Investigation and management of residual stresses in selective laser melting of maraging steel

[摘要] ENGLISH ABSTRACT: Selective Laser Melting (SLM) is a leading metal additive manufacturing process that has gained a lot of traction since the turn of the new millennium. Despite many benefits associated with SLM, a major setback that continues to impede its wider application and uptake is the inherent phenomenon of residual stresses. Although post-processing methods such as heat treatment can significantly reduce the magnitude of generated residual stresses, these methods cannot reverse the cracking, delamination and warping distortions that occur during the process. This dissertation focuses on the investigation of residual stresses and explores effective ways through which these stresses can be managed in-situ.An experimental study was conducted to establish the influence of input parameters on residual stresses and their accompanying effect on residual stresses. First, a study of the distribution of residual stresses was carried out on parts of different thickness. Secondly, scanning strategies and process parameters were studied through a structured experimental programme. Specimens were manufactured from maraging steel 300 powder on an M2 LaserCUSING as well as an EOSINT M280 machine. Residual stresses were measured using the neutron and X-ray diffraction methods whilst a coordinate measurement machine was used to measure distortions that arose from these stresses.The results show that residual stresses increase as part thickness increases, and that these stresses are not uniform, even at the same depth of measurement. From the scanning strategy perspective, reducing the scan vector length lowered residual stress magnitudes, but increased porosity significantly. Whilst rescanning lowered tensile stresses and increased the magnitude of compressive stresses, it is also clear that maintaining the same laser parameters as the initial beam pass leads to overheating and a marginal rise in porosity. An improved scanning pattern, called the successive chessboard strategy, yielded up to 40 % reduction of residual stresses against the default island scanning strategy. The correlations between input parameters and process outcomes show that increasing laser power and scanning speed increases residual stresses and distortions for the range of parameters tested. On the other hand, increasing the layer thickness from 30 to 45 μm generally reduces residual stresses and distortions but promotes porosity. However, a satisfactory process parameter combination was found at 180 W and 600 mm/s for the 45 μm layer thickness. At this point, residual stresses and distortions were reduced by 31 % and 46 % respectively, relative to the 30 μm layer at the same laser power and scanning speed. As original contribution, a method for evaluating and selecting residual stress management techniques was developed. Furthermore, new scanning sequences were developed, with the successive chessboard contributing to reduction of residual stresses and distortions. A process window was also devised for SLM of maraging steel 300. The process window demonstrates the porosity and residual stress state of final parts at different combinations of laser power and scanning speed. Finally, correlations were formulated between input parameters and the responses. This was extended to analysing the interdependencies between process outcomes, for example, residual stresses vs distortions and porosity vs distortions.

[发布机构] Stellenbosch University