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Acta Metall Sin  2016, Vol. 52 Issue (2): 184-190    DOI: 10.11900/0412.1961.2015.00212
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ELEMENT LOSS OF AZ91D MAGNESIUM ALLOY DURING SELECTIVE LASER MELTING PROCESS
Kaiwen WEI1,2,Zemin WANG2(),Xiaoyan ZENG2
1 School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
2 Wuhan National Laboratory of Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
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Abstract  

Magnesium alloys have attracted more attentions due to their low densities and excellent specific strengths. However, proper manufacturing methods are still needed to promote further applications of magnesium alloys due to the shortcomings of conventional processing methods. As one of the most promising additive manufacturing technologies, selective laser melting (SLM) was utilized to process the most commonly-used AZ91D magnesium alloy in this work. Element vaporization mechanism during the forming process and the influence of element vaporization on chemical composition, microstructure, and mechanical properties of the final products were investigated using OM, SEM, EDS, XRF and XRD. The results show that the relative content of Mg in the SLM-processed samples (86.61%~88.68%) was lower than that in the original AZ91D powders (90.63%) , whereas the relative content of Al in the former ones (10.40%~12.56%) was higher than its counterpart in the latter ones (8.97%). This variation matches well with the calculation by Langmuir model, demonstrating that element vaporization of AZ91D mainly targets at Mg. With the increase of laser energy density (EV), weight ratio of Mg to Al (η) in the SLM-processed samples first increased, then decreased and finally tended to be constant. η of the sample prepared at 55.6 J/mm3 (sample No.8) presented a smallest difference with that of the original powders. A model illustrating analytic relationship between η and EV was established by mathematical regression with the fitting index R2 being 0.858. The sample processed at 166.7 J/mm3 (sample No.1) underwent one of the most remarkable compositional variation and exhibited a typical solidified microstructure similar to the die-cast AZ91D in which net-like β-Mg17Al12 precipitates were distributed around the α-Mg matrix. However, β-Mg17Al12 content as well as solid solubility of Al in α-Mg matrix was much higher in sample No.1. The enhanced tensile strength and micro-hardness as well as the deteriorated elongation of sample No.1 could be attributed to the composition variation during SLM process.

Key words:  selective laser melting      AZ91D magnesium alloy      element loss      chemical composition      microstructure      mechanical property     
Received:  13 April 2015     
Fund: Supported by National Natural Science Foundation of China (No.51075164), High Technology Research and Development Program of China (No.2013AA031606) and Fundamental Research Funds for the Central Universities (NoHUST-2014QT006)

Cite this article: 

Kaiwen WEI,Zemin WANG,Xiaoyan ZENG. ELEMENT LOSS OF AZ91D MAGNESIUM ALLOY DURING SELECTIVE LASER MELTING PROCESS. Acta Metall Sin, 2016, 52(2): 184-190.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00212     OR     https://www.ams.org.cn/EN/Y2016/V52/I2/184

Fig.1  SEM image (a) and granulometric distribution (b) of AZ91D powders
Sample No. P / W V / (mmin-1) S / μm L / μm
1 200 20 90 40
2 200 20 110 40
3 200 30 90 40
4 200 30 110 40
5 200 40 90 40
6 200 40 110 40
7 200 50 90 40
8 200 60 90 40
9 200 50 110 40
10 140 100 80 20
11 200 60 110 40
Table 1  Parameters for selective laser melting (SLM) process
Sample No. Mg Al Zn Mn
1 86.68 12.53 0.58 0.22
2 86.61 12.56 0.61 0.22
3 86.85 12.36 0.58 0.22
4 87.70 11.48 0.60 0.22
5 87.96 11.24 0.60 0.22
6 87.57 11.61 0.61 0.21
7 88.30 10.71 0.76 0.24
8 88.68 10.40 0.70 0.22
9 88.44 10.66 0.68 0.21
10 87.66 11.39 0.67 0.19
11 87.22 11.94 0.64 0.20
Table 2  Chemical composition of the SLM-processed samples (mass fraction / %)
Fig.2  XRD spectrum of the recondensed metal dust

Ji=4.375×10-4γiXiPi0MiT12(1)

Fig.3  Burning rate of Mg (JMg) and the alloying elements ratios in AZ91D molten pools under various temperatures (JAl, JZn and JMn—burning rates of Al, Zn and Mn, respectively)
Fig.4  η of different samples and the fitted relation between η and EV (η—weight ratio of Mg to Al in SLM-processed samples, EV—laser energy density)
Fig.5  XRD spectrum of SLM-processed sample No.1
Fig.6  SEM image of SLM-processed sample No.1 (a) and OM image of die-cast AZ91D Mg alloy (b)
Fig.7  SEM images (a, b) and EDS analyses of α-Mg matrix in the rectangle areas (c, d) of SLM-processed sample No.1 (a, c) and die-cast AZ91D (b, d)
Sample Ultimate strength / MPa Yield strength / MPa Elongation / % Micro-hardness / HV
No.1 294~298 251~256 1.68~1.99 90~108
Die-cast AZ91D 230 160 3 61.3~63.7[29]
Table 3  Mechanical properties of the SLM-processed sample No.1 and die-cast AZ91D Mg alloy
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