Interface Microstructure and Strength of TLP Bonded Iron and Steel

Commercially pure iron (Fe) and stainless steel 321 (SS 321) were transient liquid phase (TLP) diffusion bonded using Cu and/or Au-12Ge interlayers. The joining pressures, temperatures and times were investigated. Au atoms appeared to diffuse back at the joining interface during slow cooling from the joining temperature. A faster cooling method prevented the back-diffusion of Au atoms from interior to the joining interface. Residual interlayer disappeared when faster cooling methods such as air cooling, water cooling or water quenching were applied. However, microcracks appeared in the joint centerline if water cooling or quenching was applied. Au-rich particles also appeared in the joint microstructure. No microcracks appeared for the samples cooled in air from the bonding temperature. The highest strength of the TLP diffusion bonded Fe reached almost same as the normalized Fe. The maximum tensile strengths (UTS) obtained for the bonded Fe were 291 +/- 2 MPa using a Cu interlayer and 315 +/- 4 MPa. The maximum strength found was 387 +/- 4 MPa for TLP diffusion bonded steel using an Au-12Ge interlayer. The maximum strength for steel was obtained for the samples bonded at 1050 degrees C for 20 h in vacuum and cooled in air. The maximum strength found was 387 +/- 4 MPa which is 70.4 % of the base alloy strength.