Papers by Author: Ai Ping Wu

Abstract: The tensile ductility of the Ti-23Al-17Nb (at.%) alloy laser welded joints is very low at 650°C, so the embrittlement of the joints at high temperature was investigated. The metastable coarse B2 phase of the as-welded metal transformed into coarse O phase in the weld zone when the joints were heated up to 650°C. This type of phase transformation was based on shear transformation. Coarse orthorhombic O phase has lower ductility compared with body-centered cubic B2 phase. Meanwhile, the shear transformation from B2 to O phase could induce stress-strain concentrations in the grain boundary, which seriously weakened the grain boundary of the columnar grains and led to the brittle fracture characteristics in the weld zone during tensile test at 650°C.

Abstract: Invar 36 alloy is increasingly used as a structural material for manufacture of liquefied natural gas (LNG) transporters and storage tanks. However, the conventional arc welding of Invar 36 alloy has high susceptibility of hot-cracking. As a high-energy-beam welding process, laser welding could be effective for producing defect-free Invar 36 weld. In the present study, defect-free Invar 36 weld was successfully produced by Nd:YAG laser welding. The microstructure and mechanical properties of weld were also tested.

Abstract: To improve the joining efficiency of Bi-Sr-Ca-Cu-O ( BSCCO) superconducting tapes, a new diffusion bonding technology with a direct uniaxial pressing at high temperature was developed to join 61-filament tapes. It was observed that bonding parameters such as bonding pressure and holding time, significantly affected the critical current ratio (CCRo). A peak CCRo value of 89 % for the lap-joined tapes was achieved at 3 MPa for 2 h when bonding temperature was 800 °C. Compared with the conventional diffusion bonding technology, this new technology remarkably shortened the fabrication period and improved the superconductivity of the joints. The bonding interface and microstructures of the joints were evaluated and correlated to the CCRo. An uniaxial pressing at high temperature was beneficial to interface bonding, and there was an optimal pressure value for the CCRo.