Terner Mathieu, Lee Jiwon, Marchese Giulio, Biamino Sara, Hong Hyun-Uk
Department of Materials Science and Engineering, Changwon National University, 20 Changwondaehak-ro, Changwon, Gyeongnam 51140, Korea.
Institut Clément Ader (ICA), Université de Toulouse, CNRS, IMT Mines Albi, INSA, ISAE-SUPAERO, UPS, Campus Jarlard, F-81013 Albi, France.
Materials (Basel). 2020 Oct 17;13(20):4643. doi: 10.3390/ma13204643.
Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h-hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150 °C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980 °C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications.
近年来，金属增材制造，尤其是激光粉末床熔融（LPBF）技术，已成为一种杰出的创新制造方法。LPBF工艺的显著特点是凝固和冷却速率极高，以及反复的急剧加热和冷却循环，这会导致各向异性微观结构和残余应力的累积。通常需要进行高温后处理去应力热处理，以释放其中的一些应力。因此，针对LPBF生产的常用合金625，研究了在不同特定温度下（固溶处理、退火、去应力和低温去应力）进行1小时保温热处理对残余应力水平的影响以及微观结构表征。残余应力的累积通过位错的形成来适应，位错会在晶粒内产生局部晶体取向错误。电子背散射衍射（EBSD）用于通过取向成像研究局部取向错误，从而评估取向错误或应变水平，进而代表材料内部的残余应力水平。研究发现，在1150°C固溶处理后，严重受限的成型材料经历了等轴晶粒的完全再结晶，由于这种晶粒重新配置，残余应力水平显著下降。然而，较低温度下的热处理，即使高达980°C的退火温度，虽在一定程度上通过明显的位错恢复有效地释放了残余应力，但仍不足以促进再结晶。通过EBSD获得的平均取向错误数据对于定性评估残余应力水平很有价值。文中讨论了不同热处理的效果，并表明LPBF生产的合金的特殊微观结构可能可以转变以适应特定应用。
