Time-resolved X-ray imaging of solidification cracking for Al-Cu alloy at the weld crater

Abstract

Synchrotron X-ray imaging was used to study the microstructural evolution of solidification cracking for an Al-Cu alloy at the weld crater without applying an external force. The distributions of strain, strain rate, Cu concentration in the liquid and solid fraction during solidification were evaluated using a quantitative analysis of the image sequences. The initiation of solidification cracking was identified by the combination of the radiographic images and the measurement of the transmitted X-ray intensities. The initial crack occurred at the local tensile strain of 1.4%, local tensile strain rate of 0.34 s−1 and solid fraction of 81% in the center of weld pool where columnar dendrites growing from the opposite sides impinged in the parallel to the growth direction. The solidification cracking evolved along the liquid film via the coalescence of small initial cracks. In addition, the solidification cracking propagated to the interdendritic region through the triple junction where the high shear strain was localized. Both the tensile and shear stresses contributed to the cracking formation at the weld crater.