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Effect of Machining Parameters on Surface Integrity in High Speed Milling of Super Alloy GH4169/Inconel 718 a
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Waseem Akhtar , Jianfei Sun & Wuyi Chen a
School of Mechanical Engineering and Automation, Beihang University, Beijing, China Accepted author version posted online: 16 Dec 2014.Published online: 16 Dec 2014.
Click for updates To cite this article: Waseem Akhtar, Jianfei Sun & Wuyi Chen (2014): Effect of Machining Parameters on Surface Integrity in High Speed Milling of Super Alloy GH4169/Inconel 718, Materials and Manufacturing Processes, DOI: 10.1080/10426914.2014.994769 To link to this article: http://dx.doi.org/10.1080/10426914.2014.994769
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Materials and Manufacturing Processes, 0: 1–8, 2015 Copyright # Taylor & Francis Group, LLC ISSN: 1042-6914 print=1532-2475 online DOI: 10.1080/10426914.2014.994769
Effect of Machining Parameters on Surface Integrity in High Speed Milling of Super Alloy GH4169/Inconel 718 Waseem Akhtar, Jianfei Sun, and Wuyi Chen
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School of Mechanical Engineering and Automation, Beihang University, Beijing, China Control of surface integrity is a vital consideration in the machining of components subjected to fatigue loading, for example, critical components of aerospace engines. In this research, three important aspects of surface integrity of a machined part—surface roughness, micro-hardness, and residual stresses—were analyzed for their variations with the cutting parameters. Finish milling of super alloy GH4169/Inconel 718 was carried out using coated cemented carbide and whisker-reinforced coated ceramic inserts. All of the three machining parameters—cutting speed, feed rate, and depth of cut—were found to have a substantial effect on the surface integrity of the finished part. Although different cutting parameters gave different effects for the two types of cutting inserts, overall better surface integrity was obtained at minimum cutting feed and medium cutting speed and depth of cut value. Moreover, carbide inserts produced better surface integrity of the finished part, whereas ceramic inserts generated very high surface tensile stresses and poor surface finish due to back striking of the adhered metal chips. Keywords Carbide; Ceramic; GH4169; Inconel 718; Integrity; Milling; Stresses; Surface.
The cutting feed was observed to be the main parameter influencing the surface finish, which was found to deteriorate as the feed rate was increased. In contrast, cutting speed did not cause any significant change in surface roughness. Sadat [2] found that both residual stresses (RS) and surface roughness decreased as the cutting speed increased and the depth of cut (DOC) decreased. Devilles et al. [3] used coated carbide tools to carry out dry turning of Inconel 718 in the finishing process. Both surface roughness and peak tensile stress reduced on increasing the cutting speed, whereas an increase in cutting feed increased the tensile stresses. Wet cutting reduced tensile stresses; however, the lessening effect was limited to cutting speeds lower than 80 m=min only. In a review on machining of super alloys by Ulutan and Ozel [4], both roughness and residual stresses were found to increase with the cutting feed and speed. Moreover, round inserts, positive rakes, big nose radius, honed edges, and application of coolant were stated to produce lower surface roughness values. In another review on surface integrity in the machining of hard-to-machine materials [5], increase in feed rate was found to cause higher surface tensile stresses and deeper compressive stresses. Sharp cutting edges caused more tensile residual stresses than honed edges, whereas chamfered cutting edges were reported to produce compressive stresses. Moreover, coated tools produced lower tensile residual stresses than uncoated ones. Pawade et al. [6] used PCBN inserts to carry out turning of Inconel 718 at high cutting speeds, they observed initial increase and then decrease in tensile residual stresses when the cutting speed, cutting feed and DOC were increased. Coelho et al. [7] investigated micro-hardness on the surface and sub-surface in turning of Inconel 718. They observed an inverse relationship between cutting speed and
INTRODUCTION Because of their high temperature strength, large strain hardening, high tool–work piece affinity, and low thermal conductivity, nickel-based super alloys fall into the category of very-hard-to-machine materials. Alternatively, their excellent creep, oxidation, and corrosion resistance along with the ability to withstand very high service temperatures make them highly suited for use in the critical sections of aerospace engines, for example, blades, blisks, and engine casings. All of these parts are very critical components and have to bear the severe conditions of thermal and mechanical fatigue. Most of the failure histories of these components have associated damage initiation to surface defects. Therefore, in order to avoid any collateral damage, a tight control of surface integrity must be insured in the finishing processes for these components. Machining is becoming more and more common as the finishing process of these components, especially in the case of blisk manufacturing. It is, therefore, very important to obtain better control of the finish machining process so that the desired surface integrity of finished parts may be achieved. A comprehensive review of the influence of machining conditions on the surface integrity of nickel-based super alloys was carried out. Sharman et al. [1] carried out turning of Inconel 718 using cemented carbide tools. Received September 23, 2014; Accepted December 1, 2014 Address correspondence to Waseem Akhtar, School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China; E-mail:
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micro-hardness. Ezugwu et al. [8], also observed a decrease in micro-hardness of the near surface layer when the cutting speed was increased. Kuo et al. [9] performed dry milling of Inconel 718 to investigate the variation of surface roughness with machining parameters, the cutting speed range employed was 50–250 m=min. Their study indicated that increase of cutting speed decreased the surface finish. Excessive adhesion of material on the cutting edge was found to deteriorate the surface for cutting speeds of greater than 150 m=min. M’Saoubi et al. [10] conducted finish-turning experiments on Inconel 718 with coated carbide and polycrystalline cubic boron nitride (PCBN) tools in a cutting speed range of 60–90 and 200–400 m=min respectively. They found that both of these tools gave comparable surface quality in their respective cutting speed ranges. ¨ zel & Ulutan [11] studied the influence of geometry O and coating of cutting tool on surface stresses in the face turning of IN100. Surface tensile stresses showed an increasing trend with an increase in cutting-edge radius; moreover, coated tools induced lower tensile stresses. In another study [12], almost similar observations were made. Bellows [13] also recommended the use of sharp tools for better surface integrity. Arunachalam et al. [14] investigated the influence of the geometry of the cutting tool on residual stresses and the surface finish of components in facing operations. Their results showed that wet machining with carbide inserts of negative type, round shape, and chamfered cutting edges resulted in compressive stresses and lesser surface roughness. This variation of the residual stresses due to cutting geometry was explained by Brinksmeier et al. [15]. According to them, cutting processes involving higher squeezing of material produced compressive residual stresses and the converse was also applicable. Yazid et al. [16] studied surface integrity in the turning of Inconel 718 using titanium aluminium nitride (TiAIN)-coated carbide tools both under dry and minimum quantity lubrication (MQL) conditions. They found that, compared to dry machining, MQL resulted in a better surface finish, lesser microstructure alteration, and lesser micro-hardness. Pusavec et al. [17] also studied the effect of various cooling, lubrication, and fluid (CLF) conditions, for example, cryogenic condition, MQL, and a combination of cryogenic and MQL in the finish turning of Inconel 718. Their results showed that cryogenic machining
outperformed other conditions and resulted in lower surface roughness, compressive residual stresses, and higher surface hardness. The above mentioned review of the literature revealed that most of the studies on surface integrity characterization of nickel-based super alloys have addressed the issues in the turning process, whereas very little is known on this subject in the finish-milling process. The application of new ceramic insert grades for milling of these high-temperature alloys have not received due attention. Moreover, there exists a difference in the findings of different studies. Keeping in view these discrepancies in the available research and the growing importance of milling processes in the machining of critical aerospace components, this study addresses the issue of surface integrity characterization in high-speed finish-milling processes using coated cemented carbide and silicon carbide (SiC) whisker-reinforced ceramic inserts. The influence of machining parameters on the surface roughness, residual stresses, and micro-hardness was analyzed in order to present a strong approach on this subject. In the end, the two types of cutting inserts were compared for their performance in terms of surface integrity (SI) of the finished part. MATERIALS AND METHODS Face milling of the nickel-based super alloy GH4169 (equivalent of Inconel 718, UNS No. 7718) was carried out under solution-treated and age-hardened conditions. The composition and mechanical properties of the material are shown in Table 1(a) and 1(b), respectively. PVD-TiAlN-coated cemented carbide inserts of grade AH725 and SiC whisker-reinforced coated ceramic inserts of grade WG600, both specialized for the machining of heat-resistant alloys (HRAs) were used in EPS11025RSB and WSRN-60002.5-RH tool holders, respectively. High-speed computerized numeric control (CNC) face milling was carried out for both types of cutting tools. Dry machining was selected for ceramic inserts, whereas wet machining using a water-soluble cutting fluid was chosen for the carbide inserts. Detailed combinations of cutting parameters are given in Table 2. Residual stress measurements in cutting (RS Cut) and feed (RS feed) direction were made using an X-Ray
TABLE 1.—Chemical composition and mechanical properties of superalloy GH4169. a. Chemical composition of GH4169 (wt.%)
C 0.052
Si 0.26
Al 0.56
Mn 0.22
Co 0.5
Ti 1.08
Cu 0.1
Mo 3.03
Nb 5.03
Ni 52.15
Cr 19.26
Fe Bal
b. Mechanical properties of GH4169 Tensile strength(MPa)
1430
Yield strength(MPa)
Elastic modulus(GPa)
Hardness (HRC)
Density (g=cm3)
Thermal conductivity(W=m C)
Melting temperature( C)
1300
204
40
8.24
14.7
1310
EFFECT OF MACHINING PARAMETERS ON SURFACE INTEGRITY IN HIGH-SPEED MILLING TABLE 2.—Cutting parameters and their levels. Cutting insert type PVD TiAlN coated carbide insert
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Exp. #
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SiC-whisker reinforced ceramic insert
Cutting speed, v (m=s)
Feed rate, f (mm=tooth)
DOC, ap (mm)
Cutting speed, v (m=s)
Feed rate, f (mm=tooth)
DOC, ap (mm)
30 45 60 75 90 60 60 60 60 60 60 60 60 60 60
0.06 0.06 0.06 0.06 0.06 0.02 0.04 0.06 0.08 0.10 0.06 0.06 0.06 0.06 0.06
0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.2 0.4 0.6 0.8 1.0
300 500 700 900 1100 700 700 700 700 700 700 700 700 700 700
0.08 0.08 0.08 0.08 0.08 0.04 0.06 0.08 0.10 0.12 0.08 0.08 0.08 0.08 0.08
0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.2 0.4 0.6 0.8 1.0
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roughness. This reasoning was also reinforced by the wear patterns of cutting edges as depicted in Fig. 2. The wear patterns shown were indicative of chipping or flaking failure of the tool edges that could be a result of dislodging of the adhered material or BUE from the tool surface.
diffraction (XRD) technique. Measurements were taken on XStress 3000 X-ray stress analyzer using manganese Ka radiation (k ¼ 0.21031 nm) at 30 kV (6.6 mA) to acquire the (h k l ¼ 3 1 1) diffraction peak at a Bragg’s angle 2h ¼ 152 . Surface topography was analyzed using a Taylor Hobson surface profiler and OLS 4100 Optical microscope, whereas surface hardness (HV0.2) was measured by a micro-hardness tester using a 200 g load for 10 s. RESULTS AND DISCUSSION Although, the lower hot hardness of cemented carbide tools limit their application in very-high-temperature machining, their high toughness makes them suitable for the machining of materials usually considered hard to machine (e.g., HRAs). The observed influence of machining parameters on surface attributes is discussed below: . Effect of Cutting Speed
Variations of layer characteristics with cutting speed is given in Fig. 1(a). The graph depicts the increase in roughness of the surface with the increase in cutting speed. Generally, surface roughness is thought to decrease when cutting speed increases. The two reasons for this behavior are: there is less scaling and material side flow at high cutting speeds and the possibility of development of built-up edge (BUE) diminishes when cutting speed increases. Formation of BUE is known to take place at medium cutting speeds; however, because of the interdependence of various cutting conditions, there is no known fixed value of medium cutting speed. In this trial, an increase in cutting speed was found to lead the cutting process toward conditions favorable for the formation of BUE, thus increasing the surface
FIGURE 1.—Variation of surface integrity attributes with machining parameters: for carbide inserts.
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FIGURE 2.—Tool wear patterns observed at different cutting speeds (a) v ¼ 60 m=s; (b) v ¼ 75 m=s; (c) v ¼ 90m=s.
Compressive residual stresses (RS) were observed at low cutting speeds and an increasing trend toward tensile direction was found as the cutting speed increased. Generally, two phenomena, mechanical compacting and thermal deformation, compete to produce compressive or tensile stresses respectively. In this case, the initial existence of compressive stresses and their subsequent move in a tensile direction indicated the dominance of thermal phenomenon at higher cutting speeds. The increase in cutting temperature caused the thermally induced residual stresses to dominate and, thus, the stresses moved from compressive toward tensile behavior. The behavior of residual stress corresponding to a cutting speed of 75 m=min was not normal, and re-measurement at this point resulted in the same high stress value. This abrupt behavior remained unexplained and, thus, was deemed to be an experimental exception. Variations of surface hardness indicated that it
decreased as the cutting speed was increased. In a machining process, the hardening or softening of a work piece is controlled by multiple factors such as plastic deformation, strain rate, cutting temperature, yield to tensile strength ratio of the material, etc. At low cutting speeds, the effect of strain hardening dominates and results in greater hardness of the material; however, at high cutting speeds, the effect of thermal softening dominates and the micro-hardness of the surface drops toward the bulk material hardness. The decrease of surface hardness in this case was also the result of increased thermal softening at high cutting speeds. . Effect of Feed Rate Because of the increased spacing between the machining marks and reduced material side flow, generally there is an increase in surface roughness with the increase in the cutting feed rate. Figure 1(b) illustrates the observed influence of
FIGURE 3.—Tool wear observed at the end of cut (a) DOC ¼ 0.2 mm; (b) DOC ¼ 0.4 mm; (c) DOC ¼ 0.8 mm.
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EFFECT OF MACHINING PARAMETERS ON SURFACE INTEGRITY IN HIGH-SPEED MILLING
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feed rate on surface integrity characteristics. An increasing trend of surface roughness was observed due to the combined effect of increased spacing between feed marks and increased tool wear rate at higher cutting feeds. Residual stresses also showed an increasing trend with the feed rate. Due to the increase in mechanical work and energy at increased feed rates, the temperature of the work piece and, therefore, residual stresses increased in the tensile direction. The material surface hardness also increased with the feed rate. An increase of the feed rate resulted in greater cutting force and deformation and, thus, caused greater work hardening. Although a thermal softening effect might also be present due to increased cutting temperature with an increase in the feed rate, the effect of deformation-induced hardening dominated and the surface hardness of the work piece was found to increase. . Effect of DOC The variation of surface characteristic with DOC is shown in Fig. 1(c). Contrary to the general perception about the minimal effect of DOC, a significant variation was observed in surface roughness and surface hardness. The increase of surface roughness with DOC was found to be caused by the increased tool wear rate, as shown in Fig. 3. Residual stresses, on the other hand, were found to be least affected by the change in DOC. Surface hardness was also observed to increase with the DOC. As there was no significant increase in the temperature of a machining process due to increase in DOC, the mechanical strain-hardening effect dominated resulting in increased hardness of the surface layer. Due to their exceptional hardness at elevated temperatures, ceramic tools offer an alluring choice for highspeed machining of hard-to-machine materials (e.g., HRAs). The advent of more thermal- and shock-resistant ceramic grades (e.g., SiAlON and SiC whisker-reinforced Al2O3) and, new tool coatings have made them suitable for use in intermittent machining applications. The effect of cutting parameters on surface integrity in the high-speed milling of the super alloy GH4169 using coated SiC whisker-reinforced Al2O3 ceramic inserts was analyzed as described further. . Effect of Cutting Speed
Excellent hot hardness of ceramic inserts make them suitable for very-high-speed (>300 m=min) machining. At high cutting speeds, thermal softening of material lowers the cutting forces and, thus, makes machining easier. The effect of cutting speed on surface integrity characteristics is shown in Fig. 4(a). It was observed that an increase in cutting speed caused a decrease in the surface roughness and surface hardness, whereas no significant influence was observed on tensile residual stresses. However, two out of five experiments
FIGURE 4.—Variation of surface integrity attributes with machining parameters: for ceramic inserts.
showed some variation in the residual stresses; these two points are on the opposite extremes and are appear to be experimental exceptions. Decreased surface roughness at higher cutting speeds is a common result. At high speeds, the
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effect of scaling and the possibility of BUE formation is decreased, causing a decrease in surface roughness. Moreover, because of the greater speed of the cutting process at higher speeds, the effect of side flow of the material is minimized, which also improves the surface finish. The decrease in surface hardness was thought to be due to two reasons; increased speed of the process at very high cutting speeds, thus making the plastic stage insignificant, and a thermal softening effect at very high temperatures. The unvarying behavior of residual stresses was due to greater heat transport at high cutting speeds. Although the temperature of cutting zone did increase at the increased cutting speeds due to higher material removal rate, less time was available for the heat to be transferred to the work piece. In this way, most of the heat of the cutting process was carried away by cutting chips and the temperature of the work piece did not increase further. . Effect of Cutting Feed Variations of surface integrity attributes with the cutting feed are given in Fig. 4(b). Surface roughness increased with the increase in cutting feed. Surface hardness, however, was found to decrease with the feed rate as a whole, but there was no simple trend. The points with increased surface hardness gave evidence of dominance of a strain-hardening effect over thermal softening and vice versa. An increase of the cutting feed was observed to initially increase and then decrease the tensile residual stresses. The initial increase in tensile residual stresses was caused by the increase in cutting temperature, whereas a later decrease was due to the reduction in work-piece temperature owing to the greater transport of heat by metal chips, limiting the amount of heat transferred to the work piece. . Effect of DOC Figure 4(c) depicts the variation of surface characteristics with DOC. Contrary to the general understanding of the minimal influence of DOC on surface integrity attributes, all three attributes showed significant variation with the increase in
the DOC. An increasing trend was observed for surface roughness and surface tensile residual stresses, whereas surface hardness was found to reduce with the increase in DOC. Both of these variations of residual stresses and surface hardness were deemed to be caused by the increase in machining temperature. A summary of the influence of machining parameters on surface integrity attributes for both types of cutting inserts is given in Table 3. A brief comparison of performance is as described here: . Surface Roughness
Although the range of cutting parameters was different for the two types of inserts, minimal surface roughness was obtained at the minimum cutting feed and with medium cutting speed and DOC values. These results of surface roughness measurements apparently indicated that these two types of cutting inserts produced comparable surface roughness values; however, this was not really the case. The measurements of surface roughness were made at a distance of 25–30 mm from the start of cut. Although the surface roughness given by the two tools was comparable at the start of the cut, ceramic tools produced a very poor surface as cutting progressed beyond a cutting length of 50 mm. A lot of material debris was seen on the surface as shown in Fig. 5; the formation of this debris was a result of back striking of the material adherent on the edges of the cutting insert. Alternatively, carbide inserts maintained a good surface finish throughout the length of the cut. It was, therefore, concluded that, in terms of surface finish in the finish-milling process, ceramic tools were not as suitable as carbide tools. . Residual Stresses In terms of residual stresses as well, carbide tools outperformed the ceramic tools. For all the cutting conditions employed, machining with ceramic tools resulted in tensile residual stresses that were severely high in most of the cases. However, for
TABLE 3.—Effect of cutting parameters on surface integrity attributes. Effect of increase in cutting parameter on surface Integrity attributes Sr. #
Cutting parameter
Carbide inserts
1
Cutting speed
2
Feed rate
3
Depth of cut
Increased surface roughness Increased tensile residual stresses Decreased surface hardness Increased surface roughness Increased tensile residual stresses Increased surface hardness Increase in surface roughness Un-affected residual stresses Increased surface hardness
Ceramic inserts
Decreased surface roughness Un-affected residual stresses Decreased surface hardness Increased surface roughness Initially increasing then decreasing tensile residual stresses Decreased surface hardness Increased surface roughness Increased tensile residual stresses Decreased surface hardness
EFFECT OF MACHINING PARAMETERS ON SURFACE INTEGRITY IN HIGH-SPEED MILLING
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FIGURE 5.—2D profile of machined surface using; (a) Ceramic insert (b) Carbide insert.
carbide tools, mostly compressive stresses were measured. Although tensile stresses were also found for carbide tools at higher cutting speeds and feed rates, their magnitude was very small as compared to that found for ceramic tools. Thus, in terms of surface residual stresses also, carbide tools outperformed the ceramic tools. . Micro-hardness In all of the cutting experiments, whether for carbide or ceramic inserts, work hardening of the work piece surface took place, resulting in increased hardness of the machined surface as compared to the bulk material. However, due to the wet cutting conditions employed for carbide tools as compared to the dry conditions for ceramic tools, the use of carbide tools always resulted in comparatively greater surface hardness. However, the difference in the maximum surface hardness value obtained for both types of the tools was not large.
. As a whole, carbide inserts generated better
surface integrity of the finished part. . Ceramic inserts gave better productivity, but
always produced very high tensile residual stresses and a poor surface finish due to adherence of material debris after 50–60 mm of the cut. . In contrast, carbide inserts generated a better surface finish, mostly compressive surface residual stresses, and comparable surface hardness values. It can thus be concluded that, for the finish milling of the nickel-based super alloy GH4169, carbide inserts generated much better surface integrity as compared to the ceramic inserts.
FUNDING This work was supported by the Science and Technology Program of Beijing, China (Grant No. Z121100001612006). REFERENCES
CONCLUSIONS This study elaborates the experimental results of variations of surface integrity with cutting parameters in high-speed milling of the super alloy GH4169=Inconel 718. Important conclusions drawn from this research are that: . All three cutting parameters had a significant
effect on surface characteristics of the finished product. Contrary to the conventional approach regarding the minimal effect of DOC, this study showed the significance of DOC as an important parameter controlling the surface attributes. . Best values of surface integrity attributes for each type of cutting insert were observed at corresponding medium cutting speed, minimum cutting feed, and medium DOC value.
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