journal of the mechanical behavior of biomedical materials 32 (2014) 335–344

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Research Paper

Microstructure, mechanical and wear properties of laser surface melted Ti6Al4V alloy Vamsi Krishna Balla1, Julie Soderlind, Susmita Bose, Amit Bandyopadhyayn W.M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, USA

art i cle i nfo

ab st rac t

Article history:

Laser surface melting (LSM) of Ti6Al4V alloy was carried out with an aim to improve

Received 23 July 2013

properties such as microstructure and wear for implant applications. The alloy substrate

Received in revised form

was melted at 250 W and 400 W at a scan velocity of 5 mm/s, with input energy of 42 J/mm2

22 November 2013

and 68 J/mm2, respectively. The results showed that equiaxed αþβ microstructure of the

Accepted 1 December 2013

substrate changes to mixture of acicular α in β matrix after LSM due to high cooling rates in

Available online 8 December 2013

the range of 2.25  10  3 K/s and 1.41  10  3 K/s during LSM. Increasing the energy input

Keywords:

increased the thickness of remelted region from 779 to 802 mm and 1173 to 1199 mm.

Ti6Al4V alloy

Similarly, as a result of slow cooling rates under present experimental conditions, the grain

Laser processing

size of the alloy increased from 4.8 μm to 154–199 μm. However, the hardness of the

Surface melting

Ti6Al4V alloy increased due to LSM melting and resulted in lowest in vitro wear rate of

Wear

3.38  10  4 mm3/Nm compared to untreated substrate with a wear rate of 6.82  10  4 mm3/

Surface modification

Nm.

1.

Introduction

Although Ti6Al4V alloy is widely used as an implant material for load-bearing and non-load-bearing implants due to its excellent biocompatibility and corrosion resistance, its low hardness and poor wear resistance is still a serious concern. Majority of intended implant applications of Ti6Al4V alloy does not require very high wear resistance. For example, the hip stem in the total hip prosthesis do not undergo any major articulation but significant amount of micro-motions can occur at two interfaces e.g., (i) femoral neck (Ti6Al4V alloy) and femoral head (CoCrMo alloy or Al2O3 or Zirconia toughened Al2O3) and (ii) hip stem and femur bone. These micro-motions can generate high wear debris, which can limit long-term n

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stability of load-bearing implants (Fukunishi, 1995; Weinans et al., 1993). The severity of this problem may be magnified several times in younger and more active patients. The wear debris generated can lead to metal sensitivity and osteolysis resulting in premature failure of the implants. It has been reported that patients with a functioning and failed metal prosthesis show relatively high metal sensitivity of 25% and 60% respectively, compared to 10–17% in general population (Hallab et al., 2001a; Frigerio et al., 2011; Thyssen et al., 2007, 2009; Granchi et al., 2006a, 2006b). Most investigators correlate the observed implant loosening, failure and shorter lifespan to wear debris and metal sensitivity (Hallab et al., 2001a; Frigerio et al., 2011; Thyssen et al., 2007, 2009; Granchi et al., 2006a, 2006b). The implant derived wear debris is one of the major

Corresponding author. Tel.: þ1509 335 4862; fax: þ1509 335 4662. E-mail address: [email protected] (A. Bandyopadhyay). 1 Current address: Central Glass & Ceramic Research Institute (CGCRI), 196, Raja S.C. Mullick Road, Kolkata 700032, India.

1751-6161/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2013.12.001

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journal of the mechanical behavior of biomedical materials 32 (2014) 335 –344

causes of implant failures due to osteolysis (Jasty, 1993; McGee et al., 2000; Edidin et al., 2001; Granchi et al., 2006a, 2006b; Hallab et al., 2001b). Since the electrochemical dissolution and wear of Ti6Al4V alloy implants originate from the implants' surfaces, it is plausible that tailoring the surface characteristics such as microstructure, composition and mechanical properties via appropriate surface modification technique can potentially minimize or even eliminate implant failures due to osteolysis and metal sensitivity. Therefore, several surface modification approaches such as laser deposition of wear resistant alloys/ composites (Das et al., 2010, 2011, 2012; Balla et al., 2012; Dittrick et al., 2011a; Bandyopadhyay et al., 2011), laser processed compositionally gradient coatings (Dittrick et al., 2011b; Balla et al., 2009a; Krishna et al., 2008), laser assisted oxidation (Balla et al., 2009b), laser nitriding (Geetha et al., 2009; Sathish et al., 2010), thermal oxidation (Dong and Bell, 2000) diamond like carbon coatings (Saikko et al., 2001), nitrogen diffusion hardening (Rodriguez et al., 1998), TiN coatings (Pappas et al., 1995; Ward et al., 1998) and ion implantation (Torregrosa et al., 1995) have been attempted to improve tribological and corrosion properties of titanium (Ti) and its alloys. Among these techniques laser based surface modification is gaining importance as a promising approach. In general laser processing provide several advantages such as refined and homogeneous microstructures, minimum dilution, small heat-affected zone, non-equilibrium microstructures, diffused interface with superior bonding and mechanical properties (Das et al., 2010, 2011, 2012; Balla et al., 2009a, 2012; Dittrick et al., 2011a; Bandyopadhyay et al., 2011; Dittrick et al., 2011b; Krishna et al., 2008; Steen and Watkins, 1993; Jiang et al., 2000). Other benefits include flexible process parameters, automation and high speed processing. Yerramareddy and Bahadur (1992) reported the influence of laser surface melting (LSM), laser nitriding and nickel alloying on tribological properties of Ti6Al4V alloy. It was reported that LSM can improve only dry sliding wear resistance (Yerramareddy and Bahadur, 1992). In another study, LSM of Ti6Al4V alloy has been shown to improve the pitting corrosion resistance and hardness (Biswas et al., 2007). Preliminary studies on surface melting of commercially pure titanium have also been reported (Cotogno et al., 2006). From these studies, it appears that research on LSM of Ti6Al4V alloy primarily for biomedical applications (Singh et al., 2006; Chikarakara et al., 2012; Biswas et al., 2007; Cotogno et al., 2006) are scarce. In this study, laser surface melting of Ti6Al4V alloy was carried out using Laser Engineered Net Shaping (LENS™) primarily to understand the influence of process parameters on microstructure and mechanical properties. The influence of laser power and number of passes on the microstructure, hardness and in vitro wear resistance was evaluated. Further, the relative concentration of α and β phases in the surface melted regions were estimated from X-ray diffraction results and were correlated with observed mechanical and tribological properties.

2.

Experimental

A Ti6Al4V alloy sheet (President Titanium, Hanson, MA, USA) with 3 mm thickness was used. The surface of Ti6Al4V

alloy sheet was melted using a 500 W continuous wave Nd: YAG laser in Laser Engineered Net Shaping (LENS™-750, Optomec Inc. Albuquerque, NM). On the substrate, several regions of ϕ 10 mm were melted at 5 mm/s scan velocity using 250 W and 400 W power for single and double passes. From the preliminary experiments, it was found that minimum laser power required for melting the Ti6Al4V alloy substrate was 250 W. Therefore, 250 W and 400 W were chosen corresponding to laser energy inputs of 42 J/mm2 and 68 J/mm2. The laser surface melting experiments were carried out in a glove box purged with argon and the oxygen content was less than 10 ppm. The melted regions were sectioned and characterized. The cross-sectional microstructures of the surface melted samples were observed, after metallographic preparation and etching, using light microscope and field emission scanning electron microscope (FEI-SIRION, 200 F). Siemens D 500 Kristalloflex diffractometer with Cu Kα radiation (1.54056 Å) at 20 kV was used between 2θ range of 30–901 to identify the constituent phases formed after laser melting and were compared with those of as-received Ti6Al4V alloy substrate. The following relationship was used to estimate the relative concentration of α and β in the laser melted regions of Ti6Al4V alloy substrate from the intensities of the reflections of X-ray diffraction data. ! IðxÞ 100 ð1Þ RðxÞ ¼ ∑IðAÞ where R(x) is the % relative intensity corresponsing to the “x” phase (in the present work it is either of α or β); I(x) is the intensity of reflection corresponding to the “x” phase; and ∑ I(A) is the sum of all intensities correspnding to reflections that are being analyzed. The relative concentration of different phases was determined as follows: RCðxÞ ¼

RðxÞ RðxsÞ

ð2Þ

where RC(x) ¼ relative concentration of α or β phase in the melted region; R(xs) ¼% relative intensity corresponsing to the “x” phase in the substrate. RC(x) more than 1 indicates relatively high concentration of “x” phase in the melted region than in the substrate. In vitro linear reciprocating ball-on-disk wear testing was performed using tribometer (NANOVEA, Microphotonics Inc., CA, USA) in freshly prepared simulated body fluids (SBF) at 37 1C. A ϕ 3 mm hardened chrome steel ball (100Cr6, 58–63 HRC) was used as counterpart. The top surface of Ti6Al4V alloy samples with and without laser melting were ground using successively finer grit SiC grinding papers and then polished on velvet cloth using a series of Al2O3 powder up to 1 μm suspended in distilled water. This procedure was followed primarily to ensure comparable surface roughness/ topography on the test surfaces. Just before testing all the samples were ultrasonically cleaned in alcohol bath for 15 mins. All the tests were performed with 2 N normal load, 10 mm linear oscillatory motion and 1200 mm/min speed. The wear rate was calculated from the wear track dimensions measured from SEM images of the worn surfaces. Three tests were performed on each sample and at least five wear track measurements were taken from each test. Thus the wear rate

journal of the mechanical behavior of biomedical materials 32 (2014) 335 –344

337

of the samples reported as mm3/Nm for 3000 m of sliding distance was average of 15 measurements. Statistical analysis was performed using Student's t-test, with Po0.05 being considered statistically significant. Supportive evidences in terms of Vickers microhardness measurements (Shimadzu, HMV-2) were also made on these samples using a 300 g load for 15 s and an average value of 10 measurements was reported.

3.

Results and discussion

3.1.

Microstructures

Laser surface melting of Ti6Al4V alloy results in thick fusion zones. Fig. 1 shows low magnification cross-sectional microstructures of laser surface melted samples. It can be seen that all melted regions exhibit very large grains which reduced in size towards the substrate. Further, the large grains were found to be embedded with randomly oriented fine needle like phase. As shown in Fig. 1e the microstructure of as-received substrate showed equiaxed two phase microstructure. The influence of laser process parameters on thickness of melted region and grain size is presented in Table 1. The melted region thickness was found to increase from 779729 mm to 1199712 mm when the laser power was increased from 250 W (42 J/mm2) to 400 W (68 J/mm2). No measurable difference in the melted region thickness was observed between single pass and double pass sample. For example, at 250 W power the single laser pass resulted in 779729 mm thick melted region and the subsequent second pass marginally increased the thickness to 802711 mm. The observed variations in the thickness can be explained based on the variations in the melt pool depth as a function of laser power or laser energy input. It is known that by increasing the laser power (and hence the laser energy input) increases the melt pool and depth (Singh et al., 2006; Chikarakara et al., 2012; Krishna and Bandyopadhyay, 2009). Therefore, in the present investigation the Ti6Al4V alloy substrates melted at 400 W showed approximately 50% increase in the melted region thickness compared to the substrates melted at 250 W. As shown in Table 1, the grains in the laser melted regions were significantly larger than in the un-melted substrate. The as-received substrate exhibited a grain size of 4.870.87 mm and after laser surface melting the grain size increased in the range of 154744 mm and 199738 mm. The observed increase in the grain size as a result of laser surface melting is presumably due to the high laser energy input used in the present investigation. In addition, multiscan treatment, to cover an area of  78 mm2 (ϕ 10 mm), in the present work results in numerous low temperature reheating cycles of earlier melted-then-solidified laser tracks by successive laser melting tracks. The reheating effect from subsequent laser melting pass could also have caused grain growth in the previously solidified tracks. Under present experimental conditions, the laser power and number of passes had no significant influence on grain size. The microstructures of melted and unmelted regions of Ti6Al4V alloy substrate were observed at high magnification using FESEM to identify the different phases based on morphology. Fig. 2 presents typical high magnification crosssectional microstructures of laser melted and as-received

Fig. 1 – Low magnification cross-sectional microstructures (a–d) laser surface melted samples, (e) as-received Ti6Al4V alloy substrate. (a) 250 W, 1 Pass, (b) 250 W, 2 Passes, (c) 400 W, 1 Pass, (d) 400 W, 2 Passes.

Ti6Al4V alloy. The microstructure of as-received Ti6Al4V alloy substrate showed typical equiaxed αþβ phase as shown in Fig. 2a. On the other hand, the laser surface melted regions of the alloy, Fig. 2b and c, showed needle like acicular α embedded in prior β matrix. The acicular α is also known as α′ martensite. The formation of acicular α in the laser melted region is intuitive as the small volume of melt treated during laser melting results in extremely high cooling rates in the range of 103–106 K/s. Formation of acicular α in the laser processed Ti alloys has been reported (Singh et al., 2006; Chikarakara et al., 2012; Krishna et al., 2007; Bandyopadhyay et al., 2010; Yerramareddy and Bahadur, 1992; Singh et al., 2006; Biswas et al., 2007). The presence of acicular α in the surface melted regions of Ti6Al4V alloy is beneficial for fatigue strength as the best fatigue resistance has been obtained in this alloy with acicular α

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Table 1 – Hardness and microstructural features of as-received and laser surface melted Ti6Al4V alloy substrates. Sample

Hardness (HV0.3)

Melted region (lm)

Grain size (lm)

Substrate 250 W-1Pass 250 W-2Pass 400 W-1Pass 400 W-2Pass

358721 413717 421716 429717 438721

– 779729 802711 1199712 117378

4.870.87 172755 154744 175736 199738

Fig. 2 – Typical high magnification FESEM microstructures of Ti6Al4V alloy (a) as-received substrate, (b) laser melted at 250 W, 2 passes, and (c) laser melted at 400 W, 2 passes.

(Cook et al., 1988). Further, the corrosion resistance of Ti6Al4V alloy was found to improve in the presence of acicular α (Singh et al., 2006; Yue et al., 2002; Biswas et al., 2007). This suggests that laser surface melted implants can have significantly better fatigue and corrosion resistance in as-processed condition than conventionally processed implants that require additional postfabrication heat treatments to achieve similar fatigue response.

3.2.

Phase analysis

Fig. 3a shows the XRD results obtained from Ti6Al4V alloy with and without laser melting. The results show that the

Ti6Al4V alloy contain α (or acicular α′) and β phases before and after laser surface melting. However, the peak intensities of β phase decreased significantly and α (or acicular α′) intensities increased after laser surface melting. The XRD results confirm the microstructural observations on transformation of microstructure from αþβ in the substrate to acicular αþβ in the laser melted region. The relative concentration of α and β phases in the melted region was estimated using Eqs. (1) and (2) and the results are presented in Fig. 3b. The results clearly show that the concentration of α is significantly high in the laser melted regions than in the asreceived substrate. Further, the samples processed at 400 W

journal of the mechanical behavior of biomedical materials 32 (2014) 335 –344

339

Fig. 3 – (a) X-ray diffraction results of Ti6Al4V alloy substrate with and without laser surface melting, (b) relative concentration of α and β estimated from XRD peak intensities. 1 P¼ single pass; 2 P¼ 2 passes. showed relatively more concentration of α than the samples processed at low laser power of 250 W. The observed increase/decrease in the concentrations of α and β phases, and formation of acicular α can be directly correlated to the achievable cooling rates under present experimental conditions. Generally at low energy input one can achieve high cooling rates due to steep/high thermal gradients near the liquid melt zone. It has been reported (Bontha et al., 2006) that within the melt zone the cooling rate, dT/dt (K/s), is expressed as the product of local temperature gradient G (K/mm) and solidification velocity R (mm/s). Further, during laser processing the R is typically of the order of laser scan velocity (v) (Gaumann et al., 1999) and G is of the order of 100 K/mm (Hofmeister et al., 2001). Therefore, in the present laser surface melting, with a scan velocity of 5 mm/s, the cooling rates in the melt zone can be around 500 K/s. The cooling rates during laser surface melting can estimated using the following Rosenthal solution for a moving point heat source, as proposed by Steen (1991)   dT V ¼  2πk ΔT2 dt Q

ð3Þ

where ‘k’ is the thermal conductivity, 6.7 W/m.K, of Ti6Al4V alloy, V is scan velocity (5 mm/s), Q laser power (250 W and 400 W) and ΔT is the range of temperature variation during cooling. For calculation the melting temperature of Ti6Al4V alloy (1660 1C) was assumed as maximum temperature in the melt pool. However, the peak melt pool temperatures in excess of melting temperature of substrates have been reported (Singh et al., 2006) and therefore the cooling rates estimated in this work would be the minimum possible cooling rates. From Eq. (3) it can be seen that by increasing the laser power, as in the present investigation, the cooling rate decreases. The cooling rates estimated using Eq. (3) for different substrate temperatures are shown in Fig. 4a. As expected the achievable cooling rates at 250 W are higher than at 400 W irrespective of initial substrate temperature. However, the difference found to decrease with increase in the substrate temperature. A maximum cooling rate of 2.25  10  3 K/s has been estimated for the substrate temperature of 298 K (25 1C), which is expected to prevail during first pass of laser surface melting at 250 W (42 J/mm2). When the laser power was increased to 400 W (68 J/mm2) the estimated cooling rate decreases to 1.41  10  3 K/s at substrate temperature of 298 K.

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Therefore, the higher cooling rates during laser melting at 250 W resulted in finer microstructural features than those obtained with 400 W. The microstructures shown in Fig. 4 b and c clearly demonstrate that the needle like acicular α is finer in the samples processed at 42 J/mm2 than in the sample processed at higher laser energy input of 68 J/mm2. Since the scale of solidification microstructures is inversely proportional to cooling rates finer microstructural features are expected with high cooling rates which primarily depends on incident laser energy input. The high thermal gradients that prevail with low energy input are responsible for high cooling rates and consequent finer microstructures observed in the present laser surface melted samples with an energy of 42 J/mm2 than in the sample processed at high laser energy of 68 J/mm2. Similar observations were made during laser welding of Ti6Al4V alloys (Mishra and DebRoy, 2004). It is believe that the estimated cooling rates of the order of 10  3 K/s are sufficient to form acicular α and further restrict the transformation of β–α during solidification. The microstructural and XRD results support the formation of acicular α and

reduction of β in the laser surface melted samples. Since the cooling rates at 250 W are relatively higher than at 400 W it can be argued that surface melting at 250 W can suppress more of the β–α transformation during cooling and results in relatively more β phase in the melted regions. This can be clearly seen from estimated relative concentrations of α and β phases in the melted regions as shown in Fig. 3b. As the laser melting proceed for second pass the substrate temperature increases, due to first laser melting pass, which decreases the heat transfer and hence the cooling rate decreases. With an initial cooling rates in the range of 2.25  10  3 K/s and 1.41  10  3 K/s during first laser pass, even if we assume a substrate temperature around 600 K (325 1C) after first laser pass the resultant cooling rates during second laser melting passes would be in the range of 1.0  10  3 K/s and 1.5  10  3 K/s. Therefore, under present experimental conditions every laser melting pass could have experienced a cooling rate of the order 10  3 K/s, which presumably resulted in the formation of acicular α and suppression of β–α transformation during solidification in the laser melted regions of Ti6Al4V alloy.

Fig. 4 – (a) Estimated cooling rates during laser surface melting of Ti6Al4V alloy at different substrate temperatures, (b–c) FESEM microstructures showing influence of cooling rate on scale of microstructural features, (b) 250 W, 1 Pass, and (c) 400 W, 1 Pass.

journal of the mechanical behavior of biomedical materials 32 (2014) 335 –344

3.3.

Hardness and in vitro wear

The hardness of laser melted Ti6Al4V alloy samples was compared with untreated substrate in Table 1. The average hardness of melted region was found to be between 413717 HV and 438717, which was 15–22% higher than the average hardness of as-received Ti6Al4V alloy substrate. It is known that the acicular α phase is harder than α phase, which is stronger than β phase (Abkowitz et al., 1955). Therefore the high hardness of laser treated samples is attributed to the formation of acicular α and reduction in amount of β phase in these samples (Yerramareddy and Bahadur, 1992; Singh et al., 2006; Biswas et al., 2007). The variation in laser power or laser energy input had very little influence on the hardness. This is presumably due to the fact that all the laser melted surfaces exhibited more or less similar microstructures. Further, as shown in Table 1, the grain size in the melted regions was also comparable among all the laser treated samples. Therefore, the hardness variation among the laser melted samples was absent. The influence of laser parameters on the in vitro wear rate of laser surface melted Ti6Al4V alloy is shown in Fig. 5. For comparison, the wear rate of untreated alloy is also shown. The experimental data show that the laser treated samples exhibit superior in vitro wear resistance compared to untreated alloy. Over all the wear rate of laser surface melted Ti6Al4V alloy was between 16% and 50% lower than that of untreated alloy. The wear rate of untreated Ti6Al4V alloy was determined to be 6.82  10  4 mm3/Nm which decreased between 3.38  10  4 mm3/Nm and 5.72  10  4 mm3/Nm due to laser surface melting. The statistical analysis using Student's t-test showed significant difference (p between 0.009 and 0.0001) in the wear rate as a result of laser surface melting and change in laser energy input. However, the influence of number of laser melting passes was found to be statistically insignificant (p between 0.075 and 1.000). It is generally agreed that the high hardness and smaller grain size improve the mechanical properties and hence the tribological performance of materials. Therefore, the comparable hardness and grain size, shown in Table 1, of laser treated samples should exhibit comparable tribological performance. In the present work, however, in

341

general, the samples treated at low energy input 42 J/mm2 (250 W) showed relatively lower wear rate than those samples treated at high energy input of 68 J/mm2 (400 W). This discrepancy is presumably due to complex nature of wear and synergetic effect of microstructural features on the wear and corrosion. It is believed that the relatively high in vitro wear resistance of laser surface melted Ti6Al4V alloy is due to changes in the microstructural constituents, hardness and their influence on electrochemical response in SBFs. Firstly, the high hardness of laser surface melted samples, due to the formation of acicular α and reduction in β phase, can improve their in vitro tribological performance compared to untreated alloy. Under present wear testing conditions it is expected that some corrosion can occur, in addition to mechanical wear, and could contribute to overall wear rate observed in the present work. The corrosion resistance of Ti6Al4V alloy was found to improve in the presence of acicular α (Singh et al., 2006; Yue et al., 2002; Biswas et al., 2007). The microstructures of laser treated samples containing acicular α and small amount of β phase can alter the corrosion behavior due to formation of passive oxide layers (Singh et al., 2006; Yue et al., 2002; Tritschler et al., 1999; Stack and Chi, 2003), which can potentially reduce the wear rate by decreasing the friction and corrosion (Dong and Bell, 1999). Fig. 6 shows the worn surfaces of Ti6Al4V alloy with and without laser surface melting after 3000 m sliding in SBF. As shown in Fig. 6a the untreated alloy showed deep grooves and, relatively smoother and shallow wear tracks were found on laser melted alloy, which are shown in Fig. 6b and c. From the wear track morphologies it appears that all surfaces undergone some kind of abrasive wear as the substrates are softer than the articulating hardened chrome steel ball with a hardness in the range of 58–63 HRC. In addition, as shown in insets of Fig. 6, all the worn surfaces exhibited deformed and smeared wear debris adhered to the surfaces. One important observation was that at some isolated location the wear tracks of laser treated samples showed thin and cracked oxide layers, which were shown in the insets of Fig. 6b and c. We hypothesize that these could be passive oxide layers that were formed during in vitro wear testing in SBF, which can potentially reduce the wear rate via providing low friction

Fig. 5 – Comparison of in vitro wear rate of Ti6Al4V alloy with and without laser surface melting.

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journal of the mechanical behavior of biomedical materials 32 (2014) 335 –344

Fig. 6 – Typical wear track morphologies of Ti6Al4V alloy (a) as-received substrate, (b) laser melted at 250 W, 2 passes, and (c) laser melted at 400 W, 2 passes. Insets show the smeared wear debris and cracked oxide scales in respective samples.

and corrosion (Tritschler et al., 1999; Stack and Chi, 2003; Dong and Bell, 1999). In summary, present laser surface melted Ti6Al4V alloy samples with high amount of acicular α potentially improve the in vitro surface dependent properties such as corrosion, fatigue and wear resistance.

4.

Conclusions

Laser surface melting (LSM) of Ti6Al4V alloy with different laser energy inputs resulted in up to 1199 mm thick melted regions on the top surface. The laser treatment resulted in formation of acicular α and suppression of β to α transformation during solidification. Under present experimental conditions the estimated cooling rates in the range of 2.25  10  3 K/s and 1.41  10  3 K/s corroborated the formation of acicular α and decrease in the relative concentration of β in the solidification microstructures. Further, relatively higher cooling rates during laser melting at 250 W resulted in finer microstructural features than those obtained at 400 W. The average hardness of melted region was 15–22% higher than the average hardness of as-received Ti6Al4V alloy substrate. A lowest in vitro wear rate of 3.38  10  4 mm3/Nm was observed on the samples

treated at 250 W compared to untreated substrate with a wear rate of 6.82  10  4 mm3/Nm. The observed improvement in the in vitro tribological performance of laser treated samples is believed to be the synergetic influence of increase in the hardness and corrosion resistance – due to formation of passive oxide layers.

r e f e r e n c e s

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Microstructure, mechanical and wear properties of laser surface melted Ti6Al4V alloy.

Laser surface melting (LSM) of Ti6Al4V alloy was carried out with an aim to improve properties such as microstructure and wear for implant application...
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