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LBW/SPF/DB Combined Processing and Microstructure of TA15 Titanium Alloy Four-Layer Sandwich Structure with Square Grid

The process combining laser beam welding, superplastic forming, anddiffusion bonding (LBW/SPF/DB) was performed to fabricate the TA15 titanium alloy sandwich structure with square grid. The microstructure of different regions before and after superplastic forming was also analyzed. The results showed that the laser welded joints of core structure own good superplastic formability under the condition that the laser welding parameters were optimized. The microstructure near the fusion line is columnar crystal and inside the columnar crystal the acicular martensite appeared. The heat-affected zone is relatively narrow and with no obvious grain growth. At 925°C, under the condition thatthe loading-time curve was optimized, the external outline of the LBW/SPF/DB formed hollow four-layer sandwich structure is clear and the strengthened ribs areupright. It shows good diffusion bonding quality between the ribs and panels. And the bonded ratio between the upright ribs which rely on each other is above 90%. After superplastic forming, the grain size of the TA15 workpieceincreases but remains equiaxed grain. The grain size of laser welded joints also significantly grew up, and the heat-affected zone almost disappeared in this zone. The typical cast microstructure of rapid cooling turned to be the deformed microstructure, creating a better overall performance of the two-state organizations.