在线磁场激励对激光选区熔化零件各向异性和残余应力的影响机制及调控规律

The parts fabricated by the selective laser melting (SLM) have outstanding problems including anisotropy and residual stress. These problems greatly hindered the application of SLM, and became the most prominent restriction on the progress of SLM technology. This proposal tends to add an online magnetic field to the SLM process, and uses the electromagnetic effects of stirring and pressing to change the solidification behaviors of melting pools during SLM process. It is expected to restrain the directional growth of microstructure and promote the equiaxed crystallization, then reduce the degree of anisotropy. Moreover, the magnetic stirring on the melting pools can produce a forced convection, which decreases the solidification speed and reduces the temperature gradient of melting pool. Besides, the Joule heat produced by magnetic fields acts on the solidified SLM parts, so that the goals of homogenization stress and crack suppression could be achieved. Two scientific issues of this study are the metallurgical mechanisms of melting pools in SLM under an online magnetic field, and the essence and regulation of residual stress for SLM parts affected by magnetic fields. Combining theoretical and experimental methods, the study will reveal the evolutions of microstructure and macroscopic properties of SLM parts, and establish the relationship between the magnetic, laser parameters and the micro- and macro- properties of SLM parts. The study is expected to achieve some innovative results on the metallurgical mechanism and property of SLM parts fabricated under a magnetic field, and to provide a new approach for improving the performance of SLM parts. Therefore, it has significant theoretical and engineering values.

激光选区熔化（SLM）零件容易产生各向异性和残余应力的突出问题，成为制约SLM技术发展的主要瓶颈之一。本项目拟在SLM成形零件的同时引入可控磁场，利用电磁搅拌和加压作用调控熔池的凝固行为，抑制组织方向性生长，促使等轴晶化，从而改善各向异性；同时，利用磁力搅拌在熔池中产生强制对流，减缓凝固速度，降低熔池各部位温差，并利用磁场焦耳加热已固化零件，达到均化应力、减少裂纹的目的。项目围绕“在线磁场激励下SLM微熔池的冶金机制”和“磁场影响SLM零件内残余应力的本质及规律”两个科学问题，结合理论和试验方法揭示在线磁场激励下SLM微观组织及宏观性能演变规律，建立磁场和激光复合工艺对SLM微宏观性能的影响关系。研究有望在激光与磁场复合作用下的SLM冶金机理及其性能特征方面获得创新性成果，并为改善SLM零件的性能提供全新途径，具有重要的理论意义和工程应用价值。

设计了在线磁场系统，对现有激光选区熔化（SLM）设备进行了改造形成了实验平台；模拟和实验研究了磁场与激光熔化复合增材新工艺。研究了稳恒磁场与激光熔化耦合数值模型，揭示了磁场对SLM熔池流动与传热传质的影响机理。优化了磁场下SLM成形GH3536镍基高温合金的工艺，探索了稳恒磁场对SLM成形GH3536微宏观性能的影响规律。定量表征了在磁场作用下其各向异性的变化程度，发现在0.3 T竖直方向稳恒磁场作用下，制件沿扫描和沉积方向的抗拉强度及延伸率各向异性程度分别减小了47.5%和55.3%。另外，研究了磁场下SLM成形AlSi7Mg合金的优化工艺，揭示了磁场强度对AlSi7Mg合金组织及性能的影响规律。发现在0.3 T水平方向稳恒磁场作用下，SLM制备AlSi7Mg合金的平均晶粒尺寸由8.35 μm 减小至7.22 μm，等轴晶所占比例由54%增加至68%，并分析和讨论了电磁力对熔池凝固以及晶粒细和等轴化的影响机制。发表论文16（第一标注12篇），授权发明专利2项、受理3项，培养博士后1人、博士生7人、硕士生7人，参加国内外会议并作报告5人次，大会特邀报告2次。研究为SLM制备合金的性能调控提供了新思路。