L-PBF process control to reduce hot-cracking and residual stress

金属的粉末床熔合（PBF）是一种增材制造工艺，其中热能选择性地熔合粉末床的区域。在激光PBF中，聚焦激光束将粉末逐层熔化以生产零件。由于其灵活性，激光PBF占当今世界所有安装的金属AM系统的约80%。然而，目前的金属增材制造机无法“第一次”就制造出正确的零件：仍需要大量的零件专用打印开发来验证新生产零件的制造策略，并且要求苛刻的应用需要在投入使用之前进行后处理零件测试和验证。热裂纹和残余应力是零件失效的主要原因，并且与粉末床中的温度分布的演变有关。随着温度在构建期间增加，激光与粉末床相互作用的条件可产生不令人满意的构建条件。热历史取决于扫描策略与正在构建的层中的部件横截面的相互作用。原则上，这些热效应是可以预测的，但目前还不可能在比物理构建更短的时间内实现这一目标。本项目将从文献中了解热裂纹和残余应力形成的物理过程，特别是不断变化的温度分布的作用。将进行对照构建实验以确认这一理解，需要将合适的热成像纳入构建室。然后将研究减少热裂纹和残余应力影响的策略。

Powder bed fusion (PBF) of metals is an additive manufacturing process in which thermal energy selectively fuses regions of a powder bed. In laser PBF, a focused laser beam melts the powder to produce parts, layer by layer. Due to its flexibility, laser PBF accounts for ~80% of all installed metal AM systems in the world today. However, current metal AM machines do not enable parts to be built 'right first time': significant part-specific print development is still required to validate the build strategy for a new production part and demanding applications require post-process part testing and verification before entering service. Hot-cracking and residual stresses are major causes of part failure and are related to the evolution of the temperature distribution in the powder bed. As the temperature increases during a build, the conditions for the interaction of the laser with the powder bed can produce an unsatisfactory build condition. The thermal history depends on the interaction of the scan strategy with the part cross-section in the layer being built. In principle, these thermal effects are predictable, but it is not currently possible to achieve that in less time than it takes to undertake the physical build.This project will understand the physical processes involved in hot-cracking and residual stress formation from the literature, in particular the role of the evolving temperature distribution. Control build experiments will be undertaken to confirm this understanding, requiring suitable thermal imaging to be incorporated into the build chamber. Strategies to reduce hot-cracking and the effects of residual stress will then be investigated.