Transient melt pool size and stress control in additive manufacturing processes

In laser- or electron beam-based additive manufacturing processes, consistent build conditions and residual stress are the primary obstacles in obtaining high precision and limiting tolerance losses in final features. The melt pool size and temperature history during deposition are key parameters linked to build conditions and residual stress respectively. This thesis, consisting of two major sections, investigates the response of melt pool size to a change of process parameters and methods for reducing residual stress by manipulating local temperature fields. The principal application of this thesis is to Laser Engineered Net Shaping (LENSRTM) process, developed at Sandia National Laboratories and currently commercialized through Optomec Inc. In the LENSRTM process, fully dense metal features are constructed by continuous injection of metallic powder into a pool of molten metal created by a focused laser beam. The feature being deposited is moved underneath the fixed laser beam along a desired computer-controlled path to trace out the geometry. In addition to the LENSRTM process, many results and insights from this thesis can be applied to any additive manufacturing process involving a moving heat source including electron beam additive manufacturing and welding processes.; In the first section of this thesis, the transient response of melt pool length or depth to abrupt changes in absorbed laser power or laser velocity are generated by finite element simulations. These simulations are of a thermal deposition process using temperature-dependent material properties of stainless-steel. A process map approach (Vasinonta, 2001) is used to present the obtained results in non-dimensional fashion which allows results over the full range of process variables to be plotted and compared. In this thesis, two fundamental geometries of thin-walled and bulky features fabricated by most laser additive processes are considered. An understanding of melt pool size response in both structures can be an important part of the development of feedback control systems and consequently, the deposition of increasingly precise final features of more complex shapes. The results presented in this thesis demonstrate that response time for changes in melt pool size is often considerable, and therefore must be addressed when developing feedback control systems. A simplified model for approximating melt pool response time for an instantaneous change in process variables is also provided based the results.; The second section of this thesis presents approaches to reduce residual stress in features deposited onto an existing part. Analytical thermal stress theory and existing work have shown that uniform preheating in the course of deposition is effective in limiting the magnitude of final residual stress in deposited parts. However, additive manufacturing processes concern deposition of relatively small features onto larger parts and thus entire part preheating is typically ineffective if not impossible. This thesis considers localized surface preheating approaches for manipulating the near-surface temperature field during the deposition process as an alternative means for achieving final residual stress reduction without whole part preheating. A series of results obtained from simplified finite element simulations provide a clear understanding of mechanisms for reducing thermal residual stress in deposited features. Results from this thesis using a simplified model of feature deposition show that a significant reduction of residual stress can be achieved for the case in which the entire top surface is preheated to a higher temperature prior to initiation of the deposition process. A more completed and sophisticated numerical model is used to verify the effectiveness and practicality of the suggested localized preheat scheme for actual additive manufacturing processes.