Continuous Drive Friction Welding Of Titanium Alloy To Steel

Titanium alloys and TiAl-based alloys have been widely applied in aerospace, shipbuilding, automotive and chemical industries, due to their properties such as high specific strength, high melting point, excellent corrosion resistance, and light specific weight, good stiffness, good high-temperature mechanical properties, respectively. Nevertheless, the extensive applications of titanium and its alloys are limited owing to their high cost. However, dissimilar metals joining of titanium or its alloys to steels can reduce cost and improve efficiencies. This paper investigated friction welding processing of titanium alloy or TiAl alloy to steels, and the mechanical properties and microstructure of the joint after post-weld heat treatment (PWHT) were examined.It was found that the tensile strength of the TC4alloy/40Cr steel joints reached766MPa and failure occurred in40Cr steel base metal in as-welded state. However, after post-welding heat treatment (PWHT) at600℃for0.5h and2h, the tensile strength of the joints decreased to675MPa and735MPa, respectively, and fracture happened through the interface with quasi-cleavage features. The bending angle of specimen was nearly9.6°in as-welded or10.6°after PWHT for0.5h, but it was improved to32.5°after PWHT for2h. The martensitic and coarse ferrite generated near the joint interface in as-welded state. After PWHT, the tempered sorbite formed near the interface, and TiC brittle phase appeared at the interface when holding time was0.5h, which deteriorated the joint strength. After PWHT for2h, carbides diffusely distributed around the interface, the grains were fine, and no TiC phase formed at the interface, improving the mechanical properties of the joint. The microhardness on the interface in as-welded state was higher than that after PWHT, indicating that the decrease of microhardness around the interface could be accompanied by degradation of the tensile strength but improvement of the ductility of the joints.For the joining of y-TiAl alloy to40Cr steel, the martensitic transformation occurred on steel side and brittle TiC phase formed near the interface due to C agglomeration, which degraded the joint strength in as-welded state. The microhardness at the interface was consequently higher in as-welded state than in PWHT state due to the existence of brittle phases and martensite. Feathery structure and widmanstatten generated near the interface on TiAl alloy side. After PWHT at580℃and630℃for2h, the tensile strength of the joint increased from86MPa in as-welded state to395MPa and330MPa, respectively. The heat-treated specimen fractured with quasi-cleavage features through the zone lmm away from the interface on TiAl alloy side, but the as-welded specimen failed through the interface. After PWHT, carbon diffusely distributed at the interface, avoiding the formation of TiC Tempered sorbite formed at steel side, and the fine grains distributed in lamellar structure, which both enhanced the joint strength.It was found that solid joint between TiAl alloy and42CrMo steel could be obtained without adding interlayer. After PWHT at580℃for2h, the tensile strength of the joint reached405MPa, and the fracture happened through the TiAl alloy substrate with quasi-cleavage features. However, the as-welded joint fractured when prepared the specimens. The phase constitution was measured by X-ray diffraction. It was found that TiFe2, TiAl, and small amount of TiC brittle phases formed at the interface, and the thickness of interfacial layer was2-5μm. The precipitated phases were1μm on average and distributed discontinuously at the interface.