激光增材制造钛合金微观组织和力学性能研究进展

激光选区熔化(SLM)技术与激光熔化沉积(LMD)技术在航空航天、生物医疗等领域的应用具有巨大潜力，但由于成形的Ti6Al4V合金构件存在较差的表面质量、较大的残余应力以及内部孔洞等问题，影响了构件的力学性能，从而制约了其大规模的应用。针对这一现状，首先概述了激光选区熔化技术与激光熔化沉积技术的制造原理，比较了2种增材制造技术的成形参数及其特点，并分析了2种不同成形技术的自身优势以及适用场合。其次，从2种增材制造技术成形钛合金的工艺参数入手，综述了激光功率、扫描速度、激光扫描间距、铺粉厚度、粉床温度等参数对SLM工艺成形钛合金的影响，以及激光功率、扫描速度、送粉速率等参数对LMD工艺成形钛合金的影响。发现成形工艺参数直接影响了粉末熔化程度、熔合质量和成形显微结构，从而影响成形件的组织与力学性能。此外，综述了不同的扫描策略对两种增材制造技术成形钛合金的表面质量与力学性能的影响，可以发现在不同扫描策略下同一试样表面的不同区域表面质量、残余应力以及抗拉强度存在较大差异，同一扫描策略下试样的不同表面之间也存在各向异性。最后，探讨了不同热处理工艺对钛合金微观组织和力学性能的影响，通过合适的热处理能够降低成形构件应力，并调控组织相变和性能。

Research Progress on Microstructure and Mechanical Properties of Titanium Alloy by Laser Additive Manufacturing

Selective laser melting (SLM) technology and laser melting deposition (LMD) technology are becoming increasingly close to the properties of manufactured titanium alloys and forgings, which have great potential for applications in aerospace, biomedical and other fields. However, the poor surface quality, large residual stresses and the presence of internal holes in the formed Ti6Al4V alloy components affect the mechanical properties of the components, thus limiting their large-scale application. To address this situation, this work firstly outlined the manufacturing principles of selective laser melting and laser melting deposition, compared the forming parameters and characteristics of the two additive manufacturing technologies, and analyzed the advantages and applications of the two different forming technologies. Since the selective laser melting technique could adjust the thickness of the laying powder, a smaller laser spot diameter was chosen to improve the surface quality and dimensional accuracy of the formed components. The laser melting and deposition technology adopted coaxial powder feeding for faster processing and was more suitable for manufacturing medium to large metal parts. Secondly, the effects of laser power, scanning speed, laser scanning pitch, powder thickness and powder bed temperature on the forming of titanium alloys by SLM process and the effects of laser power, scanning speed and powder feeding rate on the forming of titanium alloys by LMD process were reviewed from the forming process parameters of the two additive manufacturing technologies, revealing the intrinsic effects of forming parameters, microstructure and mechanical properties in the additive manufacturing process. The direct parameters of the forming process were found to affect the degree of powder melting, fusion quality and forming microstructure, thus affecting the organization and mechanical properties of the formed parts. The effect of laser power and scanning speed on the forming process was more obvious than other factors, and there was a greater correlation between them, and a combination of lower laser power and higher scanning speed could be adopted to obtain specimens with higher microhardness. In addition, the effects of different scanning strategies on the surface quality and mechanical properties of titanium alloys formed by the two additive manufacturing techniques were reviewed, and it was found that the surface quality, residual stress and tensile strength of different regions of the same specimen surface under different scanning strategies differed significantly, and anisotropy existed between different surfaces of the specimen under the same scanning strategy. Finally, the effects of different heat treatment processes on the microstructure and mechanical properties of titanium alloys were investigated, and suitable heat treatments could reduce the stresses and regulate the phase changes and properties of formed components. Two heat treatments, annealing and solution aging, can be combined to balance the strength and plasticity of the component. To summarize the research development of these two additive manufacturing technologies, it is necessary to accelerate the establishment of a complete system of methods under the forming process and forming environment, and to promote the research on the mechanism of microstructure evolution and macro mechanical properties influence.