Process maps for melt pool size and residual stress in laser-based solid freeform fabrication

In Solid Freeform Fabrication (SFF) processes involving thermal deposition, thermal control of the process is critical for obtaining consistent build conditions and limiting residual stress-induced tolerance losses. This thesis investigates the effects of changes in process variables and part geometry on melt pool size and a defined temperature gradient which are key parameters linked to build conditions and residual stress respectively. The principal application of this thesis is to Laser Engineered Net Shaping (LENS) process, the SFF process developed at Sandia National Laboratories. In LENS, fully dense metal parts are constructed by continuous injection of metallic powder into a pool of molten metal created by a focused laser beam. The laser beam is slowly moved to trace out the geometry of the desire part. In addition to LENS, the general approach and many findings of this research are applicable to any SFF process involving a moving heat source and to laser welding processes.; In this thesis, nondimensionalized plots (termed process maps) are developed from thermomechanical finite element simulations of a laser-based thermal deposition process using temperature-dependent material properties of stainless steel. Use of a normalization technique based on analytical solutions of conductive heat transfer allows a large number of numerical results to be collapsed into an easily used format. Consequently, the errors caused by temperature-dependent properties are kept within acceptable limits. Procedures for developing and using of process maps are detailed for thin-walled and bulky structures. These two structures are fundamental geometries commonly fabricated by the LENS and other SFF processes. Predictions extracted from the developed process maps are also compared against available experimental results with reasonable agreement. The results of this thesis include comprehensive guidelines for maintaining optimal build conditions while limiting residual stress throughout the building of parts, as well as strategies to be applied during the transition between the thinwalled and bulky structures. In addition, the stress results demonstrate the link between a defined temperature gradient and thermally induced residual stress and identify the mechanisms for reducing the magnitude of residual stresses.