Physical Metallurgical Mechanism Of The Zr Based Bulk Metallic Glasses During Welding Process

Because of their high strengths, high resistances to wear, high corrosion resistances, excellent soft magnetic properties and low magnetic losses, the amorphous alloys have been applied in electronic industry, mechanical industry and chemical industry. With the development of the amorphous alloys researches, their applied prospects should be more extensive. The joining of the amorphous alloys should be inevitable when they are used in engineering, especially the welding of the amorphous alloys. In this dissertation, the crystallization mechanics under the action of the thermal cycle, the methods of depressing the crystallizations and the reasons for the joining of the bulk metallic glass with friction method were studied. All of these researches are fundamental and important to the amorphous academic and engineering values.First time, the laser welding technology was used to joint the bulk metallic glassy alloy Zr₄₅Cu₄₈Al₇. The results of the micro-focused X ray diffraction indicated that the alloys in the bead and heat-affected zone (HAZ) kept amorphous and successfully jointed the bulk metallic glassy alloy Zr₄₅Cu₄₈Al₇ when the output power of 1200w, the focused spot size of 0.3mm, the welding speed of 8m/min and specimens were shielded with the argon gas. The result of DSC indicated that the thermal properties of the joint were the same with those of the parent materials. Based on the compound heat sources, the temperature field in the HAZ and the molten zone were analysed when the welding speed was 8m/min. In the HAZ and molten zone, within 0.5s, the temperature dropped to 369°C well below the glassy transition temperature (Tg=436°C) and the whole alloy in joint kept amorphous.First time, the stationary point heat source was used to heat up the bulk metallic glassy alloy. The microstructures and the morphologies of the grains in the joint, the evolutions of the construction of the joint and physical metallurgical principles were systematically analysed. The results indicated:(1) The crystallization in HAZ is about the thermal stability of the bulk metallic glass and the crystallization in molten zone is about the glass formation ability. One hand, the critical heating rate, which is about inhibiting the crystallization reactions during the bulk metallic glasses is heated up, is much faster than the critical cooling rate which is about inhibiting the crystallization reactions during the cooling process. On the other hand, in the quenched bulk metallic glass, the under-critical clusters and nucleus exist. Therefore, with the same welding conditions, the alloys in the HAZ were easier to crystallize than those in the molten zone. The key point of successfully laser welding of the bulk metallic glass is to depress the crystallization reaction in the HAZ.(2) The temperature of the maximum growth rate of the grains in the bulk metallic glass is much higher than that of the maximum nucleation rate and the different heating rate at the different point along the radius direction lead to the different nucleation rates and different growth rates at different points. When the actuation duration of laser point heating source on the bulk metallic glass was 2s, the results indicated that the size of the grains in heat-affected zone became smaller and smaller from the fusion boundary to the material. With the decrease of the actuation duration, the size of the grains in heat-affected zone became almost same. When the actuation duration was 0.04s, the alloys in the heat-affected zone kept amorphous.(3) Because of the strong ability of the bulk metallic glasses to depress the growth of the grains, the growth of the columnar grains were depressed. With the decrease of the actuation duration, the sizes of the columnar grains in the bead decreased. When the actuation duration was equal to 0.08s, the columnar grain has been vanished in the molten zone. As the actuation duration was equal to 0.06s, the fusion boundary was vanished in joint.Because the maximum heating rate of the DSC is 500K/min and the critical heating rate of the mostly bulk metallic glasses is larger than 500K/min, therefore, the continuous heating transition phases diagram of the bulk metallic glasses are difficultly obtained. Because the thermal cycle of the point at the edge of HAZ and the parent materials should be tangent with the CHT of the bulk metallic glass, a feasible way was used to obtain the CHT of the bulk metallic glasses and a difficult problem in the bulk metallic glasses field was resolved.The uniform deformation of the bulk metallic glass happens in the higher temperature (>0.70Tg, Tg the glassy transition temperature) and the deformation expresses the great super plastic. In this dissertation, the friction welding technology was used to joint the bulk glassy alloy Zr₆₅Cu_12.5Al_7.5Ni₁₅ which has large super plastic temperature interval. One specimen to be welded was rotated at the rate as fast as 2500rpm. The pressure of the friction and upsetting was 100 MPa and 150 MPa, respectively. The times of the friction and upsetting were 0.5s and 5s, respectively. The results indicated that the temperature of the contacted face was throughout under the crystallization temperature, the micro-focused X diffraction testified that the alloy in joint kept amorphous and the DSC result shown that the thermal properties of the joint were the same with those of the parent materials. The bulk glassy alloy Zr₆₅Cu_12.5Al_7.5Ni₁₅ was successfully welded with friction welding technology.