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Effect of grinding temperatures on the surface integrity of a nickel-based superalloy
Journal of Materials Processing Technology 129 (2002) 359±363
Effect of grinding temperatures on the surface integrity of a nickel-based superalloy X.P. Xua,*, Y.Q. Yua, H.J. Xub a
College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou, Fujian 362011, PR China b Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu 210016, PR China
Abstract An experimental study was carried out to investigate the in¯uence of temperatures on workpiece surface integrity in the surface grinding of a cast nickel-based superalloy with alumina abrasive wheels. The temperature response at the wheel±workpiece interface was measured using a grindable foil/workpiece thermocouple. Specimens with different grinding temperatures were obtained through changing the grinding conditions, including the depth of cut, the workpiece feed, and the coolant supply. Changes in the surface roughness, residual stress, metallography, ground surface morphology, and micro-hardness of the specimens were then analyzed. Bending fatigue tests were separately conducted at room temperature and at 950 8C in order to evaluate the in¯uence of temperatures on the service life of the ground specimens. A different burning color was found on the ground workpiece surfaces when grinding temperatures are over a critical value. Along with the emergence of a burning color, the roughness of the ground workpiece surface increased greatly compared with the surfaces without burning color, which was attributed to plastically deformed coatings on the workpiece surface occurring with elevated temperatures. Excepting the surface roughness, surface integrity of the ground workpiece was not affected by temperature, provided that grinding temperature is not high enough to cause grinding cracks. Based on the ®ndings in this study, the grinding of nickel-based superalloy can be divided into two stages in order to increase production ef®ciency, in which case the ®rst stage is to reach high material removal rate without concern for the presence of a burning color, whereas the second stage is to remove the plastically deformed coatings in order to decrease the surface roughness. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Temperature; Surface grinding; Superalloy; Surface integrity
1. Introduction Superalloys exhibit a combination of mechanical strength and resistance to surface degradation generally unmatched by other metallic compounds. However, all the excellent properties of superalloys together with their poor thermal diffusivity make them extremely dif®cult to machine and may lead to elevated temperatures at the grinding zone and possible thermal damage to the workpiece during grinding with abrasive wheels [1±3], which has been the most popular process for the machining of superalloys used in the aeronautical industry. Accordingly, extensive past research has been concerned with thermally induced grinding damage, especially for Inconel 718 and titanium alloys [1±5]. Among the commercially available superalloys used in the aeronautical industry, K417 is a relatively new kind of turbine vane materials, which is a cast nickel-based *
Corresponding author. Tel.: 86-595-2693567; fax: 86-595-2693999. E-mail address: [email protected] (X.P. Xu).
superalloy and features good comprehensive properties such as high temperature strength, high corrosion resistance, and good fatigue resistance, etc. As compared with other aeronautical superalloys, there have been virtually no profound studies reported on the thermal aspects of the grinding of K417. In a study by Osterle and Li [1], the formation of a physically deformed and a heat-affected zone during the creep feed grinding of superalloy IN738LC was investigated from the aspect of metallographic analysis. It was found that local melting at contact spots seems to be a quite common mechanism during the grinding of superalloys, thereby leading to the so-called white layer. Severe damage was observed after creep feed grinding without suf®cient coolant in their experiments. Unfortunately, the temperatures at the wheel±workpiece interface were obtained by calculation rather than by experiments in their study. Based on in-process monitoring of the temperature at the grinding zone, the present research was undertaken to experimentally examine the effect of temperature on the surface integrity of K417.
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 5 6 - 8
360
X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
2. Experimental The experimental set-up is illustrated schematically in Fig. 1. Straight surface grinding experiments were conducted on an MMD7125 precision grinder using a vitri®ed alumina grinding wheel (WA100KV) of diameter ds 300 mm and width bs 20 mm. The wheel was trued and dressed using a single-point diamond. Tests were performed in the down-cutting mode. All the grinding tests were at a peripheral cutting velocity of vs 25 m/s. The workpiece velocity vw and wheel depth of cut ap were varied, giving different machining conditions. A 3% solution of emulsion in water was used for the grinding tests. The temperature response at the wheel±workpiece interface was measured using a grindable foil/workpiece thermocouple [6,7], which was proven to be capable of providing a suf®ciently fast response for grinding conditions with a large depth of cut [7] and consisted of a Ni±Cr foil of 35 mm thickness insulated on both sides by 10 mm sheets of mica and sandwiched between two pieces of a split workpiece. The workpiece acts as the second thermocouple pole, whilst the cold junction was immersed in ice water. The signal from the foil/workpiece thermocouple was obtained by a single pass of the grinding wheel over the workpiece. The output from the foil/workpiece thermocouple was fed to a HP 3562 dynamic signal recorder. The assembly of the thermocouple is shown in Fig. 1. The workpiece material K417 was prepared according to conditions for precisely casting turbine vanes for aircrafts. In addition to Ni, other main composition elements for K417 are Cr, Co, Ti and Al. The workpiece was 30 mm along the grinding direction and 25 mm wide. The surfaces for clamping the thermocouple foil were polished and eroded for metallographic analysis. The morphologies of the ground workpiece surfaces, the metallograph of the ground surfaces and sub-surfaces, and the worn alumina abrasive grits on the grinding wheel surface were analyzed by a JSM-35C scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS). An HX-1000 micro-hardness tester was used to measure the hardness changes and an MSF-1M X-ray diffractometer (XRD) was employed to measure the residual stresses on
Fig. 1. Schematic illustration of the experimental set-up.
Table 1 Relation between surface color and maximum temperature Maximum temperature at contact zone, y (8C)
Color
95
990
1215
1440
1600
Normal
Beige
Brown
Puce
Hyacinthine
the workpiece surfaces and sub-surfaces. To obtain the residual stress distribution at different depths beneath the ground workpiece surfaces, an electrolytic etching method was used to remove the workpiece materials layer by layer. A bevel at an angle of 18300 to the workpiece ground surface was made and the values of micro-hardness at different depths beneath the surface were measured along the bevel. The fatigue behavior of the ground workpieces were investigated separately under room temperature and 950 8C, which is the working temperature for K417. SEM, EDS, XRD and hardness analysis were repeated for three separate specimens in the experiments, whereas 10 separated specimens were selected for the fatigue test. 3. Results 3.1. Surface roughness Based on the temperature responses recorded during grinding under different conditions, it was found that a different color, which is well known as the burning color in the ®eld of grinding [8], appears on the workpiece ground surfaces when the maximum temperatures at the contact zone are over 990 8C. The color generated on the workpiece surfaces at different grinding temperatures are listed in Table 1. The hyacinthine specimen was obtained by dry grinding in order to raise the temperature to an extreme situation. From Table 1 it can be seen that the higher is the maximum temperature, the deeper is the color on the specimen surface. Measured surface roughness values for the ground surfaces having different colors are plotted against
Fig. 2. Surface roughness versus maximum temperature.
X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
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the measurements were also made on the puce specimen after it had been heated to 950 8C for half-an-hour. 3.3. Residual stresses
Fig. 3. Micro-hardness depth-profile.
Residual stresses at different depths beneath ground surfaces having different colors are shown in Fig. 4, in which only the stresses in the direction perpendicular to the grinding direction are given. It can be seen that compressive stresses were generated on the ground workpiece surfaces and sub-surfaces. Away from the ground workpiece surface, the compressive stresses increase to a maximum value and then decrease to a relatively stable value identical to that of the base material. The maximum stress on the puce surface is slightly smaller than for the other two cases and the location of the maximum stress is further away from the ground surface, which is similar to what is found in CBN grinding and face milling. 3.4. Metallographic analysis
Fig. 4. Residual stress depth-profile.
the maximum temperatures in Fig. 2, in which the workpiece with a puce surface was stripped off by 0.4 mm using Ar ions. It can be seen that surface roughness increases with an increase of maximum temperature. 3.2. Micro-hardness Fig. 3 gives the measured values of micro-hardness at different depths beneath the workpiece ground surfaces with three different surface colors. It should be pointed out that
Fig. 5 shows the metallographs of different specimens. Despite the high temperatures generated on the puce surface, no obvious changes can be found compared with the original phases in the K417. For the hyacinthine specimen, however, the strengthening phase g0 decreases and does not change even after heating to 950 8C for half-an-hour. Furthermore, it can be seen from Fig. 5(e) that from the surface down to a depth of about 40 mm a grinding crack has developed along a grain boundary. 3.5. Fatigue life Tests of bending fatigue life were conducted on seven kinds of specimens to evaluate comprehensively the service life of each specimen. Experimental results are listed in Table 2. No observable differences can be found among the
Fig. 5. Metallographs for different specimens: (a) base material; (b) TEM pictures for base material; (c) normal color specimen; (d) puce specimen; (e) hyacinthine specimen; (f) hyacinthine specimen after being heated at 950 8C for 30 min.
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Table 2 Results for bending fatigue test Specimen
lg N Room temperature 950 8C
Normal
Beige
Puce
Hyacinthine
5.4534 5.4552
5.4623 5.4604
5.4506 5.4651
5.0392
fatigue life results of specimens having normal, beige, and puce color. For the specimens with grinding cracks along the grain boundary, the fatigue life dropped greatly, the reason for which will be discussed in the succeeding section. 4. Discussion It is seen from Fig. 2 that the roughness values for the surfaces having a burning color are much higher than those for the surfaces having a normal color. To account for the mechanisms for surface roughness, the morphologies of ground workpiece surfaces having different colors were examined using SEM. From the SEM pictures as shown in Fig. 6, it can be seen that the ground surface having normal color is relatively smooth and consists mostly of overlapping scratches produced by the interactions of abrasive cutting points with the workpiece. Compared with the surface having normal color, the morphology of other surfaces become more complicated, in that some kind of plastically deformed coatings appear and gradually spread on the ground workpiece surfaces with an increasing temperature. Moreover, micro-cracks are found on the workpiece surface with hyacinthine color, which is consistent with the results shown in Fig. 5(e) and indicates severe damage to the ground surface. According to previous research on metal performance, metals which are more adhesive, such as titanium and nickel-based alloys, tend to exhibit more ductility. When the interface temperature is high enough, the workpiece material at the contact zone becomes ductile enough to cause strong welds to form between the abrasive grit and workpiece, thereby resulting in the generation of plastically deformed coatings [9]. Compared with the morphology of the puce surface, the striped surface as shown in Fig. 7 becomes simple and consists of clear overlapping scratches produced by the
Fig. 7. SEM picture of the striped puce surface.
interactions of abrasive cutting points with the workpiece, further indicating the formation of a plastically deformed coating on the workpiece ground surface at high temperature. Therefore the roughness of the striped surface decreased greatly, which can be seen from Fig. 2. In addition to roughening the ground workpiece surfaces, excessive ductile deformation is also a main reason for the formation of compressive stresses on the ground surfaces, as shown in Fig. 4, since tensile stresses resulting from grinding heat might be overlaid by the compressive stresses due to the excessive ductile deformation. A previous study on grinding burning [10] has shown that the ratio of normal grinding force to tangential grinding force determines the position where plastic deformation ®rst takes place. If the ratio is less than 4, maximum stress appears at a distance further away from the ground surface with a decrease of the force ratio. In the authors' recent study it was found that the force ratio decreased with an increase in grinding temperatures at the contact zone due to the increasing tendency of adhesion between the abrasives and the superalloy at higher temperatures [9]. Therefore the location of the maximum stress is further away from the ground surface with a deeper burning color, as can be seen in Fig. 4. Combining the results of sputtering using Ar ions with Figs. 3 and 4, it can also be seen that the zones in which residual stresses and micro-hardness change obviously, are similar to the affected depth of the plastically deformed coatings, since elastic deformation can be ignored due to the extremely large elastic modulus of K417. The plastic deformation generated in the grinding process also led to a slight increase of micro-hardness in the areas near to the ground surface having a normal color, due to the processing hardening effect. As a comparison, the hardness in the areas away from the ground workpiece surfaces having a burning color are relatively lower, which can be attributed to recrystallization at higher temperatures.
Fig. 6. SEM pictures of the ground workpiece surfaces: (a) normal color specimen; (b) beige specimen; (c) puce specimen; (d) hyacinthine specimen with grinding cracks.
X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
363
can also be helpful in increasing the machining ef®ciency in the CBN grinding, which will be discussed in future papers. 5. Conclusions
Fig. 8. SEM pictures of fracture surfaces after fatigue tests: (a) fatigue crack initiation around TiC; (b) EDS data for the part with in (a); (c) fatigue crack propagation due to grinding cracks.
In order to account for the fatigue test results, the fracture surfaces of the specimens after fatigue tests were examined using SEM. Fig. 8 shows the fatigue cracks owing to the presence of TiC and grinding cracks. Based on the fracture analysis and EDS detection, it is found that the sources for fatigue crack propagation in each specimen are mainly the hardening phase TiC. Provided that the TiC does not decrease, the fatigue life of the specimens may not be affected by the temperature, which could be supported by the results listed in Table 2. For the hyacinthine specimen with grinding cracks, however, fatigue propagation is mainly due to the grinding cracks, which can be seen in Fig. 8(c). Since the decrease of g0 speeds up the propagation of fatigue cracks, the fatigue life decreases obviously, as is already shown in Table 2. Based on the above-mentioned results, it is found that except for surface roughness, the surface integrity of the ground workpiece was not affected by temperature provided that the temperature was not high enough to cause grinding cracks. This ®nding is very important for increasing the production ef®ciency in the grinding of K417, since it suggests that the grinding process can be divided into two stages. In the ®rst stage, the grinding can be conducted at a very high material removal rate without concern for the presence of a burning color. The plastically deformed coatings, which directly roughen the ground surfaces, can be removed in the second stage at a much smaller removal rate, since the thermally affected zones are within 40 mm, which is much smaller compared with the depth of cut adopted in ®ne grinding. Although CBN wheels are used widely in the grinding of superalloys and titanium alloys, the ®ndings in this paper
Excepting for surface roughness, the surface integrity of the ground workpiece is not affected by temperature provided that the temperature is not high enough to cause grinding cracks, which is a very extreme case in the present study. This ®nding is very important for increasing the production ef®ciency in the grinding of K417 since it suggests that the grinding process can be divided into two stages. In the ®rst stage, the grinding can be conducted at a very high material removal rate without concern for the presence of a burning color. The plastically deformed coatings, which can directly roughen the ground surfaces, can be removed in the second stage at a much smaller removal rate, since the thermally affected zones are within 40 mm, which is much smaller compared to the depth of cut adopted in ®ne grinding. Acknowledgements This study was supported by the Natural Science Foundation (NSF) of Fujian Province in China, by the Special Funds from Huaqiao University for Faculty Members Holding Ph.D., and partially by the Aeronautical Science Foundation of China. References [1] W. Osterle, P.X. Li, Mater. Sci. Eng. A 238 (1997) 357. [2] Q. Huang, J.X. Ren, Int. J. Fatigue 13 (1991) 322. [3] I.A. Choudhury, M.A. Eibaradle, Proc. Instn. Mech. Engrs. B 212 (1997) 195. [4] K.V. Kumar, Proceedings of the Fourth International Grinding Conference, SME Paper MR, 1990, pp. 90±505. [5] P.L. Tso, Mater. Proc. Technol. 55 (1997) 421. [6] A.Y.C. Nee, A.O. Tay, Int. J. Mach. Tool Des. Res. 21 (1981) 279. [7] X.P. Xu, S. Malkin, Trans. ASME, J. Manuf. Sci. Eng. 1239 (2001) 191. [8] R. Snoeys, Ann. CIRP 27 (1978) G19. [9] X.P. Xu, Y.Q. Yu, Surf. Interf. Anal. 33 (2002) 343. [10] Y. Furukawa, Ann. CIRP 28 (1979).
Effect of grinding temperatures on the surface integrity of a nickel-based superalloy X.P. Xua,*, Y.Q. Yua, H.J. Xub a
College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou, Fujian 362011, PR China b Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu 210016, PR China
Abstract An experimental study was carried out to investigate the in¯uence of temperatures on workpiece surface integrity in the surface grinding of a cast nickel-based superalloy with alumina abrasive wheels. The temperature response at the wheel±workpiece interface was measured using a grindable foil/workpiece thermocouple. Specimens with different grinding temperatures were obtained through changing the grinding conditions, including the depth of cut, the workpiece feed, and the coolant supply. Changes in the surface roughness, residual stress, metallography, ground surface morphology, and micro-hardness of the specimens were then analyzed. Bending fatigue tests were separately conducted at room temperature and at 950 8C in order to evaluate the in¯uence of temperatures on the service life of the ground specimens. A different burning color was found on the ground workpiece surfaces when grinding temperatures are over a critical value. Along with the emergence of a burning color, the roughness of the ground workpiece surface increased greatly compared with the surfaces without burning color, which was attributed to plastically deformed coatings on the workpiece surface occurring with elevated temperatures. Excepting the surface roughness, surface integrity of the ground workpiece was not affected by temperature, provided that grinding temperature is not high enough to cause grinding cracks. Based on the ®ndings in this study, the grinding of nickel-based superalloy can be divided into two stages in order to increase production ef®ciency, in which case the ®rst stage is to reach high material removal rate without concern for the presence of a burning color, whereas the second stage is to remove the plastically deformed coatings in order to decrease the surface roughness. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Temperature; Surface grinding; Superalloy; Surface integrity
1. Introduction Superalloys exhibit a combination of mechanical strength and resistance to surface degradation generally unmatched by other metallic compounds. However, all the excellent properties of superalloys together with their poor thermal diffusivity make them extremely dif®cult to machine and may lead to elevated temperatures at the grinding zone and possible thermal damage to the workpiece during grinding with abrasive wheels [1±3], which has been the most popular process for the machining of superalloys used in the aeronautical industry. Accordingly, extensive past research has been concerned with thermally induced grinding damage, especially for Inconel 718 and titanium alloys [1±5]. Among the commercially available superalloys used in the aeronautical industry, K417 is a relatively new kind of turbine vane materials, which is a cast nickel-based *
Corresponding author. Tel.: 86-595-2693567; fax: 86-595-2693999. E-mail address: [email protected] (X.P. Xu).
superalloy and features good comprehensive properties such as high temperature strength, high corrosion resistance, and good fatigue resistance, etc. As compared with other aeronautical superalloys, there have been virtually no profound studies reported on the thermal aspects of the grinding of K417. In a study by Osterle and Li [1], the formation of a physically deformed and a heat-affected zone during the creep feed grinding of superalloy IN738LC was investigated from the aspect of metallographic analysis. It was found that local melting at contact spots seems to be a quite common mechanism during the grinding of superalloys, thereby leading to the so-called white layer. Severe damage was observed after creep feed grinding without suf®cient coolant in their experiments. Unfortunately, the temperatures at the wheel±workpiece interface were obtained by calculation rather than by experiments in their study. Based on in-process monitoring of the temperature at the grinding zone, the present research was undertaken to experimentally examine the effect of temperature on the surface integrity of K417.
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 5 6 - 8
360
X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
2. Experimental The experimental set-up is illustrated schematically in Fig. 1. Straight surface grinding experiments were conducted on an MMD7125 precision grinder using a vitri®ed alumina grinding wheel (WA100KV) of diameter ds 300 mm and width bs 20 mm. The wheel was trued and dressed using a single-point diamond. Tests were performed in the down-cutting mode. All the grinding tests were at a peripheral cutting velocity of vs 25 m/s. The workpiece velocity vw and wheel depth of cut ap were varied, giving different machining conditions. A 3% solution of emulsion in water was used for the grinding tests. The temperature response at the wheel±workpiece interface was measured using a grindable foil/workpiece thermocouple [6,7], which was proven to be capable of providing a suf®ciently fast response for grinding conditions with a large depth of cut [7] and consisted of a Ni±Cr foil of 35 mm thickness insulated on both sides by 10 mm sheets of mica and sandwiched between two pieces of a split workpiece. The workpiece acts as the second thermocouple pole, whilst the cold junction was immersed in ice water. The signal from the foil/workpiece thermocouple was obtained by a single pass of the grinding wheel over the workpiece. The output from the foil/workpiece thermocouple was fed to a HP 3562 dynamic signal recorder. The assembly of the thermocouple is shown in Fig. 1. The workpiece material K417 was prepared according to conditions for precisely casting turbine vanes for aircrafts. In addition to Ni, other main composition elements for K417 are Cr, Co, Ti and Al. The workpiece was 30 mm along the grinding direction and 25 mm wide. The surfaces for clamping the thermocouple foil were polished and eroded for metallographic analysis. The morphologies of the ground workpiece surfaces, the metallograph of the ground surfaces and sub-surfaces, and the worn alumina abrasive grits on the grinding wheel surface were analyzed by a JSM-35C scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS). An HX-1000 micro-hardness tester was used to measure the hardness changes and an MSF-1M X-ray diffractometer (XRD) was employed to measure the residual stresses on
Fig. 1. Schematic illustration of the experimental set-up.
Table 1 Relation between surface color and maximum temperature Maximum temperature at contact zone, y (8C)
Color
95
990
1215
1440
1600
Normal
Beige
Brown
Puce
Hyacinthine
the workpiece surfaces and sub-surfaces. To obtain the residual stress distribution at different depths beneath the ground workpiece surfaces, an electrolytic etching method was used to remove the workpiece materials layer by layer. A bevel at an angle of 18300 to the workpiece ground surface was made and the values of micro-hardness at different depths beneath the surface were measured along the bevel. The fatigue behavior of the ground workpieces were investigated separately under room temperature and 950 8C, which is the working temperature for K417. SEM, EDS, XRD and hardness analysis were repeated for three separate specimens in the experiments, whereas 10 separated specimens were selected for the fatigue test. 3. Results 3.1. Surface roughness Based on the temperature responses recorded during grinding under different conditions, it was found that a different color, which is well known as the burning color in the ®eld of grinding [8], appears on the workpiece ground surfaces when the maximum temperatures at the contact zone are over 990 8C. The color generated on the workpiece surfaces at different grinding temperatures are listed in Table 1. The hyacinthine specimen was obtained by dry grinding in order to raise the temperature to an extreme situation. From Table 1 it can be seen that the higher is the maximum temperature, the deeper is the color on the specimen surface. Measured surface roughness values for the ground surfaces having different colors are plotted against
Fig. 2. Surface roughness versus maximum temperature.
X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
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the measurements were also made on the puce specimen after it had been heated to 950 8C for half-an-hour. 3.3. Residual stresses
Fig. 3. Micro-hardness depth-profile.
Residual stresses at different depths beneath ground surfaces having different colors are shown in Fig. 4, in which only the stresses in the direction perpendicular to the grinding direction are given. It can be seen that compressive stresses were generated on the ground workpiece surfaces and sub-surfaces. Away from the ground workpiece surface, the compressive stresses increase to a maximum value and then decrease to a relatively stable value identical to that of the base material. The maximum stress on the puce surface is slightly smaller than for the other two cases and the location of the maximum stress is further away from the ground surface, which is similar to what is found in CBN grinding and face milling. 3.4. Metallographic analysis
Fig. 4. Residual stress depth-profile.
the maximum temperatures in Fig. 2, in which the workpiece with a puce surface was stripped off by 0.4 mm using Ar ions. It can be seen that surface roughness increases with an increase of maximum temperature. 3.2. Micro-hardness Fig. 3 gives the measured values of micro-hardness at different depths beneath the workpiece ground surfaces with three different surface colors. It should be pointed out that
Fig. 5 shows the metallographs of different specimens. Despite the high temperatures generated on the puce surface, no obvious changes can be found compared with the original phases in the K417. For the hyacinthine specimen, however, the strengthening phase g0 decreases and does not change even after heating to 950 8C for half-an-hour. Furthermore, it can be seen from Fig. 5(e) that from the surface down to a depth of about 40 mm a grinding crack has developed along a grain boundary. 3.5. Fatigue life Tests of bending fatigue life were conducted on seven kinds of specimens to evaluate comprehensively the service life of each specimen. Experimental results are listed in Table 2. No observable differences can be found among the
Fig. 5. Metallographs for different specimens: (a) base material; (b) TEM pictures for base material; (c) normal color specimen; (d) puce specimen; (e) hyacinthine specimen; (f) hyacinthine specimen after being heated at 950 8C for 30 min.
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X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
Table 2 Results for bending fatigue test Specimen
lg N Room temperature 950 8C
Normal
Beige
Puce
Hyacinthine
5.4534 5.4552
5.4623 5.4604
5.4506 5.4651
5.0392
fatigue life results of specimens having normal, beige, and puce color. For the specimens with grinding cracks along the grain boundary, the fatigue life dropped greatly, the reason for which will be discussed in the succeeding section. 4. Discussion It is seen from Fig. 2 that the roughness values for the surfaces having a burning color are much higher than those for the surfaces having a normal color. To account for the mechanisms for surface roughness, the morphologies of ground workpiece surfaces having different colors were examined using SEM. From the SEM pictures as shown in Fig. 6, it can be seen that the ground surface having normal color is relatively smooth and consists mostly of overlapping scratches produced by the interactions of abrasive cutting points with the workpiece. Compared with the surface having normal color, the morphology of other surfaces become more complicated, in that some kind of plastically deformed coatings appear and gradually spread on the ground workpiece surfaces with an increasing temperature. Moreover, micro-cracks are found on the workpiece surface with hyacinthine color, which is consistent with the results shown in Fig. 5(e) and indicates severe damage to the ground surface. According to previous research on metal performance, metals which are more adhesive, such as titanium and nickel-based alloys, tend to exhibit more ductility. When the interface temperature is high enough, the workpiece material at the contact zone becomes ductile enough to cause strong welds to form between the abrasive grit and workpiece, thereby resulting in the generation of plastically deformed coatings [9]. Compared with the morphology of the puce surface, the striped surface as shown in Fig. 7 becomes simple and consists of clear overlapping scratches produced by the
Fig. 7. SEM picture of the striped puce surface.
interactions of abrasive cutting points with the workpiece, further indicating the formation of a plastically deformed coating on the workpiece ground surface at high temperature. Therefore the roughness of the striped surface decreased greatly, which can be seen from Fig. 2. In addition to roughening the ground workpiece surfaces, excessive ductile deformation is also a main reason for the formation of compressive stresses on the ground surfaces, as shown in Fig. 4, since tensile stresses resulting from grinding heat might be overlaid by the compressive stresses due to the excessive ductile deformation. A previous study on grinding burning [10] has shown that the ratio of normal grinding force to tangential grinding force determines the position where plastic deformation ®rst takes place. If the ratio is less than 4, maximum stress appears at a distance further away from the ground surface with a decrease of the force ratio. In the authors' recent study it was found that the force ratio decreased with an increase in grinding temperatures at the contact zone due to the increasing tendency of adhesion between the abrasives and the superalloy at higher temperatures [9]. Therefore the location of the maximum stress is further away from the ground surface with a deeper burning color, as can be seen in Fig. 4. Combining the results of sputtering using Ar ions with Figs. 3 and 4, it can also be seen that the zones in which residual stresses and micro-hardness change obviously, are similar to the affected depth of the plastically deformed coatings, since elastic deformation can be ignored due to the extremely large elastic modulus of K417. The plastic deformation generated in the grinding process also led to a slight increase of micro-hardness in the areas near to the ground surface having a normal color, due to the processing hardening effect. As a comparison, the hardness in the areas away from the ground workpiece surfaces having a burning color are relatively lower, which can be attributed to recrystallization at higher temperatures.
Fig. 6. SEM pictures of the ground workpiece surfaces: (a) normal color specimen; (b) beige specimen; (c) puce specimen; (d) hyacinthine specimen with grinding cracks.
X.P. Xu et al. / Journal of Materials Processing Technology 129 (2002) 359±363
363
can also be helpful in increasing the machining ef®ciency in the CBN grinding, which will be discussed in future papers. 5. Conclusions
Fig. 8. SEM pictures of fracture surfaces after fatigue tests: (a) fatigue crack initiation around TiC; (b) EDS data for the part with in (a); (c) fatigue crack propagation due to grinding cracks.
In order to account for the fatigue test results, the fracture surfaces of the specimens after fatigue tests were examined using SEM. Fig. 8 shows the fatigue cracks owing to the presence of TiC and grinding cracks. Based on the fracture analysis and EDS detection, it is found that the sources for fatigue crack propagation in each specimen are mainly the hardening phase TiC. Provided that the TiC does not decrease, the fatigue life of the specimens may not be affected by the temperature, which could be supported by the results listed in Table 2. For the hyacinthine specimen with grinding cracks, however, fatigue propagation is mainly due to the grinding cracks, which can be seen in Fig. 8(c). Since the decrease of g0 speeds up the propagation of fatigue cracks, the fatigue life decreases obviously, as is already shown in Table 2. Based on the above-mentioned results, it is found that except for surface roughness, the surface integrity of the ground workpiece was not affected by temperature provided that the temperature was not high enough to cause grinding cracks. This ®nding is very important for increasing the production ef®ciency in the grinding of K417, since it suggests that the grinding process can be divided into two stages. In the ®rst stage, the grinding can be conducted at a very high material removal rate without concern for the presence of a burning color. The plastically deformed coatings, which directly roughen the ground surfaces, can be removed in the second stage at a much smaller removal rate, since the thermally affected zones are within 40 mm, which is much smaller compared with the depth of cut adopted in ®ne grinding. Although CBN wheels are used widely in the grinding of superalloys and titanium alloys, the ®ndings in this paper
Excepting for surface roughness, the surface integrity of the ground workpiece is not affected by temperature provided that the temperature is not high enough to cause grinding cracks, which is a very extreme case in the present study. This ®nding is very important for increasing the production ef®ciency in the grinding of K417 since it suggests that the grinding process can be divided into two stages. In the ®rst stage, the grinding can be conducted at a very high material removal rate without concern for the presence of a burning color. The plastically deformed coatings, which can directly roughen the ground surfaces, can be removed in the second stage at a much smaller removal rate, since the thermally affected zones are within 40 mm, which is much smaller compared to the depth of cut adopted in ®ne grinding. Acknowledgements This study was supported by the Natural Science Foundation (NSF) of Fujian Province in China, by the Special Funds from Huaqiao University for Faculty Members Holding Ph.D., and partially by the Aeronautical Science Foundation of China. References [1] W. Osterle, P.X. Li, Mater. Sci. Eng. A 238 (1997) 357. [2] Q. Huang, J.X. Ren, Int. J. Fatigue 13 (1991) 322. [3] I.A. Choudhury, M.A. Eibaradle, Proc. Instn. Mech. Engrs. B 212 (1997) 195. [4] K.V. Kumar, Proceedings of the Fourth International Grinding Conference, SME Paper MR, 1990, pp. 90±505. [5] P.L. Tso, Mater. Proc. Technol. 55 (1997) 421. [6] A.Y.C. Nee, A.O. Tay, Int. J. Mach. Tool Des. Res. 21 (1981) 279. [7] X.P. Xu, S. Malkin, Trans. ASME, J. Manuf. Sci. Eng. 1239 (2001) 191. [8] R. Snoeys, Ann. CIRP 27 (1978) G19. [9] X.P. Xu, Y.Q. Yu, Surf. Interf. Anal. 33 (2002) 343. [10] Y. Furukawa, Ann. CIRP 28 (1979).