A comparison of crystallization behaviors of laser spot welded Zr-Cu-Ag-Al and Zr-Cu-Ni-Al bulk metallic glasses

A novel Ni-free Zr-Cu-Ag-Al ((Zr₄₈Cu₃₆Ag ₈Al₈)Si_0.75) and a Zr-Cu-Ni-Al ((Zr ₅₃Cu₃₀Ni₉Al₈)Si_0.5) bulk metallic glass (BMG), for comparison, were employed for Nd:YAG laser spot welding with three pre-selected energy inputs, including a low (6.2 J), a medium (8.0 J) and a high (9.2 J) energy input. After the welding process, 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 Vicker's microhardness test. The results showed the Zr-Cu-Ag-Al BMG has better weldability than the Zr-Cu-Ni-Al BMG. No crystallization was observed in the weld fusion zones (WFZs) or heat-affected zones (HAZs) of the welds under three pre-selected energy inputs. Therefore, the GFA indices of ΔT_x (ΔT_x = T_x - T_g, T_x is the crystallization temperature and T _g is the glass transition temperature), γ (γ = T _x/(T_g + T_l), T_l is the liquidus temperature) and γ_m (γ_m = (2T_x - T_g)/T_l), and the mechanical properties of the Zr-Cu-Ag-Al BMG welds did not differ significantly in comparison to the parent material (PM). For Zr-Cu-Ni-Al BMG, HAZ crystallization was unavoidable when a lower energy input was used; therefore, the GFA indices and mechanical properties of the weld were affected. Furthermore, when the GFA indices, ΔT_x, γ and γ_m, were used to predict the thermal stability of the BMG HAZs, ΔT_x seemed to correspond more directly to the HAZ crystallization behaviors in this study. It was observed that the ΔT _x value of Zr-Cu-Ag-Al BMG was about 26% higher compared to that of Zr-Cu-Ni-Al BMG.