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Thermal fatigue behavior of Mg–9Al–Zn alloy with biomimetic strengthening units processed by laser surface remelting
Optics & Laser Technology 70 (2015) 1–6
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Thermal fatigue behavior of Mg–9Al–Zn alloy with biomimetic strengthening units processed by laser surface remelting Zhihui Zhang a, Pengyu Lin a,n, Shuhua Kong b, Xiujuan Li a, Luquan Ren a a 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 b The Office of Weld Planning, Department of Planning, FAW-Volkswagen automobile company Ltd., 5 Anqing Road, Changchun 130011, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2014 Received in revised form 7 October 2014 Accepted 12 October 2014
Pulsed Nd:YAG laser was used to treat the surface of a magnesium alloy to improve the thermal fatigue resistance. The average grain size of the as-received material was reduced to 3 μm and moreover βMg17Al12 was precipitated in different forms. In addition, high density dislocation was acquired. The treated zone could efficiently prevent cracking on the sample surface. The crack growth was curbed within the treated zone and the microhardness increased by 80%. This enhancement was ascribed to the modified microstructural characteristics of the treated surface. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Laser treatment Microstructure Microhardness Fatigue Biomimetic
1. Introduction Magnesium alloys have recently been increasingly used in many industrial applications, for instance in aerospace and auto-industries [1]. These alloys are characterized by high specific strength and low density. They have the potential to replace many traditional structural metals, such as steel and iron [2 and 3]. On the other hand, due to relatively weak mechanical properties, as compared with those of steel and iron, the applications of Mg alloys are limited [4]. The resistance to thermal fatigue is an important aspect of structural materials [5–7]. Applications at elevated temperatures are especially important for light alloys; however, thermal fatigue occurs as a major failure at alternate temperatures [5]. Fatigue crack growth is easier and accelerated at elevated temperatures. Thermal fatigue behavior of Mg alloys has been studied [8 and 9]. Addition of reinforcing materials is a good method to improve mechanical properties and strength. Moreover, surface treatment is another consideration in this respect [10]. Some properties, such as corrosion and thermal fatigue and wear resistance, are strongly related to the surface microstructure. In this regard, the advantages of laser surface processing are obvious. The microstructure on the surface of nickel–aluminum bronze was modified through surface treatment [11] in which the detrimental phase in the microstructure was dissolved and the corrosion resistance was thereby enhanced.
n
Corresponding author. Tel.: þ 86 0431 8578 0434; fax: þ86 431 8509 4699. E-mail address: [email protected] (P. Lin).
http://dx.doi.org/10.1016/j.optlastec.2014.10.021 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
The fatigue property of Mg alloy AZ91 was studied by Okayasu [12]. Using various casting methods, the microstructure of the as-received material was modified. In this way, the related fatigue properties were improved owing to the formation of fine grains and spherical eutectic structures (β-Mg17Al12). Laser surface treatment is an efficient method to treat material surface [13–15]. Without processing the entire material, the treated surface can address many surface-related working conditions. Abboud et al. [13] reported the effect of laser treatment on iron. A hardened layer of 200 μm in thickness was fabricated. The resulting hardness of this layer was much higher than the substrate. In addition, Benyounis et al. [15] studied the quenching effect of surface melting on nodular cast iron and found that the original graphite was completed dissolute into the matrix and was precipitated in the form of continuous network of Fe3C. This microstructural modification largely contributed to the improved property. A biomimetic laser technique has been adopted to treat sample surface [14,16], inspired by body surfaces of some creatures, for example pangolin and sea shell, providing excellent adaptability to their respective habitats. For instance, the veins on the wings of some insects, such as fly and dragonfly, can provide essential mechanical support and protection (Fig. 1a). Because of their wings special structures, they are not easily broken by wind or water forces. Lin et al. [14] reported the thermal fatigue resistance of medium carbon steel processed through this technique. The scanning strategy was inspired by convex spots covering the body surface of beetles. Hard strengthening spots were fabricated on the sample surface, thus, cracking was essentially prevented by harder
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Fig. 1. The biological model, wing structure of dragonfly. (a) The supporting veins on the wing of dragonfly, (b) the cross-sectional morphology of the supporting veins and the wing and (c) the overall treated surface of sample (b). The veins are distributed in the form of “network”. On the other hand, the cross-sectional structure shows that the supporting veins withstand most of the mechanical load. They support the entire wing. The scanning strategy was inspired by the distribution of the supporting veins on the wing of dragonfly.
elements in the structure. In addition, Zhou et al. [16] studied the scanning strategy of “stripes”, which is similar to shell surface structures that provide shell with better adaptability to water pressure and flow stress [17]. However, the study on Mg alloys is still less carried out than that on steel and iron. As a structural material of great potential, it should be more studied in this aspect to enhance its perspective application with a new method. Gao et al. [18] reported an Al2O3 ceramic coating on AZ91D alloy processed with laser cladding, exhibiting obvious layer-like characteristics with multilayer coating. Because of significant microstructurual modification, the wear and corrosion resistance were much better than those of the as-received material. Considering these observations, laser treatment on Mg alloys is very useful in industrial applications and there is much room for further studies. In this work, the effect of laser remelting on the thermal fatigue behavior of Mg–9Al–Zn alloy was studied. The scanning strategy, microhardness, and the mechanism behind improvement were investigated.
2. Materials and methods A sample of Mg–9Al–Zn alloy was used as the parent material with a chemical composition of 8.82 wt% Al, 0.87 wt% Zn, 0.16 wt%
Mn, and Mg in balance (measured with MS9710C inductively coupled plasma technique, ICP). The as-received ingot was polished and then cut in the size of 15 10 10 mm3. The as-received sample was carefully cleansed with alcohol prior to laser surface remelting (LSR). The scanning strategy was engineered to mimic the veins on the wing of dragonfly (Fig. 1a and b) via the surface treatment. As shown in Fig. 1a, the vein paths were cross-arranged on the dragonfly wing. The cross-sectional morphology (Fig. 1b) shows the supportive effect of veins: the hard “tube” provides major mechanical and load support against water or wind so that the wing cannot be easily broken. It also provides supportive force for the high speed fly vibration. As a simplified model, in this paper, the sample surface was treated in the form of a network. Treated paths in two directions were crossed with an angle of 901. Fig. 1c shows the overall treated surface. The Nd:YAG (neodymium: yttrium–aluminum–garnet) pulsed laser was used to treat the sample surface in the protective atmosphere of high-purity argon. Previous studies showed that Mg is very susceptible to elevated temperature corrosion and oxidation [19,20]; therefore, samples should have been well protected using inert atmosphere. The wavelength of applied laser beam was 1.06 μm. The maximum power of laser was 300 W and the spot diameter of laser beam on the sample surface was 0.6 mm. In addition, the spacing between every two treated paths was varied to better understand the strengthening effect. The initial spacing was preset as 2 mm, but was varied to 3 and 4 mm. Related samples were denoted as (b), (c), and (d) for the initial spacing of 2, 3, and 4 mm, respectively. The asreceived sample was denoted as sample (a). The scanning electron microscope (SEM, JSM-5600 and ZEISSEVO I8), high resolution transmission election microscopy (HRTEM, JOEL-2100F) and X-ray diffraction (XRD, D/MAX2500PC) were used to characterize the microstructures of samples. The microhardness properties were measured using a microhardness tester (FM-700) under the load of 5 g and with the loading time of 10 s. Each data point shown is the average of five individual experimental results. The thermal fatigue tests were carried out on a self-controlled thermal fatigue testing machine. A complete cycle consisted of heating for 80 s up to the maximum temperature of 260 1C in resistance furnace and then cooling with water for 3 s to the minimum temperature of 25 1C in the water tank.
3. Results Fig. 2 shows microstructure of the as-received sample and the laser treated zone. The as-received microstructure consists of α-Mg and β-Mg17Al12 dendrites (Fig. 2a). The average grain size of Mg–9Al– Zn microstructure is 50–100 μm [21]. The β-Mg17Al12 dendrites were precipitated along the grain boundaries (GBs) and in the microstructure it existed in the appearance of coarse or intermittent dendrite or both. However, the microstructure was greatly modified as a result of laser treatment. As shown in Fig. 2b, the treated microstructure was notably refined with an average grain size of 3–5 μm. On the other hand, the shape of β-Mg17Al12 was no longer coarse dendrites and its size was smaller. The fine continuous network was uniformly distributed in the microstructure along the GBs. Fig. 2c presents the cross-sectional microstructure of the treated zone. It indicates that a good metallurgical bonding was acquired between the treated area and the original microstructure. In addition, it shows that the laser beam penetrated the surface 700 μm deep. Fig. 2d shows the HRTEM of the treated area with high dislocation density. Fig. 3 reveals the XRD patterns of the as-received and treated samples. The amount of β-Mg17Al12 in the treated zone is obviously larger than that of the original microstructure. Fig. 4 plots the microhardness profiles in various orientations. The laser treatment induced a great improvement in microhardness.
Z. Zhang et al. / Optics & Laser Technology 70 (2015) 1–6
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Fig. 2. The microstructures of the all samples. (a) The microstructure of as-received sample consisting of coarse α-Mg and β-Mg17Al12 dendrite, (b) the microstructure of treated zone of sample (b), (c) the cross-sectional morphology of the treated zone and (d) HRTEM of (b). The high dislocation density and phase distribution exhibit the sign of rapid solidification.
crack density and crack size both increased for all the samples. However, the sample (a) exhibited a significant increase in both of the examinations. On the other hand, the treated samples showed better resistance to thermal fatigue. Fig. 5b shows that, after long time fatigue test of beyond 3200 cycles, all four samples exhibited similar crack size values. Table 1 gives crack density values of the all samples.
4. Discussion
Fig. 3. XRD patterns of samples (a) and (b).
Fig. 4a gives the surface microhardness across the treated spots, which exhibited much larger values than that in the original surface. The microhardness of the original surface was only 70 HV. After laser treatment, the hardness increased up to 120 HV. In addition, the width of the treated area was observed as consistent with that of the improved microhardness. On the other hand, Fig. 4b presents the microhardness distribution in perpendicular direction. Within the treated area and heat affected zone, microhardness values were much greater than below the surface. Similarly, the depth of the treated area is consistent with that of the improved microhardness. Note that within the treated area, there is some abrupt decrease in microhardness, which is due to the porosity in the treated area led by laser. In this work, as inferred from Figs. 2 and 4, although the porosity was observed, it played minor effects on the microhardness and fatigue properties. Fig. 5 gives crack size evolution of various samples as a function of fatigue cycle. Fig. 5a shows that, as the fatigue test progressed, the
As laser reached the surface of sample, the surface was melted. The treated part was melted more rapidly. As laser moved away, the molten metal was solidified and a self-quenching effect occurred [22–24]. The heat led to the rapid heating and cooling of the treatment process, directly contributing to the microstructural modification of the treated area. In this study, a surface layer with a depth of approximately 700 μm was treated. During this process, the heat distribution was sophisticated. Zeng et al. [22] proposed the time-dependent heat conduction of laser to the sample surface as follows: 2 ∂T ∂T ∂ T ∂2 T ∂2 T ρC þυ ¼λ þ 2 þ 2 þQ ð1Þ 2 ∂t ∂y ∂x ∂y ∂z where ρ, C, λ, v and Q are material density, specific heat, thermal conductivity, velocity of laser, and power generated per unit volume, respectively. The x, y, and z are the directions within the treated area. When treating the sample surface, the power density of laser can be calculated using the following equation: I¼
4E 2
πd t
ð2Þ
where I is the power density of laser beam, d is the laser spot diameter, E is the pulse energy, and t is the pulse duration. Both E and t are key factors affecting the treatment process. Note that between the treated area and original area, there is a heat affected zone (HAZ) as shown in Fig. 2c.
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Fig. 4. Microhardness profiles of the all samples. (a) The microhardness across the surface centerline of the treated zone of sample (b) and (b) the perpendicular microhardness from the surface center of the treated zone of sample (b). The microhardness of the as-received sample was set as reference.
Inside the treated zone, the turbulent modification was also referred to as “self-quenching effect”, which is more obvious with ferrous alloys [25]. In general, soft phase, such as austenite or ferrite, is melted and then turned into martensite, coupled with many precipitates. The precipitate of the Mg–Al alloy is β-Mg17Al12 in the form of coarse dendrite and, as inferred from Fig. 2, the laser treatment greatly varied its precipitation and distribution. The microstructure of the original material is presented in Fig. 2a. β-Mg17Al12 was formed in the shape of irregular dendrite. As compared with α-Mg, β-Mg17Al12 is brittle and is a detrimental constituent to some properties. Due to existence of β-Mg17Al12, stress concentration occurs at the sharp corners of the irregular shapes, leading to early stage mechanical failure [26] and decrease in the ductility. However, the brittle β-Mg17Al12 could improve microhardness as the sole hard phase in the Mg–Al binary system [25]. The treated microstructure exhibited very different characteristics. It was observed from distribution of β-Mg17Al12 that the average grain size of the treated microstructure is much smaller than that of the asreceived counterpart. β-Mg17Al12 was precipitated in the form of a fine network. Similar to self-quenching effect on steel and iron, laser beam generated rapid solidification [27]. The melted metal can solidify within a very short time, therefore, the microstructure, in particular β-Mg17Al12, was notably refined. Different from the previous study [27], in the Mg–Al binary system, β-Mg17Al12 is the only precipitate. However, the smaller precipitates would enhance the mechanical properties. Note that the amount of β-Mg17Al12 increased with LSR (see Fig. 3). Although the time for β-Mg17Al12 precipitation
Fig. 5. Thermal fatigue behaviors of the all samples. (a) The average crack density of the all samples as the function of crack cycle and (b) the average crack size of the all samples as the function of crack cycle.
was very short, eutectic nucleation was promoted during the super cooling condition. In addition, according to the Hall–Patch relationship, the smaller grain size is a major contributor in this regard. Furthermore, the high dislocation density cannot be ignored here (Fig. 2d). As a result, the above characteristics lead to much greater microhardness value in the treated area. An abrupt decrease in microhardness was observed because only the surface was treated by laser. This decrease mainly occurred in the HAZ. The metallurgical bonding between the treated zone and original microstructure played an important role in this phenomenon. As shown in Fig. 5, the treated samples exhibited better resistance to thermal fatigue than the as-received counterpart. When fatigue cycle increased, the crack size and crack density both increased. However, the treated samples were more resistant to cracking than the as-received one. The smaller interpath spacing resulted in better fatigue resistance. Therefore, sample (b) exhibited the best fatigue resistance of all samples. Note that in the early stage of thermal fatigue, the differences in crack density and crack size were obvious, whereas in the later stage, the differences were less obvious, in particular, among the treated samples. One could anticipate that it is because the microhardness of the treated samples was insufficient to withstand late-stage fatigue. Consequently, the sample surface was badly damaged. Once the strengthening elements were damaged, the entire surface failed. Bayani et al. [8] reported the effects of β-Mg17Al12 on the fatigue resistance and observed a different result. The volume fraction of β-
Z. Zhang et al. / Optics & Laser Technology 70 (2015) 1–6
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Table 1 The true crack density and crack size of the all samples during fatigue test. L is the crack length, which falls into three categories: small crack: L 41 mm, medium crack: 1 mm o L o 2 mm, and large crack: L 42 mm. Treated samples with varied interpath spacings were all investigated. Cycle number
As-received
o400 400–800 800–1200 1200–1600 1600–2000 2000–2400 2400–2800 2800–3200 3200–4000
Lo1 7 10 15 20 27 31 34 38 44
1r L r 2 3 5 8 10 13 17 19 23 27
Interpath spacing 2 mm L42 1 1 3 4 6 7 10 12 16
Lo1 2 4 7 11 17 22 26 32 40
1r L r 2 – 2 3 5 8 10 13 18 22
Interpath spacing 3 mm L 42 – – – 1 2 4 5 7 10
Lo1 5 7 12 14 20 25 31 35 40
1r L r 2 – 2 4 7 10 12 16 20 25
Interpath spacing 4 mm L 42 – – – 2 3 6 8 9 11
Lo1 8 9 12 16 23 28 33 36 43
1r L r 2 2 3 5 11 11 15 18 21 26
L 42 – 1 1 2 4 7 8 10 13
cracks penetrate thinner veins because the veins cannot withstand the cracking/tearing force. A similar morphology is shown in Fig. 6b. Within the original surface, cracks grew randomly with ease. As aforementioned, most cracks in the alloy were initiated at the sharp corner of irregular Mg/Mg17Al12 interface, which was characterized as stress concentration sites. In the meanwhile, the soft Mg cannot curb crack growth; accordingly, cracking was promoted within the original microstructure. However, in the treated zone, the overall microhardness was much higher because of the presence of fine Mg17Al12 network and reduction of favorable crack-nucleation sites. As cracks reached the boundaries of the treated zone, they met a strong shield which was much harder to penetrate. Therefore, as shown in Fig. 6b, most cracks are stopped by the treated zone; however, some cracks still could penetrate by a depth. The network zone essentially prevented further crack growth. As compared with the as-received sample, the crack density and crack size of the treated samples were both considerably less (Table 1).
5. Conclusions
Fig. 6. The sample surfaces of pre- and post-thermal fatigue. (a) The cracking behavior of the wing of dragonfly, and (b) the cracking behavior of the original and treated areas of sample (b), after 2000 thermal fatigue cycles.
Mg17Al12 plays a major role in the fatigue resistance. The brittle Mg/ Mg17Al12 interface is detrimental to mechanical properties because the interface is a perfect site for stress concentration, leading to cracking [28]. In the present study, since the amount of Mg17Al12, which was the hard constituent in the Mg–Al binary system, was increased in the form of continuous network, the large dendrites in the irregular shape were reduced. In this way, the microhardness increased and as a result, cracking was prevented by the treated zone, which is known as the blocking effect of the hard structure [29]. However, in the late stage fatigue, the crack propagated and penetrated into the treated zone. On the other hand, fatigue cracks mainly propagate in the α-Mg grains and along high-hardness βphases in both alloys [12]. Because of its high microhardness, the direction of cracking could be changed when the cracks reached Mg17Al12. In the as-received sample, the size of Mg17Al12 was coarse, so the cracks grew with ease. In the treated zone, the distribution of Mg17Al12 was uniform and in a continuous network, thus making it hard for cracks to propagate in a straight way when they reached the refined Mg17Al12. Fig. 6 reveals the surface of the treated samples after thermal fatigue. The cracks in the wing of dragonfly (Fig. 6a) clearly grew in the wing structure. When cracks reached the supporting veins, they stopped growing, or at least, to propagate in another direction. Some
Mg–9Al–Zn was treated with Nd:YAG pulsed laser. The scanning strategy was inspired by the crossing veins throughout the wing of dragonfly to study the mechanism of curbing cracks. The following results were obtained. 1. The microstructure of the treated area was notably refined by laser beam. As a result, the grain size was reduced to 3–5 μm. On the other hand, the β-Mg17Al12 dendrites were considerably refined. 2. The Mg17Al12 dendrites were precipitated along with GBs Their irregular shapes were also varied. High density dislocation was observed in HRTEM micrographs. 3. The microhardness was increased by approximately 80%. The results of thermal fatigue examinations showed that the treated samples were more resistant to cracking. The crack density and mean crack size both were decreased. 4. There were fewer sites for crack nucleation because of the reduction in the irregular Mg17Al12 and increase in the overall microhardness. The surface treatment made it much harder for cracks to penetrate into the treated zone. Sample (b) with the interpath spacing of 2 mm had the best cracking resistance of all.
Acknowledgments This work is supported by 985 platform -Bionic Engineering Science and Technology Innovation of Jilin University, the National Natural Science Foundation of China for Youths (No. 51405186).
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References [1] Lu LW, Liu TM, Chen Y, Wang ZC. Deformation and fracture behavior of hot extruded Mg alloys AZ31. Mater Charact 2012;67:93–100. [2] Zhang SK, Xiao H, Xie HB, Gu LC. The preparation and property research of the stainless steel/iron scrap clad plate. J Mater Process Technol 2014;214:1205–10. [3] Kifle M, Engdaw D, Alemu K, Sharma HR, Amsalu S, Feleke A, et al. Work related injuries and associated risk factors among iron and steel industries workers in Addis Ababa, Ethiopia, work related injuries and associated risk factors among iron and steel industries workers in Addis Ababa, Ethiopia. Saf Sci 2014;63:211–6. [4] 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. [5] Casperson MC, Carroll JD, Lambros J, Sehitoglu H, Dodds Jr. RH. Investigation of thermal effects on fatigue crack closure using multiscale digital image correlation experiments. Int J Fatigue 2014;61:10–20. [6] Huang ZY, Wang QY, Wagner D, Bathias C. A very high cycle fatigue thermal dissipation investigation for titanium alloy TC4. Mater Sci Eng A 2014;600:153–8. [7] Paffumi E, Radu V, Nilsson KF. Thermal fatigue striping damage assessment from simple screening criterion to spectrum loading approach. Int J Fatigue 2013;53:92–104. [8] Bayani H, Saebnoori E. Effect of rare earth elements addition on thermal fatigue behaviors of AZ91 magnesium alloy. J Rare Earth 2009;27:255–8. [9] Brosi JK, Lewandowski JJ. Delamination of a sensitized commercial Al–Mg alloy during fatigue crack growth. Scr Mater 2010;63:799–802. [10] Buchbinder D, Meiners W, Pirch N, Wissenbach K, Schrage J. Investigation on reducing distortion by preheating during manufacture of aluminum components using selective laser melting. J Laser Appl 2014;26:012004. [11] Cottam R, Barry T, McDonald D, Li H, Edwards D, Majumdar A, et al. Laser processing of nickel–aluminum bronze for improved surface corrosion properties. J Laser Appl 2013;25:032009. [12] Okayasu M, Takeuchi S. Mechanical strength and failure characteristics of cast Mg–9%Al–1%Zn alloys produced by a heated-mold continuous casting process: part II: fatigue properties. Mater Sci Eng A 2015 (In press). [13] Abboud JH, Benyounis KY, Olabi AG, Hashmi MSJ. Laser surface treatments of iron-based substrates for automotive application. J Mater Process Technol 2007;182:427–31.
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Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Thermal fatigue behavior of Mg–9Al–Zn alloy with biomimetic strengthening units processed by laser surface remelting Zhihui Zhang a, Pengyu Lin a,n, Shuhua Kong b, Xiujuan Li a, Luquan Ren a a 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 b The Office of Weld Planning, Department of Planning, FAW-Volkswagen automobile company Ltd., 5 Anqing Road, Changchun 130011, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2014 Received in revised form 7 October 2014 Accepted 12 October 2014
Pulsed Nd:YAG laser was used to treat the surface of a magnesium alloy to improve the thermal fatigue resistance. The average grain size of the as-received material was reduced to 3 μm and moreover βMg17Al12 was precipitated in different forms. In addition, high density dislocation was acquired. The treated zone could efficiently prevent cracking on the sample surface. The crack growth was curbed within the treated zone and the microhardness increased by 80%. This enhancement was ascribed to the modified microstructural characteristics of the treated surface. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Laser treatment Microstructure Microhardness Fatigue Biomimetic
1. Introduction Magnesium alloys have recently been increasingly used in many industrial applications, for instance in aerospace and auto-industries [1]. These alloys are characterized by high specific strength and low density. They have the potential to replace many traditional structural metals, such as steel and iron [2 and 3]. On the other hand, due to relatively weak mechanical properties, as compared with those of steel and iron, the applications of Mg alloys are limited [4]. The resistance to thermal fatigue is an important aspect of structural materials [5–7]. Applications at elevated temperatures are especially important for light alloys; however, thermal fatigue occurs as a major failure at alternate temperatures [5]. Fatigue crack growth is easier and accelerated at elevated temperatures. Thermal fatigue behavior of Mg alloys has been studied [8 and 9]. Addition of reinforcing materials is a good method to improve mechanical properties and strength. Moreover, surface treatment is another consideration in this respect [10]. Some properties, such as corrosion and thermal fatigue and wear resistance, are strongly related to the surface microstructure. In this regard, the advantages of laser surface processing are obvious. The microstructure on the surface of nickel–aluminum bronze was modified through surface treatment [11] in which the detrimental phase in the microstructure was dissolved and the corrosion resistance was thereby enhanced.
n
Corresponding author. Tel.: þ 86 0431 8578 0434; fax: þ86 431 8509 4699. E-mail address: [email protected] (P. Lin).
http://dx.doi.org/10.1016/j.optlastec.2014.10.021 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
The fatigue property of Mg alloy AZ91 was studied by Okayasu [12]. Using various casting methods, the microstructure of the as-received material was modified. In this way, the related fatigue properties were improved owing to the formation of fine grains and spherical eutectic structures (β-Mg17Al12). Laser surface treatment is an efficient method to treat material surface [13–15]. Without processing the entire material, the treated surface can address many surface-related working conditions. Abboud et al. [13] reported the effect of laser treatment on iron. A hardened layer of 200 μm in thickness was fabricated. The resulting hardness of this layer was much higher than the substrate. In addition, Benyounis et al. [15] studied the quenching effect of surface melting on nodular cast iron and found that the original graphite was completed dissolute into the matrix and was precipitated in the form of continuous network of Fe3C. This microstructural modification largely contributed to the improved property. A biomimetic laser technique has been adopted to treat sample surface [14,16], inspired by body surfaces of some creatures, for example pangolin and sea shell, providing excellent adaptability to their respective habitats. For instance, the veins on the wings of some insects, such as fly and dragonfly, can provide essential mechanical support and protection (Fig. 1a). Because of their wings special structures, they are not easily broken by wind or water forces. Lin et al. [14] reported the thermal fatigue resistance of medium carbon steel processed through this technique. The scanning strategy was inspired by convex spots covering the body surface of beetles. Hard strengthening spots were fabricated on the sample surface, thus, cracking was essentially prevented by harder
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Fig. 1. The biological model, wing structure of dragonfly. (a) The supporting veins on the wing of dragonfly, (b) the cross-sectional morphology of the supporting veins and the wing and (c) the overall treated surface of sample (b). The veins are distributed in the form of “network”. On the other hand, the cross-sectional structure shows that the supporting veins withstand most of the mechanical load. They support the entire wing. The scanning strategy was inspired by the distribution of the supporting veins on the wing of dragonfly.
elements in the structure. In addition, Zhou et al. [16] studied the scanning strategy of “stripes”, which is similar to shell surface structures that provide shell with better adaptability to water pressure and flow stress [17]. However, the study on Mg alloys is still less carried out than that on steel and iron. As a structural material of great potential, it should be more studied in this aspect to enhance its perspective application with a new method. Gao et al. [18] reported an Al2O3 ceramic coating on AZ91D alloy processed with laser cladding, exhibiting obvious layer-like characteristics with multilayer coating. Because of significant microstructurual modification, the wear and corrosion resistance were much better than those of the as-received material. Considering these observations, laser treatment on Mg alloys is very useful in industrial applications and there is much room for further studies. In this work, the effect of laser remelting on the thermal fatigue behavior of Mg–9Al–Zn alloy was studied. The scanning strategy, microhardness, and the mechanism behind improvement were investigated.
2. Materials and methods A sample of Mg–9Al–Zn alloy was used as the parent material with a chemical composition of 8.82 wt% Al, 0.87 wt% Zn, 0.16 wt%
Mn, and Mg in balance (measured with MS9710C inductively coupled plasma technique, ICP). The as-received ingot was polished and then cut in the size of 15 10 10 mm3. The as-received sample was carefully cleansed with alcohol prior to laser surface remelting (LSR). The scanning strategy was engineered to mimic the veins on the wing of dragonfly (Fig. 1a and b) via the surface treatment. As shown in Fig. 1a, the vein paths were cross-arranged on the dragonfly wing. The cross-sectional morphology (Fig. 1b) shows the supportive effect of veins: the hard “tube” provides major mechanical and load support against water or wind so that the wing cannot be easily broken. It also provides supportive force for the high speed fly vibration. As a simplified model, in this paper, the sample surface was treated in the form of a network. Treated paths in two directions were crossed with an angle of 901. Fig. 1c shows the overall treated surface. The Nd:YAG (neodymium: yttrium–aluminum–garnet) pulsed laser was used to treat the sample surface in the protective atmosphere of high-purity argon. Previous studies showed that Mg is very susceptible to elevated temperature corrosion and oxidation [19,20]; therefore, samples should have been well protected using inert atmosphere. The wavelength of applied laser beam was 1.06 μm. The maximum power of laser was 300 W and the spot diameter of laser beam on the sample surface was 0.6 mm. In addition, the spacing between every two treated paths was varied to better understand the strengthening effect. The initial spacing was preset as 2 mm, but was varied to 3 and 4 mm. Related samples were denoted as (b), (c), and (d) for the initial spacing of 2, 3, and 4 mm, respectively. The asreceived sample was denoted as sample (a). The scanning electron microscope (SEM, JSM-5600 and ZEISSEVO I8), high resolution transmission election microscopy (HRTEM, JOEL-2100F) and X-ray diffraction (XRD, D/MAX2500PC) were used to characterize the microstructures of samples. The microhardness properties were measured using a microhardness tester (FM-700) under the load of 5 g and with the loading time of 10 s. Each data point shown is the average of five individual experimental results. The thermal fatigue tests were carried out on a self-controlled thermal fatigue testing machine. A complete cycle consisted of heating for 80 s up to the maximum temperature of 260 1C in resistance furnace and then cooling with water for 3 s to the minimum temperature of 25 1C in the water tank.
3. Results Fig. 2 shows microstructure of the as-received sample and the laser treated zone. The as-received microstructure consists of α-Mg and β-Mg17Al12 dendrites (Fig. 2a). The average grain size of Mg–9Al– Zn microstructure is 50–100 μm [21]. The β-Mg17Al12 dendrites were precipitated along the grain boundaries (GBs) and in the microstructure it existed in the appearance of coarse or intermittent dendrite or both. However, the microstructure was greatly modified as a result of laser treatment. As shown in Fig. 2b, the treated microstructure was notably refined with an average grain size of 3–5 μm. On the other hand, the shape of β-Mg17Al12 was no longer coarse dendrites and its size was smaller. The fine continuous network was uniformly distributed in the microstructure along the GBs. Fig. 2c presents the cross-sectional microstructure of the treated zone. It indicates that a good metallurgical bonding was acquired between the treated area and the original microstructure. In addition, it shows that the laser beam penetrated the surface 700 μm deep. Fig. 2d shows the HRTEM of the treated area with high dislocation density. Fig. 3 reveals the XRD patterns of the as-received and treated samples. The amount of β-Mg17Al12 in the treated zone is obviously larger than that of the original microstructure. Fig. 4 plots the microhardness profiles in various orientations. The laser treatment induced a great improvement in microhardness.
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Fig. 2. The microstructures of the all samples. (a) The microstructure of as-received sample consisting of coarse α-Mg and β-Mg17Al12 dendrite, (b) the microstructure of treated zone of sample (b), (c) the cross-sectional morphology of the treated zone and (d) HRTEM of (b). The high dislocation density and phase distribution exhibit the sign of rapid solidification.
crack density and crack size both increased for all the samples. However, the sample (a) exhibited a significant increase in both of the examinations. On the other hand, the treated samples showed better resistance to thermal fatigue. Fig. 5b shows that, after long time fatigue test of beyond 3200 cycles, all four samples exhibited similar crack size values. Table 1 gives crack density values of the all samples.
4. Discussion
Fig. 3. XRD patterns of samples (a) and (b).
Fig. 4a gives the surface microhardness across the treated spots, which exhibited much larger values than that in the original surface. The microhardness of the original surface was only 70 HV. After laser treatment, the hardness increased up to 120 HV. In addition, the width of the treated area was observed as consistent with that of the improved microhardness. On the other hand, Fig. 4b presents the microhardness distribution in perpendicular direction. Within the treated area and heat affected zone, microhardness values were much greater than below the surface. Similarly, the depth of the treated area is consistent with that of the improved microhardness. Note that within the treated area, there is some abrupt decrease in microhardness, which is due to the porosity in the treated area led by laser. In this work, as inferred from Figs. 2 and 4, although the porosity was observed, it played minor effects on the microhardness and fatigue properties. Fig. 5 gives crack size evolution of various samples as a function of fatigue cycle. Fig. 5a shows that, as the fatigue test progressed, the
As laser reached the surface of sample, the surface was melted. The treated part was melted more rapidly. As laser moved away, the molten metal was solidified and a self-quenching effect occurred [22–24]. The heat led to the rapid heating and cooling of the treatment process, directly contributing to the microstructural modification of the treated area. In this study, a surface layer with a depth of approximately 700 μm was treated. During this process, the heat distribution was sophisticated. Zeng et al. [22] proposed the time-dependent heat conduction of laser to the sample surface as follows: 2 ∂T ∂T ∂ T ∂2 T ∂2 T ρC þυ ¼λ þ 2 þ 2 þQ ð1Þ 2 ∂t ∂y ∂x ∂y ∂z where ρ, C, λ, v and Q are material density, specific heat, thermal conductivity, velocity of laser, and power generated per unit volume, respectively. The x, y, and z are the directions within the treated area. When treating the sample surface, the power density of laser can be calculated using the following equation: I¼
4E 2
πd t
ð2Þ
where I is the power density of laser beam, d is the laser spot diameter, E is the pulse energy, and t is the pulse duration. Both E and t are key factors affecting the treatment process. Note that between the treated area and original area, there is a heat affected zone (HAZ) as shown in Fig. 2c.
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Fig. 4. Microhardness profiles of the all samples. (a) The microhardness across the surface centerline of the treated zone of sample (b) and (b) the perpendicular microhardness from the surface center of the treated zone of sample (b). The microhardness of the as-received sample was set as reference.
Inside the treated zone, the turbulent modification was also referred to as “self-quenching effect”, which is more obvious with ferrous alloys [25]. In general, soft phase, such as austenite or ferrite, is melted and then turned into martensite, coupled with many precipitates. The precipitate of the Mg–Al alloy is β-Mg17Al12 in the form of coarse dendrite and, as inferred from Fig. 2, the laser treatment greatly varied its precipitation and distribution. The microstructure of the original material is presented in Fig. 2a. β-Mg17Al12 was formed in the shape of irregular dendrite. As compared with α-Mg, β-Mg17Al12 is brittle and is a detrimental constituent to some properties. Due to existence of β-Mg17Al12, stress concentration occurs at the sharp corners of the irregular shapes, leading to early stage mechanical failure [26] and decrease in the ductility. However, the brittle β-Mg17Al12 could improve microhardness as the sole hard phase in the Mg–Al binary system [25]. The treated microstructure exhibited very different characteristics. It was observed from distribution of β-Mg17Al12 that the average grain size of the treated microstructure is much smaller than that of the asreceived counterpart. β-Mg17Al12 was precipitated in the form of a fine network. Similar to self-quenching effect on steel and iron, laser beam generated rapid solidification [27]. The melted metal can solidify within a very short time, therefore, the microstructure, in particular β-Mg17Al12, was notably refined. Different from the previous study [27], in the Mg–Al binary system, β-Mg17Al12 is the only precipitate. However, the smaller precipitates would enhance the mechanical properties. Note that the amount of β-Mg17Al12 increased with LSR (see Fig. 3). Although the time for β-Mg17Al12 precipitation
Fig. 5. Thermal fatigue behaviors of the all samples. (a) The average crack density of the all samples as the function of crack cycle and (b) the average crack size of the all samples as the function of crack cycle.
was very short, eutectic nucleation was promoted during the super cooling condition. In addition, according to the Hall–Patch relationship, the smaller grain size is a major contributor in this regard. Furthermore, the high dislocation density cannot be ignored here (Fig. 2d). As a result, the above characteristics lead to much greater microhardness value in the treated area. An abrupt decrease in microhardness was observed because only the surface was treated by laser. This decrease mainly occurred in the HAZ. The metallurgical bonding between the treated zone and original microstructure played an important role in this phenomenon. As shown in Fig. 5, the treated samples exhibited better resistance to thermal fatigue than the as-received counterpart. When fatigue cycle increased, the crack size and crack density both increased. However, the treated samples were more resistant to cracking than the as-received one. The smaller interpath spacing resulted in better fatigue resistance. Therefore, sample (b) exhibited the best fatigue resistance of all samples. Note that in the early stage of thermal fatigue, the differences in crack density and crack size were obvious, whereas in the later stage, the differences were less obvious, in particular, among the treated samples. One could anticipate that it is because the microhardness of the treated samples was insufficient to withstand late-stage fatigue. Consequently, the sample surface was badly damaged. Once the strengthening elements were damaged, the entire surface failed. Bayani et al. [8] reported the effects of β-Mg17Al12 on the fatigue resistance and observed a different result. The volume fraction of β-
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Table 1 The true crack density and crack size of the all samples during fatigue test. L is the crack length, which falls into three categories: small crack: L 41 mm, medium crack: 1 mm o L o 2 mm, and large crack: L 42 mm. Treated samples with varied interpath spacings were all investigated. Cycle number
As-received
o400 400–800 800–1200 1200–1600 1600–2000 2000–2400 2400–2800 2800–3200 3200–4000
Lo1 7 10 15 20 27 31 34 38 44
1r L r 2 3 5 8 10 13 17 19 23 27
Interpath spacing 2 mm L42 1 1 3 4 6 7 10 12 16
Lo1 2 4 7 11 17 22 26 32 40
1r L r 2 – 2 3 5 8 10 13 18 22
Interpath spacing 3 mm L 42 – – – 1 2 4 5 7 10
Lo1 5 7 12 14 20 25 31 35 40
1r L r 2 – 2 4 7 10 12 16 20 25
Interpath spacing 4 mm L 42 – – – 2 3 6 8 9 11
Lo1 8 9 12 16 23 28 33 36 43
1r L r 2 2 3 5 11 11 15 18 21 26
L 42 – 1 1 2 4 7 8 10 13
cracks penetrate thinner veins because the veins cannot withstand the cracking/tearing force. A similar morphology is shown in Fig. 6b. Within the original surface, cracks grew randomly with ease. As aforementioned, most cracks in the alloy were initiated at the sharp corner of irregular Mg/Mg17Al12 interface, which was characterized as stress concentration sites. In the meanwhile, the soft Mg cannot curb crack growth; accordingly, cracking was promoted within the original microstructure. However, in the treated zone, the overall microhardness was much higher because of the presence of fine Mg17Al12 network and reduction of favorable crack-nucleation sites. As cracks reached the boundaries of the treated zone, they met a strong shield which was much harder to penetrate. Therefore, as shown in Fig. 6b, most cracks are stopped by the treated zone; however, some cracks still could penetrate by a depth. The network zone essentially prevented further crack growth. As compared with the as-received sample, the crack density and crack size of the treated samples were both considerably less (Table 1).
5. Conclusions
Fig. 6. The sample surfaces of pre- and post-thermal fatigue. (a) The cracking behavior of the wing of dragonfly, and (b) the cracking behavior of the original and treated areas of sample (b), after 2000 thermal fatigue cycles.
Mg17Al12 plays a major role in the fatigue resistance. The brittle Mg/ Mg17Al12 interface is detrimental to mechanical properties because the interface is a perfect site for stress concentration, leading to cracking [28]. In the present study, since the amount of Mg17Al12, which was the hard constituent in the Mg–Al binary system, was increased in the form of continuous network, the large dendrites in the irregular shape were reduced. In this way, the microhardness increased and as a result, cracking was prevented by the treated zone, which is known as the blocking effect of the hard structure [29]. However, in the late stage fatigue, the crack propagated and penetrated into the treated zone. On the other hand, fatigue cracks mainly propagate in the α-Mg grains and along high-hardness βphases in both alloys [12]. Because of its high microhardness, the direction of cracking could be changed when the cracks reached Mg17Al12. In the as-received sample, the size of Mg17Al12 was coarse, so the cracks grew with ease. In the treated zone, the distribution of Mg17Al12 was uniform and in a continuous network, thus making it hard for cracks to propagate in a straight way when they reached the refined Mg17Al12. Fig. 6 reveals the surface of the treated samples after thermal fatigue. The cracks in the wing of dragonfly (Fig. 6a) clearly grew in the wing structure. When cracks reached the supporting veins, they stopped growing, or at least, to propagate in another direction. Some
Mg–9Al–Zn was treated with Nd:YAG pulsed laser. The scanning strategy was inspired by the crossing veins throughout the wing of dragonfly to study the mechanism of curbing cracks. The following results were obtained. 1. The microstructure of the treated area was notably refined by laser beam. As a result, the grain size was reduced to 3–5 μm. On the other hand, the β-Mg17Al12 dendrites were considerably refined. 2. The Mg17Al12 dendrites were precipitated along with GBs Their irregular shapes were also varied. High density dislocation was observed in HRTEM micrographs. 3. The microhardness was increased by approximately 80%. The results of thermal fatigue examinations showed that the treated samples were more resistant to cracking. The crack density and mean crack size both were decreased. 4. There were fewer sites for crack nucleation because of the reduction in the irregular Mg17Al12 and increase in the overall microhardness. The surface treatment made it much harder for cracks to penetrate into the treated zone. Sample (b) with the interpath spacing of 2 mm had the best cracking resistance of all.
Acknowledgments This work is supported by 985 platform -Bionic Engineering Science and Technology Innovation of Jilin University, the National Natural Science Foundation of China for Youths (No. 51405186).
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