| Abstract Scope |
In laser powder bed fusion (LPBF) additive manufacturing, a laser beam selectively scans closely packed layers of powders and creates a molten pool, which after solidification, forms the part. Gas bubbles can originate inside the molten pool from shielding gas, metal vapors, and gases trapped inside powders during atomization. If these gas bubbles cannot escape from the molten pool before solidification, they can result in gas porosity and degrade the tensile and fatigue properties of parts. Post-process hot isostatic pressing can reduce pores but adds cost. The trial-and-error approach to optimize the processing conditions to reduce porosity is time-consuming and expensive. We have combined mechanistic modeling and experimental data analysis to predict and control gas porosity during LPBF of stainless steel 316, Ti-6Al-4V, Inconel 718, and AlSi10Mg. Temperature and molten pool dimensions calculated using a mechanistic model are used to derive and compute a gas porosity index for predicting and controlling gas porosity. The index is defined as the ratio of time needed for a gas bubble to rise and escape out of the molten pool and the solidification time of the pool. The gas porosity index has two main utilities. First, it can predict if porosity will form or not under a given set of processing conditions for a particular alloy. Second, if porosity forms, the gas porosity index can provide an approximate quantitative idea of its amount. AlSi10Mg has the largest molten pool under the same processing conditions among the four alloys and thus it is the most susceptible alloy to porosity. The process map for gas porosity developed here demonstrates that, for a certain alloy, fast scanning and low laser power can produce a small pool that quickly solidifies, preventing the escape of gas bubbles and increasing the possibility of gas porosity in parts. |