Effective diffusion coefficient for Cu in steel joined by transient liquid phase bonding

Seamless carbon steel tubes were joined by the transient liquid phase (TLP) bonding process using Cu foils as interlayers. Bonding was performed at 1300°C for 7min with an applied uniaxial pressure of 5MPa. The completion of isothermal solidification was not systematically achieved along the joint, leading to the presence of athermally solidified liquid (ASL). Consequently, the ability to predict the time to complete isothermal solidification — and therefore its kinetics — is of great interest. For this purpose, a one-dimensional model using the finite element method was employed to simulate the TLP bonding. In particular, regions where the completion of the process was not achieved provided a source for the effective diffusion coefficient (Def) for the bonding process. By knowing Def, different cases were considered within the proposed model, from the remaining time to complete the bonding process for an observed ASL, to the maximum liquid gap that can be isothermally solidified for a selected holding time. The validity of the utilization of Def was confirmed by analysing the diffusion kinetic regime of Cu in steel.