Melt Pool Size and Stress Control for Laser-Based Deposition Near a Free Edge

Abstract

Thermomechanical models developed in this research address two experimental observations made during the deposition of thin-walled structures by the LENSTM process. The first observation (via thermal imaging) is of substantial increases in melt pool size as a vertical free edge is approached under conditions of constant laser power and velocity. The second observation (via neutron diffraction) is of large tensile stresses in the vertical direction at vertical free edges, after deposition is completed and the wall is allowed to cool to room temperature. At issue is how to best control melt pool size as a free edge is approached and whether such control will also reduce observed free edge stresses. Thermomechanical model results are presented which demonstrate that power reduction curves suggested by process maps for melt pool size under steady-state conditions can be effective in controlling melt pool size as a free edge is approached. However, to achieve optimal results it is important that power reductions be initiated before increases in melt pool size are observed. Stress simulations indicate that control of melt pool size can reduce free-edge stresses; however, the primary cause of these stresses is a constraint effect which is independent of melt pool size.