Laser melting deposition of fine-grained 90W-7Ni-3Fe alloys using pre-sintered granulated powder

Abstract

Reducing the grain sizes of tungsten (W) in tungsten heavy alloys (WHAs) is pivotal for achieving high strength. Traditional liquid phase sintering (LPS) often results in the growth of W grains, due to the long sintering time and slow cooling rates. To achieve a high cooling rate, a laser melting deposition (LMD) additive manufacturing technique was employed in this study to prepare 90W-7Ni-3Fe alloys with unique pre-sintered powder prepared using a spray granulation method. The pre-sintered granulated powder particles, each consisting of many fine elemental particles, exhibit a spherical shape with a size up to 100 μm, contributing to optimal flowability. Thin plate samples were prepared using laser powers ranging from 500 W to 900 W. The results show that with laser powers below 600 W, WHAs with fine W grain sizes of 5 μm can be successfully obtained, representing a record of the finest grain size ever reported for the laser additive manufactured WHAs. On the other hand, at 900 W, the W grain size increases significantly to 17 μm, comparable to that of traditional LPS samples. The volume fraction of W grains increases with increasing laser power due to the Ni and Fe evaporation, and at laser power exceeding 700 W, this fraction aligns with 95WNiFe alloys. The fine grains and/or the increased volume fractions of W grains contribute to a higher micro-hardness in these samples compared to LPS samples. Nonetheless, high porosities are also observed in these samples, which leads to a diminished micro-hardness compared to previous LMD 90W-7Ni-3Fe alloys prepared with mixed elemental powder. This study underscores the potential of the LMD method using pre-sintered powder to fabricate fine-grained WHAs, paving the way for the fabrication of high-strength and high-ductility WHAs using LMD.