Preparation And Application In Welding Of Bulk Mg-based Amorphous Alloys

Mg₆₅ Cu₂₂ Ni₃Y_{10 -x}Nd_x bulk amorphous alloys with x=0,2,4,5,6,and 8 were prepared by casting directly into a wedge-shaped copper mold method. The glass forming ability and thermal stability of the alloys were analyzed by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) analysis respectively. It was shown that Nd addition could effectively enhance the thermal stability and glass-forming ability (GFA) of the alloys when x=2,4．The bulk amorphous alloy with the largest thermal stability exhibited maximum supercooled liquid regionΔT_x as 61．5 K when x=2．Though the supercooled liquid region was the narrowest (ΔT_x=48．5K) when x=5,the alloy had greatest glass forming ability (T_rg=T_g/T₁=0．568) and maximum thickness (Tmax=3．8 mm) for BMG formation. With the increase of Nd content (x>5) ,the GFA of the amorphous alloy decreased.A welding experiment was performed using Mg-based alloys AM60B as base metal and bulk Mg₆₅ Cu₂₂ Ni₃Y₁₀ amorphous alloy as interlayer between 420℃and 540℃. Micro morphology and elements distribution of the brazed joints were analyzed through metallographic microscopy, SEM and EDS. The results were as following: a sound bonded interface was obtained without sheet inclusion, pore and unbonding. Microstructure were consisted of bulkα-Mg phase and needle compound in the weld metal, The element Al diffused along the grain boundary of the substrate into welding line. A little Y diffused into the base metal. A little Mn,Cu segregated and the element Ni distributed homogeneously and there was no segregation in weld area.A TLP diffusion and bonding experiment was performed using Mg-based alloys AZ91D as base metal and bulk Mg₆₅ Cu₂₂ Ni₃Y₅Nd₅ amorphous alloy with 2 mm thickness as interlayer. The minimum temperature guaranteeing TLP diffusion and bonding valid connecting was 400℃. With the rise of welding temperature, weld width gradually reduced; Microstructure of weld altered from straight to twists and turns; Weld metal developed from center to boundary as the shape of dendrite ;Liquid interlayer diffused along the grain boundary of the substrate into welding line, there was a drastic mutual diffuses taken place between liquid interlayer and base metal; The microstructure of welding district remainder mesosphere had a conversion taking place with massive (500℃)→short rod (520℃)→needle (540℃and 560℃);And element Mg had a degressive changing direction ,A1 ascended , Zn didn't variety ultimately and the other element had a little variation also. Temperature had a great impact on the joint strength, the higher the temperature, the greater its joint strength. At the condition of 560℃and 15min, the shear strength got maximum with 76MPa; Holding time and temperature effects on the weld seam to the same effect; The appropriate welding time was an important factor to form an excellent joint; At 520℃, holding the welding temperature unchanged and increasing pre-pressure holding time, the mechanical properties of the joints had an obvious improvement; At high temperature improving the pre-pressure there was little effect on the mechanical properties.