Gas bubble coalescence in laser directed energy deposition of irregular HDH titanium alloy powder feedstock

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In this study, the synchrotron X-ray imaging technique was used to investigate the coalescence of gas bubbles during laser directed energy deposition (L-DED) of irregular hydride-dehydride (HDH) titanium alloy particles onto a titanium alloy substrate. The objective is to better understand the coalescing mechanism of gas bubbles during the L-DED process of unique feedstock materials. The coalescence frequency of gas bubbles is the percentage of bubbles merging and is dependent on collision frequency and coalescence efficiency. Forces in the melt pool flow such as Marangoni forces, buoyancy force, vaporization pressure, and acoustic waves, along with the number of generated bubbles, were the major factors for increasing the collision frequency between bubbles. In addition, the random forest model determined that the diameter of the smaller bubble during collision was the most significant factor impacting the efficiency of coalescence, which was equal to 48% when the smaller diameter was larger than a threshold of 76 μm and 3.5% and when the diameter was smaller than 76 μm. The results also showed that 6.5% of the formed bubbles in the melt pool led to coalescence. Overall, this work can help mitigate pores, verify simulation models, and promote irregular or recycled powder feedstock as effective replacement to atomized feedstock.