Effect of annealing time on residual stress in selective laser melted Co-Cr-Mo components

Abstract

Additive manufacturing (AM) is a three-dimensional (3D) manufacturing technique that allows the user to manufacture designs with complex geometries. Selective Laser Melting (SLM) is an AM technique where a layer-wise process of melting metal powder is used to manufacture 3D objects. A major disadvantage of SLM is the deformation of parts during the print cycle or after the part is removed from the build plate. The thermal inconsistencies inherent in this layer-wise manufacturing process cause residual stress build-up that may exceed the yield stress of the material and lead to print failures. The residual stress in AM parts manufactured with metal powder is similar to the residual stress that builds up in cold-worked metal parts due to the applied stresses stored within the material. The residual stress is stored in the form of dislocations and stress-relief annealing is commonly used to minimize this residual stress. Several studies have investigated the effect of stress-relief annealing on the microstructure, mechanical properties and residual stress in AM components. The research indicates that stress-relief annealing can change the microstructure, improve mechanical properties and reduce residual stress. Limited knowledge is available on the effect stress-relief annealing hold time at maximum temperature has on the residual stress in AM parts. This study aimed to investigate the effect of stress-relief anneal time on the residual stress in Co-Cr-Mo components manufactured with SLM with the use of neutron diffraction techniques and Simufact Additive software. Neutron diffraction is a non-destructive method for determining residual stress in a material. Simufact Additive software is simulation-based software that uses thermo-mechanical simulations to analyse the AM process. Six Co-Cr-Mo samples were manufactured on the ORLAS Creator with SLM and five of the samples were stress-relief annealed using varying time profiles. The annealing times and process parameters were chosen from the literature to provide comparable results. The samples were annealed at 1065oC with annealing times of 15 minutes, 1 hour, 2 hours, and 3 hours. One sample was annealed at 750oC for 1 hour. The results obtained through simulation and neutron diffraction analysis showed good correlation. The samples heat treated to 1065oC showed more residual relief than 750oC indicating that the heat treatment temperature had a significant effect on the degree of residual stress relief. The results related to samples heat treated to 1065oC indicated that shorter annealing times resulted in near- identical residual stress values than longer annealing times. This discovery indicated that residual stress relief occurred in a shorter time frame than anticipated and that heat treatment duration longer than 15 minutes did not result in improved residual stress relief results.