Effect of selective laser melting process on phase transition and tensile properties of NiTi shape memory alloys
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摘要: 采用选区激光熔化成形工艺制备了NiTi形状记忆合金，研究了不同工艺参数组合下激光能量密度对NiTi合金相变、显微组织、拉伸性能和形状记忆性能的影响。结果表明：激光能量密度在45～85 J·mm−3时，试样相对密度均在99.5%以上。随激光能量密度增大，试样中NiTi（B2）相含量有所减少，相变温度逐渐提高。在打印试样中均存在纳米Ti2Ni相，随激光能量密度增大，析出相从均匀点状分布变为半网状分布。激光能量密度为47.62 J·mm−3的试样具有最优综合性能，样品的抗拉强度为(783±3) MPa，断后伸长率为(13.9±0.2)%，室温循环拉伸20次经热回复后回复率可达100%，可回复应变为2.75%。Abstract: TiNi shape memory alloys were prepared by selective laser melting (SLM). The effects of laser energy density on the phase transformation, microstructure, tensile properties, and shape memory properties of the NiTi alloys under the different printing combinations of process parameters were studied. The results show that, when the laser energy density is between 45 J·mm‒3 to 85 J·mm−3, the relative density of the samples is above 99.5%. With the increase of laser energy density, the content of NiTi(B2) phase in the SLM-formed NiTi alloys decreases, and the phase transition temperature increases gradually. Nano Ti2Ni precipitates exist in all the printed samples. With the increase of laser energy density, the precipitates change from the uniform point distribution to the semi-reticular distribution. The SLM-formed samples at the laser energy density of 47.62 J·mm−3 show the best comprehensive performance as the tensile strength of (783±3) MPa, the elongation after fracture of (13.9±0.2)%, the recovery rate of 100%, and the recoverable strain of 2.75% after 20 times of cyclic tensile at room temperature.
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Key words:
- selective laser melting /
- NiTi alloys /
- shape memory effect /
- phase transformation /
- tensile properties
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图 1 NiTi合金粉末形貌及粒径分布：（a）粉末形貌；（b）粒径分布
Figure 1. Morphology and particle size distribution of the NiTi powders: (a) powder morphology; (b) particle size distribution
图 2 SLMed-NiTi合金块体和拉伸试样尺寸：（a）、（c）不同打印参数下的块体试样；（b）拉伸试样尺寸
Figure 2. Dimensions of the SLMed-NiTi bulk samples and tensile samples: (a) and (c) bulk samples with the different building parameters; (b) tensile samples
图 3 NiTi合金粉末及SLMed-NiTi试样示差扫描量热曲线（a）及相变温度随能量密度变化曲线（b）
Figure 3. DSC curves (a) and the phase transformation temperatures as the function of energy density (b) of the NiTi alloy powders and the SLMed-NiTi samples
图 4 NiTi合金粉末及SLMed-NiTi试样X射线衍射谱图（a）和SLMed-NiTi试样物相质量分数随能量密度变化曲线（b）
Figure 4. XRD patterns of the NiTi alloy powders and the SLMed-NiTi samples (a) and the phase content as the function of energy density of the SLMed-NiTi samples (b)
图 5 SLMed-NiTi抛光光学显微镜图像及显微组织：（a）1#试样；（b）2#试样；（c）3#试样；（d）4#试样
Figure 5. Optical images and microstructure of the SLMed-NiTi samples: (a) 1#; (b) 2#; (c) 3#; (d) 4#
图 6 不同能量密度下制备的SLMed-NiTi试样扫描电子显微形貌：（a）、（c）4#试样；（b）、（d）1#试样
Figure 6. SEM morphology of the SLMed-NiTi samples: (a), (c) 4#; (b), (d) 1#
图 7 1#试样透射电镜和高分辨率透射电镜形貌：（a）晶粒形貌；（b）位错网络；（c）Ti2Ni析出相分布；（d）Ti2Ni/NiTi(B2)界面的高分辨率透射电镜形貌及相应的快速傅立叶变换(fast fourier transform，FFT)图像
Figure 7. TEM images and HRTEM images of the 1# samples: (a) grain morphology; (b) dislocation network; (c) Ti2Ni precipitation phase distribution; (d) HRTEM image of Ti2Ni/NiTi (B2) interface and the corresponding FFT image
图 8 不同能量密度下制备的SLMed-NiTi合金拉伸应力–应变曲线（a）和合金拉伸性能比较（b）
Figure 8. Tensile stress‒strain curves of the SLMed-NiTi alloys prepared by the different energy densities(a) and the tensile property comparison of the NiTi alloys (b)
图 9 SLMed-NiTi合金各试样循环拉伸曲线：（a）、（b）1#；（c）、（d）2#；（e）、（f）3#；（g）、（h）4#
Figure 9. Circular tensile curves of the SLMed-NiTi alloys: (a), (b) 1#; (c), (d) 2#; (e), (f) 3#; (g), (h) 4#
图 10 SLMed-NiTi合金各试样RSE、RSME及Rirrec随能量密度变化趋势图
Figure 10. Trend of RSE, RSEM, and Rirrec with the energy density of the SLMed-NiTi alloys
表 1 选区激光熔化制备NiTi合金工艺参数
Table 1. SLM process parameters of the NiTi alloys
实验编号 激光功率 / W 扫描速率 / (mm·s‒1) 粉末层厚 / μm 扫描间距 / μm 能量密度 / (J·mm‒3) 1# 80 700 30 80 47.62 2# 100 700 30 80 59.52 3# 80 500 30 80 66.67 4# 100 500 30 80 83.33 
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表 2 不同能量密度下制备SLMed-NiTi合金的临界应力、抗拉强度和断后伸长率
Table 2. Critical stress, tensile strength, and plasticity of the SLMed-NiTi alloys formed under the different energy densities
试样 临界应力 / MPa 抗拉强度 / MPa 断后伸长率 / % 1# 430±3 783±3 13.9±0.2 2# 377±2 675±4 11.4±0.3 3# 302±3 752±3 13.0±0.2 4# 144±4 579±5 11.9±0.2
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