The Research On Friction Stir Welding Of Cu-Zr Based Bulk Metallic Glass

Due to its special structural characteristics,the bulk metallic glasses have excellent physical,mechanical,and chemical properties.For example,high intension,large elasticity limitation,good magnetism property and corrosion resistance.BMGs can be used as a key material for new technologies such as electric power industry,national defense industry and precision instrument.However,the three-dimensional size of the bulk metallic glasses?BMGs?prepared is small at present,which greatly limits the application of BMGs in engineering materials.Friction stir welding?FSW?is a solid phase welding technology,which can effectively increase the size of amorphous alloy.Because of its low heat input in the welding process,it can easily maintain the metastable structure of amorphous alloy in welding process.In addition,the amorphous alloy has good plastic flow capacity in the supercooled liquid region.Therefore,FSW has become an ideal technology for amorphous alloy to expand in size.In this paper,The bulk metallic glasses of Cu₃₆Zr₄₈Ag₈Al₈ are taken as the research object,and friction stir welding is carried out in supercooled liquid region.through the analysis of XRD and non-isothermal DSC with different heating rates of cast samples,we found that with the increase of heating rate,the supercooled liquid phase region of the bulk metallic glasses shifts to higher temperature and the width of the supercooled liquid phase is also larger,which shows that the heating auxiliary device with very high heating rate is beneficial to welding of the BMGs in the supercooled liquid phase region.By isothermal DSC,it was found that at the temperature of the supercooled liquid phase,the time required for the inoculation period of the bulk metallic glasses before crystallization is 13 minutes.This requires that the BMGs'welding process be completed within 13 minutes.Based on the above analysis results,the heated auxiliary device,the size of the stirring head and the process parameters of the FSW were designed.The selected shoulder is 12 mm,and the stirring needle is a round table-shaped stirring head.At the same time,it was found that when the welding speed is 20 mm/min,the rotation speed is 400 rpm,and the lower pressure is 0.1 mm,the weld with the bulk metallic glasses is the best.The welding process parameters were welded at preheating temperatures of 100°C,200°C,and 300°C.and it was found that the amorphous welded j oint was successfully obtained when the preheating temperature was 100°C.In addition,the bulk metallic glasses undergoes crystallization at the preheating temperatures of200°C.and 300°C,and the order of the precipitated phases during it s crystallization is:amorphous?Cu₁₀Zr₇+Cu Zr₂+amorphous?Cu₁₀Zr₇+Cu Zr₂+Al₂Zr₃+Amorphous.Fro m the Vickers hardness,it was found that semicircular shear band around the zonal indentation,and the hardness at the weld is higher than that of the non welded area.DSC of the welded specimens revealed that the number of semicircular shear bands around the Vickers indentation was proportional to the free volume of the amorphous alloy specimen.Finally,we performed nano indentation tests on the amorphous welded joint and the cast sample.It was found that during the loading stage,the viscoplastic deformation of the amorphous welded joint specimen was less than that of the cast samples;Moreover,with the increase of loading rate,the viscoplastic deformation decreases at loading stage.In the hold phase,the creep displacement increases correspondingly with the increase of the loading rate,and the creep displacement after welding is less than that before welding.Fitting by EVEV model shows that the creep compliance of amorphous welded joints is less than the creep compliance of as-cast samples;there are two peaks in the retardation spectrum,and the delay spectrum of amorphous welded joints is two.The peaks are sharper than the as-cast specimens,indicating that there are two different types of relaxation processes inside the two specimens,and the relaxation process at the welded joints is more severe than that of the as-cast specimens.