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 ((Zr48Cu36Ag8Al8)Si0.75) and a Zr–Cu–Ni–Al ((Zr53Cu30Ni9Al8)Si0.5) 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 Vickerʹs 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 ΔTx (ΔTx = Tx − Tg, Tx is the crystallization temperature and Tg is the glass transition temperature), γ (γ = Tx/(Tg + Tl), Tl is the liquidus temperature) and γm (γm = (2Tx − Tg)/Tl), 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, ΔTx, γ and γm, were used to predict the thermal stability of the BMG HAZs, ΔTx seemed to correspond more directly to the HAZ crystallization behaviors in this study. It was observed that the ΔTx value of Zr–Cu–Ag–Al BMG was about 26% higher compared to that of Zr–Cu–Ni–Al BMG.