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Influence of laser surface melting on the properties of MB26 and AZ80 magnesium alloys
Surface & Coatings Technology 378 (2019) 124964
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Influence of laser surface melting on the properties of MB26 and AZ80 magnesium alloys Yuhang Lia,1, Srinivasan Arthanaria,b,1, Yingchun Guana,b,c,
T
⁎
a
School of Mechanical Engineering and Automation, Beihang University, 37 Xueyuan Road, Beijing 100191, PR China Hefei Innovation Research Institute of Beihang University, Xinzhan Hi-Tech District, Anhui 230012, PR China c National Engineering Laboratory of Additive Manufacturing for Large Metallic Components, Beihang University, Beijing 100083, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium alloys Laser surface melting Hardness Corrosion Polarization
In the present investigation, laser surface melting (LSM) was performed on MB26 and AZ80 magnesium alloys and their surface properties were evaluated. The finite element method (FEM) was used to calculate the resulting temperature during the laser surface melting process. The melted layer average thickness values were about 296 and 190 μm for MB26 and AZ80 alloys, respectively and the variation in the thickness is mainly attributed to the absorption in laser energy. The average grain size of the LSMed MB26 alloy was decreased about 14.5 times and AZ80 had very finer grains than the MB26 along with the fine distribution of second phases. Further, the X-ray diffraction results revealed that the second phase intensities were decreased in both the alloys due to their dispersion. The laser absorption efficiency of MB26 was higher due to the higher plasma attachment during LSM process, thereby has higher melt layer thickness compared to AZ80. The refined microstructure of the melted layer resulted in an increase of micro-hardness up to 110 Hv. Potentiodynamic polarization test results revealed that the corrosion current density (icorr) values of the LSMed MB26 and AZ80 alloys were decreased about 1.8 and 2.5 times, respectively compared to the as-received alloys. A variation in solidification rates of the melt pool due to the alloying elements were attributed to an improvement in the surface and electrochemical properties.
1. Introduction
processes to achieve the grain refinement and phase segregation on the surfaces of Mg alloys without altering their bulk properties [11–13]. The surface wettability and corrosion resistance of LSMed Mg-Zn-Dy alloy were improved in Hank's balanced salt solution [14]. A rapid heating and cooling cycles during LSM treatment produced significant variation in the grain refinement and surface roughness values as the laser energy density was varied. As a result, the surface wettability was improved and the in-vitro degradation rate was also reduced. Recently, Zhang et al. reported an improvement in the mechanical properties and biocompatibility of laser surface processed Mg-Gd-Ca alloy [15,16]. The β-Mg5Gd phase was disappeared due to the higher solubility of Gd during laser melting process and the hardness value was also substantially increased. Laser-induced periodic surface structure (LIPSS) on laser melted surface had better biocompatibility compared to the microgrooved surface. Laser re-melting of Mg-Zn-Ca alloy was carried out by Zhang et al. and the amorphous structure was formed at a scan speed over 1000 mm/min and the corrosion resistance was increased with scan speed [17]. Laser surface melting produced finer and cellular microstructures on MgeZn alloy and higher treatment depths had
Mg alloys are used in several applications involving external stress and aqueous environments, therefore, improvement of their mechanical properties and corrosion resistance is indeed important [1]. The controlled degradation rate of Mg-based degradable implant materials in the physiological environments is also essential for better osseointegration [2]. It is well-known that Mg possesses higher electrode potential therefore easily oxidizes and the surface is covered with loosely bound hydroxide/oxide layers [3,4]. Nevertheless, a surface layer formed on the Mg alloys is not as stable as in the case of aluminum and titanium alloys [5]. Therefore, surface properties of Mg alloys need to be altered to establish a stable surface layer formation. The variation in the microstructures such as grain refinement and phase segregation would be beneficial to simultaneously enhance the mechanical as well as corrosion properties [6]. Surface modification of Mg alloys, in particular, laser-based processing are generally applied in recent days to enhance their surface-related properties without altering the bulk properties of the material [7–10]. LSM is one of the precise and rapid
⁎
Corresponding author at: School of Mechanical Engineering and Automation, Beihang University, 37 Xueyuan Road, Beijing 100191, PR China. E-mail address: [email protected] (Y. Guan). 1 Equally contributed. https://doi.org/10.1016/j.surfcoat.2019.124964 Received 11 July 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online 07 September 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 378 (2019) 124964
Y. Li, et al.
element is a cube with a size of 30 × 30 × 30 μm. To simplify the simulation process, it was assumed that: a) laser absorption does not change with temperature, b) neglecting the flow of melting pool and evaporation of the MB26 and AZ80 alloys at boiling point and c) the MB26 and AZ80 alloys are continuous and isotropic. The basic heat transfer control equation can be expressed as follows:
better corrosion resistance. Furthermore, the in-vitro degradation and hydrogen evolution tests in Hank's balanced salt solution revealed that the laser melted and polished samples had lower corrosion rate (< 1 mm/y) [13]. The surface hardness and tensile properties at room and elevated temperatures were increased for the LSMed AZ91 D alloy, however, the ductility was decreased [18]. Based on the available literature it could be understood that the LSM significantly altered the grain refinement and second phase distribution in the melt layer. The melting and solidification behaviors of various alloying elements and their diffusion in the melt pool differ during LSM process due to the variation in laser absorption efficiencies, which affects the melted layer properties. Furthermore, the solidification behavior of second phases varied as the cooling rates changed with varying the laser process parameters; thereby affect the mechanical as well as corrosion behavior. Therefore, the presence of alloying elements plays a key role in determining the melt layer properties and this phenomenon needs to be investigated. Hence, in the present investigation, LSM was carried out on two different magnesium alloys such as MB26 and AZ80 to understand the laser surface melting on the microstructural, hardness and electrochemical corrosion behavior. The finite element method (FEM) was also used to simulate the temperature change during the LSM process and correlated with the experimental results.
∂ ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ ∂T + + k k k + q = ρc ∂x ⎝ ∂x ⎠ ∂y ⎝ ∂y ⎠ ∂z ⎝ ∂z ⎠ ∂t ⎜
⎟
(1)
where ρ is the material density; c is the specific heat capacity of material; k is the material thermal conductivity; T is the temperature; q is the heat generated by heat transfer media per unit volume and t is time. In order to obtain the differential equation, the fixed boundary conditions are applied on both ends of the alloys resembling the experimental laser surface melting situation. The temperature of the alloys at the initial moment is considered as the ambient temperature. The density of the input heat flow rate of each facula was calculated by the Gaussian function and it is consistent with the laser heat source used in the LSM experiment. Besides, the exhaustive explanation of its convergence criteria, boundary conditions, and governing equations were reported elsewhere [20]. 2.4. Electrochemical characterization
2. Experimental work
Electrochemical corrosion studies of as-received and LSMed alloys were carried out in 3.5 wt% NaCl solution using a CH Instruments 660E electrochemical workstation. As received, and LSMed alloys were successively abraded with SiC papers up to 2500# to ensure similar surface roughness values prior to the corrosion test. Electrochemical tests were carried out at ambient temperature and the cell was comprised of as received or LSMed alloys as working electrode (1 cm2), saturated calomel (SCE) and carbon rod were used as reference and counter electrodes, respectively. Potentiodynamic polarization studies were performed in the potential range OCP ± 0.200 V at a scan rate of 1 mV/s after 15 min of exposure. In order to confirm the reproducibility of the results, two samples in each condition were tested under the same experimental condition.
2.1. Laser surface melting of Mg alloys Extruded MB26 (5.6 wt% Zn, 1.2 wt% Y, 0.66 wt% Zr and Mg balance) and AZ80 (9.1 wt% Al, 0.42 wt% Zn, 0.03 wt% Mn, 0.01 wt% Si, ≤0.005 Fe, ≤0.05 Cu, ≤0.005 Ni and Mg balance) alloys were used for the laser surface melting process. Samples were cut from the extruded plates with a dimension of 50 × 40 × 8 mm3, abraded with silicon carbide (SiC) emery papers to remove the surface contaminants and washed with ethanol. A nanosecond (ns) pulsed fiber laser with the wavelength of 1060 nm was used for the LSM process. The process parameters such as pulse duration, repetition rate, and spot size were 220 ns, 500 kHz, and 44 μm, respectively. Alloys were irradiated with a laser power density of 1.20 × 107 W/cm2 and at a scanning speed of 200 mm/s with 50% beam bath overlapping. The LSM process was carried out under Ar gas protection.
3. Results and discussion 3.1. Surface characterization of laser surface melted alloys
2.2. Surface characterization Fig. 1(a–d) shows the surface roughness profiles of the as-received and LSMed alloys. As can be seen, after the LSM process, the average roughness (Ra) value was decreased from 0.525 to 0.166 μm for the MB26 alloy and the value was marginally decreased from 0.515 to 0.308 μm for the AZ80 alloy (Table 1). The smooth surface obtained for the MB26 alloy could be mainly attributed to several reasons including surface tension, convection, alloying elements (Zn and Y), etc. Marangoni effect suggests that the motion of melt pool is mainly happening due to the surface tension gradient and thermocapillary force is responsible to pull the material from the hotter region to the cooler regions, which could affect the surface roughness values. Furthermore, chemicapillary force is during the laser melting process could move the material towards the high surface energy region [21]. The Ra roughness values were decreased by about 3 and 1.6 times for the MB26 and AZ80 alloys, respectively and the smooth surface was achieved for the MB26 alloy could be attributed to the higher surface tension gradient existed between the melt pool and substrate during the LSM process. AZ80 alloy had a relatively rough surface compared to the MB26 alloy, which could also be attributed to the variation in the solubility of added alloying elements mainly Al and evaporation of Mg. In addition to that, the overlap ratio used for the LSM affects the average surface roughness values. In the present investigation, the overlap ratio was similar for both the alloys and only the alloying composition is different which
Surface topography of the as-received and LSMed alloys was observed using a 3D laser scanning confocal microscope (VK100, Keyence, Osaka, Japan). Cross-sectional microstructures of the melted layers were observed using a Nikon LV150N optical microscope (OM). Besides, the SU8010 scanning electron microscope with energy dispersive X-ray analysis (SEM-EDAX) was utilized for microstructural and elemental chemical composition analysis. X-ray diffraction (XRD) patterns of the as-received and laser melted surfaces were obtained using a Rigaku D/max2200PC instrument in the 2θ range between 20° and 80° at a scan speed of 0.03°. Micro-hardness measurements were carried out across the LSMed surfaces using FUTURE-TECH FM-800 Vickers instrument with the loading force of 0.98 N at holding time of 10 s. 2.3. Numerical simulation The temperature profiles of the MB26 and AZ80 alloys during the LSM process was calculated using a finite element commercial software ABAQUS (ABAQUS 2018, SIMULIA, Rhode Island, RI, USA). The thermal modeling was established by solving the general heat transfer equation for conduction [19]. The calculation of the two models (MB26 and AZ80) has been carried out on a rectangle substrate with dimensions of 3 × 1 × 0.5 mm3 and the minimum differential volume 2
Surface & Coatings Technology 378 (2019) 124964
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Fig. 1. Surface roughness profiles of (a) as-received MB26, (b) laser surface melted MB26, (c) as-received AZ80 and (d) laser surface melted AZ80 magnesium alloys.
Fig. 3a & b shows the microstructures and EDAX elemental composition of the LSMed and as-received MB26 alloy. The melted surface has mainly consisted of fine dendrites-like second phases along the grain boundaries and the particulate like second phases was also identified in the matrix of the surface. The chemical composition of the particulate like phase (P1) consisted of small amounts of Zn and Y elements and rich of Mg which could be attributed to the Mg(ZnY) phase. Interestingly, the chemical composition of the second phase along the grain boundaries (P2) has consisted of about 75.68, 21.36 and 2.08 at.% of Mg, Zn, and Y respectively. The matrix has consisted of a small amount of Zn and Y, further, the amount of Y was marginally increased at the matrix of the melted layer compared to the as-received alloy. The coarse second phase (P4) was identified on the as-received alloy and the phase was rich in Zn and Y. The average melted layer thickness values were measured from the cross-sectional morphologies of AZ80 alloy (Fig. 4b) and the average thickness value was 190 ± 22 μm. Furthermore, the melted layer has consisted of very fine microstructure and the uniform distribution of fine second phases could also be identified along the grain boundaries. Whereas the average grain size was 28.1 ± 5 μm for as-received alloy and coarse second phases were also identified at the grain boundaries. The temperature contour (Fig. 4a) reveals that the maximum temperature was 2205 °C and the temperature in melt pool and HAZ was predicted as 1590 and 644 °C, respectively. It could also be noticed that the maximum temperature of the AZ80 alloy was marginally lower compared with the MB26 alloy furthermore, the melt pool temperature was also lower. The laser processing parameters were the same for both the alloys, however alloying elements resulted in a variation in the melt pool temperature and the further solidification behavior, which resulted in different microstructures and the second phase segregation. The chemical composition at the second phase and matrix was measured using EDAX and are shown in Fig. 5a & b. The white second phases (P1) were rich in Al and Mg and could be attributed to the
Table 1 Comparison of average surface roughness values of MB26 and AZ80. Alloy
Condition
Ra (μm)
Rq (μm)
MB26
As received Laser melted As received Laser melted
0.525 0.166 0.515 0.308
0.693 0.222 0.676 0.416
AZ80
would be a key factor to affect the surface roughness values. Cross-sectional morphologies and the corresponding simulated thermal contour of the LSMed MB26 alloy is shown in Fig. 2a & b and the direction of laser re-melting is along the Y-axis. In addition to that, the microstructures at different regions across the melted surfaces (A–D) are also included in Fig. 2. The average melted layer thickness value was measured using Image J software and the value was 296 ± 21 μm for the MB26 alloy. The average grain size at the melted layer was 9.2 ± 1.6 μm, while the grain size at the as-received alloy was 133 ± 33 μm. In addition to that, the melted layer was well attached to the substrate and no defects were noticed. It can be seen from the temperature contour (Fig. 2a) that the high-temperature contour extended inside the melt layer, and a sharp thermal gradient was emerged in the vicinity of the laser source, in particular in front of the melt zone. The molten pool was the elliptical shape and the maximum temperature was 2335 °C for the MB26 alloy. The melt pool and heataffected zone temperature were predicted about 1780 and 1590 °C, respectively and these results are in accordance with Yilbas et al. [22]. The higher temperature gradients exist between the surface temperature and the melt pool, thus results in high cooling rate (1.209 × 104 K/ s) on the thin layers of the alloy substrate was mainly responsible for the grain refinement in the melted layer [23]. Furthermore, the average grain size marginally varied from the bottom of the melted layer to the top surface due to the variation in the cooling rate. 3
Surface & Coatings Technology 378 (2019) 124964
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Fig. 2. Cross-sectional morphologies of laser surface melted MB26 magnesium alloys.
The variation in melted layer thickness between MB26 and AZ80 could be attributed to the difference in cooling rate and the smaller ratio of a temperature gradient to solidification rate (G/R) [24,25]. Furthermore, it can be seen from FEM results that the temperature of the molten pool is > 1780 °C for MB26 alloy which is higher than the AZ80 (1590 °C). The phase diagrams of MgeZn and MgeAl systems revealed that the phase transition temperature of MB26 alloy is
Mg17Al12 phase. Besides, the second phases are uniformly distributed in the melted layer, while the coarse phase was noticed at the as-received surface (Fig. 5b). The finer grains with uniform distribution of the Mg17Al12 phase along the grain boundaries are due to the rapid heating and cooling process during LSM process. Al content was varied in the matrix at the as-received and melted layers could be attributed to the dispersion of Al in the matrix during the laser melting process. 4
Surface & Coatings Technology 378 (2019) 124964
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Element Mg K Zn K YL Zr L Mn K
(a) P2
P1
Wt.% 80.34 6.20 8.67 4.05 0.74
At.% 92.70 2.99 2.75 1.18 0.38
P3
P1
5 µm Element Mg K Zn K YL Zr L Mn K
Wt.% 52.2 40.24 5.30 1.11 0.73
At.% 75.68 21.36 2.08 0.43 0.47
Element Mg K Zn K YL Zr L Mn K
Wt.% 96.95 2.77 0.15 0.99 0.14
keV At.% 98.57 1.06 0.04 0.27 0.06
P2
P3 Element Mg K Zn K YL Zr L Mn K
keV
Wt.% 96.71 2.19 0.45 0.32 0.33
At.% keV 98.80 0.83 0.13 0.09 0.15
(b)
P5 P4 Element Mg K Zn K YL Zr L Mn K
P4
Wt.% 94.46 4.73 0.07 0.55 0.19
keV At.% 97.92 1.82 0.02 0.15 0.09
5 µm
P5 keV Fig. 3. EDAX chemical composition of (a) laser surface melted and (b) as received MB26 magnesium alloy.
phase present in the AZ80 alloy [28]. Moreover, during the hightemperature LSM process the elements present in the second phases could dissolve in the matrix and the coarse phases are redistributed as fine particles in the melted layer after the LSM process. It is also presumed that due to these phenomena more plasma spatter could attach
598–689 °C which is also higher than the phase transition temperature of AZ80 alloy (437–450 °C) [26]. The results show that MB26 need to absorb more energy than AZ80 alloy to produce phase transition. In addition to that, the melting point of Mg3Zn6Y phase in the MB26 alloy is 445 °C [27] which is marginally lower than the β-Mg17Al12 (455 °C) 5
Surface & Coatings Technology 378 (2019) 124964
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Fig. 4. Cross-sectional morphologies of laser surface melted AZ80 magnesium alloys.
during laser processing. The absorption energies of the melted state of untreated WE43 and ZE41 alloys were about 40 and 30%, respectively and the energy needed to melt the ZE41 alloy was lower than the WE43 [30]. In both LSMed MB26 and AZ80 alloys, the grain refinement was higher at the top surface and varied as the melt depth was increased, which could be attributed to the decrease in cooling rate of the solidifying material during LSM process [31] and these results were further
to the MB26 surface during the LSM process and higher plasma attachments could increase the laser absorption efficiency of MB26, thereby has a higher melt layer thickness compared to AZ80 [29]. Ignat et al. reported the variation in the absorption efficiencies of pulsed Nd:YAG laser (λ = 1.06 μm) treated WE43 and ZE41 alloys before and after the anodizing and mordancage processes. The surface layer covered on the surface significantly varied the absorption efficiencies 6
Surface & Coatings Technology 378 (2019) 124964
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Element Mg K Al K Zn L Mn K
(a)
Wt.% 78.64 20.18 1.09 0.09
At.% 80.85 18.69 0.42 0.04
P1
P1 Element Mg K Al K Zn L Mn K
P2
Wt.% 94.58 4.93 0.19 0.11
At.% keV 95.25 4.47 0.07 0.05
5 µm
P2
Element Mg K Al K Zn L Mn K
Wt.% 67.98 28.73 3.00 0.31
keV At.% 71.57 27.12 1.17 0.14
(b)
P3
P3
P4 Element Mg K Al K Zn L Mn K
Wt.% 90.55 9.29 0.08 0.08
keV At.% 91.49 8.45 0.03 0.04
5 µm
P4 keV Fig. 5. EDAX chemical composition of (a) laser surface melted and (b) as received AZ80 magnesium alloys.
3.2. Surface micro-hardness of laser surface melted alloys
substantiated by the FEM results. XRD patterns of the as-received and LSMed alloys are shown in Fig. 6a & b. As can be seen, MB26 alloy has mainly consisted of primary α-Mg and Mg3Zn6Y phases (Fig. 6a). The second phase peak intensities of both as-received and LSMed alloys appeared almost the same and no significant change was noticed. It is presumed that during laser melting process some new phases could be formed, however, their amounts are less than the detection limits of XRD. As received AZ80 alloy has mainly consisted of β-Mg17Al12 phase and the intensities of these peaks were significantly reduced for the laser-melted surfaces due to the heat accumulation during the laser re-melting process. It is also presumed that the LSM resulted in the dispersion of the second phase, which is evident from the cross-sectional morphologies of melted AZ80 alloy (Fig. 2b).
The micro-hardness profiles of LSMed MB26 and AZ80 alloys were measured across the surface and are shown in Fig. 7a & b. Laser surface melting generally enhances the surface microhardness of the LSMed layer due to the grain refinement and solid solution strengthening, however, the magnitude of increment varied depending upon the laser processing conditions and alloys. The microhardness value of the LSMed MB26 alloy was increased from about 70 to 105 Hv. Besides, the fluctuations in the hardness values are seen, which could be attributed to the presence of dispersed second phases. The value was increased from about 80 to 100 Hv for the laser melted AZ80 alloy. The uniform distribution of the Mg17Al12 phase in the AZ80 alloy can have a pinning 7
Surface & Coatings Technology 378 (2019) 124964
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-Mg Mg3Zn6Y
Intensity (a.u.)
(a)
Laser melted
As received
20
30
40
50
60
70
80
2 (degree) -Mg -Mg17Al12
Intensity (a.u.)
(b)
Laser melted
As received
20
30
40
50
60
70
80
2 (degree) Fig. 6. XRD patterns of the as received and laser surface melted (a) MB26 and (b) AZ80 magnesium alloys.
Fig. 7. Micro hardness values across the surface melted layer of (a) MB26 and (b) AZ80 magnesium alloys.
effect on the dislocation, therefore, improve the strength and microhardness of the melted surface. However, the hardness value of LSMed AZ80 was lower than the MB26 alloy. It is also seen from the crosssectional morphologies that, the melted layer thickness is less for the AZ80 alloy compared to MB26, which could also be a reason for the higher hardness values of MB26 alloy. Further, it is confirmed from the Hall-Petch equation that, the grain refinement significantly enhances the microhardness values of magnesium alloys and support the present results. The microhardness values of the melted layers are compared with the reported values and are shown in Table 2. Results showed that the values achieved in the present investigation are comparable with the hardness values of AZ91 alloy, which is most commonly used in industrial applications due to its higher fluidity during the casting process.
Table 2 Comparison of micro hardness values of various laser surface melted Mg alloys. Alloy
Hardness (Hv)
Ref.
MEZ AZ91D AZ91D AZ31B (laser melting at Ar environment) AZ31B (laser melting at liquid N2 environment) AZ91 AZ91D Mg-Zn-Dy AZ91D MB26 AZ80
100 113 100 90 148 93 112 94 70.8 110 100
[8] [36] [37] [24] [31] [18] [14] [38] Present study
Furthermore, it is observed from the cross-sectional morphologies that, the melted layer thickness on MB26 alloy was about 100 μm higher than the AZ80 alloy, which could also be responsible for the shift in the Ecorr value. The corrosion potential of the LSMed WE43 alloy was shifted about 0.141 mV in the positive direction compared to the asreceived alloy [32]. The shift in Ecorr values was varied as the lasermelting environment was varied, the positive shift in Ecorr values were 0.124 and 0.098 V for the AZ91 alloy treated with liquid nitrogen and Ar gas environments. The difference in the cooling rate of the melted layer was responsible for the variation in grain refinement, thus shifted the Ecorr values in the nobler direction [24]. In the present investigation, variation in the melted layer thickness is mainly attributed to the
3.3. Electrochemical corrosion behavior Electrochemical corrosion behavior of the as-received and LSMed MB26 and AZ80 alloys were studied in 3.5% NaCl solution and the potentiodynamic polarization curves are shown in Fig. 8a & b. Potentiodynamic polarization parameters are listed in Table 3. The corrosion potential (Ecorr) values were shifted about 0.122 and 0.06 V in the positive direction for the LSMed MB26 and AZ80 alloys, respectively compared to as-received alloys. A positive shift in Ecorr values indicated that the melted layer is thermodynamically stable in the aqueous environment and hence control the Cl− ions penetration. 8
Surface & Coatings Technology 378 (2019) 124964
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alloys. The icorr values were about 31.2 and 37.9 μA/cm2 (Table 3) for the laser melted MB26 and AZ80 alloys, respectively. The reduction in the icorr values of LSMed alloys were about 1.8 and 2.5 times compared to as-received MB26 (56.8 μA/cm2) and AZ80 (96.0 μA/cm2) alloys, respectively. A decrease in icorr values clearly indicated the better corrosion resistance of the LSMed alloys. Interestingly, the reduction in icorr value was higher for the AZ80 alloy, which could be attributed to the presence of uniformly distributed grain containing β-Mg17Al12 phases, which could act as a barrier to the corrosive ions penetration and help to establish the stable surface layer [33–35]. Furthermore, rapid solidification of the Al-containing Mg alloys also improves the passivation behavior. Microstructural studies further confirmed the presence of β-phase Mg17Al12 with larger grain sizes at the as-received conditions. The random distribution of the β-phase Mg17Al12 could act as a cathode to the α-Mg matrix, thus accelerate the galvanic corrosion and not acting as a barrier to Mg dissolution. After the laser surface melting, the grain sizes were uniform and the distribution of Mg17Al12 phases, which was confirmed from the less intense peak found at XRD patterns. This could accelerate the stable surface layer formation on the surface during exposure to the corrosive medium. Lower icorr value of the MB26 alloy could also be attributed to the uniform microstructures and fine distribution of the second phases which could establish a stable surface layer formation. However, the variation in the icorr value of the MB26 alloy was less compared to the AZ80 alloy, which could be attributed to the galvanic influence of the Mg3Zn6Y phase, which is nobler than the Mg phase and acts as a cathode to accelerate the Mg dissolution. Furthermore, the presence of alloying elements such as Al helps to enhance the performance of Mg alloys due to its fine distribution in the re-melted layers. This reveals that the presence of alloying elements in the Mg alloys plays a key role during laser surface melting and resulting grain refinement. The icorr value of the as-received AZ80 alloy was higher than the MB26 alloy indicating its lower corrosion resistance due to the presence of coarse βphase. It is confirmed from the results that the LSM significantly affected the surface electrochemical activity by altering the microstructure and second phase distribution. Corrosion current (icorr) densities of various LSMed Mg alloys are compared with the present results and are listed in Table 4. The results showed that the icorr values of LSMed MB26 and AZ80 are lower than the LSMed AZ91D alloy. The variation in the corrosion resistance of the LSMed alloys is mainly attributed to the laser treatment condition and alloying composition.
Fig. 8. Potentiodynamic polarization curves of as received and laser surface melted (a) MB26 and (b) AZ80 magnesium alloys.
Table 3 Potentiodynamic polarization parameters of as received and laser surface melted MB26 and AZ80. Alloy
Condition
Ecorr (VSCE)
icorr (μA/cm2)
MB26
As-received Laser melted As-received Laser melted
−1.568 −1.446 −1.595 −1.535
56.8 31.2 96.0 37.9
AZ80
4. Conclusions Laser surface melting of MB26 and AZ80 magnesium alloys were carried and their surface and electrochemical corrosion properties were systematically evaluated; 1. Surface roughness values of LSMed alloys were reduced in particular, the reduction was relatively higher for MB26 alloy due to the variation in cooling rate during laser processing. 2. Cross-sectional morphologies revealed that LSMed layer thickness was significantly varied and melted layer thickness value of AZ80
nobler shift in the Ecorr values. The current density values at cathodic regions were lower for the LSMed alloys compared to the as-received alloys indicating the reduction in cathodic reaction kinetics and hydrogen evolution reaction. Further, the reduction was higher for the LSMed AZ80 compared to MB26 alloy, could be attributed mainly to the dispersed second phase in the melted zone. Laser melted surface does not show any sign for the presence of a passive region in the anodic potential sweep. The current was also rapidly increased as the potential was increased after the Ecorr value. Nevertheless, the current density values in the anodic region were lower compared to the as-received alloy shows the lower dissolution rate of Mg. Furthermore, the anodic slope of the LSMed alloys is marginally higher than the as-received alloys could be attributed to the slower dissolution rate. Corrosion current density (icorr) is a key parameter deciding the corrosion performances of the various metals/
Table 4 Comparison of corrosion current density (icorr) values of various laser surface melted Mg alloys.
9
Alloy
Corrosion medium
icorr (μA/cm2)
Ref.
MEZ AM60B WE43 Mg-Ca-Zn AZ91D MB26 AZ80
3.5% NaCl 3.5% NaCl 3.5% NaCl Hank's solution 3.5% NaCl 3.5% NaCl 3.5% NaCl
1500 0.598 8.51 9.3 68.7 31.2 37.9
[8] [39] [32] [40] [38] Present study
Surface & Coatings Technology 378 (2019) 124964
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alloy was less compared with MB26. This could be attributed to the difference in the melting behavior of various constituent alloying elements and the evaporation of alloying elements. The FEM temperature contour results also further substantiated the experimental results. 3. XRD patterns clearly indicated the decrease in the peak intensities of the second phases after LSM confirming the dispersion of the second phases in the matrix due to the high-temperature laser processing, which affected the mechanical as well as corrosion behavior. 4. Microhardness value of LSMed MB26 alloy was higher compared with AZ80 alloy is due to the higher melt layer thickness; furthermore, the fluctuations in the values across the surface could be due to the presence of segregated second phases. 5. Potentiodynamic polarization studies clearly indicated the better corrosion resistance of the melted layers. In particular, the improvement in corrosion resistance was higher for the LSMed AZ80 alloy compared with MB26. The fine microstructure along with a uniform distribution of β-phase Mg17Al12 could be responsible for the better corrosion resistance. The galvanic influence of second phase present in the LSMed MB26 alloy marginally decreased the improvement of corrosion resistance compared with LSMed AZ80 alloy.
[7] A. Singh, S.P. Harimkar, JOM 64 (2012) 716–733. [8] J. Dutta Majumdar, R. Galun, B.L. Mordike, I. Manna, Mater. Sci. Eng. A 361 (2003) 119–129. [9] P. Chakraborty Banerjee, R.K. Singh Raman, Y. Durandet, G. McAdam, Metall. Mater. Trans. A 44 (2013) 2346–2357. [10] M.A. Melia, D.C. Florian, F.W. Steuer, B.F. Briglia, M.K. Purzycki, J.R. Scully, J.M. Fitz-Gerald, Surf. Coat. Technol. 325 (2017) 157–165. [11] G. Abbas, Z. Liu, P. Skeldon, Appl. Surf. Sci. 247 (2005) 347–353. [12] Y. Guan, S. Zhang, J. Li, X. Tian, H. Zheng, Laser surface processing of magnesium alloys, in: D. Carou, J.P. Davim (Eds.), Machining of Light Alloys, Aluminum, Titanium, and Magnesium, CRC Press, 2019, pp. 125–150. [13] B. Manne, H. Thiruvayapati, S. Bontha, R. Motagondanahalli Rangarasaiah, M. Das, V.K. Balla, Surf. Coat. Technol. 347 (2018) 337–349. [14] R. K.R, S. Bontha, R. M.R, M. Das, V.K. BallaAppl. Surf. Sci., 480 (2019) 70–82. [15] J. Zhang, Y. Guan, W. Lin, X. Gu, Surf. Coat. Technol. 362 (2019) 176–184. [16] J. Zhang, W. Lin, Y. Guan, X. Gu, Journal of Laser Applications 31 (2019) 022510. [17] D. Zhang, Y. Qin, W. Feng, M. Huang, X. Wang, S. Yang, Surf. Coat. Technol. 363 (2019) 87–94. [18] C. Meng, Z. Chen, G. Li, P. Dong, J. Alloys Compd. 711 (2017) 258–266. [19] E. Ukar, A. Lamikiz, I. Tabernero, F. Liebana, D.d. Pozo, AIP Conference Proceedings 1181 (2009) 474–483. [20] X. He, J.W. Elmer, T. DebRoy, J. Appl. Phys. 97 (2005) 84909. [21] T.D. Bennett, D.J. Krajnovich, C.P. Grigoropoulos, P. Baumgart, A.C. Tam, J. Heat Transf. 119 (1997) 589–596. [22] B.S. Yilbas, S.S. Akhtar, C. Karatas, Opt. Lasers Eng. 48 (2010) 740–749. [23] N. Li, S. Huang, G. Zhang, R. Qin, W. Liu, H. Xiong, G. Shi, J. Blackburn, Journal of Materials Science & Technology 35 (2019) 242–269. [24] Z. Cui, H. Shi, W. Wang, B. Xu, Trans. Nonferrous Metals Soc. China 25 (2015) 1446–1453. [25] X. Duan, S. Gao, Q. Dong, Y. Zhou, M. Xi, X. Xian, B. Wang, Surf. Coat. Technol. 291 (2016) 230–238. [26] M.B. Djurdjević, S. Manasijević, Z. Odanović, N. Dolić, Adv. Mater. Sci. Eng. 2013 (2013) 1–8. [27] J. Meng, W. Sun, Z. Tian, X. Qiu, D. Zhang, Corrosion Prevention of Magnesium Alloys: 2. Corrosion Performance of Magnesium (Mg) Alloys Containing Rare-Earth (RE) Elements, Elsevier Science, 2013. [28] C. Taltavull, B. Torres, A.J. López, P. Rodrigo, E. Otero, J. Rams, Mater. Lett. 85 (2012) 98–101. [29] R. Teghil, A. Santagata, V. Marotta, S. Orlando, G. Pizzella, A. Giardini-Guidoni, A. Mele, Appl. Surf. Sci. 90 (1995) 505–514. [30] S. Ignat, P. Sallamand, D. Grevey, M. Lambertin, Appl. Surf. Sci. 233 (2004) 382–391. [31] J. Iwaszko, M. Strzelecka, Opt. Lasers Eng. 81 (2016) 63–69. [32] C. Liu, Q. Li, J. Liang, J. Zhou, L. Wang, RSC Adv. 6 (2016) 30642–30651. [33] G.L. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11–33. [34] L. Wang, B.-P. Zhang, T. Shinohara, Mater. Des. 31 (2010) 857–863. [35] T. Zhang, Y. Shao, G. Meng, Z. Cui, F. Wang, Corros. Sci. 53 (2011) 1960–1968. [36] C. Taltavull, B. Torres, A.J. López, P. Rodrigo, J. Rams, Surf. Coat. Technol. 222 (2013) 118–127. [37] K. Wei, M. Gao, Z. Wang, X. Zeng, Mater. Sci. Eng. A 611 (2014) 212–222. [38] J. Zhou, J. Xu, S. Huang, Z. Hu, X. Meng, X. Feng, Surf. Coat. Technol. 309 (2017) 212–219. [39] C. Liu, J. Liang, J. Zhou, L. Wang, Q. Li, Appl. Surf. Sci. 343 (2015) 133–140. [40] J. Park, H.-S. Han, J. Park, H. Seo, J. Edwards, Y.-C. Kim, M.-R. Ok, H.-K. Seok, H. Jeon, Appl. Surf. Sci. 448 (2018) 424–434.
Acknowledgements This work was financially supported by National Natural Science Foundation of China under Grant 51705013; National Key Research and Development Program of China under Grants 2018YFB1107400, 2018YFB1107700, and 2016YFB1102503; National Key Basic Research Program of China under Grant 2015CB059900. References [1] A.A. Luo, A.K. Sachdev, Applications of magnesium alloys in automotive engineering, in: C. Bettles, M. Barnett (Eds.), Advances in Wrought Magnesium Alloys, Woodhead Publishing, 2012, pp. 393–426. [2] S. Agarwal, J. Curtin, B. Duffy, S. Jaiswal, Mater. Sci. Eng. C 68 (2016) 948–963. [3] M. Esmaily, J.E. Svensson, S. Fajardo, N. Birbilis, G.S. Frankel, S. Virtanen, R. Arrabal, S. Thomas, L.G. Johansson, Prog. Mater. Sci. 89 (2017) 92–193. [4] A. Atrens, G.-L. Song, M. Liu, Z. Shi, F. Cao, M.S. Dargusch, Adv. Eng. Mater. 17 (2015) 400–453. [5] A. Atrens, G.L. Song, Z. Shi, A. Soltan, S. Johnston, M.S. Dargusch, Understanding the corrosion of Mg and Mg alloys, in: K. Wandelt (Ed.), Encyclopedia of Interfacial Chemistry, Elsevier, Oxford, 2018, pp. 515–534. [6] Y. Ding, C. Wen, P. Hodgson, Y. Li, J. Mater. Chem. B 2 (2014) 1912–1933.
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Influence of laser surface melting on the properties of MB26 and AZ80 magnesium alloys Yuhang Lia,1, Srinivasan Arthanaria,b,1, Yingchun Guana,b,c,
T
⁎
a
School of Mechanical Engineering and Automation, Beihang University, 37 Xueyuan Road, Beijing 100191, PR China Hefei Innovation Research Institute of Beihang University, Xinzhan Hi-Tech District, Anhui 230012, PR China c National Engineering Laboratory of Additive Manufacturing for Large Metallic Components, Beihang University, Beijing 100083, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium alloys Laser surface melting Hardness Corrosion Polarization
In the present investigation, laser surface melting (LSM) was performed on MB26 and AZ80 magnesium alloys and their surface properties were evaluated. The finite element method (FEM) was used to calculate the resulting temperature during the laser surface melting process. The melted layer average thickness values were about 296 and 190 μm for MB26 and AZ80 alloys, respectively and the variation in the thickness is mainly attributed to the absorption in laser energy. The average grain size of the LSMed MB26 alloy was decreased about 14.5 times and AZ80 had very finer grains than the MB26 along with the fine distribution of second phases. Further, the X-ray diffraction results revealed that the second phase intensities were decreased in both the alloys due to their dispersion. The laser absorption efficiency of MB26 was higher due to the higher plasma attachment during LSM process, thereby has higher melt layer thickness compared to AZ80. The refined microstructure of the melted layer resulted in an increase of micro-hardness up to 110 Hv. Potentiodynamic polarization test results revealed that the corrosion current density (icorr) values of the LSMed MB26 and AZ80 alloys were decreased about 1.8 and 2.5 times, respectively compared to the as-received alloys. A variation in solidification rates of the melt pool due to the alloying elements were attributed to an improvement in the surface and electrochemical properties.
1. Introduction
processes to achieve the grain refinement and phase segregation on the surfaces of Mg alloys without altering their bulk properties [11–13]. The surface wettability and corrosion resistance of LSMed Mg-Zn-Dy alloy were improved in Hank's balanced salt solution [14]. A rapid heating and cooling cycles during LSM treatment produced significant variation in the grain refinement and surface roughness values as the laser energy density was varied. As a result, the surface wettability was improved and the in-vitro degradation rate was also reduced. Recently, Zhang et al. reported an improvement in the mechanical properties and biocompatibility of laser surface processed Mg-Gd-Ca alloy [15,16]. The β-Mg5Gd phase was disappeared due to the higher solubility of Gd during laser melting process and the hardness value was also substantially increased. Laser-induced periodic surface structure (LIPSS) on laser melted surface had better biocompatibility compared to the microgrooved surface. Laser re-melting of Mg-Zn-Ca alloy was carried out by Zhang et al. and the amorphous structure was formed at a scan speed over 1000 mm/min and the corrosion resistance was increased with scan speed [17]. Laser surface melting produced finer and cellular microstructures on MgeZn alloy and higher treatment depths had
Mg alloys are used in several applications involving external stress and aqueous environments, therefore, improvement of their mechanical properties and corrosion resistance is indeed important [1]. The controlled degradation rate of Mg-based degradable implant materials in the physiological environments is also essential for better osseointegration [2]. It is well-known that Mg possesses higher electrode potential therefore easily oxidizes and the surface is covered with loosely bound hydroxide/oxide layers [3,4]. Nevertheless, a surface layer formed on the Mg alloys is not as stable as in the case of aluminum and titanium alloys [5]. Therefore, surface properties of Mg alloys need to be altered to establish a stable surface layer formation. The variation in the microstructures such as grain refinement and phase segregation would be beneficial to simultaneously enhance the mechanical as well as corrosion properties [6]. Surface modification of Mg alloys, in particular, laser-based processing are generally applied in recent days to enhance their surface-related properties without altering the bulk properties of the material [7–10]. LSM is one of the precise and rapid
⁎
Corresponding author at: School of Mechanical Engineering and Automation, Beihang University, 37 Xueyuan Road, Beijing 100191, PR China. E-mail address: [email protected] (Y. Guan). 1 Equally contributed. https://doi.org/10.1016/j.surfcoat.2019.124964 Received 11 July 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online 07 September 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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element is a cube with a size of 30 × 30 × 30 μm. To simplify the simulation process, it was assumed that: a) laser absorption does not change with temperature, b) neglecting the flow of melting pool and evaporation of the MB26 and AZ80 alloys at boiling point and c) the MB26 and AZ80 alloys are continuous and isotropic. The basic heat transfer control equation can be expressed as follows:
better corrosion resistance. Furthermore, the in-vitro degradation and hydrogen evolution tests in Hank's balanced salt solution revealed that the laser melted and polished samples had lower corrosion rate (< 1 mm/y) [13]. The surface hardness and tensile properties at room and elevated temperatures were increased for the LSMed AZ91 D alloy, however, the ductility was decreased [18]. Based on the available literature it could be understood that the LSM significantly altered the grain refinement and second phase distribution in the melt layer. The melting and solidification behaviors of various alloying elements and their diffusion in the melt pool differ during LSM process due to the variation in laser absorption efficiencies, which affects the melted layer properties. Furthermore, the solidification behavior of second phases varied as the cooling rates changed with varying the laser process parameters; thereby affect the mechanical as well as corrosion behavior. Therefore, the presence of alloying elements plays a key role in determining the melt layer properties and this phenomenon needs to be investigated. Hence, in the present investigation, LSM was carried out on two different magnesium alloys such as MB26 and AZ80 to understand the laser surface melting on the microstructural, hardness and electrochemical corrosion behavior. The finite element method (FEM) was also used to simulate the temperature change during the LSM process and correlated with the experimental results.
∂ ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ ∂T + + k k k + q = ρc ∂x ⎝ ∂x ⎠ ∂y ⎝ ∂y ⎠ ∂z ⎝ ∂z ⎠ ∂t ⎜
⎟
(1)
where ρ is the material density; c is the specific heat capacity of material; k is the material thermal conductivity; T is the temperature; q is the heat generated by heat transfer media per unit volume and t is time. In order to obtain the differential equation, the fixed boundary conditions are applied on both ends of the alloys resembling the experimental laser surface melting situation. The temperature of the alloys at the initial moment is considered as the ambient temperature. The density of the input heat flow rate of each facula was calculated by the Gaussian function and it is consistent with the laser heat source used in the LSM experiment. Besides, the exhaustive explanation of its convergence criteria, boundary conditions, and governing equations were reported elsewhere [20]. 2.4. Electrochemical characterization
2. Experimental work
Electrochemical corrosion studies of as-received and LSMed alloys were carried out in 3.5 wt% NaCl solution using a CH Instruments 660E electrochemical workstation. As received, and LSMed alloys were successively abraded with SiC papers up to 2500# to ensure similar surface roughness values prior to the corrosion test. Electrochemical tests were carried out at ambient temperature and the cell was comprised of as received or LSMed alloys as working electrode (1 cm2), saturated calomel (SCE) and carbon rod were used as reference and counter electrodes, respectively. Potentiodynamic polarization studies were performed in the potential range OCP ± 0.200 V at a scan rate of 1 mV/s after 15 min of exposure. In order to confirm the reproducibility of the results, two samples in each condition were tested under the same experimental condition.
2.1. Laser surface melting of Mg alloys Extruded MB26 (5.6 wt% Zn, 1.2 wt% Y, 0.66 wt% Zr and Mg balance) and AZ80 (9.1 wt% Al, 0.42 wt% Zn, 0.03 wt% Mn, 0.01 wt% Si, ≤0.005 Fe, ≤0.05 Cu, ≤0.005 Ni and Mg balance) alloys were used for the laser surface melting process. Samples were cut from the extruded plates with a dimension of 50 × 40 × 8 mm3, abraded with silicon carbide (SiC) emery papers to remove the surface contaminants and washed with ethanol. A nanosecond (ns) pulsed fiber laser with the wavelength of 1060 nm was used for the LSM process. The process parameters such as pulse duration, repetition rate, and spot size were 220 ns, 500 kHz, and 44 μm, respectively. Alloys were irradiated with a laser power density of 1.20 × 107 W/cm2 and at a scanning speed of 200 mm/s with 50% beam bath overlapping. The LSM process was carried out under Ar gas protection.
3. Results and discussion 3.1. Surface characterization of laser surface melted alloys
2.2. Surface characterization Fig. 1(a–d) shows the surface roughness profiles of the as-received and LSMed alloys. As can be seen, after the LSM process, the average roughness (Ra) value was decreased from 0.525 to 0.166 μm for the MB26 alloy and the value was marginally decreased from 0.515 to 0.308 μm for the AZ80 alloy (Table 1). The smooth surface obtained for the MB26 alloy could be mainly attributed to several reasons including surface tension, convection, alloying elements (Zn and Y), etc. Marangoni effect suggests that the motion of melt pool is mainly happening due to the surface tension gradient and thermocapillary force is responsible to pull the material from the hotter region to the cooler regions, which could affect the surface roughness values. Furthermore, chemicapillary force is during the laser melting process could move the material towards the high surface energy region [21]. The Ra roughness values were decreased by about 3 and 1.6 times for the MB26 and AZ80 alloys, respectively and the smooth surface was achieved for the MB26 alloy could be attributed to the higher surface tension gradient existed between the melt pool and substrate during the LSM process. AZ80 alloy had a relatively rough surface compared to the MB26 alloy, which could also be attributed to the variation in the solubility of added alloying elements mainly Al and evaporation of Mg. In addition to that, the overlap ratio used for the LSM affects the average surface roughness values. In the present investigation, the overlap ratio was similar for both the alloys and only the alloying composition is different which
Surface topography of the as-received and LSMed alloys was observed using a 3D laser scanning confocal microscope (VK100, Keyence, Osaka, Japan). Cross-sectional microstructures of the melted layers were observed using a Nikon LV150N optical microscope (OM). Besides, the SU8010 scanning electron microscope with energy dispersive X-ray analysis (SEM-EDAX) was utilized for microstructural and elemental chemical composition analysis. X-ray diffraction (XRD) patterns of the as-received and laser melted surfaces were obtained using a Rigaku D/max2200PC instrument in the 2θ range between 20° and 80° at a scan speed of 0.03°. Micro-hardness measurements were carried out across the LSMed surfaces using FUTURE-TECH FM-800 Vickers instrument with the loading force of 0.98 N at holding time of 10 s. 2.3. Numerical simulation The temperature profiles of the MB26 and AZ80 alloys during the LSM process was calculated using a finite element commercial software ABAQUS (ABAQUS 2018, SIMULIA, Rhode Island, RI, USA). The thermal modeling was established by solving the general heat transfer equation for conduction [19]. The calculation of the two models (MB26 and AZ80) has been carried out on a rectangle substrate with dimensions of 3 × 1 × 0.5 mm3 and the minimum differential volume 2
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Fig. 1. Surface roughness profiles of (a) as-received MB26, (b) laser surface melted MB26, (c) as-received AZ80 and (d) laser surface melted AZ80 magnesium alloys.
Fig. 3a & b shows the microstructures and EDAX elemental composition of the LSMed and as-received MB26 alloy. The melted surface has mainly consisted of fine dendrites-like second phases along the grain boundaries and the particulate like second phases was also identified in the matrix of the surface. The chemical composition of the particulate like phase (P1) consisted of small amounts of Zn and Y elements and rich of Mg which could be attributed to the Mg(ZnY) phase. Interestingly, the chemical composition of the second phase along the grain boundaries (P2) has consisted of about 75.68, 21.36 and 2.08 at.% of Mg, Zn, and Y respectively. The matrix has consisted of a small amount of Zn and Y, further, the amount of Y was marginally increased at the matrix of the melted layer compared to the as-received alloy. The coarse second phase (P4) was identified on the as-received alloy and the phase was rich in Zn and Y. The average melted layer thickness values were measured from the cross-sectional morphologies of AZ80 alloy (Fig. 4b) and the average thickness value was 190 ± 22 μm. Furthermore, the melted layer has consisted of very fine microstructure and the uniform distribution of fine second phases could also be identified along the grain boundaries. Whereas the average grain size was 28.1 ± 5 μm for as-received alloy and coarse second phases were also identified at the grain boundaries. The temperature contour (Fig. 4a) reveals that the maximum temperature was 2205 °C and the temperature in melt pool and HAZ was predicted as 1590 and 644 °C, respectively. It could also be noticed that the maximum temperature of the AZ80 alloy was marginally lower compared with the MB26 alloy furthermore, the melt pool temperature was also lower. The laser processing parameters were the same for both the alloys, however alloying elements resulted in a variation in the melt pool temperature and the further solidification behavior, which resulted in different microstructures and the second phase segregation. The chemical composition at the second phase and matrix was measured using EDAX and are shown in Fig. 5a & b. The white second phases (P1) were rich in Al and Mg and could be attributed to the
Table 1 Comparison of average surface roughness values of MB26 and AZ80. Alloy
Condition
Ra (μm)
Rq (μm)
MB26
As received Laser melted As received Laser melted
0.525 0.166 0.515 0.308
0.693 0.222 0.676 0.416
AZ80
would be a key factor to affect the surface roughness values. Cross-sectional morphologies and the corresponding simulated thermal contour of the LSMed MB26 alloy is shown in Fig. 2a & b and the direction of laser re-melting is along the Y-axis. In addition to that, the microstructures at different regions across the melted surfaces (A–D) are also included in Fig. 2. The average melted layer thickness value was measured using Image J software and the value was 296 ± 21 μm for the MB26 alloy. The average grain size at the melted layer was 9.2 ± 1.6 μm, while the grain size at the as-received alloy was 133 ± 33 μm. In addition to that, the melted layer was well attached to the substrate and no defects were noticed. It can be seen from the temperature contour (Fig. 2a) that the high-temperature contour extended inside the melt layer, and a sharp thermal gradient was emerged in the vicinity of the laser source, in particular in front of the melt zone. The molten pool was the elliptical shape and the maximum temperature was 2335 °C for the MB26 alloy. The melt pool and heataffected zone temperature were predicted about 1780 and 1590 °C, respectively and these results are in accordance with Yilbas et al. [22]. The higher temperature gradients exist between the surface temperature and the melt pool, thus results in high cooling rate (1.209 × 104 K/ s) on the thin layers of the alloy substrate was mainly responsible for the grain refinement in the melted layer [23]. Furthermore, the average grain size marginally varied from the bottom of the melted layer to the top surface due to the variation in the cooling rate. 3
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Fig. 2. Cross-sectional morphologies of laser surface melted MB26 magnesium alloys.
The variation in melted layer thickness between MB26 and AZ80 could be attributed to the difference in cooling rate and the smaller ratio of a temperature gradient to solidification rate (G/R) [24,25]. Furthermore, it can be seen from FEM results that the temperature of the molten pool is > 1780 °C for MB26 alloy which is higher than the AZ80 (1590 °C). The phase diagrams of MgeZn and MgeAl systems revealed that the phase transition temperature of MB26 alloy is
Mg17Al12 phase. Besides, the second phases are uniformly distributed in the melted layer, while the coarse phase was noticed at the as-received surface (Fig. 5b). The finer grains with uniform distribution of the Mg17Al12 phase along the grain boundaries are due to the rapid heating and cooling process during LSM process. Al content was varied in the matrix at the as-received and melted layers could be attributed to the dispersion of Al in the matrix during the laser melting process. 4
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Element Mg K Zn K YL Zr L Mn K
(a) P2
P1
Wt.% 80.34 6.20 8.67 4.05 0.74
At.% 92.70 2.99 2.75 1.18 0.38
P3
P1
5 µm Element Mg K Zn K YL Zr L Mn K
Wt.% 52.2 40.24 5.30 1.11 0.73
At.% 75.68 21.36 2.08 0.43 0.47
Element Mg K Zn K YL Zr L Mn K
Wt.% 96.95 2.77 0.15 0.99 0.14
keV At.% 98.57 1.06 0.04 0.27 0.06
P2
P3 Element Mg K Zn K YL Zr L Mn K
keV
Wt.% 96.71 2.19 0.45 0.32 0.33
At.% keV 98.80 0.83 0.13 0.09 0.15
(b)
P5 P4 Element Mg K Zn K YL Zr L Mn K
P4
Wt.% 94.46 4.73 0.07 0.55 0.19
keV At.% 97.92 1.82 0.02 0.15 0.09
5 µm
P5 keV Fig. 3. EDAX chemical composition of (a) laser surface melted and (b) as received MB26 magnesium alloy.
phase present in the AZ80 alloy [28]. Moreover, during the hightemperature LSM process the elements present in the second phases could dissolve in the matrix and the coarse phases are redistributed as fine particles in the melted layer after the LSM process. It is also presumed that due to these phenomena more plasma spatter could attach
598–689 °C which is also higher than the phase transition temperature of AZ80 alloy (437–450 °C) [26]. The results show that MB26 need to absorb more energy than AZ80 alloy to produce phase transition. In addition to that, the melting point of Mg3Zn6Y phase in the MB26 alloy is 445 °C [27] which is marginally lower than the β-Mg17Al12 (455 °C) 5
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Fig. 4. Cross-sectional morphologies of laser surface melted AZ80 magnesium alloys.
during laser processing. The absorption energies of the melted state of untreated WE43 and ZE41 alloys were about 40 and 30%, respectively and the energy needed to melt the ZE41 alloy was lower than the WE43 [30]. In both LSMed MB26 and AZ80 alloys, the grain refinement was higher at the top surface and varied as the melt depth was increased, which could be attributed to the decrease in cooling rate of the solidifying material during LSM process [31] and these results were further
to the MB26 surface during the LSM process and higher plasma attachments could increase the laser absorption efficiency of MB26, thereby has a higher melt layer thickness compared to AZ80 [29]. Ignat et al. reported the variation in the absorption efficiencies of pulsed Nd:YAG laser (λ = 1.06 μm) treated WE43 and ZE41 alloys before and after the anodizing and mordancage processes. The surface layer covered on the surface significantly varied the absorption efficiencies 6
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Element Mg K Al K Zn L Mn K
(a)
Wt.% 78.64 20.18 1.09 0.09
At.% 80.85 18.69 0.42 0.04
P1
P1 Element Mg K Al K Zn L Mn K
P2
Wt.% 94.58 4.93 0.19 0.11
At.% keV 95.25 4.47 0.07 0.05
5 µm
P2
Element Mg K Al K Zn L Mn K
Wt.% 67.98 28.73 3.00 0.31
keV At.% 71.57 27.12 1.17 0.14
(b)
P3
P3
P4 Element Mg K Al K Zn L Mn K
Wt.% 90.55 9.29 0.08 0.08
keV At.% 91.49 8.45 0.03 0.04
5 µm
P4 keV Fig. 5. EDAX chemical composition of (a) laser surface melted and (b) as received AZ80 magnesium alloys.
3.2. Surface micro-hardness of laser surface melted alloys
substantiated by the FEM results. XRD patterns of the as-received and LSMed alloys are shown in Fig. 6a & b. As can be seen, MB26 alloy has mainly consisted of primary α-Mg and Mg3Zn6Y phases (Fig. 6a). The second phase peak intensities of both as-received and LSMed alloys appeared almost the same and no significant change was noticed. It is presumed that during laser melting process some new phases could be formed, however, their amounts are less than the detection limits of XRD. As received AZ80 alloy has mainly consisted of β-Mg17Al12 phase and the intensities of these peaks were significantly reduced for the laser-melted surfaces due to the heat accumulation during the laser re-melting process. It is also presumed that the LSM resulted in the dispersion of the second phase, which is evident from the cross-sectional morphologies of melted AZ80 alloy (Fig. 2b).
The micro-hardness profiles of LSMed MB26 and AZ80 alloys were measured across the surface and are shown in Fig. 7a & b. Laser surface melting generally enhances the surface microhardness of the LSMed layer due to the grain refinement and solid solution strengthening, however, the magnitude of increment varied depending upon the laser processing conditions and alloys. The microhardness value of the LSMed MB26 alloy was increased from about 70 to 105 Hv. Besides, the fluctuations in the hardness values are seen, which could be attributed to the presence of dispersed second phases. The value was increased from about 80 to 100 Hv for the laser melted AZ80 alloy. The uniform distribution of the Mg17Al12 phase in the AZ80 alloy can have a pinning 7
Surface & Coatings Technology 378 (2019) 124964
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-Mg Mg3Zn6Y
Intensity (a.u.)
(a)
Laser melted
As received
20
30
40
50
60
70
80
2 (degree) -Mg -Mg17Al12
Intensity (a.u.)
(b)
Laser melted
As received
20
30
40
50
60
70
80
2 (degree) Fig. 6. XRD patterns of the as received and laser surface melted (a) MB26 and (b) AZ80 magnesium alloys.
Fig. 7. Micro hardness values across the surface melted layer of (a) MB26 and (b) AZ80 magnesium alloys.
effect on the dislocation, therefore, improve the strength and microhardness of the melted surface. However, the hardness value of LSMed AZ80 was lower than the MB26 alloy. It is also seen from the crosssectional morphologies that, the melted layer thickness is less for the AZ80 alloy compared to MB26, which could also be a reason for the higher hardness values of MB26 alloy. Further, it is confirmed from the Hall-Petch equation that, the grain refinement significantly enhances the microhardness values of magnesium alloys and support the present results. The microhardness values of the melted layers are compared with the reported values and are shown in Table 2. Results showed that the values achieved in the present investigation are comparable with the hardness values of AZ91 alloy, which is most commonly used in industrial applications due to its higher fluidity during the casting process.
Table 2 Comparison of micro hardness values of various laser surface melted Mg alloys. Alloy
Hardness (Hv)
Ref.
MEZ AZ91D AZ91D AZ31B (laser melting at Ar environment) AZ31B (laser melting at liquid N2 environment) AZ91 AZ91D Mg-Zn-Dy AZ91D MB26 AZ80
100 113 100 90 148 93 112 94 70.8 110 100
[8] [36] [37] [24] [31] [18] [14] [38] Present study
Furthermore, it is observed from the cross-sectional morphologies that, the melted layer thickness on MB26 alloy was about 100 μm higher than the AZ80 alloy, which could also be responsible for the shift in the Ecorr value. The corrosion potential of the LSMed WE43 alloy was shifted about 0.141 mV in the positive direction compared to the asreceived alloy [32]. The shift in Ecorr values was varied as the lasermelting environment was varied, the positive shift in Ecorr values were 0.124 and 0.098 V for the AZ91 alloy treated with liquid nitrogen and Ar gas environments. The difference in the cooling rate of the melted layer was responsible for the variation in grain refinement, thus shifted the Ecorr values in the nobler direction [24]. In the present investigation, variation in the melted layer thickness is mainly attributed to the
3.3. Electrochemical corrosion behavior Electrochemical corrosion behavior of the as-received and LSMed MB26 and AZ80 alloys were studied in 3.5% NaCl solution and the potentiodynamic polarization curves are shown in Fig. 8a & b. Potentiodynamic polarization parameters are listed in Table 3. The corrosion potential (Ecorr) values were shifted about 0.122 and 0.06 V in the positive direction for the LSMed MB26 and AZ80 alloys, respectively compared to as-received alloys. A positive shift in Ecorr values indicated that the melted layer is thermodynamically stable in the aqueous environment and hence control the Cl− ions penetration. 8
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alloys. The icorr values were about 31.2 and 37.9 μA/cm2 (Table 3) for the laser melted MB26 and AZ80 alloys, respectively. The reduction in the icorr values of LSMed alloys were about 1.8 and 2.5 times compared to as-received MB26 (56.8 μA/cm2) and AZ80 (96.0 μA/cm2) alloys, respectively. A decrease in icorr values clearly indicated the better corrosion resistance of the LSMed alloys. Interestingly, the reduction in icorr value was higher for the AZ80 alloy, which could be attributed to the presence of uniformly distributed grain containing β-Mg17Al12 phases, which could act as a barrier to the corrosive ions penetration and help to establish the stable surface layer [33–35]. Furthermore, rapid solidification of the Al-containing Mg alloys also improves the passivation behavior. Microstructural studies further confirmed the presence of β-phase Mg17Al12 with larger grain sizes at the as-received conditions. The random distribution of the β-phase Mg17Al12 could act as a cathode to the α-Mg matrix, thus accelerate the galvanic corrosion and not acting as a barrier to Mg dissolution. After the laser surface melting, the grain sizes were uniform and the distribution of Mg17Al12 phases, which was confirmed from the less intense peak found at XRD patterns. This could accelerate the stable surface layer formation on the surface during exposure to the corrosive medium. Lower icorr value of the MB26 alloy could also be attributed to the uniform microstructures and fine distribution of the second phases which could establish a stable surface layer formation. However, the variation in the icorr value of the MB26 alloy was less compared to the AZ80 alloy, which could be attributed to the galvanic influence of the Mg3Zn6Y phase, which is nobler than the Mg phase and acts as a cathode to accelerate the Mg dissolution. Furthermore, the presence of alloying elements such as Al helps to enhance the performance of Mg alloys due to its fine distribution in the re-melted layers. This reveals that the presence of alloying elements in the Mg alloys plays a key role during laser surface melting and resulting grain refinement. The icorr value of the as-received AZ80 alloy was higher than the MB26 alloy indicating its lower corrosion resistance due to the presence of coarse βphase. It is confirmed from the results that the LSM significantly affected the surface electrochemical activity by altering the microstructure and second phase distribution. Corrosion current (icorr) densities of various LSMed Mg alloys are compared with the present results and are listed in Table 4. The results showed that the icorr values of LSMed MB26 and AZ80 are lower than the LSMed AZ91D alloy. The variation in the corrosion resistance of the LSMed alloys is mainly attributed to the laser treatment condition and alloying composition.
Fig. 8. Potentiodynamic polarization curves of as received and laser surface melted (a) MB26 and (b) AZ80 magnesium alloys.
Table 3 Potentiodynamic polarization parameters of as received and laser surface melted MB26 and AZ80. Alloy
Condition
Ecorr (VSCE)
icorr (μA/cm2)
MB26
As-received Laser melted As-received Laser melted
−1.568 −1.446 −1.595 −1.535
56.8 31.2 96.0 37.9
AZ80
4. Conclusions Laser surface melting of MB26 and AZ80 magnesium alloys were carried and their surface and electrochemical corrosion properties were systematically evaluated; 1. Surface roughness values of LSMed alloys were reduced in particular, the reduction was relatively higher for MB26 alloy due to the variation in cooling rate during laser processing. 2. Cross-sectional morphologies revealed that LSMed layer thickness was significantly varied and melted layer thickness value of AZ80
nobler shift in the Ecorr values. The current density values at cathodic regions were lower for the LSMed alloys compared to the as-received alloys indicating the reduction in cathodic reaction kinetics and hydrogen evolution reaction. Further, the reduction was higher for the LSMed AZ80 compared to MB26 alloy, could be attributed mainly to the dispersed second phase in the melted zone. Laser melted surface does not show any sign for the presence of a passive region in the anodic potential sweep. The current was also rapidly increased as the potential was increased after the Ecorr value. Nevertheless, the current density values in the anodic region were lower compared to the as-received alloy shows the lower dissolution rate of Mg. Furthermore, the anodic slope of the LSMed alloys is marginally higher than the as-received alloys could be attributed to the slower dissolution rate. Corrosion current density (icorr) is a key parameter deciding the corrosion performances of the various metals/
Table 4 Comparison of corrosion current density (icorr) values of various laser surface melted Mg alloys.
9
Alloy
Corrosion medium
icorr (μA/cm2)
Ref.
MEZ AM60B WE43 Mg-Ca-Zn AZ91D MB26 AZ80
3.5% NaCl 3.5% NaCl 3.5% NaCl Hank's solution 3.5% NaCl 3.5% NaCl 3.5% NaCl
1500 0.598 8.51 9.3 68.7 31.2 37.9
[8] [39] [32] [40] [38] Present study
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alloy was less compared with MB26. This could be attributed to the difference in the melting behavior of various constituent alloying elements and the evaporation of alloying elements. The FEM temperature contour results also further substantiated the experimental results. 3. XRD patterns clearly indicated the decrease in the peak intensities of the second phases after LSM confirming the dispersion of the second phases in the matrix due to the high-temperature laser processing, which affected the mechanical as well as corrosion behavior. 4. Microhardness value of LSMed MB26 alloy was higher compared with AZ80 alloy is due to the higher melt layer thickness; furthermore, the fluctuations in the values across the surface could be due to the presence of segregated second phases. 5. Potentiodynamic polarization studies clearly indicated the better corrosion resistance of the melted layers. In particular, the improvement in corrosion resistance was higher for the LSMed AZ80 alloy compared with MB26. The fine microstructure along with a uniform distribution of β-phase Mg17Al12 could be responsible for the better corrosion resistance. The galvanic influence of second phase present in the LSMed MB26 alloy marginally decreased the improvement of corrosion resistance compared with LSMed AZ80 alloy.
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