Rapid prediction of real-time thermal characteristics, solidification parameters and microstructure in laser directed energy deposition (powder-fed additive manufacturing)

Abstract

The localized thermal characteristics, solidification parameters and the corresponding microstructure depend highly on the process parameters in the laser powder-fed additive manufacturing (LPF-AM) process. However, undesirable and inconsistent microstructure may potentially be induced due to the environmental disturbances or improper setting of process parameters. This research correlates the process parameters to the localized transient thermal characteristics, i.e. temperature and cooling rate, and solidification parameters, namely thermal gradient (G) and solidification rate (R) for rapid prediction of the microstructural evolution. The built correlation can also be used for parameter optimization and could be potentially employed for in-situ microstructural control. Firstly, the thermal characteristics and the solidification parameters were resolved from a three-dimensional analytical thermal model that couples the powder mass flow and the laser heat flux. Subsequently, the calculated solidification parameters were combined with the substructure scale solutions for microstructure prediction with respect to the solidification map. In the end, experiments of LPF-AM were conducted with stainless steel 316L (SS 316L) and Inconel 625 powders to validate the built correlation. It was noticed that the calculated real-time melt pool peak temperatures match well with the experimental results at different laser scanning speeds and diverse energy densities in the SS 316L deposition. The predicted microstructural evolutions show reasonable agreement with the experimental observations for both SS 316L and Inconel 625 depositions under different scanning speeds. In addition, it was found that the combination of a higher scanning speed with a lower laser power results in a finer microstructure, but the combinations should be kept within the melt pool temperature thresholds for effective deposition. Moreover, the G × R values increase from the bottom to the top of the melt pool bead, leading to a finer microstructure at the top zone for both SS 316L (5.5 μm average primary dendrite arm spacing) and Inconel 625 (3 μm average secondary dendrite arm spacing) deposits. On the contrary, the G/R values decrease from the bottom to the top of the melt pool bead, which in turn, give rise to the gradual transition of the substructure morphology from columnar dendritic to equiaxed dendritic for the Inconel 625 deposits.