Special Welding Behaviors Of A Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀ Bulk Metallic Glass

Due to the limited glass formation ability, Ti-based alloy can be only cast into glassysamples with small size via conventional casting process. In the present dissertation,Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG samples with small-size have been successfully welded intolarge-size Ti-based BMG by different welding methods. The microstructure andmechanical behaviors of welded BMG samples have been systematically investigated.The underlying mechanism for the maintenance of a glassy phase in the welded BMGsample has been also discussed.The microstructures and mechanical properties of BMG samples welded by variousprocessing parameters have been investigated, based on the rapid heating and coolingcharacteristics of laser welding. The results indicated that a joint with fully amorphousfeature could be achieved using a welding speed of10m/min and a laser power of3.5kW. The sound joint exhibits a high tensile strength,93%of the base alloy,demonstrating a perfect bonding. Well-developed vein patterns, typical of the fracturefeature for amorphous metals, were observed over the whole fractured surfaces. For thecase of the samples welded from other welding parameters, crystalline phases wereobserved both in weld fusion zone and heat affected zone. The critical heating ratekeeping Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG without crystallization during the heating processwas calculated. The calculated critical heating rate was much lower than the heating rateexperienced during laser welding, suggesting that no devitrification occurs duringheating process for Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG. The cooling rate in the weld fusion zonehas been further calculated to be780K/s, much larger than the critical cooling rate ofcast BMG sample. This phenomenon can be attributed to the fact that that the laserwelding was carried out under atmosphere condition. Thus, the introduction of oxygencould seriously suppress the formation of amorphous phase, leading to the highercritical cooling rate for glass formation during laser welding.Temperature fields during laser welding have been studied by numericalsimulations, in order to further discuss the mechanism of successful welding of theTi₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG. Heat source model with the cylinder and Gaussian wasselected. Material properties were detected as temperature-dependent functions, afterconsidering the effect of convection and radiation on heat transfer. Calculation resultsagree well with the experimental data through comparing of the weld width and poolshape, confirming the accuracy of heat source model. Based on the model, thetemperature fileds of the Ti-based BMG at different parameters were calculated. Thecooling rates measured around the fusion line, and near the weld fusion zone accordwell with the calculation values. For heat affected zone, the heating rate of sample was also much larger than the critical heating rate.Friction welding of Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG rods have been performed, utilizingthe unique superplastic deformation feature within supercooled liquid region of BMGs.Joint with fully amorphous structure can be obtained by selecting proper friction timeand rotational speed under constant upsetting force and upsetting time. The glassy jointobtained exhibits a high tensile strength （1580MPa）,90%of the base alloy. Withincreasing the rotational speed, numerous crystals embedded in the interface wereobserved under different friction times, the crystal phases are indentified as β-（Ti, Zr）.The influence of welding parameters on the formation of protrusion has been studied.The friction time and upsetting force exert no influence on the thickness of theprotrusion, whereas the rotational speed has great influence on the thickness of theprotrusion. However, the volume of protrusion is proportional to the friction time. Themechanism for the maintenance of a glassy phase in joint has been interpreted. Due tothe decrease of the viscosity resulting from the deformation of supercooled liquid andthe rotational speed, the temperature of the interface can be self-controlled so as not toover-heat. Based on the analysis of the temperature distribution during the frictionwelding, an equation mentioning the critical friction time for retaining amorphousstructure has been established. The experimental results varify the reasonability of theequation. Under constant rotational speed, the joint with fully amorphous structure canbe obtained when the friction time is lower than the critical friction time.Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀metallic glass has been welded by using resistance spotwelding. Under5kA welding current and2welding cycles, the nugget zone was foundto retain its original amorphous structure. However, crystals embeded in the matrix wereobserved in heat affected zone. Moreover, fully crystallized Ti-based alloy sheets wereprepared and then welded under5kA welding current and2welding cycles. Amorphousfeature was also obtained from the WFZ of the crystallized Ti-based alloy sample,during the tensile-shearing tests, the fracture occurs following three distinctive fracturemodes: interfacial failure, partial failure, and button pullout failure. Meanwhile, thefracture mainly took place via partial failure mode. The mechanical performance of thejoint was closely related to the nugget size and crystallization volume fraction.Moreover, the critical cooling rate for preserving the amorphous phase has beenobtained by calculating the cooling rate around the weld line.