alain Bernard2 1
Indian Institute of technology Bombay, India Ecole Centrale de Nantes, France
2
While CNC machining, the subtractive method, is the only option when it comes to high quality components, the need for human intervention to generate the CNC programs makes it a slow and costly route. On the other hand, Rapid Prototyping (RP), the additive method, is able to convert the design into the physical objects without any human intervention but, its total automation comes with compromises in the qualities of geometry and material. A balance between these two extremes is Hybrid Rapid Manufacturing (HRM). In HRM, the near-net shape of the component is built in layers (additive method) and the same is finish-machined (subtractive method). While the priority during material addition is material integrity and speed, the same is on geometric quality and speed during material subtraction. The existing HRM processes for metallic objects are reviewed in this paper followed by a brief description of ArcHLM under development at IIT Bombay. Keywords: Rapid Prototyping, Rapid Casting, CNC machining, Visibility.
1. Introduction Automation has two parts, one is the process automation and the other is the process planning automation. CNC machining has only automation of the process and its process planning still requires extensive human intervention. Computer-Aided Process Planning (CAPP) is an active area of research to automate process planning but it had little success for subtractive manufacturing. CAPP became successful for additive manufacturing in a layer-by-layer manner which led to the development of a number of Rapid Prototyping (RP) processes since 1987. RP can produce objects directly from their CAD models without the use of any tooling specific to the geometry of the objects being produced. RP adopts a divide-and-conquer approach in which the complex 3D object is split into several 2D slices that are simple enough to automatically manufacture. As the object grows from bottom up, the chances of collisions are eliminated. RP revolutionized the way products are designed and manufactured today. Its ability to realize conformal cooling channels and gradient objects are its most significant applications. RP cuts down product development time and Rapid Tooling (RT) cuts down productionizing time. Therefore, Rapid Prototyping & Tooling (RP&T) is an effective tool where time to market matters. However, use of RP&T is still limited to the manufacture of only prototypes, mostly of non-metallic materials. The dream of manufacturing engineers today is to extend the total automation to functional and full-life components. In other words, RP is evolving into Rapid Manufacturing (RM). The following bottlenecks are the hindrance to this evolution:
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Poor quality Short life Long cycle time High cost.
Figure 1 Classification of RM Processes
Slicing that contributed to RP's total automation is responsible for these problems too. Today’s rapid prototypes leave much to be desired in terms of the quality of geometry (accuracy and surface finish) and material (strength, variety, homogeneity, size, proprietary nature etc.). One can produce parts out of any material on a CNC machine by using appropriate cutting parameters. On the other hand, if a new material is to be used on a RP machine, elaborate experimentation is required to fine-tune the process for the new material. Due to this as well as the commercial interests, the raw materials for all RP machines are proprietary and have limited shelf life. Furthermore, all RP processes inherently exhibit anisotropy. Due to these quality restrictions, rapid prototypes have poor life. The cycle time of a production part through the use of appropriate tooling is orders of magnitude shorter than that of the prototype. Therefore, existing RP processes are unacceptably slow and costly for regular production. Furthermore, in order to cater to the numerous combinations of materials, quality levels, production volumes, cost sensitivity etc., a unique strategy like slicing may not be sufficient for RM; it will require multi-faceted and hybrid approaches. An elaborate classification of these RM processes is given in Figure 1. All these approaches to RM fall into the following six groups of technologies for metallic objects [1]: i. ii. iii. iv. v.
CNC machining Laminated Manufacturing Powder-bed technologies Deposition technologies Hybrid technologies
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vi. Rapid Casting technologies. The first is purely subtractive and the next three are purely additive processes. Hybrid RM combines the benefits of additive and subtractive routes. It combines the second, third or fourth with the first. These first five technologies are direct RM and the last is indirect. The status of Hybrid RM processes will be reviewed. This will be followed by a brief presentation of ArcHLM, an arc welding based HRM process under development at IIT Bombay.
2. Hybrid Rapid Manufacturing Machining has been integral to enhance the accuracy of the prototypes in a few popular RP processes like Laminated Object Manufacturing (LOM), Solid Ground Curing (SGC) and Sander’s ModelMaker II. Therefore, the hybrid approach is not new to RP. In Hybrid Rapid Manufacturing (HRM), the metallic object is realized in the desired geometric and material qualities by a combination of additive and subtractive processes. The additive process focuses on speed while ensuring the desired material integrity. The resulting object is only near-net as no attention to the geometric quality is paid at the time of building it in layers. The inherently fast CNC machining, the subtractive process that follows, ensures the desired geometric quality. As the focuses in both these steps are different, they are individually very fast. One may subject the near-net shape to stress relieving or heat treatment as required before finish-machining. As mentioned earlier, the layered building of the near-net shape can be achieved through (i) Laminated Manufacturing, (ii) a powder-bed technology or (iii) a deposition technology. These three groups of HRM processes are discussed in the following sub-sections. It may be noted that only very few processes exist in which the subtractive manufacturing is integrated seamlessly with the additive manufacturing on the same platform.
2.1 Hybrid RM Using Laminated Manufacturing and CNC Machining All the Laminated Manufacturing processes inherently are hybrid since each laminate undergoes machining before its joining. Even before the advent of RP, Nakagawa had used this concept to build tools for sheet metal manufacture [2]. These Laminated Tools were realized by tying the laminates using long struts (Figure 2). The laser is used not only for cutting but also for hardening the edges through laser quenching and engraving the details like layer number [3]. The raw material utilization of the laminated dies for blanking and punching is almost 100% and tying of the laminates is adequate for these dies. The thickness of these laminates is about 0.5mm. For obvious reasons, only cut-then-paste approach is suitable for laminated tools as against the paste-then-cut approach of LOM. As injection molding, pressure die casting or forging involves liquid or semi-solid material which may seep inbetween the laminates, tying the laminates is not suitable for these applications. Therefore, there have been many attempts to develop other joining methods such as adhesive bonding, brazing, ultrasonic welding and diffusion bonding [4, 5]. Efforts were also made to minimize the stair step effect through 4-axis machining/laser cutting of the edges of the laminates or finish-machining the assembled dies [6, 7]. However, only case studies have been reported and no integrated machine for manufacturing
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these laminated tools has been commercialized. Apart from laminated metallic tools and objects, automated/semi-automated machines based on Laminated Manufacturing to make wooden furniture and Expanded Polystyrene (EPS) patterns/models have also been reported. These nonmetallic objects can be used as patterns in a suitable casting process to obtain the metallic objects.
Figure 2 Laminated Dies for Punching and Blanking [3]
(a) Integrated machine for Ultrasonic Consolidation
(c) A die pair made using Ultrasonic consolidation
(b) Ultrasonic Consolidation process
Figure 3 Ultrasonic Consolidation [8]
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A recent significant process of this category is Ultrasonic Consolidation (UC) developed by Solidica, USA [8]. It makes use of ultrasonic welding for joining the laminates. Hence, it can build dense tools useful for injection molding etc. This process has substantial material saving as it uses a metallic tape of 25mm width and 0.150mm thick to construct the required layer rather than cutting it from a large rectangular sheet. UC is available as an integrated machine which is a milling machine fitted with the ultrasonic tape cladding system on its spindle head along with the necessary software for creating the NC program for controlling both the tape cladding and the milling operations (Figure 3). Milling may be done after every layer or after a few layers depending on the nature of the contour. An advantage of UC over deposition technologies is its ability to build overhangs due to the adequate stiffness of the metallic tape. The geometry obtained from this integral machine will be free from staircase error. UC has been found suitable for joining Al and Cu strips as well as making Metal MatriX (MMX) objects. Of late, UC is finding applications in seamless embedding of RF sensors for RFID applications. It also finds applications in satellites to embed electronics (wiring, sensors, fibers and other functional devices) within a fully dense aluminum structure.
Figure 4 Lumex 25, HLM Using a Laser-based Powder-Bed Technology and CNC Machining [1]
2.2 Hybrid RM Using Powder-Bed Technology and CNC Machining A powder-bed technology is a layered manufacturing process in which each layer is realized by first spreading a rectangular layer of powder and then joining the particles constituting the layer using a focused tool along an appropriate path [1, 9, 10]. This focused tool may be an energy source such as a laser (as in SLS), an electron beam (as in EBM) or simply an electric arc (no process so far) or a jet of liquid binder (as in 3DP). Most powder-bed technologies for metallic objects require post-processing in a furnace for removing the binder and completing the sintering. Another furnace process for copper-impregnation is required to close the voids closer to the boundary. Copper impregnation improves only the polishability essential in dies and molds and does not significantly contribute to its densification; densification can be achieved through Hot Iso-static Pressing (HIP). The size of the particles in the powder limits the layer thickness
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and hence the accuracy of the objects obtained using the powder-bed technologies. As a result, the metallic objects obtained through powder-bed technologies more often require finishmachining or at least polishing. In all these popular processes mentioned above, all the post-processing operations including machining or polishing occur at different stations and require extensive human intervention. To our knowledge, the only integral hybrid process based on a powder-bed technology is Lumex 25C, developed by Matsuura Machinery Corporation in Japan (Figure 4). It successfully combines a laser sintering technology and high-speed milling [1]. It uses a powder mixture of 90% steel and 10% copper and sinters it with a 300 to 500 W CO2 laser. The edges of the part are milled after sintering five layers. A surface roughness of as low as 15 µm can be obtained.
2.3 Hybrid RM Using Deposition Technology and CNC Machining In this group of hybrid RP processes, weld-deposition of metal is used to build the near-net object in layers and it is finish-machined. Stress relieving or heat treatment may be optionally included before finish-machining. Table 1 gives a comparison of the powder-bed and deposition technologies. Ability to build fully dense as well as gradient objects makes the deposition technologies more attractive. The only problem with the deposition technologies is the need for a sacrificial support mechanism. Table 1 Comparison of the Deposition and Powder-bed Technologies Deposition Technology Powder-bed Technology Explicit support mechanism is required Inherent support mechanism in the form of the remaining powder Very suitable for gradient objects Not suitable for gradient objects* Fully dense as melting is involved. Porous structure as the powder is not melted but only sintered. Examples: LENS, LAM, POM, … Examples: SLS, 3DP, Arcam * Exception is 3D Printing which is unique for material addition from two sources.
As depicted in Figure 1, both the deposition as well as the powder-bed technologies employ laser, electron beam or electric arc as the sources of thermal energy for sintering/melting, in the order of their present popularity. Table 2 gives a comparison of the three types of energy sources. Laser has been the most popular due to its ‘precision’. However, it has very poor energy efficiency (2-5%). Of late, electron beam is becoming popular for these applications due to its better energy efficiency (15-20%) but it requires high vacuum for the working environment [911]. Table 2 Comparison of the three energy sources Characteristic Laser Beam Source Photon Particle/Electro-Magnetic Wave (EMW) EMW Energy density Less (106 W/mm2) Efficiency Poor (2-5%) Vacuum Not required
Electron Beam Electron Not yet clear More (108 W/mm2) Good (15-20%) Required
Arc Metallic ion Particle Very high Excellent Not required
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Laser-based processes: Laser Engineered Net-Shaping (LENS) originally developed at Sandia National Laboratory, USA, and further developed and marketed by OptoMec, USA, is the most popular commercial RM process capable of handling a variety of metallic powders including Ti [12]. Its deposition head uses 1 kW or 2 kW laser which is at the centre. It is surrounded by 2 or 4 nozzles (Figure 5a). This head is mounted on a XYZ manipulator. When the head is moved over a substrate, it creates a moving weld pool into which the powders from the nozzles dive and get integrated. As the powder used is fine, fluidized feeding using Argon is employed. By moving the welding head along appropriate raster and contouring paths, the object is built in layers. It permits usage of different powders through different nozzles with the ability to control their flow rates independently. Thus, LENS is capable of building gradient objects. This machine also comes with other types of manipulators upto 5-axis capability. LENS has been successfully tested not only for fresh objects but also for repair of aerospace components (Figure 5b-d).
(a) LENS process
(b) A pair of dies made using LENS
(c) A turbine blade made using LENS (d) LENS used for refurbishing a turbine Figure 5 Laser Engineered Net Shaping (LENS)
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LENS is a pure additive process and hence produces only a near-net shape which requires finish-machining on a separate machine. Furthermore, building the near-net object is totally automatic only for the objects free from undercuts. LENS does not use any support mechanism. Therefore, objects with undercuts and intricate shapes have to be built by suitably orienting the substrate; however, the required complex motion planning for such cases have to be created by the user. Apart from poor energy efficiency, this and other powder-deposition processes also suffer from poor powder efficiency of 10-15% and slow deposition rates of 6-10 gm/min. There are several processes similar to LENS with subtle differences among them. LaserAugmented Manufacturing (LAM) developed by Aeromet, USA, was used successfully to build Ti components for Boeing [13]. The undercuts of the CAD model were suppressed and suitable machining allowance was added. This near-net shape was built used an 18kW laser on a 2.5 axis machine as shown in Figure 6a. This was stress relieved and heat treated and then finishmachined using a 5-axis CNC machine. It used this process to build large Ti components such as keels and spars (Figure 6b&c). Although this process could successfully produce the Ti components to serve the full life and technically proved as good as the machined components, it was a commercial failure which led to the closure of the company.
CAD model of a bracket
(b) Building a Ti keel
Its prepreg (a) A root fitting
Finished component
(c) Building a Ti spar
Figure 6 Functional Aerospace Components Made by AeroMet Corporation, Minneapolis
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Direct Metal Deposition (DMD) developed at Michigan University, USA and marketed through Precision Optical Manufacturing Inc. (POM) uses optical feedback to ensure the integrity of deposition [14]. Trumpf, a German laser company, adopted this process for their laser. This machine shown in Figure 7 is known as TrumaForm DMD 505. Its laser deposition head has a very high powder utilization of over 90%. It is mounted onto a 6-axis robot so that the cladding can be done not only in planar layers but also over a freeform surface. Apart from repair, this machine can be used for cladding and hard facing over a contoured surface.
Schematic Actual Figure 7 POM-Trumpf Laser Deposition Head
LENS(OptoMec) – Laser source
DLF(FhG, Dresden) – Laser
Arcam – EB
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LAM(AeroMet) – Laser
HLM(IIT Bombay) – Arc
Figure 8 Comparison of Surface Quality of RM Processes Using Laser, EB and Arc Sources
All these popular systems do not have material addition and subtraction occurring at the same platform and hence are not truly hybrid.
Electron Beam-based processes: Electron beam welding has been in use for very demanding nuclear and aerospace applications. It is more often a fusion joining process without filler carried out under high vacuum. Electron beam has not been tried so far for building the objects, perhaps due to the techno-commercial difficulties in producing the high vacuum and in incorporating supports. Arcam, Sweden, is the only electron beam based commercial process, but it is a powder-bed process. However, it is just a matter of time before more electron beam based processes of both powder-bed and deposition types emerge due to its attraction of higher energy efficiency over laser.
Arc-based processes: Figure 8 shows some metallic objects realized using different RM processes. As it is obvious from these figures, all of them produce only near-net shapes and rough surfaces. These cannot be used unless they undergo finish machining. Therefore, there are no major differences among laser, electron beam and arc welding in terms of the finish and material integrity. Arc welding has the added advantages of higher deposition rates, lower costs and safer operation. Deposition rate of laser or electron beam is of the order of 2-10 g/min, whereas deposition rates of 50130g/min have been reported in arc-based RM [15] and it can reach as much as 800g/min with proper heat management. Table 3 gives the comparison of arc welding based RM systems with laser/ electron beam based RM systems. It is clear that the use of laser or electron beam may be overkill for the tooling and component applications barring a few exceptions. Unfortunately the RM community has stopped further exploitation of arc welding for this application. IIT Bombay appears to be the only research group still exploring arc welding based RM. Interestingly whatever is done for arc welding head can be adapted to laser or electron beam heads. In the subsequent sections, some of the RM processes that used arc welding are reviewed. Table 3 Comparison of the three energy sources used for RM Characteristic Laser or Electron Beam Quality of Near-net
Arc Near-net
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Geometry Quality of material Power efficiency Material utilization Speed
2-5% for lasers and about 15-20% for EB Powder efficiency of 10-15%. Only Trumpf’s DMD has close to 50%. 2-10g/min
Same > 80% Close to 100% as wire is used.
Cost
Too expensive. A 1kW RM machine costs about Rs. 3.5 crore.
Size Safety
Bulky Strict safety regulations to be followed
50-130 g/min. It is possible to reach about 800 g/min with proper heat management The welding system costs from Rs. 4-8 lakhs. It is integrated with a CNC machine or a robot costing about Rs. 15 lakhs. Compact Safe
3D Welding developed at the University of Nottingham and Shape Deposition Manufacturing (SDM) developed at CMU are some of the earliest promising examples of using arc welding for RM of metallic objects [16, 17]. But these research works were not continued to the level of commercialization. Many such research groups including Southern Methodist University (SMU), USA and Korea Institute of Science and Technology (KIST) were working on arc welding for layered deposition (which is also known as shape welding in the welding parlance) in the late 1990s [18, 19]. Interestingly most of these processes had integrated machining on the same platform. Unfortunately, two factors caused the exodus of these researchers into laser-based RM. One is the thermal issues leading to excessive distortion and homogeneity variations. The other is that laser welding in conjunction with multiple powder feeding can be used to build FGM components which had many military applications with good funding prospects. Therefore, the RP/RM research community left arc welding prematurely and failed to exploit its full potentials.
3. ArcHLM A hybrid RM process using arc welding under development in IIT Bombay is called Arc Hybrid Layered Manufacturing (ArcHLM). A significant advantage of ArcHLM is its availability as a retrofitment to any existing CNC machine. Figure 9 shows a 3-axis arcHLM machine by integrating a 12-year old Argo 1050P 3-axis CNC machine and a Fronius TPS 4000 pulsed synergic MIG welding equipment. The integration was achieved through the relay responsible for the coolant function of the CNC machine. In other words, M08/M09 in the NC program does welding on/off in arcHLM [20].
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Figure 9 ArcHLM Machine at IIT Bombay
ArcHLM’s techno-economic viability has been established for injection molds [20]. Figure 10a shows the CAD models of an egg template of a refrigerator and its injection molds. The near-net shape of these injection molds were made by alternately weld-depositing a layer and face milling. Face milling was required to remove the oxide layer, to ensure Z accuracy and to have a flat surface for the next deposition. Each layer was substantially thick, of the order of 1.500 to 3.000mm as compared to the existing RP processes which is of the order of 0.100 to 0.250mm. These molds had already been manufactured through the subtractive route by the company and their cost and time were compared with those of HLM route. ArcHLM resulted in savings of 37.5% in time and 22.3% in cost. These savings arise from the following sources: • •
Elimination of roughing operation: Savings of machine hours and programming time. Material saving: Material is added only where required.
(a) CAD models of Egg Template and its Injection molds
(b) Near-net molds
(c) Finished molds
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Figure 10 5-Axis Hybrid Layered Manufacturing Machine for Component Manufacture
The present focus is at establishing the methodology of making components using ArcHLM. For this purpose, Hermle C30U 5-axis CNC machine has been procured.
4. Conclusions RP is evolving into RM. In order to cater to a number of combinations of materials, quality specifications, production volume, sensitivity to cost etc., a unique strategy like slicing is not sufficient for RM; it will require multi-faceted and hybrid approaches. An elaborate classification of these processes was presented. Among the six major groups of RM processes, the hybrid approach that combines the best features of material addition and subtraction was elaborated in this paper. In these hybrid processes, material addition is achieved using Laminated Manufacturing, a powder-bed technology or a deposition technology. These three groups of hybrid RM processes were reviewed. The energy source used in RM was also compared and the potential of using arc welding was brought out. Finally, arcHLM, a hybrid RM process under development at IIT Bombay was discussed.
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