M. Langerman, U. Korde, V. Kalanovic, South Dakota School of Mines and Technology, Rapid City, SD

Summary:

Laser based solid free-form fabrication is an emerging metallurgical forming process aimed at rapid production of high quality, near net shape products directly from starting powders. Laser powder deposition (LPD) shares, with other free-form technologies, the common characteristic that part fabrication occurs directly from a 3-D computer aided design (CAD) model. The SDSMT Additive Laser Laboratory (AML) contains a continuous wave (CW) 3 kW Nd: YAG laser equipped with two metal powder-feed systems and mounted on a Fanuc 16Mi robot. This equipment allows for direct laser deposition, solid freeform fabrication, and graded alloy research and development.

The microstructure evolution and resulting material properties (strength, ductility, etc.) of the component part fabricated using laser deposition are dependent upon the thermal history that the part undergoes during the process. The difficulty in measuring the transient temperatures during the process has lead to the need to develop accurate and reliable thermal models. These models can be used to assess the effects of various parameters on the process thermal efficiency and subsequent thermal residual stresses within the base and build.

A thermal model of the LPD process has been developed and tested at AML. Unlike many of the models described in the literature, this model is a fully transient description of the process, accounting for phase change, substrate boundary conditions, and the effects of heat losses at all boundaries. The results obtained from the model have been compared to results from both analytic solutions and to process experimental data. Results from these comparisons have been published and show excellent agreement.

The AML model is currently being modified to account for melt pool dynamics including Marangoni effects. It is also being applied to assess the effects of selective deposition paths on subsequent part deformation. For the application here, the model is used to predict heat losses from the base and sides of a thin-walled build. This phenomenon has been investigated by others with the conclusion that heat loss through the base substrate dominates and heat loss from the sides of the build can be neglected. The work presented here will show that this conclusion is inappropriate for layered builds over 10 mm in height.