Accelerative effects of diffusion bonding of recrystallization with reversion of deformation-induced martensite in SUS304

Diffusion bonding at low temperature is needed to improve the functional and mechanical properties of diffusion-bonded products. For example, metal micro pumps fabricated by diffusion bonding of stacked metal foils experience performance loss caused by a loss of yield strength due to grain growth during bonding. Accelerating the recrystallization by using pre-distorted base metals effectively decreases the diffusion-bonding temperature of metal materials. Pre-distorted SUS304 contains deformation-induced martensite before bonding, and then during diffusion bonding recrystallizes with reversion of this martensite. This study evaluates the effect of bonding time on bonding state and on recrystallization with the reversion of SUS304 that has deformation-induced martensite, and discusses the relationship between bonding behavior and microstructure change. Results revealed that the bonding area (evaluated based on line profile of pixel intensity on the bonding interface in a cross-sectional image) exceeded 70% in a specimen bonded at 973 K bonding temperature, 50 MPa bonding pressure, and 60 s bonding time. Results also showed that a bonding time longer than 900 s was necessary to obtain bonding strength comparable to that of the base material. On the other hand, analysis of the bonding area of pre-distorted SUS304 that had severely deformed austenite revealed recrystallized grains at the bonded area. Recrystallization of SUS304 that has deformation-induced martensite starts homogeneously due to reversion of this martensite. Therefore the most likely effect of reversion in diffusion bonding is that diffusion is promoted homogeneously in the bonding interface because recrystallization starts homogeneously during the bonding process.