Measurement and Modeling of Residual Stress-Induced Warping in Direct Metal Deposition Processes

Tolerance loss due to residual stress-induced warping is a major concern in solid freeform fabrication (SFF) processes. 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 microcasting and welding processes used in Shape Deposition Manufacturing (SDM). Results from these experiments give insight into differences between the two processes, 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 predictions from two types of residual stress models. While the models effectively predict warping magnitudes and the effects of various thermal conditions, they are unable to capture some of the more subtle trends in the experiments. 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.