Efficacy of elemental mixing of in situ alloyed Al-33wt%Cu during laser powder bed fusion

Challenges in developing novel alloys, specifically for use in laser powder bed fusion, may be overcome by in situ alloying of elemental powders during laser melting. This process could expedite prototyping of various alloy compositions and alleviate the restrictions and cost barriers of creating custom made alloy powder. In this research, the efficacy of in situ alloying is studied with respect to particle size distributions of the powder blends and the laser process parameters. The microstructure of the Al-Cu eutectic system is used here as an indicator of mixing quality of the constituent elements during laser melting of the particles. Hypo- or hypereutectic regions are readily visible through the dendritic growth of the α or θ phases, indicating regions where the solute concentration deviates from the nominal eutectic composition. Samples were built from four powder blends at a range of scan speeds and powers to show how mixing is affected at different processing parameters. Image analysis and Vickers microhardness tests are both used to characterize the degree of mixing within the samples. Results of this study show that poor mixing can occur due to segregation of elemental powder within the powder blend. This produces local build compositions different than the mean powder composition when the mean particle volume is large enough that the melt pool encompasses too few particles to be a stochastic representation of the blend. Liquid phase intermixing limitations within the pool are thought to be less important than the melt pool volume itself.