Fatigue crack growth in Al-Zn-Mg (7075-T651) welds obtained by modified indirect and gas metal arc welding techniques

The modified indirect electric arc welding (MIEA) and the conventional gas metal arc welding (GMAW) processes were used to weld 7075-T651 aluminum alloy plates. The fatigue life behavior was studied for three different zones in the welded joints (base metal, heat affected zone and weld metal). Compact-type (CT) specimens manufactured from the 7075-T651 aluminum alloy welded plates were subjected to fatigue tests with constant amplitude cyclic loading. The CT specimens corresponding to MIEA weld metal shown the largest fatigue life (number of cycles to reach the stable crack propagation), which was four times larger than the weld metal obtained by GMAW. This improvement was attributed to the high tensile residual stresses induced by the application of three welding beads of the GMAW process in contrast with the stresses induced by the application of only one welding bead in the MIEA. The tensile residual stresses were of no consequence in terms of fatigue life for the heat affected zone (HAZ). At this zone, the microstructural transformation (soft zone formation in the HAZ) induces a similar fatigue life behavior. For the weld metal produced by the MIEA welding process, the fatigue crack growth tends to increase in comparison with the GMAW, whereas for the HAZ no difference was detected. Fracture surfaces of all materials were analyzed by using scanning electron microscopy to correlate the fatigue crack growth condition with the striations formation.