Residual stress-induced warping in direct metal solid freeform fabrication

Tolerance loss due to residual stress-induced warping is a major concern in solid freeform fabrication (SFF) processes, particularly those which involve direct deposition of molten metals. An understanding of how residual stresses develop and how they lead to tolerance loss is a key issue in advancing these processes. In this paper, results are presented from warping experiments on plate-shaped specimens created by two direct metal deposition methods, which are utilized by a particular SFF process termed shape deposition manufacturing (SDM). Results from these experiments give insight into the differences between the two deposition methods, the role of preheating and insulating conditions during manufacture and the influence of deposition path on magnitudes and distributions of warping displacements. Results are then compared to numerical predictions from both one and two-dimensional residual stress models, which are applicable to SDM and similar direct metal deposition processes. Results from the experiments and numerical models suggest that a combination of initial substrate preheating and part insulation can be applied to SDM and similar SFF processes to limit warping deflections, which is substantially simpler than active control of part temperatures during manufacture. Results also suggest that 3-D mechanical constraints are important in achieving precise control of warping behavior in SFF processes.