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Significant improvement of corrosion resistance of biodegradable metallic implants processed by laser shock peening
CIRP Annals - Manufacturing Technology 61 (2012) 583–586
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Significant improvement of corrosion resistance of biodegradable metallic implants processed by laser shock peening Yuebin Guo a,*, Michael P. Sealy a, Changsheng Guo (2)b a b
Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA United Technologies Research Center, East Hartford, CT 06118, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Surface integrity Laser Corrosion
Biodegradable magnesium–calcium alloys are attractive new orthopedic biomaterials compared to conventional permanent implant alloys. However, magnesium–calcium alloys corrode too fast in human body fluids. This study explores the process capability of laser shock peening (LSP) to control the corrosion of magnesium–calcium implants by tailoring the surface integrity. LSP induced unique surface topographies, highly compressive residual stresses, and extended strain hardening significantly enhanced the corrosion resistance of the alloy by more than 100-fold in simulated body fluid. Furthermore, corrosion of the peened implants was controllable by varying the laser power and peening overlap ratio. ß 2012 CIRP.
1. Introduction Conventional orthopedic permanent implant materials such as stainless steel, titanium, and cobalt–chromium alloys used in fixation devices not only induce stress shielding but also need secondary removal surgery [1,2]. Magnesium–calcium (MgCa) alloys are very promising biodegradable implant materials that avoid issues inherent to permanent metallic alloys. However, the major challenge for MgCa implants is rapid corrosion within the human body. Thus, how to improve the corrosion resistance is the critical technical barrier to realizing its great socioeconomic benefits [3,4]. Calcium alloying and coatings have shown to slightly improve the corrosion resistance in MgCa alloys [5,6]. However, neither alloying nor coatings are capable of imparting a beneficial surface integrity, such as compressive residual stress that is critical to corrosion performance. Machining and burnishing were reported to improve the corrosion resistance by imparting deep compressive residual stresses [2,7]. On the other hand, shot peening, a well known method for imparting compressive residual stress, was found to decrease the corrosion resistance [3,8]. The cause was attributed to surface contamination from the peening media. Machining may also produce surface contamination that cannot be removed by normal cleaning, which could consequently alter the surface biochemistry [9]. Also, burnishing is limited to simple implant geometries due to the physical size constraints of a burnishing tool. An alternative non-contacting surface treatment is laser shock peening (LSP) to cause plastic deformation of a metal via shock pressure wave loading [10]. LSP improves both the mechanical
* Corresponding author. 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.125
properties that control implant degradation and imparts a physiologically favorable surface topography that is better suited for cell attachment and bone ingrowth. Also, LSP is a precision process capable of treating complex implant geometries. The distinctive benefit of LSP over other surface treatments is the capability to produce a gradient surface integrity. This allows for an additional level of functionality that could be tailored to specific applications. Nevertheless, there is a very limited understanding on the role LSP has on controlling the degradation of MgCa implants through adjusting surface integrity. Therefore, the objective of this study is to explore the LSP process capability to significantly improve the corrosion resistance of MgCa implants by tailoring the surface integrity.
2. Laser shock peening (LSP) experimental procedure Cylindrical MgCa0.8 (wt%) samples of 25 mm diameter were LSP processed as shown in Fig. 1. A Continuum1 SureliteTM Nd:YAG laser (wavelength 1064 nm, pulse width 5–7 ns, repetition rate 30 Hz) was used to peen an area of 8 mm 8 mm on the sample surface. The samples were polished to a mirror finish before peening. LSP was performed with three different overlap ratios (25%, 50%, and 75%) and two laser power levels (3 W and 8 W). The corresponding laser power densities were 5.1 GW/cm2 and 13.5 GW/cm2, respectively. The overlap ratio was defined as the percentage of overlap (d) among consecutive peening diameters (Fig. 2). The peening feed (f) was the center-to-center distance between consecutive peens. Table 1 lists the peening feeds for each overlap ratio. By varying the laser power in conjunction with peening overlap ratio, a unique surface integrity for each sample was created to consequently alter in vitro corrosion rates of the MgCa alloy.
Y. Guo et al. / CIRP Annals - Manufacturing Technology 61 (2012) 583–586
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Fig. 3. Surface topography of peened MgCa surfaces.
Fig. 1. LSP experimental setup.
Fig. 2. Theoretical and experimental LSP patterns with different peening overlap ratios.
Representative scans of the surface topography are presented in Fig. 3. As expected, a laser power of 8 W always produced deeper, more compressed surfaces compared to 3 W. Alternatively, changing the overlap ratio did not significantly affect the maximum depth at either power level. Instead, the surface became more compressed as the overlap ratio increased. Surface roughness (Ra) is plotted in Fig. 4a. Since bone cells favor attaching to rougher surfaces, Ra to some degree is indicative of bone’s ability to attach to a rough surface [1]. Therefore, Ra as a surface integrity parameter could potentially be used as a metric to design for biocompatibility. The unpeened surface roughness was 0.15 0.07 mm (95% confidence level). After peening, Ra increased nonlinearly with laser power and peening overlap ratio. Mean amplitude (Rc) is plotted in Fig. 4b. Rc is an average measure of the peak to valley distance. Rc is another potential surface integrity metric that could be incorporated into the design for biocompatibility. Surfaces that exhibit a higher Rc indicate a greater surface area which is favorable for cell adhesion and bone ingrowth [1,3,11]. Increasing the laser power was found to increase Rc, while increasing the overlap ratio generally decreased Rc. This indicated that the height of the pile up region around a peening dent was significantly reduced since average valley depths remained stable across all dent overlap ratios. Reducing the height of the pile-up region is critical to controlling the corrosion performance [6]. Therefore, an optimal value could exist that balances cell adhesion with corrosion performance.
3. Surface integrity characterization
3.2. Microhardness
3.1. Surface topography
Microhardness was measured using a Buehler Hardness Tester and is presented in Fig. 5. When microhardness increases from surface treatments such as LSP, a thin layer of compressed grains form enhancing surface properties such as strength, fatigue life, wear resistance, and corrosion resistance. Each peening condition exhibited an increased hardness relative to the unpeened sample.
The surface topography of an orthopedic implant can critically alter the cellular response at the tissue-implant interface [1]. By varying laser power and peening feed, unique surface topography parameters such as roughness and amplitude were controllable. Surface topography was measured using an Olympus LEXT OLS4000 laser profilometer with a sampling length of 8 mm.
Table 1 Overlap vs. laser power. Overlap (%)
25 50 75
Peening feeds, f (mm) 3W
8W
1.067 0.712 0.356
1.326 0.884 0.442
Fig. 4. (a) Surface roughness (Ra) and (b) mean amplitude (Rc) of peened MgCa surfaces.
Y. Guo et al. / CIRP Annals - Manufacturing Technology 61 (2012) 583–586
585
Fig. 5. Vickers hardness (VHN) of peened MgCa surfaces: (a) top surface and (b) subsurface. Fig. 7. Residual stress characteristics by (a) LSP vs. (b) other processes.
As the peening overlap ratio increased, the magnitude and depth of the microhardness increased. This indicated that with more overlapping peens, the surface became more compressed as well as any existing tensile regions were either reduced or eliminated. Furthermore, an increase in laser power also increased the magnitude and depth of the microhardness. With more laser power, the magnitude of the pressure wave was much higher allowing for more severe plastic deformation. Depending on the peening condition, the microhardness extended 350–600 mm in the subsurface. Another important aspect is the location of the maximum microhardness. At 3 W the maximum hardness was 100 mm in the subsurface for 25% and 50% overlap. At 8 W with 25% and 50% overlap, the subsurface microhardness remained relatively constant for 100 mm before approaching the bulk microhardness. However, for 75% overlap the maximum microhardness shifted to the top surface. 3.3. Microstructures The representative subsurface microstructure of peened MgCa is shown in Fig. 6. Grain sizes ranged from 100 mm to 700 mm. For unpeened vs. peened samples, there were no distinguishable differences in the microstructure. The visible pits in the subsurface are Mg2Ca precipitates formed during the material fabrication process. 3.4. Residual stress The residual stress profile depends on the surface treatment and the work material. Different processes produce unique stress fields that can drastically affect the corrosion resistance. The representative residual stress produced by LSP is shown in Fig. 7a. Residual stresses were measured using a Bruker XRD with a Cu Xray source (l = 0.1542 nm) and applying 35 mA and 40 kV power. Crystallographic plane [1 2 3] corresponding to 2u = 118.488 was used to measure residual strains and then calculate residual stresses by the sin2 c technique. Modulus of elasticity and Poisson’s ratio of MgCa0.8 alloy were 45 GPa and 0.33, respectively. The maximum residual stress in the planar direction occurred on the top surface. In the peening direction 3, the maximum residual stress was in the subsurface. Also, the
Fig. 6. Cross-section microstructure of MgCa before and after peening.
maximum residual stresses in the planar directions 1 and 2 were much higher than in the peening direction 3. For comparative purposes, the characteristic residual stress profiles are shown in Fig. 7b. Residual stress from turning was minimal compared to burnishing or peening. Burnishing and shot peening generate maximum compressive residual stress in the subsurface instead of the top surface [2,12]. The characteristics of residual stress by each process may not change. However, the magnitude of residual stress may change significantly depending on the loading for individual processes. 4. Corrosion performance To determine the effect of LSP on the corrosion rate of MgCa, potentiodynamic tests using a three-electrode cell were conducted on the peened surfaces in LonzaTM Hank’s balanced salt solution (HBSS) 10-527F as shown in Fig. 8. When Mg is exposed to a solution for long periods of time, the pH is capable of rising. However, since the exposure time for each sample was short enough to not elicit a significant pH change, a buffer solution was not used. The average pH among all corrosion tests was 7.55 0.27. Also, the solution was stirred in order to prevent localized pH imbalances. Corrosion rates were measured using an EG&G Potentiostat Model 273A. The scan increment and scan rate were 1 mV and 5 mV/s, respectively. Multiple concurrent potentiodynamic scans were performed for each peening condition since the damage from polarization tests was initially nondestructive. Each scan was approximately 30 min and allowed time for open circuit potential to stabilize. The corrosion rate was determined using the polarization resistance method. SoftCorr II was used to analyze the corrosion data. The corrosion rates and potential (i = 0) of a peened MgCa surface is presented in Fig. 9. The unpeened surface had a corrosion rate of 17 1.2 mm/year. Fig. 9a shows the corrosion rate of a peened surface drastically reduced by at least 100-fold and depended heavily on the laser power and peening overlap. This proves LSP was an enabling process to significantly reduce and control the degradation of MgCa implants.
Fig. 8. Three-electrode potentiodynamic corrosion test.
Y. Guo et al. / CIRP Annals - Manufacturing Technology 61 (2012) 583–586
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Fig. 9. (a) Corrosion rate (CR) and (b) potential of peened MgCa surfaces.
decreased with the peening overlap ratio. While the hardness increased with both laser power and peening overlap, peening overlap had a more predominate influence. The subsurface can be strain hardened up to 500 mm. The maximum residual stress occurred on the top surface in the planar direction and extended approximately 500 mm in the subsurface. Microstructural deformation was not observed due to the large grain size of the MgCa alloy. By correlating individual surface integrity factors with corrosion data, a certain relationship can be established. For surface topography, the corrosion rate was inversely related to the surface roughness (Ra), while it was proportional to the mean amplitude (Rc). The trends for surface hardness as a function of peening process parameters were also inversely related to the corrosion rate. The maximum compressive residual stress of the peened surface in the planar direction may significantly contribute to the decrease of corrosion rates. It is expected that these surface integrity factors have a compound effect on the corrosion performance and cannot be individually isolated. Acknowledgment
Fig. 10. (a) Element wt%; (b) SEM image of unpeened corroded MgCa.
LSP was capable of creating a unique surface integrity to control corrosion initialization and kinetics. For example, surfaces peened at 8 W had higher corrosion rates compared to surfaces peened at 3 W. By comparing the surface integrity at different laser powers, the higher surface roughness (Ra) caused a lower corrosion rate. Also, the corrosion rate significantly reduced when peening at 75% overlap compared to 25% overlap. This suggests that peening patterns can produce a unique surface integrity to control corrosion behavior. Fig. 9b demonstrates a peened surface had less potential to initiate corrosion. The potential to corrode generally decreased by more than 50% compared to the unpeened surface. Energy dispersive spectroscopy (EDS) analysis on the representative peened and corroded surfaces and an SEM image of a corroded surface is shown in Fig. 10. The unpeened corroded sample exhibited a thick uniform layer of corrosion products consisting mainly of oxygen, calcium, and phosphorus. The peened corroded samples had a thinner layer of corrosion products but still retained several localized deposits of oxygen. 5. Conclusions Laser shock peening of a biodegradable MgCa alloy has been initiated to create a superior surface integrity that significantly improves corrosion resistance. The surface integrity was tuned by peening at different laser powers and overlap ratios. The surface integrity was characterized by surface topography, microhardness, residual stress, and subsurface microstructure. The surface roughness (Ra) increased and the mean amplitude (Rc)
This research is based upon the work supported by NSF under Grant No. CMMI-1000706.
References [1] Kieswetter K, Schwartz Z, Dean DD, Boyan BD (1996) The Role of Implant Surface Characteristics in the Healing of Bone. Critical Reviews in Oral Biology & Medicine 7(4):329–345. [2] Denkena B, Lucas A (2007) Biocompatible Magnesium Alloys as Absorbable Implant Materials – Adjusted Surface and Subsurface Properties by Machining Processes. Annals of the CIRP 56(1):113–116. [3] Bruzzone AAG, Costa HL, Lonardo PM, Lucca DA (2008) Advances in Engineered Surfaces for Functional Performance. Annals of the CIRP 57(2):750–769. [4] Staiger MP, Pietak AM, Huadmai J, Dias G (2006) Magnesium and its Alloys as Orthopedic Biomaterials: A Review. Biomaterials 27(9):1728–1734. [5] Kirkland NT, Birbilis N, Walker J, Woodfield T, Dias GJ, Staiger MP (2010) In vitro Dissolution of Magnesium–Calcium Binary Alloys: Clarifying the Unique Role of Calcium Additions in Bioresorbable Magnesium Implant Alloys. Journal of Biomedical Materials Research Part B Applied Biomaterials 95B(1):91–100. [6] Yang J, Cui F, Lee IS (2011) Surface Modifications of Magnesium Alloys for Biomedical Applications. Annals of Biomedical Engineering 39(7):1857–1871. [7] Guo YB, Salahshoor M (2010) Process Mechanics and Surface Integrity by HighSpeed Dry Milling of Biodegradable Magnesium–Calcium Implant Alloys. Annals of the CIRP 59(1):151–154. [8] Dindorf C, Mu¨ller C (2005) Corrosion and Fatigue Behaviour of the Magnesium Die-Cast Alloy AZ91 hp after Surface Treatment, Magnesium. in Kainer KU, (Ed.) Proceedings of the 6th International Conference Magnesium Alloys and Their Applications, Wiley-VCH, Weinheim575–579. [9] Brinksmeier E, Lucca DA, Walter A (2004) Chemical Aspects of Machining Processes. Annals of the CIRP 53(2):685–699. [10] Fairand BP, Wilcox BA, Gallagher WJ, Williams DN (1972) Laser Shock-Induced Microstructure and Mechanical Property Changes in 7075 Aluminum. Journal of Applied Physics 43:3893–3895. [11] Ramsden JJ, Allen DM, Stephenson DJ, Alcock JR, Peggs GN, Fuller G, Goch G (2007) The Design and Manufacture of Biomedical Surfaces. Annals of the CIRP 56(2):687–710. [12] Zhang P, Lindemann J, Leyens C (2010) Shot Peening on the High-Strength Wrought Magnesium Alloy AZ80 – Effect of Peening Media. Journal of Materials Processing Technology 210:445–450.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Significant improvement of corrosion resistance of biodegradable metallic implants processed by laser shock peening Yuebin Guo a,*, Michael P. Sealy a, Changsheng Guo (2)b a b
Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA United Technologies Research Center, East Hartford, CT 06118, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Surface integrity Laser Corrosion
Biodegradable magnesium–calcium alloys are attractive new orthopedic biomaterials compared to conventional permanent implant alloys. However, magnesium–calcium alloys corrode too fast in human body fluids. This study explores the process capability of laser shock peening (LSP) to control the corrosion of magnesium–calcium implants by tailoring the surface integrity. LSP induced unique surface topographies, highly compressive residual stresses, and extended strain hardening significantly enhanced the corrosion resistance of the alloy by more than 100-fold in simulated body fluid. Furthermore, corrosion of the peened implants was controllable by varying the laser power and peening overlap ratio. ß 2012 CIRP.
1. Introduction Conventional orthopedic permanent implant materials such as stainless steel, titanium, and cobalt–chromium alloys used in fixation devices not only induce stress shielding but also need secondary removal surgery [1,2]. Magnesium–calcium (MgCa) alloys are very promising biodegradable implant materials that avoid issues inherent to permanent metallic alloys. However, the major challenge for MgCa implants is rapid corrosion within the human body. Thus, how to improve the corrosion resistance is the critical technical barrier to realizing its great socioeconomic benefits [3,4]. Calcium alloying and coatings have shown to slightly improve the corrosion resistance in MgCa alloys [5,6]. However, neither alloying nor coatings are capable of imparting a beneficial surface integrity, such as compressive residual stress that is critical to corrosion performance. Machining and burnishing were reported to improve the corrosion resistance by imparting deep compressive residual stresses [2,7]. On the other hand, shot peening, a well known method for imparting compressive residual stress, was found to decrease the corrosion resistance [3,8]. The cause was attributed to surface contamination from the peening media. Machining may also produce surface contamination that cannot be removed by normal cleaning, which could consequently alter the surface biochemistry [9]. Also, burnishing is limited to simple implant geometries due to the physical size constraints of a burnishing tool. An alternative non-contacting surface treatment is laser shock peening (LSP) to cause plastic deformation of a metal via shock pressure wave loading [10]. LSP improves both the mechanical
* Corresponding author. 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.125
properties that control implant degradation and imparts a physiologically favorable surface topography that is better suited for cell attachment and bone ingrowth. Also, LSP is a precision process capable of treating complex implant geometries. The distinctive benefit of LSP over other surface treatments is the capability to produce a gradient surface integrity. This allows for an additional level of functionality that could be tailored to specific applications. Nevertheless, there is a very limited understanding on the role LSP has on controlling the degradation of MgCa implants through adjusting surface integrity. Therefore, the objective of this study is to explore the LSP process capability to significantly improve the corrosion resistance of MgCa implants by tailoring the surface integrity.
2. Laser shock peening (LSP) experimental procedure Cylindrical MgCa0.8 (wt%) samples of 25 mm diameter were LSP processed as shown in Fig. 1. A Continuum1 SureliteTM Nd:YAG laser (wavelength 1064 nm, pulse width 5–7 ns, repetition rate 30 Hz) was used to peen an area of 8 mm 8 mm on the sample surface. The samples were polished to a mirror finish before peening. LSP was performed with three different overlap ratios (25%, 50%, and 75%) and two laser power levels (3 W and 8 W). The corresponding laser power densities were 5.1 GW/cm2 and 13.5 GW/cm2, respectively. The overlap ratio was defined as the percentage of overlap (d) among consecutive peening diameters (Fig. 2). The peening feed (f) was the center-to-center distance between consecutive peens. Table 1 lists the peening feeds for each overlap ratio. By varying the laser power in conjunction with peening overlap ratio, a unique surface integrity for each sample was created to consequently alter in vitro corrosion rates of the MgCa alloy.
Y. Guo et al. / CIRP Annals - Manufacturing Technology 61 (2012) 583–586
584
Fig. 3. Surface topography of peened MgCa surfaces.
Fig. 1. LSP experimental setup.
Fig. 2. Theoretical and experimental LSP patterns with different peening overlap ratios.
Representative scans of the surface topography are presented in Fig. 3. As expected, a laser power of 8 W always produced deeper, more compressed surfaces compared to 3 W. Alternatively, changing the overlap ratio did not significantly affect the maximum depth at either power level. Instead, the surface became more compressed as the overlap ratio increased. Surface roughness (Ra) is plotted in Fig. 4a. Since bone cells favor attaching to rougher surfaces, Ra to some degree is indicative of bone’s ability to attach to a rough surface [1]. Therefore, Ra as a surface integrity parameter could potentially be used as a metric to design for biocompatibility. The unpeened surface roughness was 0.15 0.07 mm (95% confidence level). After peening, Ra increased nonlinearly with laser power and peening overlap ratio. Mean amplitude (Rc) is plotted in Fig. 4b. Rc is an average measure of the peak to valley distance. Rc is another potential surface integrity metric that could be incorporated into the design for biocompatibility. Surfaces that exhibit a higher Rc indicate a greater surface area which is favorable for cell adhesion and bone ingrowth [1,3,11]. Increasing the laser power was found to increase Rc, while increasing the overlap ratio generally decreased Rc. This indicated that the height of the pile up region around a peening dent was significantly reduced since average valley depths remained stable across all dent overlap ratios. Reducing the height of the pile-up region is critical to controlling the corrosion performance [6]. Therefore, an optimal value could exist that balances cell adhesion with corrosion performance.
3. Surface integrity characterization
3.2. Microhardness
3.1. Surface topography
Microhardness was measured using a Buehler Hardness Tester and is presented in Fig. 5. When microhardness increases from surface treatments such as LSP, a thin layer of compressed grains form enhancing surface properties such as strength, fatigue life, wear resistance, and corrosion resistance. Each peening condition exhibited an increased hardness relative to the unpeened sample.
The surface topography of an orthopedic implant can critically alter the cellular response at the tissue-implant interface [1]. By varying laser power and peening feed, unique surface topography parameters such as roughness and amplitude were controllable. Surface topography was measured using an Olympus LEXT OLS4000 laser profilometer with a sampling length of 8 mm.
Table 1 Overlap vs. laser power. Overlap (%)
25 50 75
Peening feeds, f (mm) 3W
8W
1.067 0.712 0.356
1.326 0.884 0.442
Fig. 4. (a) Surface roughness (Ra) and (b) mean amplitude (Rc) of peened MgCa surfaces.
Y. Guo et al. / CIRP Annals - Manufacturing Technology 61 (2012) 583–586
585
Fig. 5. Vickers hardness (VHN) of peened MgCa surfaces: (a) top surface and (b) subsurface. Fig. 7. Residual stress characteristics by (a) LSP vs. (b) other processes.
As the peening overlap ratio increased, the magnitude and depth of the microhardness increased. This indicated that with more overlapping peens, the surface became more compressed as well as any existing tensile regions were either reduced or eliminated. Furthermore, an increase in laser power also increased the magnitude and depth of the microhardness. With more laser power, the magnitude of the pressure wave was much higher allowing for more severe plastic deformation. Depending on the peening condition, the microhardness extended 350–600 mm in the subsurface. Another important aspect is the location of the maximum microhardness. At 3 W the maximum hardness was 100 mm in the subsurface for 25% and 50% overlap. At 8 W with 25% and 50% overlap, the subsurface microhardness remained relatively constant for 100 mm before approaching the bulk microhardness. However, for 75% overlap the maximum microhardness shifted to the top surface. 3.3. Microstructures The representative subsurface microstructure of peened MgCa is shown in Fig. 6. Grain sizes ranged from 100 mm to 700 mm. For unpeened vs. peened samples, there were no distinguishable differences in the microstructure. The visible pits in the subsurface are Mg2Ca precipitates formed during the material fabrication process. 3.4. Residual stress The residual stress profile depends on the surface treatment and the work material. Different processes produce unique stress fields that can drastically affect the corrosion resistance. The representative residual stress produced by LSP is shown in Fig. 7a. Residual stresses were measured using a Bruker XRD with a Cu Xray source (l = 0.1542 nm) and applying 35 mA and 40 kV power. Crystallographic plane [1 2 3] corresponding to 2u = 118.488 was used to measure residual strains and then calculate residual stresses by the sin2 c technique. Modulus of elasticity and Poisson’s ratio of MgCa0.8 alloy were 45 GPa and 0.33, respectively. The maximum residual stress in the planar direction occurred on the top surface. In the peening direction 3, the maximum residual stress was in the subsurface. Also, the
Fig. 6. Cross-section microstructure of MgCa before and after peening.
maximum residual stresses in the planar directions 1 and 2 were much higher than in the peening direction 3. For comparative purposes, the characteristic residual stress profiles are shown in Fig. 7b. Residual stress from turning was minimal compared to burnishing or peening. Burnishing and shot peening generate maximum compressive residual stress in the subsurface instead of the top surface [2,12]. The characteristics of residual stress by each process may not change. However, the magnitude of residual stress may change significantly depending on the loading for individual processes. 4. Corrosion performance To determine the effect of LSP on the corrosion rate of MgCa, potentiodynamic tests using a three-electrode cell were conducted on the peened surfaces in LonzaTM Hank’s balanced salt solution (HBSS) 10-527F as shown in Fig. 8. When Mg is exposed to a solution for long periods of time, the pH is capable of rising. However, since the exposure time for each sample was short enough to not elicit a significant pH change, a buffer solution was not used. The average pH among all corrosion tests was 7.55 0.27. Also, the solution was stirred in order to prevent localized pH imbalances. Corrosion rates were measured using an EG&G Potentiostat Model 273A. The scan increment and scan rate were 1 mV and 5 mV/s, respectively. Multiple concurrent potentiodynamic scans were performed for each peening condition since the damage from polarization tests was initially nondestructive. Each scan was approximately 30 min and allowed time for open circuit potential to stabilize. The corrosion rate was determined using the polarization resistance method. SoftCorr II was used to analyze the corrosion data. The corrosion rates and potential (i = 0) of a peened MgCa surface is presented in Fig. 9. The unpeened surface had a corrosion rate of 17 1.2 mm/year. Fig. 9a shows the corrosion rate of a peened surface drastically reduced by at least 100-fold and depended heavily on the laser power and peening overlap. This proves LSP was an enabling process to significantly reduce and control the degradation of MgCa implants.
Fig. 8. Three-electrode potentiodynamic corrosion test.
Y. Guo et al. / CIRP Annals - Manufacturing Technology 61 (2012) 583–586
586
Fig. 9. (a) Corrosion rate (CR) and (b) potential of peened MgCa surfaces.
decreased with the peening overlap ratio. While the hardness increased with both laser power and peening overlap, peening overlap had a more predominate influence. The subsurface can be strain hardened up to 500 mm. The maximum residual stress occurred on the top surface in the planar direction and extended approximately 500 mm in the subsurface. Microstructural deformation was not observed due to the large grain size of the MgCa alloy. By correlating individual surface integrity factors with corrosion data, a certain relationship can be established. For surface topography, the corrosion rate was inversely related to the surface roughness (Ra), while it was proportional to the mean amplitude (Rc). The trends for surface hardness as a function of peening process parameters were also inversely related to the corrosion rate. The maximum compressive residual stress of the peened surface in the planar direction may significantly contribute to the decrease of corrosion rates. It is expected that these surface integrity factors have a compound effect on the corrosion performance and cannot be individually isolated. Acknowledgment
Fig. 10. (a) Element wt%; (b) SEM image of unpeened corroded MgCa.
LSP was capable of creating a unique surface integrity to control corrosion initialization and kinetics. For example, surfaces peened at 8 W had higher corrosion rates compared to surfaces peened at 3 W. By comparing the surface integrity at different laser powers, the higher surface roughness (Ra) caused a lower corrosion rate. Also, the corrosion rate significantly reduced when peening at 75% overlap compared to 25% overlap. This suggests that peening patterns can produce a unique surface integrity to control corrosion behavior. Fig. 9b demonstrates a peened surface had less potential to initiate corrosion. The potential to corrode generally decreased by more than 50% compared to the unpeened surface. Energy dispersive spectroscopy (EDS) analysis on the representative peened and corroded surfaces and an SEM image of a corroded surface is shown in Fig. 10. The unpeened corroded sample exhibited a thick uniform layer of corrosion products consisting mainly of oxygen, calcium, and phosphorus. The peened corroded samples had a thinner layer of corrosion products but still retained several localized deposits of oxygen. 5. Conclusions Laser shock peening of a biodegradable MgCa alloy has been initiated to create a superior surface integrity that significantly improves corrosion resistance. The surface integrity was tuned by peening at different laser powers and overlap ratios. The surface integrity was characterized by surface topography, microhardness, residual stress, and subsurface microstructure. The surface roughness (Ra) increased and the mean amplitude (Rc)
This research is based upon the work supported by NSF under Grant No. CMMI-1000706.
References [1] Kieswetter K, Schwartz Z, Dean DD, Boyan BD (1996) The Role of Implant Surface Characteristics in the Healing of Bone. Critical Reviews in Oral Biology & Medicine 7(4):329–345. [2] Denkena B, Lucas A (2007) Biocompatible Magnesium Alloys as Absorbable Implant Materials – Adjusted Surface and Subsurface Properties by Machining Processes. Annals of the CIRP 56(1):113–116. [3] Bruzzone AAG, Costa HL, Lonardo PM, Lucca DA (2008) Advances in Engineered Surfaces for Functional Performance. Annals of the CIRP 57(2):750–769. [4] Staiger MP, Pietak AM, Huadmai J, Dias G (2006) Magnesium and its Alloys as Orthopedic Biomaterials: A Review. Biomaterials 27(9):1728–1734. [5] Kirkland NT, Birbilis N, Walker J, Woodfield T, Dias GJ, Staiger MP (2010) In vitro Dissolution of Magnesium–Calcium Binary Alloys: Clarifying the Unique Role of Calcium Additions in Bioresorbable Magnesium Implant Alloys. Journal of Biomedical Materials Research Part B Applied Biomaterials 95B(1):91–100. [6] Yang J, Cui F, Lee IS (2011) Surface Modifications of Magnesium Alloys for Biomedical Applications. Annals of Biomedical Engineering 39(7):1857–1871. [7] Guo YB, Salahshoor M (2010) Process Mechanics and Surface Integrity by HighSpeed Dry Milling of Biodegradable Magnesium–Calcium Implant Alloys. Annals of the CIRP 59(1):151–154. [8] Dindorf C, Mu¨ller C (2005) Corrosion and Fatigue Behaviour of the Magnesium Die-Cast Alloy AZ91 hp after Surface Treatment, Magnesium. in Kainer KU, (Ed.) Proceedings of the 6th International Conference Magnesium Alloys and Their Applications, Wiley-VCH, Weinheim575–579. [9] Brinksmeier E, Lucca DA, Walter A (2004) Chemical Aspects of Machining Processes. Annals of the CIRP 53(2):685–699. [10] Fairand BP, Wilcox BA, Gallagher WJ, Williams DN (1972) Laser Shock-Induced Microstructure and Mechanical Property Changes in 7075 Aluminum. Journal of Applied Physics 43:3893–3895. [11] Ramsden JJ, Allen DM, Stephenson DJ, Alcock JR, Peggs GN, Fuller G, Goch G (2007) The Design and Manufacture of Biomedical Surfaces. Annals of the CIRP 56(2):687–710. [12] Zhang P, Lindemann J, Leyens C (2010) Shot Peening on the High-Strength Wrought Magnesium Alloy AZ80 – Effect of Peening Media. Journal of Materials Processing Technology 210:445–450.