Effects of laser printing parameters on molten pool formation, microstructure evolution and mechanical properties of laser directed energy deposition of difficult-to-process tungsten heavy-alloy

Abstract

Laser directed energy deposition (LDED) additive manufacturing is promising in the fabrication of high-performance W-based heavy-alloys which are normally difficult-to-process due to their unique nature such as the extremely high melting point and high ductile-brittle transition temperature. In this work, a novel 95 W heavy alloy modified by nanosized Ni and Fe particles was manufactured by LDED process. The effects of laser power on the formation mechanisms of molten pool were investigated. It was found that a relatively high laser power could promote the complete melting of W powder and formation of regular molten pool morphology. The microstructures of laser-processed W heavy alloy showed columnar grains, cellular grains and equiaxed grains from bottom to top within the molten pool along the building direction. The three-dimensional finite element simulation revealed the temperature gradient ( G ), cooling rate ( C ) and solidification velocity ( S ) within the molten pool were different, which was responsible for the microstructural development. The bulk-form 95 W heavy-alloy part free of defects was produced at the laser power of 1400 W. The optimally processed 95 W heavy-alloy by LDED showed a high average nanohardness of 8.6 GPa and a sound wear performance with a low coefficient of friction (COF) of 0.5. The tensile tests also revealed the achievement of a combination of an excellent tensile strength of 541 MPa and an elongation of 0.99%. This work demonstrated that LDED was able to manufacture high-performance tungsten heavy-alloys in bulk form with outstanding mechanical properties.