Crystallization Behavior of Laser Solid Forming of Annealed Zr₅₅Cu₃₀Al₁₀Ni₅ Powder

Laser solid forming (LSF) provides an innovative way in building the bulk metallic glasses (BMGs) due to its inherently rapid heating and cooling process and point by point additive manufacturing process, which can eliminate the limitation of critical casting size of BMGs. The annealed powder has been demonstrated to be applicable to the preparation of BMGs with high content of amorphous phase using LSF. In this work, the plasma rotating electrode processed (PREPed) Zr₅₅Cu₃₀Al₁₀Ni₅ (Zr55) powders annealed at 1000 K are used for LSF of Zr55 BMGs. The influences of powder size and laser processing parameter on the crystallization characteristic of the deposit are investigated, and the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) is analyzed. It is found that the microstructures of the pre-annealed Zr55 powders are composed of the Al₅Ni₃Zr₂, CuZr₂ and Al₂Zr₃ phases. As the heat input increases from 7.0 J/mm to 15.7 J/mm, the every deposited layer presents a periodic repeating gradient microstructure (amorphous, NiZr₂ nanocrystal, CuZr₂+ZrCu dendrite-like eutectic, CuZr₂+ZrCu spherulite-like eutectic) from the molten pool to the HAZ. The size of the eutectic phase in the HAZ decreases as the increase of distance from the featureless amorphous zone. On condition that the laser heat input is less than 7.0 J/mm, the deposits contain a high content of amorphous phase. As the increase of laser heat input, the crystallization degree of HAZ does not increase obviously for the deposit prepared by the powder with size range of 75~106 mm. However, the crystallization degree of HAZ increases significantly for the deposit prepared by the powder with size range of 106~150 mm. That is because the lower overheating temperature and shorter existing time of the molten pool enhances the heredity of Al₅Ni₃Zr₂ clusters and other intermetallic clusters in remelted alloy melt during LSF of coarser powder, which decreases the thermal stability of the already-deposited layer and induces the severe crystallization. It is deduced that the raw state of annealed powders has a minimal impact on the crystallization behavior of the Zr55 deposited layers when the content of Al₅Ni₃Zr₂ phase is same in different sizes of annealed powders. The thermal history of RZ and HAZ during deposition is the primary factor to affect the crystallization behavior in the Zr55 deposits fabricated by different powder sizes.