Thermo-mechanical modeling of metallic substrates around laser-induced melt pools

Additive manufacturing (AM) has introduced new possibilities of creating sophisticated designs and structures. Selective Laser Melting (SLM) is an AM technique where structures are fabricated by selectively melting and fusing powder layers. In SLM, melt pools are induced by a laser beam moving on the top surface of a substrate submerged in a powder bed. Mechanical properties of additively manufactured metallic parts are known to be strongly affected by thermal histories, and residual stresses arise due to large temperature gradients. Thermo-mechanical models would help to gain information about both, which is usually hard to obtain. This thesis focuses on examining how well thermal histories and residual stresses in metallic substrates around laser-induced melt pools can be computed by thermo-mechanical models, through experiments on substrates of 17-4PH Stainless Steel (SS) and Ti-6Al-4V. In the first set of experiments, one of two different laser beams moves with constant velocity and power over substrates of 17-4PH SS or Ti-6Al-4V. The substrates are sectioned and etched to expose melt pool traces. In the second set of experiments, single-pass lasers move with constant velocity and power on top surfaces of 17-4PH SS substrates. The time evolution of the deflection of substrates are recorded with a high speed camera. Two types of heat transfer models (accounting for and not accounting for convective heat transfer through fluid flow) reproduced the melt pool traces in the first set of experiments. Predicted thermal histories were critically analyzed. As an extension, how well the model accounting for convective heat transfer reproduced the effect of a substrate edge on the melt pool was examined. Later, the model without convective heat transfer was applied to real-time ultrasonic monitoring of a melt pool in metallic substrates. For the second set of experiments, the model based on heat conduction and elasto-viscoplasticity reproduced the time evolution of deflection of 17-4PH SS substrates. The contributions of this thesis are as follows. Through experiments with various combinations of laser power, scanning speed, power density distribution and metallic material, we show that simply reproducing melt pool traces is insufficient to determine thermal histories. Specifically, for a non-axisymmetric laser beam, three-dimensional melt pool shapes can be disparate even if their two-dimensional traces are very similar. Convective heat transfer in laser-induced melt pools cannot be completely ignored, otherwise there may be inconsistencies between the model and experiment conditions, as well as distortion of thermal histories related to phase transformation. With experiments of laser melting tracks near edges of substrates, we demonstrate that the model accounting for convective heat transfer can consistently reproduce melt pool traces affected by a substrate's edge. We have proven the existence of scattering waves by the presence of a melt pool through simulation, for a possibility of monitoring the state of laser-induced melt pool in real-time with ultrasound. We have designed deflection experiments of metallic substrates monitored by a high-speed camera, which would benefit calibrating thermo-mechanical models for residual stresses because of the substrate's simple thermal and mechanical history. By reproducing the deflection experiments with the model based on heat conduction and elasto-viscoplasticity, we conclude that the solid state phase transformation plays an indispensable role in the evolution of residual stresses of 17-4PH SS. We also highlight the necessity of monitoring time evolution instead of the end state when evaluating models for residual stress of alloys with volume change during phase transformation

Keywords: Mechanical Engineering.
Advisor: Lew Adrian 
Cai Wei
Pinsky Peter M. (Peter Michael)