Numerical and experimental study of molten pool behaviour and defect formation in multi-material and functionally graded materials laser powder bed fusion

Abstract Laser powder bed fusion (LPBF) of multi-material and functionally graded materials (FGM) has attracted significant research interest due to its ability to fabricate components with superior performance compared with those manufactured with single powder material. However, the forming mechanisms of various defects remain unknown. In this paper, a DEM-CFD model was first established to obtain an in-depth understanding of this process. It was discovered that the defects including partially melted and un-melted Invar36 powder were embedded in the lower level of the powder layer; this was attributed to the low laser absorptivity, low melting point and high thermal conductivity of the Cu10Sn powder. Inter-layer defects were more likely to occur with an increased powder layer thickness. In addition, the scanned track width was found related to an equilibrium achieved among the thermal properties of the powder mixture. Process parameters were optimised to obtain FGM structures without defects in both horizontal and vertical directions. Invar36/Cu10Sn samples were fabricated with a multi-material LPBF system using different mixed powder contents and laser volumetric energy densities (VEDs). By increasing the VED, fewer defects were observed between the interface of two processed powder layers, which had a good agreement with the modelling results.