| In order to effective metallurgical bonding of dissimilar metal, diffusion bonding is processed at high temperature for long time. Resultly, thicker intermetallic compounds is yielded easily on the bonding interface, crystal grains grow significantly in the diffusion layer, and innerstress is large in the joint because of tremendous differences of physical, chemical and mechanical property, which deteriorated badly mechanical character of joint. For the sake of putting the axe in the helve, in the paper, surface self-nanocrystallization (SSNC) was apllied to synthesize nanostructured layer on the bonded surfaces of 0Cr18Ni9Ti stainless steel and TA17 titanium alloy bars by means of high energy shot peening (HESP), which aims at increasing atomic diffusion coefficient, reducing bonding temperature and restraining of brittle intermetallic compounds at bonding surface, betterment of microstructure in the diffusion layer, advancing of tensile strength. Because a large volume fraction of grain boundaries in nanostructure may act as fast atomic diffusion channels, and large number of nonequilibrium defects with high stored energy may reduce activation energy, which may increase atomic diffusion coefficient while diffusion bonding.After samples SSNCed, microstructures were characterized in the deformation layer, and thermal stability of nanostructure was put on test. Constant temperature and pressure diffusion bonding (CTPDB) and pressure and pulse pressure diffusion bonding (PPDB) were applied to produce SSNCed 0Cr18Ni9Ti stainless steel/TA17 titanium alloy joints. Microstructure observation, micro-hardness testing, scanning electron microscope (SEM) observation, energy dispersive spectroscope (EDS) and X-ray diffraction (XRD) analysis were employed to investigate the structure and performance of the joints.After SSNCed for 5min by means of HESP, surface nanostructured layer about 70μm and 50μm thickness were obtained on the stainless steel and titanium alloy ends respectively. Refinement mechanism of coarse grains for metal depend strongly on the lattice structure and the stacking fault energy while SSNCed. For 0Cr18Ni9Ti stainless steel, refinement mechanism is dislocation slip and twinning deformation and strain-induced martensitic transformation, and twinning deformation and dislocation slip for the titanium alloy. Stress and strain rate is reducing with increasing of depth to top surface, resulting that size of grains is increasing in the deformation layer and transition to that of matrix gradually.Surface nano-microstructures keep good thermal stability in the stainless steel and titanium alloy when annealing temperature is no more than 550℃and 650℃respectively. Furthermore, nano grains size of stainless steel and titanium alloy don't exceed 80nm and 100nm respectively even if annealed at 850℃for 30min.CTPDB was applied to produce joints of SSNCed TA17/0Cr18Ni9Ti in the temperature rang of 800～900℃under a uniaxial load of 8MPa in vacuum for 20min, and effective joints were formed. When bonding temperature was 850℃, the maximum tensile strength of joint was as high as 327MPa. At the same diffusion bonding technology, the value is higher above 60MPa than that of their conventional coarsegrained bonded joint.PPDB was applied to prepare joints of SSNCed TA17/0Cr18Ni9Ti in the temperature rang of 650～750℃in vacuum, and joints with certain tensile strength were achieved. For the first time, joint of titanium alloy/stainless steel was obtained at below 800℃in this paper. While heating rate before pulse and cooling velocity after pulse were 5℃/s, pulse frequency was 0.5Hz, the optimized parameters for PPDB below 800℃were gained as following: bonding temperature was 750℃, pulse pressure was 80～150MPa, pulse pressuring times was 400 cycles. The joint strength under the optimum condition was 262.0MPa.By means of PPDB, SSNCed TA17/0Cr18Ni9Ti was bonded at 850℃for 80s. The pulse pressure is 8～50MPa, pulse frequency is 0.5Hz and the cycles is 40times. The joint tensile strength of 384.0MPa was achieved. At the same diffusion bonding technology, the value also is higher above 60MPa than that of their conventional coarsegrained bonded joint.Microstructures of joints by mentioned above diffusion bonding technology were researched. The results showed that multi-microstructures, which are in turnγ-Fe, brittle intermetallic compounds,β-Ti andα-Ti from stainless steel side to titanium alloy side, were formed on the joint. The intermetallic compounds are mainly FeTi,Fe2Ti andσphases. Thickness of compounds is decreasing with reducing of bonding temperature. Relativing to conventional coarse-grained joints, the thickness of compounds in SSNCed joints are thinner at the same bonding technology. Research of joint fractures showed theβ-Ti beared principal tensile load while joints were tensile test, and the brittle intermetallic compound is the prime reason of fracture. Diffusion coefficient and activation energy of atoms in the diffusion layer were calculated, the results showed that the activation energy is far lower than that in coarse-grained samples, and the diffusion coefficients is larger than that in coarsegrained samples.In the paper, research results showed that application SSNC to diffusion bonding of titanium alloy and stainless steel decreased activation energy of diffusion atoms significantly, increased diffusion coefficient atoms, suppressing remarkably growth of intermetallic compounds, improved microstructures of joints, increased tensile strength of joints. With SSNC treatment before bonding, the temperature and time of diffusion bond is lowered, the performance of joint of is improved, and the negative effects of intermetallic compounds for mechanical behaviour of joint was reduced as possible as. Through this research, an improved technology for the bonding of dissimilar materials was developed.
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