中文摘要：

利用OM,SEM,EDS,XRF和XRD等方法研究了AZ91D镁合金在激光选区熔化中的元素烧损机制以及烧损对成形试样化学成分、显微组织及力学性能的影响.结果表明,成形试样中Mg的相对含量（86.61%-88.68%）低于粉末原料中Mg的相对含量（90.63%）,而其Al的相对含量（10.40%-12.56%）则高于后者（8.97%）.该结果与基于Langmuir模型的计算结果相符,表明在激光作用下主要是Mg发生了烧损.成形试样的Mg与Al质量比h随激光体能量密度EV的增加呈现先上升后下降并最终趋于稳定的演变规律.采用55.6 J/mm^3的EV所成形试样（试样No.8）的h值最接近粉末原料.使用回归分析法建立了h与EV的解析关系,其拟合度指标系数R^2为0.858.成分变化最为显著之一的成形试样No.1（采用166.7 J/mm^3的EV所成形）与压铸态AZ91D的组织特征相似,均为b-Mg17Al12相呈网状分布于a-Mg基体间的典型凝固组织.但成形试样No.1的bMg17Al12相含量及其a-Mg基体中Al的固溶量明显高于压铸态AZ91D.成分变化导致成形试样No.1的拉伸强度及显微硬度得到提升,但使其延伸率有所下降.

英文摘要：

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 77 and Ev was established by mathematical regression with the fitting index R^2 being 0.858. The sample processed at 166.7 J/mm^3 （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