| The so-called thermohydrogen processing (THP) is a technique in which hydrogen is used as a temporary alloying element in titanium alloys by controlling the microstructure and phase structure to improve the mechanical working properties. Since the appearance of THP, it has gained considerable interests. A large number of researches have been done including the basic theory of hydrogenation and dehydrogenation, the enhancement of plastic working properties and different THP methods used to modify the microstructure. However, little work has been reported about diffusion bonding of hydrogenated titanium alloys. The investigation about diffusion bonding with or without interlayer of hydrogenated TC4 titanium alloys was carried out. The phase structure and microstructure evolution of the substrates were researched, the interface structure of diffusion bonding with or without interlayer was determined, the effect of processing parameter on interface structure was analyzed, the nonisothermal dehydrogenation kinetics of the TiH2 powders and the TC4 alloy with high hydrogen content was established, and the positive effect of hydrogen on diffusion bonding was investigated.The phase structure and microstructure evolution of the substrates were researched. The platelike substrate before hydrogenation had a (α+β) dual phase structure with rolling direction. With the increase of the hydrogen theβH andδphases gradually formed, and the rolling direction gradually disappeared with the hydrogen content increasing. The needleα′martensite appeared when the hydrogen content was up to 0.4wt.%. The rodlike substrate before hydrogenation had an equiaxed (α+β) dual phase structure. When the hydrogen content was 0.15wt.%, the primaryαphase started to nucleate and grow up in the boundary ofβphase and the hydrogen inβphase led to the formation of lamellar (βH+α). The high hydrogen content resulted in the formation of (α+δ) andα′phase when the hydrogen content was 0.3wt.%.The diffusion bonding without interlayer was carried out, and the interface structure and the effect of processing parameter were analyzed. The voids were only observed in the interface, and the quantity of the voids gradually decreased with the increase of bonding temperature, holding time, bonding pressure and heating rate. Moreover, the bonding temperature decreased by 150℃under high hydrogen content at the fast heating rate. The phases with hydrogen sequential decomposed at low heating rate, and the elements in alloy sufficiently diffused. However, the phases with hydrogen decomposed at the same time at fast heating rate, and the elements in alloy insufficiently diffused and the fast cooling rate led to the formation ofα′phase. The diffusion bonding with different interlayers was carried out, and the interface structure was analyzed. The reaction layers in the interface structure with Ni interlayer was theβ(Ni) phase and the Ti2Ni/TiNi/TiNi3 intermetallics, the reaction layers in the interface structure with Al interlayer was the TiAl3 intermetallics, and the diffusion layer in the interface structure with Nb interlayer was the solid solution.The thickness of layers gradually increased with the hydrogen content increasing when bonded at the same processing parameter. But the interface structure with Ti interlayer was only composed of the voids, and the quantity of the voids gradually decreased with the increase of hydrogen content at the same processing parameter. The nonisothermal dehydrogenation of hydrogenated TC4 alloys happened between 600℃and 950℃, and the hydrogen in the TC4 alloy with high hydrogen content mainly located inδtitanium hydride. The nonisothermal dehydrogenation kinetics of the TiH2 powders was established. The results show the nonisothermal dehydrogenation occurred in a four-step process: TiH2→TiH1.5+H2↑;δ→βH+H2↑;βH→βH+H2↑andβH→αH,βH→αH+H2↑. Also the dehydrogenation kinetics of the TC4 alloy with 0.5wt.% hydrogen was established. It consisted of TiH1.5～2→TiH1.5+H2↑,δ→βH+H2↑,βH→βH+H2↑andβH→αH+H2↑. Moreover, the positive effect of hydrogen on diffusion bonding was investigated. The enhancement of diffusion bonding quality was attributed to the increase of diffusion coefficient and the improvement of creep deformation and distortion. The fast heating prevented the most hydrogen escaping during rise in temperature, which helped to improve the diffusion bonding quality. In addition, the hydrogen in titanium alloys arouses the interdiffusion enhancenment for the diffusion bonding joints with reaction layer.
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