Non-cryatallization Controlling Of Zr-based Bulk Metallic Classes During Welding Process

Bulk metallic glasses have excellent physical and chemical properties, which results in important value in scientific research and engineering applications. However, bulk metallic glasses have small dimension due to the limited glass formation ability. Furthermore, the shear brand in bulk metallic glass trends to expand rapidly along a single direction and then leads to brittle failure of material. These problems greatly limit the practical application value of bulk metallic glasses. In order to solve these problems, we investigate the welding process for Zr-based bulk metallic glass using laser welding and diffusion bonding methods in the present dissertation. The main contributions are as follows.The welding mechanism and theoretical basis were analyzed for laser welding of bulk metallic glasses, then the welding modes of low speed and high speed were used to join the Zr41Ti14Cu12Ni10Be23and Zr55Cu30Ni5Al10bulk metallic glass respectively, and the successful welding joints were obtained. The inspection results show that the high welding speed is helpful to avoid the crystallization or obtain nanocrystals based on the full penetration. First time, we used laser beam to welding Zr-based amorphous alloy and pure zirconium. Due to the rapid cooling, the liquid metals do not mix sufficiently. However, the microhardness of material in the welding fusion zone is as high as the parent metal of bulk metallic glass.First time, we welded the annealed Zr55Cu30Ni5Al10plates to simulate the laser welding of multi-layer amorphous micro-parts. The morphology, micro-structure and mechanical property were examined. The results demonstrate that the joints are welded together successfully without visible defects and the annealing conditions affect the inner structure of joints obviously. The proper annealing pretreatment is helpful to inhibit the crystallization and obtain nanocrystals during laser welding, also improve the welding quality and mechanical property. For Zr55Cu30Ni5Al10bulk metallic glass, the proper anneling temperature should be chose near the glass transition temperature, and the annealing time is approximately equally to the hot-embossing time of amorphous part.We used finite element method to simulate the laser welding of Zr-based bulk metallic glasses, and then obtained the distributions of temperature field and thermal cycle curves. The continuous heating transformation curve from Kissinger fitting was combined with the thermal cycle curves to predict the crystallization of laser welding of bulk metallic glass. The prediction results are consistent with the welding experiments, In addition, the simulation results prove that the material in the heat affected zone is easier to crystallize than material in welding fusion zone. By calculating the critical heating rate of heat affected zone, it can be found that the crystallization occurs mainly in the cooling process, which is consistent with the simulations. It is concluded that the crystallization prediction is helpful to optimize the welding process and guide the laser welding of bulk metallic glass.The diffusion welding method was used to join the Zr-based bulk metallic glass and crystallization metal for fabricating composite material. The Zr55Cu30Ni5Al10bulk metallic glass and Al alloy were connected together successfully and the joints with two layers and three layers were obtained. First time, the diffusion bonding between Zr-based bulk metallic glass and copper was carried out in steps using Al alloy as interlayer, and the welding samples were obtained. The examination results show that all samples display fully bonding and no welding defects are observed. The amorphous material in welding joints maintains its glassy nature except very few nanocrystals embedded in the glass matrix. At last, a method for fabricating multi-layer amorphous micro-parts by diffusion welding was proposed. When validated, the big part can duplicate the pattern of small part well in the supercooled liquid region. It is confirmed that the diffusion welding is a feasible approach to fabricate micro-scale complex amorphous parts under proper conditions.