A pragmatic continuum level model for the prediction of the onset of keyholing in laser powder bed fusion

Laser powder bed fusion (L-PBF) is a complex process involving a range of multi-scale and multi-physical phenomena. There has been much research involved in creating numerical models of this process using both high and low fidelity modelling approaches where various approximations are made. Generally, to model single lines within the process to predict melt pool geometry and mode, high fidelity computationally intensive models are used which, for industrial purposes, may not be suitable. The model proposed in this work uses a pragmatic continuum level methodology with an ablation limiting approach at the mesoscale coupled with measured thermophysical properties. This model is compared with single line experiments over a range of input parameters using a modulated yttrium fibre laser with varying power and line speeds for a fixed powder layer thickness. A good trend is found between the predicted and measured width and depth of the tracks for 316L stainless steel where the transition into keyhole mode welds was predicted within 13% of experiments. The work presented highlights that pragmatic reduced physics-based modelling can accurately capture weld geometry which could be applied to more practical based uses in the L-PBF process

Internal Stress Evolution and Subsurface Phase Transformation in Titanium Parts Manufactured by Laser Powder Bed Fusion—An In Situ X‐Ray Diffraction Study

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology,which enables the manufacturing of complex geometries for various metals andalloys. Herein, parts made from commercially pure titanium are studied usingin situ synchrotron radiation diffraction experiments. Both the phase transformationand the internal stress buildup are evaluated depending on the processingparameters. For this purpose, evaluation approaches for both temperatureand internal stresses from in situ diffraction patterns are presented. Four differentparameter sets with varying energy inputs and laser scanning strategiesare investigated. A combination of a low laser power and scanning speed leads toa more homogeneous stress distribution in the observed gauge volumes. Theresults show that the phase transformation is triggered during the primarymelting and solidification of the powder and subsurface layers. Furthermore, thestress buildup as a function of the part height during the manufacturing processis clarified. A stress maximum is formed below the part surface, extending intodeeper layers with increasing laser power. A temperature evaluation approach forabsolute internal stresses shows that directional stresses decrease sharply duringlaser impact and reach their previous magnitude again during cooling