Impression creep behavior of different zones in friction stir welded ZE41 magnesium-rare earth alloy

Abstract

The ability of the impression tests to obtain localized creep data is exploited to study the creep behavior of the friction stir welded (FSWelded) joints made on 15 mm thick plates of ZE41 magnesium alloy. The three zones of the FSWelded joint, Nugget zone (NZ), Thermo-Mechanically Affected Zone (TMAZ), and Heat Affected Zone (HAZ), were divided into ten different parts and their local steady-state creep behavior was studied. Impression tests were carried out at stresses of 350, 400, and 450 MPa and temperatures of 513 K, 533 K, and 553 K. Optical and SEM micrographs were used to study the microstructure of various zones of the FSWelded joint both before and after the impression test. Power law was used to obtain the creep exponent (n) and activation energy (Q) for different parts of the weld and the weld zones as a whole. The creep exponent was found to vary from 3.6 to 10.45 and the activation energy from ∼81 kJ/mol to ∼226 kJ/mol. The creep rate was found to vary between the upper and lower surfaces of the weld zones and also among the advancing and retreating sides. The weak zone was found to be temperature-dependent and varied when the temperature or stress changes. Within the parameters studied, the lower part of the TMAZ on the AS showed poor creep resistance at a lower temperature, while the lower part of TMAZ on the RS showed poor creep resistance at higher temperatures. The upper part of the TMAZ on the RS showed better creep resistance throughout the range of temperatures studied. To rationalize the high n and Q values, the threshold stress approach and the Eyring relationship were adopted. Using the threshold approach, creep exponents of 4.87 and 5.17 were obtained for TMAZ and HAZ, respectively. Using Eyring hyperbolic sine relationship, single activation energies of 143.5 kJ/mol and 261.5 kJ/mol were obtained for TMAZ and HAZ, respectively. Based on the obtained n and Q values, it is proposed that two mechanisms, namely climb and cross-slip of dislocations, are operative in the creep of the ZE41 FS weld joint. However, the dominant/rate controlling mechanism could not be arrived at conclusively.