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Wear resistance of AZ91D magnesium alloy processed by improved laser surface remelting
Optics and Lasers in Engineering 55 (2014) 237–242
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Wear resistance of AZ91D magnesium alloy processed by improved laser surface remelting Zhihui Zhang, Pengyu Lin n, Luquan Ren The Key Laboratory of Engineering Bionics (Ministry of Education, China) and the College of Biological and Agricultural Engineering, Jilin University (Nanling Campus), 5988 Renmin Street, Changchun 130025, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2013 Received in revised form 13 November 2013 Accepted 22 November 2013 Available online 20 December 2013
We investigate the effects of laser surface remelting on the wear resistance of the AZ91D magnesium alloy. A CO2 continuous laser was used to treat the samples. The sample surface was strengthened by laser-treated area in the shape of stripes. The wear resistance was improved by marked microstructural characteristics, due to the notable effect of laser treatment, such as the grain refinement and very small sized β-Mg17Al12 dendrite. The microhardness was increased up to 130 HV. And the wear volume loss was reduced by 70%. The variations in some laser parameters, the output power and the laser scanning speed, also influenced the microstructure and related wear resistance. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Laser treatment Microstructure Magnesium alloys Wear
1. Introduction Magnesium alloys (Mg alloy) have been considered as very useful structural materials due to their excellent physical properties, e.g. the low density and high specific strength. However, because of their relatively poor mechanical and chemical properties (especially for the Mg–Al alloy system), their application is significantly limited [1–4]. As it is all known [5], in general, these properties depend on alloy microstructure. Therefore, recent studies have been focusing on improving the microstructure and thus enhancing related mechanical properties [6–9]. Amongst Mg alloys, AZ91D has merged to have significant importance in a wide range of industrial applications. This type has balanced strength and ductility. Its usage is also stable. However, as industrial technology progresses, it needs corresponding development. Previous studies adopted many effective methods to improve its mechanical properties [10–17]. Wang et al. [10] used a rare earth element yttrium (Y) to improve the corrosion residual strength of AZ91D. 1.0 wt% Y considerably promotes the overall corrosion resistance. However, pitting corrosion still occurred even after adding Y into the alloy, due to the nucleation and propagation of corrosion pits on alloy surface. As a matter of fact, the rare earth elements also are studied elsewhere [13], where, a coating technique using rare earth improved the corrosion resistance of AZ91D. Furthermore, Zhang et al. [14]
n
Corresponding author. Tel.: þ 86 431 8578 0434; fax: þ86 431 8509 4699. E-mail address: [email protected] (P. Lin).
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.11.014
studied the effects of electroless plating pretreatment and Ni– TiO2 composite coating on the corrosion resistance of AZ91D. The results indicate that the pretreatment can better protect the inner alloy. In addition to this, the microhardness is also significantly improved. Many applications, to large extent, depend on surface properties, such as wear [18] and corrosion [5]. Laser treatment has recently been studied to improve the surface microstructure and mechanical properties of alloys and steels [19–22]. Laser surface remelting (LSR) is a major method in all laser treatments. It can treat a layer on surface, and thus generate fine grains and short dendrites due to its self-quenching effect, which is ascribed to improve the properties. Iordanova et al. [21] studied the microstructure and mechanical properties of a cold rolled low-carbon steel surface-treated by a Nd:glass pulsed laser. As a result, a surface layer with the depth of 80 μm was fabricated by it. In the meanwhile, the corresponding microhardness was increased by 30%. It is interesting to note that the change of laser parameters, such as, frequency and scanning speed, can also affect test results (e.g., microstructure, mechanical properties). Zhang et al. [23] reported that after varying laser parameters in a certain range (electrical current 200–300 A, pulse duration 5–15 ms, frequency 4–10 Hz and scanning speed 0.24–0.72 mm s 1), the effective treatment depth and the surface roughness both were changed. Within the ranges of the parameters, the optimization of parameters and properties was acquired. Therefore, it is of great importance to investigate the effects of laser parameters on alloy properties.
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Many previous studies [21,24] focused on the effects of laser treatment on steels. However, the effects of laser treatment on Mg alloys are relatively less studied. This treatment might be an effective method to improve the surface properties of Mg alloy and then to improve its applications under some conditions. Therefore, we conduct this study to provide useful information about the effects of LSR on the microstructure and the resistances to wear and thermal fatigue of AZ91D, in order to enhance its application. In addition, the related mechanism is discussed.
2. Materials and methods In the present study, commercially available AZ91D alloy is considered as the base material (or the as-received sample). Its chemical compositions are as follows: 8.90 wt% Al, 0.74 wt% Zn, 0.17 wt% Mn, and Mg in balance, which were measured by the MS9710C inductively coupled plasma (ICP). The original size of the sample is 15 mm 10 mm 10 mm. A JW-CO2 laser was employed to treat the surfaces of samples. Laser treatment was once investigated in our previous studies [23,25]. The as-received sample was carefully cleansed with alcohol before the LSR processing. Here, in order to study the effects of laser parameters on the microstructure and mechanical properties, different laser parameters were adopted: (1) the scanning speeds are 300 mm/min, 600 mm/min and 900 mm/ min (Here, the output power was 1800 W); (2) the laser output power are 1400 w, 1600 w and 1800 W (The scanning speed was 600 mm/min). The laser beam has a diameter of 1.5 mm. The sample was laser-treated in stripes. The spacing of every two treated stripes is 2 mm. In the present study, the sample processed by LSR is referred to as the LSR sample. Note that the traditional laser treatment was carried out in a protective air [21,24,25]. This is because the laser melted the treated area, and the protective air (e.g. Ar) is to prevent oxidation. However, in this work, we adopted an improved laser treatment: the LSR here was carried out in deionized water at room
temperature. The samples were immersed into deionized water in a container. The distance between the top of the water and the top surface of the sample is 2 mm. It is known that a major characteristic of laser treatment is its self-quenching effect which led to very high cooling speed [21]. Here, the water cooling condition was to strengthen this effect and to better prevent the oxidation in air, since Mg alloy is more prone to oxidation than steels and irons [24]. The LSR and as-received samples were cleaned and then polished after LSR. The optical microscope (OM, JSP-3C) and the scanning electron microscope (SEM, JSM-5600 & ZEISS-EVO I8) were used to characterize the microstructure of all samples. The Xray diffraction (XRD, D/MAX2500PC) was used to detect the phases in the sample microstructure. The microhardness was measured using a microhardness tester (FM-700), under the load of 5 g and with the loading time of 10 s. Each data shown later is the average of five individual experimental results. The dry sliding wear tests were carried out using an MM-200 block-on-ring wear tester under the ambient condition. The specimens were polished to a roughness of 0.1 mm before the tests. The wear tests were carried out again a rotating no. 45 steel plate (50 mm in diameter, and with a hardness of 600 HV), under the working load of 10 kg. The rotating speed was 400 rpm and the sliding duration was 10 min. The size of the wear sample is 10 10 14 mm3. Each data shown in our study is the average of six individual experimental results. The volume loss of the sample was used to analyze the resistance to wear.
3. Results Fig. 1 shows treated microstructure, with the scanning speed of 600 mm/min and the output power of 1800 W. Fig. 1a is the crosssectional morphology of the laser-treated area. Fig. 1b shows the obvious boundary of the laser-treated and the original areas. As inferred from it, good metallurgical bonding was acquired. Fig. 1c and d demonstrates the laser-treated and the original microstructures,
Fig. 1. The cross-sectional microstructures of the LSR sample, with the scanning speed of 600 mm/min and the output power of 1800 W: (a) the overall morphology of the laser-treated area; (b) the boundary between the laser-treated area and the original one; and (c) the microstructure of the laser-treated area; (d) the microstructure of the original area.
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Fig. 2. The microstructures of the LSR samples with different output powers: (a) 1400 W; (b) 1600 W; and (c) 1800 W.
Fig. 3. The XRD patterns of the LSR samples with varied output powers.
respectively. Both microstructures comprise α-Mg and β-Mg17Al12. Note that the treatment depth is 1000 μm. The microstructures with varied output powers are given in Fig. 2. It is obvious that as the output power is 1400 W (Fig. 2a), the β-Mg17Al12 dendrite is the smallest of all. The specimen microstructure was notable refined. Larger-sized dendrites were acquired with increasing output power. In addition to it, as the output power reached up to 1800 W, the corresponding microstructure is the most homogeneous of all (Fig. 2c). Fig. 3 presents the XRD patterns of the LSR samples in Fig. 2. The amount of β-Mg17Al12 increased with output power. The output power of 1800 W led to the largest amount of β-Mg17Al12 in the all microstructures. Fig. 4 demonstrates the relationship between the depth from the sample surface (in the perpendicular direction to the lasertreated surface) and the microhardness of all the three LSR samples. The microhardness increased with output power. Meanwhile, for all
Fig. 4. The microhardness profiles of the LSR samples, in the perpendicular direction to the laser-treated area. All the three LSR samples with varied output powers were shown.
samples, the laser-treated area has higher microhardness than the original part. The LSR sample with the output power of 1800 W exhibited the highest microhardness of all samples. As the depth increased up to 800 μm, the LSR could no longer improve the microhardness. Fig. 5 gives the SEM of all the LSR samples with varied scanning speeds. As the scanning speed was lower, the β-Mg17Al12 exhibited larger dendrites. As the scanning speed increased, the dendrites were smaller. At the scanning speed of 900 mm/min, the smallestsized dendrite was acquired, amongst the all LSR samples, as shown in Fig. 5c. The XRD patterns of the LSR samples with different scanning speeds are shown in Fig. 6. All three samples consist of α-Mg and β-Mg17Al12. The amount of β-Mg17Al12 was decreased with increasing scanning speed. Fig. 7 presents the microhardness of all the three samples, (in the perpendicular direction to the laser-treated surface). The sample microhardness increased with the scanning speed.
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Fig. 5. The microstructures of the LSR samples with varied scanning speeds: (a) 300 mm/min; (b) 600 mm/min; and (c) 900 mm/min.
Fig. 6. The XRD patterns of the LSR samples with varied scanning speeds.
The LSR sample, with the scanning speed of 900 mm/min, exhibited the highest microhardness of all. Meanwhile, the microhardness of surface is always higher than that of the inner material. The worn surface morphologies of the as-received sample and its LSR counterpart are presented in Fig. 8. As shown in Fig. 8a, the surface of the LSR sample has higher ability to withstand wear, due to its higher microhardness. Therefore, its surface is smoother. However, the surface of the as-received sample (Fig. 8b) is much worse than that of the LSR sample. Fig. 8c demonstrates the relationship of wear volume loss with the output power and the scanning speed. It was reduced by 70%, as the output power or the scanning speed was increased up to 1800 W or 900 mm/min, respectively.
4. Discussion As shown in our previous study [26], LSR is an effective method to treat steel surface. However, in the present study, we adopted a
Fig. 7. The microhardness profiles of samples, in the perpendicular direction to the laser-treated area, with varied scanning speeds.
continuous laser on a different material (The base material in [26] was graphite cast iron.). Fig. 1 demonstrates the notable microstructural modification led by LSR. The distribution of α-Mg and βMg17Al12 both were obviously varied. For AZ91D, the β-Mg17Al12 is the hard microstructural constituent. The dendrites were greatly refined and then uniformly distributed throughout the treated area. Therefore, it is sensible to ascribe the improvement in microhardness (Figs. 4 and 7) to the modified microstructure. A notably modified microstructure of Mg alloy was once reported elsewhere [27]. The sample was treated by means of rapid solidification (RS), using a single-roller melt-spinner. Here, the microstructure in Fig. 1b and c is similar with that in Ref. [27], with very small grain size and refined β-Mg17Al12 dendrite. This is because the LSR can generate a self-quenching effect on the sample surface [28], which has very high cooling speed. The average grain size of the LSR sample is also notably smaller than that of its as-received counterpart, due to this effect.
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Fig. 8. The results of the wear test to all the as-received and LSR samples: (a) the worn surface morphology of the LSR sample; (b) the worn surface morphologies of the as-received sample; and (c) the relationship of the wear volume loss with output power and scanning speed.
The amount of β-Mg17Al12 increased with the output power, and moreover, its distribution was more uniform. Higher temperature was induced with increasing the output power. Solidification also took more time. As a result, the treated surface is better homogenized. On the other hand, the amount of β-Mg17Al12 was increased. As inferred from the Figs. 2 and 3, the microstructure was better homogenized with higher output power. Therefore, one can anticipate that the LSR sample with the output power of 1800 W has the highest microhardness of all, as evidenced by Fig. 4. As a matter of fact, the microhardness was increased by 60%. On the other hand, the microstructure was influenced with varied scanning speeds. The amount of β-Mg17Al12 was decreased with increasing scanning speed. As the scanning speed was increased up to 900 mm/min, as shown in Figs. 5 and 6, the microstructure has the smallest amount of β-Mg17Al12 of all. Meanwhile, as the scanning speed increased, the size of the Mg17Al12 dendrite was smaller. As shown in Fig. 7, the higher scanning speed can give rise to higher microhardness. This is because when the higher scanning speed provided higher cooling speed. It demonstrates better selfquenching effect. Whereas the amount of β-Mg17Al12 was decreased with increasing scanning speed, more Al element was melted into the α-Mg solid solution, which resulted in solid solution strengthening. Moreover, the growth of β-Mg17Al12 was also curbed. As a result, the scanning speed of 900 mm/min led to the highest microhardness of all. The time-dependent heat conduction was given in Zeng et al. [29]: 2 ∂T ∂T ∂ T ∂2 T ∂2 T þυ ¼λ þ þ ð1Þ þQ ; ρC ∂t ∂y ∂x2 ∂y2 ∂z2 where ρ, C, λ, υ and Q are the material density, the specific heat, the thermal conductivity (it significantly influences the cooling speed), the
velocity of the laser beam (scanning speed) and the output power per unit volume, respectively. Moreover, Q is a parameter directly related to the output power. The laser heat source was treated as a Gaussian plane distribution model of absorbed laser heat flux q(x, y) and is given as: 2ηP 2ðx2 þ y2 Þ qðx; yÞ ¼ 2 exp ð2Þ 2 πr r where η, P and r refer to the average absorptivity of materials, the laser power and the radius of laser spot, respectively. Eqs. (1) and (2) give the relationship of heat with output power and scanning speed. It is interesting to note that, from the sample surface to the inner part, the decrease in microhardness is continuous, as shown in Figs. 4 and 7. In contrast, our previous studies [23,29] show that at the boundary between the laser-treated and the original areas, the decrease in microhardness is abrupt. However, the present study is different. This might be due to the nature of different materials. For AZ91D in this work, the transition area (namely the boundary between the two areas) is relatively larger than that of steel and iron in Refs. [23,28,30]. Basu et al. [31] studied the relationship between microhardness and wear resistance by means of laser treatment. Here, the worn morphologies (Fig. 8a and b) coincide with the microhardness examination, which agrees well with our earlier discussion. The laser-treated surface was better-protected than its as-received counterpart. After wear test, wear grooves were thus generated. However, the as-received sample exhibited serious surfacial spallation and oxidation. This is because the soft surface of AZ91D cannot hold material transfer, and then notable material loss occurred in the contact area. The peeled material was also oxidized. On the other hand, since the microhardness of the laser-treated area was greatly enhanced, it could provide sufficient
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mechanical support to hold the material transfer, as shown in Fig. 8a. As a result, the oxidation was curbed, and moreover, the worn surface was smoother. Furthermore, the variation in the wear volume loss is also in good agreement with the microstructural modification. It better demonstrates the relationship of microhardness and wear resistance with different laser-treated microstructures. This is because Mg alloy is ductile. The increase in microhardness could directly contribute to its wear behavior. The harder surface can better withstand wear. 5. Conclusions In summary, LSR was carried out on the surface of AZ91D Mg alloy, under the water-cooling condition. Two laser parameters, output power and scanning speed, were varied, to study their effects on microstructure and property. The microstructure was greatly refined. Moreover, the growth of β-Mg17Al12 dendrites was obviously curbed. Significant property improvements were acquired, which essentially lie in the laser treatment. Microhardness increased with the output power. This is because the amount of β-Mg17Al12 increased with output power. The increase in the hard microstructural constituent was beneficial for microhardness. On the other hand, the change in scanning speed can also affect microstructural characteristics. As the scanning speed increased, the amount of β-Mg17Al12 was decreased. However, the dendrites were better refined. Therefore, the microhardness increased with it. The wear resistance directly depended on the microhardness. The surface of original AZ91D was badly damaged after wear test. The soft Mg cannot hold the material transfer. Oxidation took place during wear. However, the laser-treated area can withstand wear, due to higher microhardness. Material transfer was held in the contact area. The surface was thus well protected. The wear volume loss was reduced with higher microhardness. Acknowledgments This work is supported by Project 985-Bionic Engineering Science and Technology Innovation of Jilin University, the National Natural Science Foundation for Youths (No.51005097), and the Research Fund of Young Scholars for the Doctoral Program of Higher Education of China (No. 20100061120074). References [1] Wu YJ, Xu C, Zheng FY, Peng LM, Zhang Y, Ding WJ. Formation and characterization of microstructure of as-cast Mg–6Gd–4Y–xZn–0.5Zr (x¼ 0.3, 0.5 and 0.7 wt%) alloys. Mater. Charact. 2013;79:93–9. [2] Lu LW, Liu TM, Chen Y, Wang ZC. Deformation and fracture behavior of hot extruded Mg alloys AZ31. Mater. Charact. 2012;67:93–100. [3] Ye LY, Hu JL, Tang CP, Zhang XM, Deng YL, Liu ZY. Modification of Mg2Si in Mg–Si alloys with gadolinium. Mater. Charact. 2013;79:1–6. [4] Liu WJ, Cao FH, Jia BL, Zheng LY, Zhang JQ, Cao CN, et al. Corrosion behaviour of AM60 magnesium alloys containing Ce or La under thin electrolyte layers. Part 2: Corrosion product and characterization. Corros. Sci. 2010;52:639–50.
[5] Lin PY, Zhou H, Li W, Li WP, Sun N, Yang R. Interactive effect of cerium and aluminum on the ignition point and the oxidation resistance of magnesium alloy. Corros. Sci. 2008;50:2669–75. [6] Bi GL, Fang DQ, Zhao L, Zhang QX, Lian JS, Jiang Q, et al. Double-peak ageing behavior of Mg–2Dy–0.5Zn alloy. J. Alloys Compd. 2011;509:8268–75. [7] Kim YK, Sohn SW, Kim DH, Kim WT, Kim DH. Role of icosahedral phase in enhancing the strength of Mg–Sn–Zn–Al alloy. J. Alloys Compd. 2013;549:46–50. [8] Son HT, Lee JS, Kim DG, Yoshimi K, Maruyama K. Effects of samarium (Sm) additions on the microstructure and mechanical properties of as-cast and hot-extruded Mg–5 wt%Al–3 wt%Ca-based alloys. J. Alloys Compd. 2009;473: 446–52. [9] Ahlatci H. Production and corrosion behaviours of the Al–12Si–XMg alloys containing in situ Mg2Si particles. J. Alloys Compd. 2010;503:122–6. [10] Wang Q, Liu YH, Fang SJ, Song YL, Zhang DW, Zhang LN, et al. Evaluating the improvement of corrosion residual strength by adding 1.0 wt% yttrium into an AZ91D magnesium alloy. Mater. Charact. 2010;61:674–82. [11] Zhang S, Cao FH, Chang LR, Zheng JJ, Zhang Z, Zhang JQ, et al. Electrodeposition of high corrosion resistance Cu/Ni–P coating on AZ91D magnesium alloy. Appl. Surf. Sci. 2011;257:9213–20. [12] Amira S, Dubé D, Tremblay R, Ghali E. Microstructure of die cast and thixocast ZAEX10430 (Mg–10Zn–4Al–3Ce–0.3Ca) alloy. Mater. Charact. 2011;257:48–54. [13] Laleh M, Kargar F, Rouhaghdam AS. Investigation of rare earth sealing of porous micro-arc oxidation coating formed on AZ91D magnesium alloy. J. Rare Earth 2012;30:1293–7. [14] Zhang SY, Li Q, Yang XK, Zhong XK, Dai Y, Luo F. Corrosion resistance of AZ91D magnesium alloy with electroless plating pretreatment and Ni–TiO2 composite coating. Mater. Charact. 2010;61:269–76. [15] Hu LF, Chen SP, Miao Y, Meng QS. Die-casting effect on surface characteristics of thin-walled AZ91D magnesium components. Appl. Surf. Sci. 2012;261:851–6. [16] Hu LF, Meng QS, Chen SP, Wang H. Effect of Zn content on the chemical conversion treatments of AZ91D magnesium alloy. Appl. Surf. Sci. 2012;259: 816–23. [17] Zhao ZD, Chen Q, Wang YB, Shu DY. Microstructures and mechanical properties of AZ91D alloys with Y addition. Mater. Sci. Eng. A 2009;515:152–61. [18] Chen TJ, Ma Y, Li B, Li YD, Hao Y. Effects of processing parameters on wear behaviors of thixoformed AZ91D magnesium alloys. Mater. Des. 2009;30:235–44. [19] Yilbas BS, Akhtar S, Karatas C. Laser surface treatment of pre-prepared Rene 41 surface. Opt. Lasers Eng. 2012;50:1533–7. [20] Lambarri J, Leunda J, Navas VG, Soriano C, Sanz C. microstructural and tensile characterization of Inconel 718 laser coatings for aeronautic components. Opt. Lasers Eng. 2013;51:813–21. [21] Iordanova I, Antonov V, Gurkovsky S. Changes of microstructure and mechanical properties of cold-rolled low carbon steel due to its surface treatment by Nd:glass pulsed laser. Surf. Coat. Technol. 2002;153:267–75. [22] Viswanathan A, Sastikumar D, Kumar H, Nath AK. Laser processed TiC–Al13Fe4 composite layer formation on Al–Si alloy. Opt. Lasers Eng. 2012;50:1321–9. [23] Zhang ZH, Ren LQ, Zhou T, Han ZW, Zhou H, Chen L, et al. Optimization of laser processing parameters and their effect on penetration depth and surface roughness of biomimetic units on the surface of 3Cr2W8V steel. J. Bionic Eng. 2010;7:67–76. [24] Dobrzański LA, Bonek M, Hajduczek E, Klimpel A, Lisiecki A. Comparison of the structures of the hot-work tool steels laser modified surface layers. J. Mater. Process. Technol. 2005;164:1014–24. [25] Wang CT, Zhou H, Lin PY, Sun N, Yu JX, Zhao Y, et al. Fabrication of nano-sized grains by pulsed laser surface melting. J. Phys. D.: Appl. Phys. 2010;43:095402. [26] Guo QC, Zhou H, Wang CT, Zhang W, Lin PY, Sun N, et al. Effect of medium on friction and wear properties of compacted graphite cast iron processed by biomimetic coupling laser remelting process. Appl. Surf. Sci. 2009;255:6266–73. [27] Lin PY, Zhou H, Sun N, Li WP, Wang CT, Wang MX, et al. Influence of cerium addition on the resistance to oxidation of AM50 alloy prepared by rapid solidification. Corros. Sci. 2010;52:416–21. [28] Lin PY, Zhu YF, Zhou H, Wang CT, Ren LQ. Wear resistance of a bearing steel processed by laser surface remelting cooled by water. Scr. Mater. 2010;63:839–42. [29] Zeng C, Tian W, Hua L. A comprehensive study of thermal damage consequent to laser surface treatment. Mater. Sci. Eng. A 2013;564:381–8. [30] Zhang ZH, Zhou H, Ren LQ, Tong X, Shan HY, Li XZ. Surface morphology of laser tracks used for forming the non-smooth biomimetic unit of 3Cr2W8V steel under different processing parameters. Appl. Surf. Sci. 2008;254:2548–55. [31] Basu A, Chakraborty J, Shariff SM, Padmanabham G, Joshi SV, Sundararajan G, et al. Laser surface hardening of austempered (bainitic) ball bearing steel. Scr. Mater. 2007;56:887–90.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Wear resistance of AZ91D magnesium alloy processed by improved laser surface remelting Zhihui Zhang, Pengyu Lin n, Luquan Ren The Key Laboratory of Engineering Bionics (Ministry of Education, China) and the College of Biological and Agricultural Engineering, Jilin University (Nanling Campus), 5988 Renmin Street, Changchun 130025, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2013 Received in revised form 13 November 2013 Accepted 22 November 2013 Available online 20 December 2013
We investigate the effects of laser surface remelting on the wear resistance of the AZ91D magnesium alloy. A CO2 continuous laser was used to treat the samples. The sample surface was strengthened by laser-treated area in the shape of stripes. The wear resistance was improved by marked microstructural characteristics, due to the notable effect of laser treatment, such as the grain refinement and very small sized β-Mg17Al12 dendrite. The microhardness was increased up to 130 HV. And the wear volume loss was reduced by 70%. The variations in some laser parameters, the output power and the laser scanning speed, also influenced the microstructure and related wear resistance. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Laser treatment Microstructure Magnesium alloys Wear
1. Introduction Magnesium alloys (Mg alloy) have been considered as very useful structural materials due to their excellent physical properties, e.g. the low density and high specific strength. However, because of their relatively poor mechanical and chemical properties (especially for the Mg–Al alloy system), their application is significantly limited [1–4]. As it is all known [5], in general, these properties depend on alloy microstructure. Therefore, recent studies have been focusing on improving the microstructure and thus enhancing related mechanical properties [6–9]. Amongst Mg alloys, AZ91D has merged to have significant importance in a wide range of industrial applications. This type has balanced strength and ductility. Its usage is also stable. However, as industrial technology progresses, it needs corresponding development. Previous studies adopted many effective methods to improve its mechanical properties [10–17]. Wang et al. [10] used a rare earth element yttrium (Y) to improve the corrosion residual strength of AZ91D. 1.0 wt% Y considerably promotes the overall corrosion resistance. However, pitting corrosion still occurred even after adding Y into the alloy, due to the nucleation and propagation of corrosion pits on alloy surface. As a matter of fact, the rare earth elements also are studied elsewhere [13], where, a coating technique using rare earth improved the corrosion resistance of AZ91D. Furthermore, Zhang et al. [14]
n
Corresponding author. Tel.: þ 86 431 8578 0434; fax: þ86 431 8509 4699. E-mail address: [email protected] (P. Lin).
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.11.014
studied the effects of electroless plating pretreatment and Ni– TiO2 composite coating on the corrosion resistance of AZ91D. The results indicate that the pretreatment can better protect the inner alloy. In addition to this, the microhardness is also significantly improved. Many applications, to large extent, depend on surface properties, such as wear [18] and corrosion [5]. Laser treatment has recently been studied to improve the surface microstructure and mechanical properties of alloys and steels [19–22]. Laser surface remelting (LSR) is a major method in all laser treatments. It can treat a layer on surface, and thus generate fine grains and short dendrites due to its self-quenching effect, which is ascribed to improve the properties. Iordanova et al. [21] studied the microstructure and mechanical properties of a cold rolled low-carbon steel surface-treated by a Nd:glass pulsed laser. As a result, a surface layer with the depth of 80 μm was fabricated by it. In the meanwhile, the corresponding microhardness was increased by 30%. It is interesting to note that the change of laser parameters, such as, frequency and scanning speed, can also affect test results (e.g., microstructure, mechanical properties). Zhang et al. [23] reported that after varying laser parameters in a certain range (electrical current 200–300 A, pulse duration 5–15 ms, frequency 4–10 Hz and scanning speed 0.24–0.72 mm s 1), the effective treatment depth and the surface roughness both were changed. Within the ranges of the parameters, the optimization of parameters and properties was acquired. Therefore, it is of great importance to investigate the effects of laser parameters on alloy properties.
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Z. Zhang et al. / Optics and Lasers in Engineering 55 (2014) 237–242
Many previous studies [21,24] focused on the effects of laser treatment on steels. However, the effects of laser treatment on Mg alloys are relatively less studied. This treatment might be an effective method to improve the surface properties of Mg alloy and then to improve its applications under some conditions. Therefore, we conduct this study to provide useful information about the effects of LSR on the microstructure and the resistances to wear and thermal fatigue of AZ91D, in order to enhance its application. In addition, the related mechanism is discussed.
2. Materials and methods In the present study, commercially available AZ91D alloy is considered as the base material (or the as-received sample). Its chemical compositions are as follows: 8.90 wt% Al, 0.74 wt% Zn, 0.17 wt% Mn, and Mg in balance, which were measured by the MS9710C inductively coupled plasma (ICP). The original size of the sample is 15 mm 10 mm 10 mm. A JW-CO2 laser was employed to treat the surfaces of samples. Laser treatment was once investigated in our previous studies [23,25]. The as-received sample was carefully cleansed with alcohol before the LSR processing. Here, in order to study the effects of laser parameters on the microstructure and mechanical properties, different laser parameters were adopted: (1) the scanning speeds are 300 mm/min, 600 mm/min and 900 mm/ min (Here, the output power was 1800 W); (2) the laser output power are 1400 w, 1600 w and 1800 W (The scanning speed was 600 mm/min). The laser beam has a diameter of 1.5 mm. The sample was laser-treated in stripes. The spacing of every two treated stripes is 2 mm. In the present study, the sample processed by LSR is referred to as the LSR sample. Note that the traditional laser treatment was carried out in a protective air [21,24,25]. This is because the laser melted the treated area, and the protective air (e.g. Ar) is to prevent oxidation. However, in this work, we adopted an improved laser treatment: the LSR here was carried out in deionized water at room
temperature. The samples were immersed into deionized water in a container. The distance between the top of the water and the top surface of the sample is 2 mm. It is known that a major characteristic of laser treatment is its self-quenching effect which led to very high cooling speed [21]. Here, the water cooling condition was to strengthen this effect and to better prevent the oxidation in air, since Mg alloy is more prone to oxidation than steels and irons [24]. The LSR and as-received samples were cleaned and then polished after LSR. The optical microscope (OM, JSP-3C) and the scanning electron microscope (SEM, JSM-5600 & ZEISS-EVO I8) were used to characterize the microstructure of all samples. The Xray diffraction (XRD, D/MAX2500PC) was used to detect the phases in the sample microstructure. The microhardness was measured using a microhardness tester (FM-700), under the load of 5 g and with the loading time of 10 s. Each data shown later is the average of five individual experimental results. The dry sliding wear tests were carried out using an MM-200 block-on-ring wear tester under the ambient condition. The specimens were polished to a roughness of 0.1 mm before the tests. The wear tests were carried out again a rotating no. 45 steel plate (50 mm in diameter, and with a hardness of 600 HV), under the working load of 10 kg. The rotating speed was 400 rpm and the sliding duration was 10 min. The size of the wear sample is 10 10 14 mm3. Each data shown in our study is the average of six individual experimental results. The volume loss of the sample was used to analyze the resistance to wear.
3. Results Fig. 1 shows treated microstructure, with the scanning speed of 600 mm/min and the output power of 1800 W. Fig. 1a is the crosssectional morphology of the laser-treated area. Fig. 1b shows the obvious boundary of the laser-treated and the original areas. As inferred from it, good metallurgical bonding was acquired. Fig. 1c and d demonstrates the laser-treated and the original microstructures,
Fig. 1. The cross-sectional microstructures of the LSR sample, with the scanning speed of 600 mm/min and the output power of 1800 W: (a) the overall morphology of the laser-treated area; (b) the boundary between the laser-treated area and the original one; and (c) the microstructure of the laser-treated area; (d) the microstructure of the original area.
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Fig. 2. The microstructures of the LSR samples with different output powers: (a) 1400 W; (b) 1600 W; and (c) 1800 W.
Fig. 3. The XRD patterns of the LSR samples with varied output powers.
respectively. Both microstructures comprise α-Mg and β-Mg17Al12. Note that the treatment depth is 1000 μm. The microstructures with varied output powers are given in Fig. 2. It is obvious that as the output power is 1400 W (Fig. 2a), the β-Mg17Al12 dendrite is the smallest of all. The specimen microstructure was notable refined. Larger-sized dendrites were acquired with increasing output power. In addition to it, as the output power reached up to 1800 W, the corresponding microstructure is the most homogeneous of all (Fig. 2c). Fig. 3 presents the XRD patterns of the LSR samples in Fig. 2. The amount of β-Mg17Al12 increased with output power. The output power of 1800 W led to the largest amount of β-Mg17Al12 in the all microstructures. Fig. 4 demonstrates the relationship between the depth from the sample surface (in the perpendicular direction to the lasertreated surface) and the microhardness of all the three LSR samples. The microhardness increased with output power. Meanwhile, for all
Fig. 4. The microhardness profiles of the LSR samples, in the perpendicular direction to the laser-treated area. All the three LSR samples with varied output powers were shown.
samples, the laser-treated area has higher microhardness than the original part. The LSR sample with the output power of 1800 W exhibited the highest microhardness of all samples. As the depth increased up to 800 μm, the LSR could no longer improve the microhardness. Fig. 5 gives the SEM of all the LSR samples with varied scanning speeds. As the scanning speed was lower, the β-Mg17Al12 exhibited larger dendrites. As the scanning speed increased, the dendrites were smaller. At the scanning speed of 900 mm/min, the smallestsized dendrite was acquired, amongst the all LSR samples, as shown in Fig. 5c. The XRD patterns of the LSR samples with different scanning speeds are shown in Fig. 6. All three samples consist of α-Mg and β-Mg17Al12. The amount of β-Mg17Al12 was decreased with increasing scanning speed. Fig. 7 presents the microhardness of all the three samples, (in the perpendicular direction to the laser-treated surface). The sample microhardness increased with the scanning speed.
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Fig. 5. The microstructures of the LSR samples with varied scanning speeds: (a) 300 mm/min; (b) 600 mm/min; and (c) 900 mm/min.
Fig. 6. The XRD patterns of the LSR samples with varied scanning speeds.
The LSR sample, with the scanning speed of 900 mm/min, exhibited the highest microhardness of all. Meanwhile, the microhardness of surface is always higher than that of the inner material. The worn surface morphologies of the as-received sample and its LSR counterpart are presented in Fig. 8. As shown in Fig. 8a, the surface of the LSR sample has higher ability to withstand wear, due to its higher microhardness. Therefore, its surface is smoother. However, the surface of the as-received sample (Fig. 8b) is much worse than that of the LSR sample. Fig. 8c demonstrates the relationship of wear volume loss with the output power and the scanning speed. It was reduced by 70%, as the output power or the scanning speed was increased up to 1800 W or 900 mm/min, respectively.
4. Discussion As shown in our previous study [26], LSR is an effective method to treat steel surface. However, in the present study, we adopted a
Fig. 7. The microhardness profiles of samples, in the perpendicular direction to the laser-treated area, with varied scanning speeds.
continuous laser on a different material (The base material in [26] was graphite cast iron.). Fig. 1 demonstrates the notable microstructural modification led by LSR. The distribution of α-Mg and βMg17Al12 both were obviously varied. For AZ91D, the β-Mg17Al12 is the hard microstructural constituent. The dendrites were greatly refined and then uniformly distributed throughout the treated area. Therefore, it is sensible to ascribe the improvement in microhardness (Figs. 4 and 7) to the modified microstructure. A notably modified microstructure of Mg alloy was once reported elsewhere [27]. The sample was treated by means of rapid solidification (RS), using a single-roller melt-spinner. Here, the microstructure in Fig. 1b and c is similar with that in Ref. [27], with very small grain size and refined β-Mg17Al12 dendrite. This is because the LSR can generate a self-quenching effect on the sample surface [28], which has very high cooling speed. The average grain size of the LSR sample is also notably smaller than that of its as-received counterpart, due to this effect.
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Fig. 8. The results of the wear test to all the as-received and LSR samples: (a) the worn surface morphology of the LSR sample; (b) the worn surface morphologies of the as-received sample; and (c) the relationship of the wear volume loss with output power and scanning speed.
The amount of β-Mg17Al12 increased with the output power, and moreover, its distribution was more uniform. Higher temperature was induced with increasing the output power. Solidification also took more time. As a result, the treated surface is better homogenized. On the other hand, the amount of β-Mg17Al12 was increased. As inferred from the Figs. 2 and 3, the microstructure was better homogenized with higher output power. Therefore, one can anticipate that the LSR sample with the output power of 1800 W has the highest microhardness of all, as evidenced by Fig. 4. As a matter of fact, the microhardness was increased by 60%. On the other hand, the microstructure was influenced with varied scanning speeds. The amount of β-Mg17Al12 was decreased with increasing scanning speed. As the scanning speed was increased up to 900 mm/min, as shown in Figs. 5 and 6, the microstructure has the smallest amount of β-Mg17Al12 of all. Meanwhile, as the scanning speed increased, the size of the Mg17Al12 dendrite was smaller. As shown in Fig. 7, the higher scanning speed can give rise to higher microhardness. This is because when the higher scanning speed provided higher cooling speed. It demonstrates better selfquenching effect. Whereas the amount of β-Mg17Al12 was decreased with increasing scanning speed, more Al element was melted into the α-Mg solid solution, which resulted in solid solution strengthening. Moreover, the growth of β-Mg17Al12 was also curbed. As a result, the scanning speed of 900 mm/min led to the highest microhardness of all. The time-dependent heat conduction was given in Zeng et al. [29]: 2 ∂T ∂T ∂ T ∂2 T ∂2 T þυ ¼λ þ þ ð1Þ þQ ; ρC ∂t ∂y ∂x2 ∂y2 ∂z2 where ρ, C, λ, υ and Q are the material density, the specific heat, the thermal conductivity (it significantly influences the cooling speed), the
velocity of the laser beam (scanning speed) and the output power per unit volume, respectively. Moreover, Q is a parameter directly related to the output power. The laser heat source was treated as a Gaussian plane distribution model of absorbed laser heat flux q(x, y) and is given as: 2ηP 2ðx2 þ y2 Þ qðx; yÞ ¼ 2 exp ð2Þ 2 πr r where η, P and r refer to the average absorptivity of materials, the laser power and the radius of laser spot, respectively. Eqs. (1) and (2) give the relationship of heat with output power and scanning speed. It is interesting to note that, from the sample surface to the inner part, the decrease in microhardness is continuous, as shown in Figs. 4 and 7. In contrast, our previous studies [23,29] show that at the boundary between the laser-treated and the original areas, the decrease in microhardness is abrupt. However, the present study is different. This might be due to the nature of different materials. For AZ91D in this work, the transition area (namely the boundary between the two areas) is relatively larger than that of steel and iron in Refs. [23,28,30]. Basu et al. [31] studied the relationship between microhardness and wear resistance by means of laser treatment. Here, the worn morphologies (Fig. 8a and b) coincide with the microhardness examination, which agrees well with our earlier discussion. The laser-treated surface was better-protected than its as-received counterpart. After wear test, wear grooves were thus generated. However, the as-received sample exhibited serious surfacial spallation and oxidation. This is because the soft surface of AZ91D cannot hold material transfer, and then notable material loss occurred in the contact area. The peeled material was also oxidized. On the other hand, since the microhardness of the laser-treated area was greatly enhanced, it could provide sufficient
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mechanical support to hold the material transfer, as shown in Fig. 8a. As a result, the oxidation was curbed, and moreover, the worn surface was smoother. Furthermore, the variation in the wear volume loss is also in good agreement with the microstructural modification. It better demonstrates the relationship of microhardness and wear resistance with different laser-treated microstructures. This is because Mg alloy is ductile. The increase in microhardness could directly contribute to its wear behavior. The harder surface can better withstand wear. 5. Conclusions In summary, LSR was carried out on the surface of AZ91D Mg alloy, under the water-cooling condition. Two laser parameters, output power and scanning speed, were varied, to study their effects on microstructure and property. The microstructure was greatly refined. Moreover, the growth of β-Mg17Al12 dendrites was obviously curbed. Significant property improvements were acquired, which essentially lie in the laser treatment. Microhardness increased with the output power. This is because the amount of β-Mg17Al12 increased with output power. The increase in the hard microstructural constituent was beneficial for microhardness. On the other hand, the change in scanning speed can also affect microstructural characteristics. As the scanning speed increased, the amount of β-Mg17Al12 was decreased. However, the dendrites were better refined. Therefore, the microhardness increased with it. The wear resistance directly depended on the microhardness. The surface of original AZ91D was badly damaged after wear test. The soft Mg cannot hold the material transfer. Oxidation took place during wear. However, the laser-treated area can withstand wear, due to higher microhardness. Material transfer was held in the contact area. The surface was thus well protected. The wear volume loss was reduced with higher microhardness. Acknowledgments This work is supported by Project 985-Bionic Engineering Science and Technology Innovation of Jilin University, the National Natural Science Foundation for Youths (No.51005097), and the Research Fund of Young Scholars for the Doctoral Program of Higher Education of China (No. 20100061120074). References [1] Wu YJ, Xu C, Zheng FY, Peng LM, Zhang Y, Ding WJ. Formation and characterization of microstructure of as-cast Mg–6Gd–4Y–xZn–0.5Zr (x¼ 0.3, 0.5 and 0.7 wt%) alloys. Mater. Charact. 2013;79:93–9. [2] Lu LW, Liu TM, Chen Y, Wang ZC. Deformation and fracture behavior of hot extruded Mg alloys AZ31. Mater. Charact. 2012;67:93–100. [3] Ye LY, Hu JL, Tang CP, Zhang XM, Deng YL, Liu ZY. Modification of Mg2Si in Mg–Si alloys with gadolinium. Mater. Charact. 2013;79:1–6. [4] Liu WJ, Cao FH, Jia BL, Zheng LY, Zhang JQ, Cao CN, et al. Corrosion behaviour of AM60 magnesium alloys containing Ce or La under thin electrolyte layers. Part 2: Corrosion product and characterization. Corros. Sci. 2010;52:639–50.
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