Effect of Inhomogeneous Distribution of Alloying Elements on Integrity of Al-2.1 wt.% Mg Alloy Tubes and Welds

Abstract

Al-2.1 wt.% Mg alloy is an important nuclear research reactor material. The tubular products of the alloy are usually made by port-hole die extrusion process and several quality control steps are involved during fabrication to assess the quality of the weld joint. The paper describe two cases of  failures of the alloy during fabrication. In one case, a thin wall tube failed during hydro-test at the weld-line while in another, a through-wall crack is observed at the heat affected zone (HAZ) of the weld joint between the thin tube and the tie-plate. In both the cases, the fracture surfaces have the appearance of brittle failure without any gross plastic deformation. Visual inspection, liquid penetrant testing, optical microscopy (OM), scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), Electron back scattered diffraction (EBSD) and micro-hardness measurement have been carried out for root cause analysis of the failures. EDS analysis at low KV indicates high Magnesium (Mg) and Silicon (Si) content at the fracture surface in both the cases. In the first case, segregation of these alloying elements at the weld-line in the port hole extruded tube has been observed. In the second case, the microstructure at the HAZ location shows grain boundary precipitation of low melting  phase containing Mg, Si and Fe. Presence of Mg and Si reduce the solidus temperature of the grain boundary phase, which is responsible for HAZ liquation leading to failure under tensile stress during cooling.