- Email: [email protected]
Surface Layer Modifications in Co-Cr-Mo Biomedical Alloy from Cryogenic Burnishing
Available online at www.sciencedirect.com
Procedia Engineering 00 (2012) 000–000 Procedia Engineering 19 (2011) 383 – 388
Procedia Engineering www.elsevier.com/locate/procedia
1st CIRP Conference on Surface Integrity (CSI)
Surface Layer Modifications in Co-Cr-Mo Biomedical Alloy from Cryogenic Burnishing S. Yanga*, D. A. Puleob, O. W. Dillon, Jr.a, I. S. Jawahira a* a
Department of Mechanical Engineering, Institute for Sustainable Manufacturing, University of Kentucky, Lexington 40506 USA; b Center for Biomedical Engineering, University of Kentucky, Lexington 40506, USA
Abstract Severe plastic deformation (SPD) processes have been used to modify the surface integrity properties of many materials by generating ultrafine or even nanometer-sized grains in the surface and subsurface region. These fine grained materials created by SPD and dynamic recrystallization (DRX) usually have higher hardness and frequently exhibit enhanced mechanical properties (wear resistance, corrosion resistance, fatigue life, etc.). A novel burnishing SPD process, cryogenic burnishing, was used to improve the surface integrity of a Co-Cr-Mo biomedical alloy. Results show that grain refinement in the surface region was achieved through burnishing-induced SPD and DRX. The SPD layer thickness was shown to be increased by over 170% with the application of liquid nitrogen compared to the equivalent dry burnished workpiece. Microstructure refinement with a final grain size of 300 nm was obtained from cryogenic burnishing, which reflects a reduction from an initial grain size of about 80 μm. Microhardness measurements indicate that the hardness in the small grained SPD layer created by cryogenic burnishing was increased by 87% relative to the bulk value. Current results show that cryogenic burnishing could be an effective processing method to modify the surface integrity of Co-Cr-Mo biomedical alloy.
© 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of Prof E. Brinksmeier Keywords: Processing, Surface integrity, Microstructure, Cryogenic burnishing
1. Introduction The contact surface layer properties of metal on metal type joint replacements in biomedical applications significantly determine the functional performance of the implants. Severe plastic deformation (SPD) processes have been extensively reported for modifying surface region properties by creating ultrafine or nanometer-sized grains and grain size gradients in the surface region of many products. These nano-grained materials are partially due to the high strain/strain-rate involved in the
* Corresponding author. tel.: +18594940541 E-mail address: [email protected].
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.129
384
S. Yang et al. / Procedia Engineering 19 (2011) 383 – 388 S. Yang et al./ Procedia Engineering 00 (2012) 000–000
processing. Much evidence has shown that SPD induced nano-sized/ultrafine grained materials possess appealing surface integrity properties compared with their coarse-grained counterparts [1-3]. Burnishing is an SPD process with high strain and strain-rate [4]. Significant grain refinement has been reported in the processed surface layers of various metals and alloys. Pu et al. [5] created ultrafine grained surface layers in AZ31 Mg alloy using cryogenic burnishing. Their refined surface layers show better corrosion resistance than their large grain counterparts. Prevey and co-workers [6, 7] showed that low plasticity burnishing can provide a layer of compressive residual stress with sufficient depth to effectively increase the fatigue life of many materials. After roller burnishing of structural Rb40 steel, Hamadache et al. [8] observed 89% wear improvement and up to 9% hardness improvement compared to the sample made by turning. A 40% reduction of wear rate was presented by Radziejewska et al. [9] from burnishing after laser alloying of carbon steels with cobalt stellite. Cryogenic SPD has been reported to successfully introduce thicker surface layers consisting of ultrafine/nano-grained microstructures compared to normal or elevated temperature processing. By using surface mechanical grinding treatment at cryogenic temperatures, Li et al. [10] synthesized a grain size gradient in the nano-micro-structure in the surface layer of bulk pure copper. The average grain sizes varied from about 22 nm in the topmost layer to submicrometer at a 200 μm depth. Ni et al. [11] also reported that by using a cooling fluid during machining, grain size in the secondary deformation zone was reduced from 1.2 μm to 360 nm. The aim of the present study is to investigate the effect of cryogenic burnishing on the surface integrity changes (i.e., grain size, hardness, and phase) of Co-Cr-Mo alloys. A further aim of this study is to establish relationships among burnishing conditions, surface integrity properties and the wear performance of this Co-Cr-Mo alloy. 2. Experimental Work 2.1. Work Material The material used in the present investigation was BioDur CCM alloy, which is a high nitrogen, low carbon wrought version of ASTM F75 Cast Alloy used in biomedical applications. In order to fully ◦ investigate the effects of burnishing, the as-received material was annealed at 1100 C for 1 hour followed by air cooling. The annealed Co-Cr-Mo alloy sample hardness is measured to be 300 HV in average. Disk samples which have a diameter of 50.8 mm and a thickness of 3 mm were cut from such an annealed bar. Although grain refinement is an effective property-improving method that is independent of alloy design, there have been limited reports of the application of SPD techniques to Co-Cr-Mo alloys. This is because Co-Cr-Mo alloy forms a dual-phase microstructure due to a martensitic transformation during cooling to room temperature and straining. This dual phase consists of a metastable fcc-γ phase and an hcp-ε phase and it suffers from poor ductility [12]. Moreover, according to the Co-Cr-Mo ternary phase diagram [13], when Co-Cr-Mo alloy is held at high temperatures, there is a high probability that it will precipitate the σ phase. This can act as a starting point of fracture, making it difficult to simultaneously suppress the σ phase, using microstructure control in conjunction with static recrystallization (SRX), and perform grain refinement. 2.2. Burnishing Process Burnishing experiments were conducted on a Mazak CNC lathe along with ICEFLYTM cryogenic equipment. Liquid nitrogen as a cryogenic coolant was applied to the workpiece on the flank side of the tool-work contact. The specially designed and fabricated burnishing tool with a fixed burnishing roller is shown in Fig. 1. The generated forces were collected by a KISTLER 3-Component Tool Dynamometer.
2
385 3
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An uncoated carbide roller with a diameter of 14.3 mm was chosen as the burnishing tool for the current study. The hardness and surface roughness of the used roller was measured to be 1000 HV and 0.01 μm (Ra), respectively. The roller head was fixed in order to induce enough shear stress and strain to the surface region to cause grain refinement by means of SPD and dynamic recrystallization (DRX). Dry and cryogenic burnishing experiments were conducted with conditions as: feed rate = 0.025 mm/rev, depth of penetration = 0.25 mm, burnishing speed = 100 m/min and burnishing stay time (tool stays time after reaching the desired depth of penetration) = 10 seconds. Nozzle for liquid nitrogen
a
b
c
Fixed roller Co-Cr-Mo discs Tangential Force Fig. 1. (a) Application of liquid nitrogen during cryogenic burnishing; (b) experiment setup for cryogenic burnishing; (c) schematic illustration of cryogenic burnishing
2.3. Material Characterization Measurements of the materials’ microhardness and microstructure in the surface region were conducted before and after the processing. Microhardness measurements of the Co-Cr-Mo specimens were made by using a Vickers indenter on a CLARK Digital Microhardness Tester with 100gf applied load. Microstructure analysis was conducted by using an Olympus BX41 optical microscope, an S-4300 Hitachi scanning electron microscope (SEM) and a Bruker AXS D8 X-ray equipment. 3. Results and Discussion 3.1. Burnishing Forces Cryogenic burnishing produced 13% higher radial and 10% higher tangential forces compared to dry burnishing. In the current study, liquid nitrogen was applied on the tool flank side toward the workpiece surface, when the temperature is reduced, the workpiece material has a higher yield stress and is harder [14], and in turn, the burnishing forces increase. 3.2. Microstructure and Microhardness The initial microstructure prior to burnishing is shown in Fig. 2(a); the grain boundaries are clearly visible throughout the specimen with an average grain size of 80 µm. Fig. 2(b) and 2(c) show the microstructure of the surface regions after dry and cryogenic burnishing, respectively. Within these regions, SPD layers where the grain boundaries are not discernable (Fig. 2b and c) are created. This SPD layer with indiscernible grain boundaries was also reported in other materials after burnishing [10, 15]. The measured SPD layer thicknesses are shown on Fig. 2. It is evident that with the application of liquid nitrogen (Fig. 2(c)), the layer thickness was increased by over 170% compared to the dry burnished workpiece (Fig. 2(b)). This suggests that cryogenic cooling has substantial influence on the surface layer developed during SPD processing. For dry burnishing, the effects of plastic deformation on the surface layer are compromised by the large amount of heat generated during processing. On the other hand, liquid
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S. Yang et al. / Procedia Engineering 19 (2011) 383 – 388 S. Yang et al./ Procedia Engineering 00 (2012) 000–000
4
nitrogen application effectively suppresses the temperature changes and increases the process influenced depth to a larger extent. The subsurface microhardness variations obtained from different cooling conditions are illustrated in Fig. 2(d). The average measured microhardness of the virgin disk is 300 HV. The hardness decreases significantly with increase in depth, which coincided with the microstructure observed in these samples. Compared to the initial disc hardness, an increase of up to 87% was achieved using cryogenic burnishing. Based on the well-known Hall-Petch relationship between yield stress and grain size as well as the close interrelations among hardness, yield stress, and residual stresses, high hardness values often indicate fine grain size and large residual stresses. In our current work, microstructure changes due to cryogenic cooling were observed to a 105 μm depth from the surface. In contrast, microhardness differences were measured to the depth of over 1000 μm. It is reasonable to state that the variations in microhardness were likely due to the different residual stresses being generated during processing. The residual stresses of the processed samples will be measured using X-ray diffraction techniques to validate this hypothesis. a
b
d
c
SPD layer = 38 μm SPD layer = 105 μm
Fig. 2. Microstructures of Co-Cr-Mo discs: (a) initial microstructure, (b) after dry burnishing, (c) after cryogenic burnishing; (d) Subsurface microhardness profiles for burnished Co-Cr-Mo discs
Scanning electron microscopy (SEM) pictures of the typical microstructures near the topmost surface are shown in Figure 3. While the average longitudinal axis grain size is 80 µm for the initial material, the grain size near the topmost surface (shown in Fig. 3(b)) was reduced to submicron scale after cryogenic burnishing. The smallest grain size achieved was 298 nm. As the depth from the surface increases, the amount of ultrafine grains decreases. It was reported that the volume fraction of dynamically recrystallized grains increased with strain in a sigmoidal scheme [16]. Since the strain induced by burnishing decreases with distance from the surface to the bulk material, it is expected that the amount of ultrafine grains decreases with increasing depth. The grains beneath the SPD layer, especially for the cryogenic burnished sample (Fig. 2(c)), show heavy twinning and many stacking-faults compared to the initial sample surface (Fig. 2(a)). Moreover, smaller grain size was achieved by cryogenic burnishing. Fig. 3 shows the longitudinal axis grain size distributions from dry and cryogenic burnishing. More than a
Dry
5 μm
b
Cryogenic
5 μm
Fig. 3. SEM micrographs and longitudinal axis grain size distributions of burnished Co-Cr-Mo discs: (a) after dry burnishing; (b) after cryogenic burnishing
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70% of the grains from cryogenic burnishing falls into the range from 300 nm to 600 nm, while about 80% of the grains from dry burnishing is around an one-micron scale. Therefore, it is evident that thicker surface layers with remarkable grain refinement occurred due to the SPD-induced DRX and effective liquid nitrogen cooling. Moreover, comparing to dry burnishing, the larger burnishing force from cryogenic burnishing also lead to more severe plastic deformation. 3.3. Phase Analysis by X-ray Diffraction (XRD) Method The XRD 2θ scans were carried out from 39o to 100o. Fig. 4 shows the detailed slow XRD 2θ scan profiles from 39o to 53o. A comparison of these XRD patterns shows that in the initial sample, both fcc-γ phase and hcp-ε phase are observed in the surface. On the other hand, the XRD patterns taken from the dry and cryogenic burnished surface consist mainly of fcc-γ phase. The formation of the hcp-ε phase is totally suppressed by the burnishing process (both dry and cryogenic). The suppression of martensitic-ε transformation [17] is conjectured to be a result of grain refinement to smaller than 10 μm. Moreover, no peaks indicating the precipitation of the σ phase were observed. 111γ 0002ε 200γ
1010ε
Fig. 4. XRD spectra of initial and burnished samples
Table 1 shows the full width at half maximum (FWHM) results of burnished and initial samples; the FWHMs of burnished samples were much larger than that of the initial sample, and the FWHMs of the cryogenic burnished sample are slightly larger than the ones from dry burnishing. It is reported that the peak broadening is contributed by decreasing grain size and increasing dislocation density [18], which is consistent with the microstructure observations. The dislocation density effect will be revealed by residual stress measurements soon. Table 1. FWHMs of different measured peaks Peak
Samples
FWHM (o)
Peak
Samples
FWHM(o)
111γ
Initial Dry burnished Cryogenic burnished
0.101 0.371 0.380
200γ
Initial Dry burnished Cryogenic burnished
0.174 0.385 0.390
4. Conclusions Burnishing experiments were conducted on annealed Co-Cr-Mo alloy using a novel burnishing tool with liquid nitrogen applied to the flank side of the tool-workpiece contact surface during burnishing. Experimental results showed that the microstructure of Co-Cr-Mo alloy can be significantly improved by changing cooling conditions. Compared to dry conditions, the created SPD layer thickness was increased up to 170% with the application of liquid nitrogen. Grain refinement in the surface region was achieved through burnishing-induced SPD. Microstructure refinement with a smallest grain size of 298 nm was obtained following cryogenic burnishing, which is a reduction from an initial grain size of 80 μm.
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Microhardness measurements indicated that the hardness in the SPD layer from cryogenic burnishing was increased up to 87% relative to the bulk value. The present results demonstrate that cryogenic burnishing can significantly modify the surface properties of Co-Cr-Mo alloy. Acknowledgements The authors sincerely thank to Air Products and Chemicals for supplying the equipment for liquid nitrogen application. References [1] Wang ZB, Tao NR, Li S, Wang W, Liu G, Lu J, Lu K, Effect of surface nanocrystallization on friction and wear properties in low carbon steel, Mater. Sci. Eng. 2003, 352:144-49. [2] Shi YN, Han Z, Tribological behaviors of nanostructured surface layer processed by means of surface mechanical attrition treatment, Key Eng. Mater. 2008, 384:321-34. [3] Qi Z, Jiang J, Meletis EI, Wear mechanism of nanocrystalline metals, J Nanosci Nanotechnol. 2009, 9:4227-32. [4] Skalski K, Morawski A, Przybylski W, Analysis of contact elastic-plastic strains during the process of burnishing, Int. J. Mech. Sci. 1995, 37:461–72. [5] Pu Z, Yang S, Song G-L, Dillon OW, Puleo DA and Jawahir IS, Ultrafine-grained surface layer on Mg–Al–Zn alloy produced by cryogenic burnishing for enhanced corrosion resistance, Scripta Materialia, 65:520-23. [6] Prevey P, Cammett J, Low cost corrosion damage mitigation and improved fatigue performance of low plasticity burnished 7075-T6, J. Mater. Eng. Perform. 2001, 10:548–55. [7] Prevey P, Telesman J, Gabb T, Kantzos P, FOD resistance and fatigue crack arrest in low plasticity burnished IN718, In: Proceedings of the 5th Nat. Turbine Eng. HCF Conf., 2000. [8] Hamadache H, Laouar L, Zeghib NE, Chaoui K, Characteristics of Rb40 steel superficial layer under ball and roller burnishing, J. Mater. Process. Technol. 2006, 180:130-36. [9] Radziejewska J, Nowicki B, Kalita W, Hybrid method for modification of surface layer, In: Proc. 13th Intern. Sci. Conf. CO-MAT-TECH, Trnava, Slovakia 2005, p. 111–16. [10] Li WL, Tao NR, Lu K, Fabrication of a gradient nano-micro-structured surface layer on bulk copper by means of a surface mechanical grinding treatment, Scripta Material 2008, 59:546–49. [11] Ni H, Elmadagli M, Alpas AT, Mechanical properties and microstructures of 1100 aluminum subjected to dry machining, Mater. Sci. Eng. A 2004, 385:267–78. [12] Yamanaka K, Mori M, Kurosu S, Matsumoto H, Chiba A, Ultrafine grain refinement of biomedical Co-29Cr-6Mo alloy during conventional hot-compression deformation, Metallurgical and Mater. Trans. A 2009, 40:1980-94. [13] Sitdikov O, Sakai T, Avtokratova E, Kaibyshev R, Tsuzaki K, and Watanabe Y, Microstructure behavior of Al-Mg-Sc alloy processed by ECAP at elevated temperature, Acta Mater. 2008, 56:821–34. [14] Zhao Z, Hong SY, Cooling strategies for cryogenic machining from a materials viewpoint, J. Mater. Eng. Perform. 1992, 1 (5):669–78. [15] Nalla RK, Altenberger I, Noster U, Liu GY, Scholtes B, Ritchie RO, On the influence of mechanical surface treatmentsdeep rolling and laser shock peening on the fatigue behavior of Ti-6Al-4V at ambient and elevated temperatures, Mater. Sci. Eng. A 2003, 355:216-30. [16] Fatemi-Varzaneh SM, Zarei-Hanzaki A, Beladi H, Dynamic recrystallization in AZ31 magnesium alloy, Mater. Sci. Eng. A 2007, 456:52-7. [17] Huang P, Lopez HF, Effects of grain size on development of athermal and strain induced ε martensite in Co–Cr–Mo implant alloy, Mach. Sci. and Technol. 1999, 15:157-64. [18] Jiang XP, Wang XY, Li JX, Li DY, Man C-S, Shepard MJ, Zhai T, Enhancement of fatigue and corrosion properties of pure Ti by sandblasting, Mater. Sci. Eng. A 2006, 429:30-5.
6
Procedia Engineering 00 (2012) 000–000 Procedia Engineering 19 (2011) 383 – 388
Procedia Engineering www.elsevier.com/locate/procedia
1st CIRP Conference on Surface Integrity (CSI)
Surface Layer Modifications in Co-Cr-Mo Biomedical Alloy from Cryogenic Burnishing S. Yanga*, D. A. Puleob, O. W. Dillon, Jr.a, I. S. Jawahira a* a
Department of Mechanical Engineering, Institute for Sustainable Manufacturing, University of Kentucky, Lexington 40506 USA; b Center for Biomedical Engineering, University of Kentucky, Lexington 40506, USA
Abstract Severe plastic deformation (SPD) processes have been used to modify the surface integrity properties of many materials by generating ultrafine or even nanometer-sized grains in the surface and subsurface region. These fine grained materials created by SPD and dynamic recrystallization (DRX) usually have higher hardness and frequently exhibit enhanced mechanical properties (wear resistance, corrosion resistance, fatigue life, etc.). A novel burnishing SPD process, cryogenic burnishing, was used to improve the surface integrity of a Co-Cr-Mo biomedical alloy. Results show that grain refinement in the surface region was achieved through burnishing-induced SPD and DRX. The SPD layer thickness was shown to be increased by over 170% with the application of liquid nitrogen compared to the equivalent dry burnished workpiece. Microstructure refinement with a final grain size of 300 nm was obtained from cryogenic burnishing, which reflects a reduction from an initial grain size of about 80 μm. Microhardness measurements indicate that the hardness in the small grained SPD layer created by cryogenic burnishing was increased by 87% relative to the bulk value. Current results show that cryogenic burnishing could be an effective processing method to modify the surface integrity of Co-Cr-Mo biomedical alloy.
© 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of Prof E. Brinksmeier Keywords: Processing, Surface integrity, Microstructure, Cryogenic burnishing
1. Introduction The contact surface layer properties of metal on metal type joint replacements in biomedical applications significantly determine the functional performance of the implants. Severe plastic deformation (SPD) processes have been extensively reported for modifying surface region properties by creating ultrafine or nanometer-sized grains and grain size gradients in the surface region of many products. These nano-grained materials are partially due to the high strain/strain-rate involved in the
* Corresponding author. tel.: +18594940541 E-mail address: [email protected].
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.129
384
S. Yang et al. / Procedia Engineering 19 (2011) 383 – 388 S. Yang et al./ Procedia Engineering 00 (2012) 000–000
processing. Much evidence has shown that SPD induced nano-sized/ultrafine grained materials possess appealing surface integrity properties compared with their coarse-grained counterparts [1-3]. Burnishing is an SPD process with high strain and strain-rate [4]. Significant grain refinement has been reported in the processed surface layers of various metals and alloys. Pu et al. [5] created ultrafine grained surface layers in AZ31 Mg alloy using cryogenic burnishing. Their refined surface layers show better corrosion resistance than their large grain counterparts. Prevey and co-workers [6, 7] showed that low plasticity burnishing can provide a layer of compressive residual stress with sufficient depth to effectively increase the fatigue life of many materials. After roller burnishing of structural Rb40 steel, Hamadache et al. [8] observed 89% wear improvement and up to 9% hardness improvement compared to the sample made by turning. A 40% reduction of wear rate was presented by Radziejewska et al. [9] from burnishing after laser alloying of carbon steels with cobalt stellite. Cryogenic SPD has been reported to successfully introduce thicker surface layers consisting of ultrafine/nano-grained microstructures compared to normal or elevated temperature processing. By using surface mechanical grinding treatment at cryogenic temperatures, Li et al. [10] synthesized a grain size gradient in the nano-micro-structure in the surface layer of bulk pure copper. The average grain sizes varied from about 22 nm in the topmost layer to submicrometer at a 200 μm depth. Ni et al. [11] also reported that by using a cooling fluid during machining, grain size in the secondary deformation zone was reduced from 1.2 μm to 360 nm. The aim of the present study is to investigate the effect of cryogenic burnishing on the surface integrity changes (i.e., grain size, hardness, and phase) of Co-Cr-Mo alloys. A further aim of this study is to establish relationships among burnishing conditions, surface integrity properties and the wear performance of this Co-Cr-Mo alloy. 2. Experimental Work 2.1. Work Material The material used in the present investigation was BioDur CCM alloy, which is a high nitrogen, low carbon wrought version of ASTM F75 Cast Alloy used in biomedical applications. In order to fully ◦ investigate the effects of burnishing, the as-received material was annealed at 1100 C for 1 hour followed by air cooling. The annealed Co-Cr-Mo alloy sample hardness is measured to be 300 HV in average. Disk samples which have a diameter of 50.8 mm and a thickness of 3 mm were cut from such an annealed bar. Although grain refinement is an effective property-improving method that is independent of alloy design, there have been limited reports of the application of SPD techniques to Co-Cr-Mo alloys. This is because Co-Cr-Mo alloy forms a dual-phase microstructure due to a martensitic transformation during cooling to room temperature and straining. This dual phase consists of a metastable fcc-γ phase and an hcp-ε phase and it suffers from poor ductility [12]. Moreover, according to the Co-Cr-Mo ternary phase diagram [13], when Co-Cr-Mo alloy is held at high temperatures, there is a high probability that it will precipitate the σ phase. This can act as a starting point of fracture, making it difficult to simultaneously suppress the σ phase, using microstructure control in conjunction with static recrystallization (SRX), and perform grain refinement. 2.2. Burnishing Process Burnishing experiments were conducted on a Mazak CNC lathe along with ICEFLYTM cryogenic equipment. Liquid nitrogen as a cryogenic coolant was applied to the workpiece on the flank side of the tool-work contact. The specially designed and fabricated burnishing tool with a fixed burnishing roller is shown in Fig. 1. The generated forces were collected by a KISTLER 3-Component Tool Dynamometer.
2
385 3
S.S.Yang et et al.al./ / Procedia – 388 Yang ProcediaEngineering Engineering1900(2011) (2012)383 000–000
An uncoated carbide roller with a diameter of 14.3 mm was chosen as the burnishing tool for the current study. The hardness and surface roughness of the used roller was measured to be 1000 HV and 0.01 μm (Ra), respectively. The roller head was fixed in order to induce enough shear stress and strain to the surface region to cause grain refinement by means of SPD and dynamic recrystallization (DRX). Dry and cryogenic burnishing experiments were conducted with conditions as: feed rate = 0.025 mm/rev, depth of penetration = 0.25 mm, burnishing speed = 100 m/min and burnishing stay time (tool stays time after reaching the desired depth of penetration) = 10 seconds. Nozzle for liquid nitrogen
a
b
c
Fixed roller Co-Cr-Mo discs Tangential Force Fig. 1. (a) Application of liquid nitrogen during cryogenic burnishing; (b) experiment setup for cryogenic burnishing; (c) schematic illustration of cryogenic burnishing
2.3. Material Characterization Measurements of the materials’ microhardness and microstructure in the surface region were conducted before and after the processing. Microhardness measurements of the Co-Cr-Mo specimens were made by using a Vickers indenter on a CLARK Digital Microhardness Tester with 100gf applied load. Microstructure analysis was conducted by using an Olympus BX41 optical microscope, an S-4300 Hitachi scanning electron microscope (SEM) and a Bruker AXS D8 X-ray equipment. 3. Results and Discussion 3.1. Burnishing Forces Cryogenic burnishing produced 13% higher radial and 10% higher tangential forces compared to dry burnishing. In the current study, liquid nitrogen was applied on the tool flank side toward the workpiece surface, when the temperature is reduced, the workpiece material has a higher yield stress and is harder [14], and in turn, the burnishing forces increase. 3.2. Microstructure and Microhardness The initial microstructure prior to burnishing is shown in Fig. 2(a); the grain boundaries are clearly visible throughout the specimen with an average grain size of 80 µm. Fig. 2(b) and 2(c) show the microstructure of the surface regions after dry and cryogenic burnishing, respectively. Within these regions, SPD layers where the grain boundaries are not discernable (Fig. 2b and c) are created. This SPD layer with indiscernible grain boundaries was also reported in other materials after burnishing [10, 15]. The measured SPD layer thicknesses are shown on Fig. 2. It is evident that with the application of liquid nitrogen (Fig. 2(c)), the layer thickness was increased by over 170% compared to the dry burnished workpiece (Fig. 2(b)). This suggests that cryogenic cooling has substantial influence on the surface layer developed during SPD processing. For dry burnishing, the effects of plastic deformation on the surface layer are compromised by the large amount of heat generated during processing. On the other hand, liquid
386
S. Yang et al. / Procedia Engineering 19 (2011) 383 – 388 S. Yang et al./ Procedia Engineering 00 (2012) 000–000
4
nitrogen application effectively suppresses the temperature changes and increases the process influenced depth to a larger extent. The subsurface microhardness variations obtained from different cooling conditions are illustrated in Fig. 2(d). The average measured microhardness of the virgin disk is 300 HV. The hardness decreases significantly with increase in depth, which coincided with the microstructure observed in these samples. Compared to the initial disc hardness, an increase of up to 87% was achieved using cryogenic burnishing. Based on the well-known Hall-Petch relationship between yield stress and grain size as well as the close interrelations among hardness, yield stress, and residual stresses, high hardness values often indicate fine grain size and large residual stresses. In our current work, microstructure changes due to cryogenic cooling were observed to a 105 μm depth from the surface. In contrast, microhardness differences were measured to the depth of over 1000 μm. It is reasonable to state that the variations in microhardness were likely due to the different residual stresses being generated during processing. The residual stresses of the processed samples will be measured using X-ray diffraction techniques to validate this hypothesis. a
b
d
c
SPD layer = 38 μm SPD layer = 105 μm
Fig. 2. Microstructures of Co-Cr-Mo discs: (a) initial microstructure, (b) after dry burnishing, (c) after cryogenic burnishing; (d) Subsurface microhardness profiles for burnished Co-Cr-Mo discs
Scanning electron microscopy (SEM) pictures of the typical microstructures near the topmost surface are shown in Figure 3. While the average longitudinal axis grain size is 80 µm for the initial material, the grain size near the topmost surface (shown in Fig. 3(b)) was reduced to submicron scale after cryogenic burnishing. The smallest grain size achieved was 298 nm. As the depth from the surface increases, the amount of ultrafine grains decreases. It was reported that the volume fraction of dynamically recrystallized grains increased with strain in a sigmoidal scheme [16]. Since the strain induced by burnishing decreases with distance from the surface to the bulk material, it is expected that the amount of ultrafine grains decreases with increasing depth. The grains beneath the SPD layer, especially for the cryogenic burnished sample (Fig. 2(c)), show heavy twinning and many stacking-faults compared to the initial sample surface (Fig. 2(a)). Moreover, smaller grain size was achieved by cryogenic burnishing. Fig. 3 shows the longitudinal axis grain size distributions from dry and cryogenic burnishing. More than a
Dry
5 μm
b
Cryogenic
5 μm
Fig. 3. SEM micrographs and longitudinal axis grain size distributions of burnished Co-Cr-Mo discs: (a) after dry burnishing; (b) after cryogenic burnishing
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70% of the grains from cryogenic burnishing falls into the range from 300 nm to 600 nm, while about 80% of the grains from dry burnishing is around an one-micron scale. Therefore, it is evident that thicker surface layers with remarkable grain refinement occurred due to the SPD-induced DRX and effective liquid nitrogen cooling. Moreover, comparing to dry burnishing, the larger burnishing force from cryogenic burnishing also lead to more severe plastic deformation. 3.3. Phase Analysis by X-ray Diffraction (XRD) Method The XRD 2θ scans were carried out from 39o to 100o. Fig. 4 shows the detailed slow XRD 2θ scan profiles from 39o to 53o. A comparison of these XRD patterns shows that in the initial sample, both fcc-γ phase and hcp-ε phase are observed in the surface. On the other hand, the XRD patterns taken from the dry and cryogenic burnished surface consist mainly of fcc-γ phase. The formation of the hcp-ε phase is totally suppressed by the burnishing process (both dry and cryogenic). The suppression of martensitic-ε transformation [17] is conjectured to be a result of grain refinement to smaller than 10 μm. Moreover, no peaks indicating the precipitation of the σ phase were observed. 111γ 0002ε 200γ
1010ε
Fig. 4. XRD spectra of initial and burnished samples
Table 1 shows the full width at half maximum (FWHM) results of burnished and initial samples; the FWHMs of burnished samples were much larger than that of the initial sample, and the FWHMs of the cryogenic burnished sample are slightly larger than the ones from dry burnishing. It is reported that the peak broadening is contributed by decreasing grain size and increasing dislocation density [18], which is consistent with the microstructure observations. The dislocation density effect will be revealed by residual stress measurements soon. Table 1. FWHMs of different measured peaks Peak
Samples
FWHM (o)
Peak
Samples
FWHM(o)
111γ
Initial Dry burnished Cryogenic burnished
0.101 0.371 0.380
200γ
Initial Dry burnished Cryogenic burnished
0.174 0.385 0.390
4. Conclusions Burnishing experiments were conducted on annealed Co-Cr-Mo alloy using a novel burnishing tool with liquid nitrogen applied to the flank side of the tool-workpiece contact surface during burnishing. Experimental results showed that the microstructure of Co-Cr-Mo alloy can be significantly improved by changing cooling conditions. Compared to dry conditions, the created SPD layer thickness was increased up to 170% with the application of liquid nitrogen. Grain refinement in the surface region was achieved through burnishing-induced SPD. Microstructure refinement with a smallest grain size of 298 nm was obtained following cryogenic burnishing, which is a reduction from an initial grain size of 80 μm.
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