Effects of Heat Treatment on Residual Stresses in Additive Manufacturing

Abstract

Metal additive manufacturing (AM) is constantly developing and expanding into new segments. The automotive industry has so far not implemented AM to any larger extent. One reason is the relatively high cost of AM-components, but mainly due to the materials that are available is not used to a larger extent within the automotive sector e.g. titanium and stainless steel. Powder Bed Fusion–Laser Beam (PBF– LB), the most developed metal AM-method had, until recently, a limited range of materials available. However, recently several low-alloyed carbon-containing steels, that are commonly used in vehicles and machined components have been developed. One issue with increased carbon content is that the material has shown to be difficult to manufacture with PBF–LB. Moreover, heat treatment and its influence on residual stresses, microstructure and hardness are not yet known. This thesis focuses on the residual stresses and how they are affected by the heat treatment on samples printed in a low alloy medium carbon steel, 42CrMo4 (AISI 4140). The heat treatments applied in this thesis were not intended to be optimised for 42CrMo4, but rather investigate a more industrial approach using relatively standard heat treatment cycles. After analysing the different heat treated samples it was found that in a regular quench and temper cycle, the parts achieved similar hardness as conventionally manufactured 42CrMo4 using the same heat treatment. Samples exposed to a direct temper cycle, hence just tempered as-printed, showed promising results in terms of residual stresses. This knowledge may serve as a basis to further research and development of heat treatment cycles to better utilise 42CrMo4 steels in the automotive sector