| An investigation is performed of the thermally induced residual stresses in parts made by layered manufacturing methods. The results are applicable to material deposition processes that involve molten metal on a solid substrate, such as Laser Engineered Net Shaping and Directed Light Fabrication. The motivation comes from Shape Deposition Manufacturing (SDM), a solid freeform fabrication process, and the process used in SDM to build layers of a part from superheated metal droplets that free-fall onto a substrate.; The broad residual stress problem is divided into a sequence of models of increasing complexity, the simplest being deposition of a single layer or droplet, and progresses to successive deposition of layers, droplets, and rows. Each model type is analyzed with a commercial finite element package by solving the heat transfer problem first and then applying the thermal loads to the mechanical problem. The thermomechanical modeling uses temperature-dependent material properties of carbon steel and stainless steel.; All model types examined indicate that transient compressive yield is followed by final high tensile stresses in the deposit and top of the substrate. The single droplet model shows that substantial stress reduction is achievable through moderate substrate preheating, while the single layer model shows that part constraint during manufacture is extremely favorable in reducing the final stresses. Models of successive deposition of layers and droplets piled up show that intense heating from newly deposited material wipes out initial stresses in underlying material so that the initial stress state at the substrate top is of minimal importance. The models of successive ad acent rows show that preheating from adjacent rows can be large. Thus while an initial substrate preheat is useful to reduce stress at the start of deposition, later heating by deposited material acts to reduce stresses and curvature regardless of initial substrate temperature. In addition, it is advantageous to place new rows next to recently deposited rows and to avoid paths over unheated regions. The successive row models also show that constraints, both physical and in modeling, strongly influence the three dimensional behavior and are even more critical when large volumes undergo plastic deformation. |
