Investigation Of Diffusion Bonding Of Surface Self-Nanocrystallined Stainless Steel And High Strength Aluminum Alloy

It’s difficult to achieve an effective joint in diffusion bonding process due to the large difference of physical and chemical properties between stainless steel and aluminum as well as the Fe-Al intermetallic compound generated in the connection had bad influence on mechanical properties of joints. Thus, this study seeks to improve the overall performance of joints by surface grain refinement adopting surface self-nanocrystalline of stainless steel and aluminum before diffusion bonding.Surface mechanical attrition technology (SMAT) and supersonic fine particles bombarding(SFPB) were used respectively for 304 stainless steel and 7A04 aluminum alloy. A variety of test methods were used to characterize structure on the surface, and results show that nanocrystals had been prepared in a certain thickness from the surface of stainless steel and aluminum alloy. In order to determine the operating temperature of nanocrystals so that technological parameters of diffusion bonding can be worked out, isothermal annealing at different temperatures were carried out to study their thermal stability. Using optical microscopy (OM), X-ray diffraction (XRD) and microhardness to study thermal stability of two metals. Study on organization and performance of joints were carried out through scanning electron microscopy, microhardness, shear strength and X-ray diffraction analysis.Results show that after surface treatment, a thickness of 100μm and 20μm of nanocrystals were obtained on 304 stainless steel and 7A04 aluminum alloy (solution-aged) respectively. Grain refinement mechanism of stainless steel are comprehensive effect of mechanical twinning, dislocation motion and strain-induced martensitic transformation, but for the aluminum is the result of dislocation motion. Surface self-nanocrystalline produces the effect of grain refinement as well as the dramatical increase of grain boundaries and defect density which can play an important role in diffusion bonding process.Thermal stability analysis shows that 304 stainless steel after surface treatment can be maintained in nanometer below 600℃ while aluminum can only be maintained within the range of 1OOnm below 500℃.The vacuum diffusion bonding test of surface self-nanocrystalline stainless steel and aluminum were carried out at a temperature of 400℃,425℃,450℃,475℃ and 500℃ for 0.5h-4h under the pressure of 6MPa and degree of vacuum at about 1Pa. Shear strength test were conducted on samples after connection, and shear strength rises first and then falls with increase of temperature and connection time. The maximum appears when connecting at a temperature of 450℃ for 2h with strength of 55MPa. From the analysis of phase composition after different processes, it can be found that the resulting metal compound are FeAl, Al3Fe, AlFe3 and Fe2Al5In summary, surface self-nanocrystalline can effectively improve the diffusion coefficient and lower activation energy of atoms, and enable efficient connection of two materials at a lower temperature. Finally improve the mechanical properties of connections.