Laser Engineered Net Shaping (LENS) modeling using welding simulation concepts

This study focuses on obtaining residual stresses and molten pool details for LENS (Laser Engineered Net Shaping) produced parts by using finite element analysis. Obtaining residual stresses and deformation is very important for the LENS process since this information is used for improving quality and life of the LENS produced parts. In addition, this information can be used in order to understand and relate input parameters to the results such as stresses and deformation. LENS process is basically a laser welding process, therefore welding simulation concepts provides the background for simulating LENS process. Basics and theory for the welding simulation along with two applications are presented. These applications are; (1) Numerical Simulation of Welding Induced Residual Stresses in a Circular Hollow Section T-joint, and (2) Laser welding of a thin aluminum plate. The difficulties associated with simulating the LENS process are summarized and a "wall" configuration consisting of one hundred deposited layers is calculated using finite element analysis. Two different materials are used, one being a single phase material and the other being a four phase material where phase transformations are taken into account. The stress fields for these two materials are found to be quite different. The results for the single phase material are compared with published experimental results and some agreement is observed. For all welding simulations a welding specific finite element package, SYSWELD, is used.;The heat transfer details in the melt pool such as temperature gradient have very little affect on the overall mechanical results. For example, this was demonstrated on a simple bead on a plate problem by calculating the melt pool from four different heat source definitions and comparing the residual stress levels for each melt pool. However, it has been known that temperature gradients are very important if a certain solidification pattern is desired. Repairing single crystal turbine blades by using a LENS machine, requires precise control of the temperature gradient in order to prevent columnar to equiaxed transition (CET) during solidification. Computational fluid dynamics is used in order to model the problem and predict the temperature gradient within the melt pool. The solid-liquid boundary is also calculated. Energy, momentum and continuity equations are solved in order to accurately take fluid flow into account. The Marangoni force is found to be the main driving force. This force is caused by the temperature dependent surface tension on the free surface. Fluid flow calculations were performed by using the FIDAP computational fluid dynamics code.