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金属学报  2016, Vol. 52 Issue (2): 184-190    DOI: 10.11900/0412.1961.2015.00212
  论文 本期目录 | 过刊浏览 |
AZ91D镁合金在激光选区熔化成形中的元素烧损*
魏恺文1,2,王泽敏2(),曾晓雁2
1 华中科技大学光学与电子信息学院, 武汉 430074
2 华中科技大学武汉光电国家实验室, 武汉 430074
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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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η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/mm3EV所成形试样(试样No.8)的η值最接近粉末原料. 使用回归分析法建立了ηEV的解析关系, 其拟合度指标系数R2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/mm3EV所成形)与压铸态AZ91D 的组织特征相似, 均为β-Mg17Al12相呈网状分布于α-Mg基体间的典型凝固组织. 但成形试样No.1的β-Mg17Al12相含量及其α-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.

关键词 激光选区熔化AZ91D镁合金元素烧损化学成分显微组织力学性能    
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 wordsselective laser melting    AZ91D magnesium alloy    element loss    chemical composition    microstructure    mechanical property
收稿日期: 2015-04-13     
基金资助:* 国家自然科学基金项目51075164, 国家高技术研究发展计划项目2013AA031606, 以及中央高校基本科研业务费项目HUST-2014QT006资助

引用本文:

魏恺文,王泽敏,曾晓雁. AZ91D镁合金在激光选区熔化成形中的元素烧损*[J]. 金属学报, 2016, 52(2): 184-190.
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.

链接本文:

https://www.ams.org.cn/CN/10.11900/0412.1961.2015.00212      或      https://www.ams.org.cn/CN/Y2016/V52/I2/184

图1  AZ91D粉末的SEM像和粒径分布 采用自主研发的LSNF-I平台作为SLM设备, 该平台包含一台IPG-YLR-200光纤激光器(波长1070 nm, 最大输出功率200 W, 光斑直径100 μm). 平台具体细节及SLM加工流程见文献[18,19]. 根据前期基础工艺实验结果, 选择表1所示参数, 成形尺寸为5 mm×5 mm×5 mm的系列块体试样以及依据ASTM (B557M-10)标准设计的拉伸试样(标距25 mm, 标距段横截面尺寸6 mm×2 mm). 在成形过程中使用高纯Ar气作为保护气氛, 以避免粉末氧化或燃烧.
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
表1  激光选区熔化(SLM)成形参数
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
表2  成形试样化学成分
图2  蒸发烟尘的XRD谱 通常来说, 选择性元素烧损由不同合金元素间烧损速度的差异所导致. 根据气体动力学与热力学理论, 合金熔体中某一元素i的烧损速率Ji (gcm-2s-1)可由Langmuir方程计算[20]:
图3  AZ91D熔池内Mg在不同温度下的烧损速率及Mg与其它元素的烧损速率比
图4  样品中Mg和Al质量比(η)以及η与激光体能量密度(EV)的拟合曲线
图5  成形试样No.1的XRD谱 图6为成形试样No.1与压铸态AZ91D的显微组织. 可见, 富Al相β-Mg17Al12呈网状分布于α-Mg基体间. 但由于成形试样No.1中Al相对含量较高, 其β-Mg17Al12含量明显高于铸态样品. 利用金相定量体视法[26], 根据图6a和b所估算出的成形试样No.1与压铸态AZ91D中β-Mg17Al12相体积分数分别为34.2%与28.9%. 此外, 与铸态样品相比, 成形试样No.1的组织明显细化. 成形试样No.1中β-Mg17Al12相的分布更为弥散均匀, 其平均宽度仅为亚微米级, 而压铸态样品中β-Mg17Al12相的平均宽度则为微米级. 这是因为, 一方面, SLM技术固有的高冷却速率促进了这一转变; 另一方面, 考虑到Al在AZ91D合金中有一定晶粒细化作用[27,28], Al相对含量的增高亦有助于成形试样No.1的组织细化. 图7为成形试样No.1和压铸态AZ91D的高倍SEM像及相应位置的EDS分析. 可见, 成形试样No.1的α-Mg基体中Al的固溶含量远高于铸态AZ91D的对应值. 除了SLM较高的冷却速率导致的溶质捕获效应之外, Al相对含量的提升显然也有助于成形件中Al固溶量的提高.
图6  成形试样No.1和压铸态AZ91D的显微组织
图7  成形试样No.1和压铸态AZ91D的SEM像及相应位置EDS分析
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]
表3  成形试样No.1及压铸态AZ91D的力学性能
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