A study on overlapping effect of melt pools in powder bed fusion based on a novel prediction method

The unique advantages distinguishing additive manufacturing from other processing technologies, such as casting, forging, powder metallurgy and laser remelting originate from multiple melting and resulting overlapping of melt pools. However, it is difficult to predict or reconstruct the state of melt pool overlapping because melt pools are affected in each processing stage (deposition, melting, and solidification). Moreover, the impact of melt pool overlapping on microstructure and properties is still unclear . Here, a novel reconstruction model was established for melt pool overlapping by using Python during laser powder bed fusion. Melt-pool dimensions used in this model can be derived using experiments or predicted through normalized enthalpy. The “overlapping unit” was proposed to be a result of the periodic regulation of deposition, which helps in simplifying the microstructure formed through multiple melting and layer-wise deposition. Subsequently, the overlapping effect and its impact on elemental losses, defect formation, and microstructure evolution were investigated; the results were elucidated based on the average number of melting cycles (NMC) of the “overlapping unit” and the calculated thermal history. It can be concluded that the loss of elements with low boiling points (Mg in our alloy) is positively correlated with NMC due to repeated vaporization. Most incomplete defects are formed in regions melted less than twice, while hole defects are found predominantly in regions melted more than six times. Furthermore, the fine-grained area (average diameter ≤ 1 µm) tends to vary with both melt pool overlapping and heat accumulation. Moreover, an increase in sample temperature due to overlapping can augment the number and size of precipitates. Overall, this work offers a new way to improve the microstructure and mechanical properties of materials by optimizing overlapping conditions.