Vacuum Brazing Of Titanium Alloy To Austenitic Stainless Steel

Welding structure of Titanium and stainless steel dissimilar metals are widely used in aerospace, shipbuilding and so on, but the physical, chemical and mechanical properties of dissimilar metals are different, a brittle compound will be formed in the interface, which affected the quality of welding seriously. The vacuum brazing of TA31 and A304 with the brazing fillers of BAg72Cu, BNi-2 and BA188SiMg foil was researched. Microstructure morphology, element diffusion, mechanical property and fracture morphology under different process parameters were analyzed by means of optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), micro-hardness, tensile testing and so on. Preliminary following results were obtained:(1) Using silver-based solder, the joint with dense organization could be obtained in brazing TA31 and A304, the central area of brazing seam is mainly silver-based solid solution, diffusion layer of the interface generated TiCu、TiCu2 inter-metallic compounds. With the increase of holding time, the width of brazing seam and the thickness of diffusion layer increased. Hardness of joints is low in the central of brazing seam, while that of joints is becoming higher and higher gradually from center to interface, which is related to Intermetallic of TiCu and TiCu2. Brazing temperature at 850℃, holding time extended from lOmin to 30min, a tensile strength from 50MPa up to 110MPa, and fracture morphology were the mixed fracture of tear and dimple.(2) Metallurgical bonding can be occurred between BNi-2 brazing filler and both sides of the base material, and generate a certain thickness of diffusion layer, more brittle intermetallic compounds of TiFe and Ti2Fe will be formed in the central of brazing seam. Hardness of the center of brazing seam with Nickel-based solder is significantly higher than both sides of base metal material, while that of joints is becoming higher and higher gradually from both sides of base material to the center, Mainly due to the generation of intermetallic compounds with Ti-Fe. Brazing temperature at 1050℃, holding time extended from 10min to 60min, a tensile strength from 32MPa up to 60MPa. Fracture morphology were the river-like pattern which have the characteristic of cleavage fracture, this was the typical brittle fracture.(3) With BA188SiMg foil as brazing filler metals, metallurgy reaction can be happened in the brazing interfacial zone, but the metallurgic reaction is inadequate, and there are more holes, gaps and loose defects. Mainly due to the larger solid-liquid contraction ratio of aluminum, liquid solder contracted after cooling, and formed a larger hole after brazing.Considering the quality of the joint and the mechanical properties and so on, selecting the BAg72Cu silver-based solder of the three brazing filler which brazing temperature at 1050℃, holding time at 30min, it was more suitable for vacuum brazing TA31 titanium alloy and stainless steel A304.