Microstructure evolution of the laser spot welded Ni-free Zr-based bulk metallic glass composites

A novel Ni-free (Zr ₄₈Cu ₃₂Al ₈Ag ₈,Ta ₄)Si _0.75 bulk metallic glass composite (BMG-C), showing an excellent combination of high strength and remarkable ductility, was laser spot welded with the pre-selected laser welding parameters. After welding, the microstructure evolution, glass forming ability (GFA) and mechanical properties of the welded samples were determined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Vickers microhardness test. Test results showed that the parent material (PM), heat affected zone (HAZ) and weld fusion zone (WFZ) in the welds all consist of an amorphous matrix with two kinds of Ta particles: micro-sized and nano-sized. In the WFZ, during the rapid melting, the partial dissolution of micro-sized Ta particles resulted in the reduction of their volume fraction. After subsequent rapid cooling, partial micro-sized Ta particles were transformed to nano-sized particles or nano-sized particle accumulations. This transformation resulted in a slightly higher magnitude of hardness in the WFZ. Furthermore, it was found that the surrounding area of micro-sized Ta in the WFZ has better resistance to the etchant solution. In the HAZ, small amounts of Zr ₂Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface, and may act as heterogeneous nucleation sites. However, the small amount of precipitation in the HAZ and the micro-sized Ta transformation in the WFZ did not significantly affect the magnitude of the GFA indices, ΔT _x, γ and γ _m, when compared to that of un-welded BMG-C.