Study of fatigue crack growth of al 6061-T6 welds obtained by gas metal arc welding along longitudinal direction

The behavior of fatigue crack growth(FCG) plays a vital role in the design and performance of modern structural materials like Al 6061-T6 alloy. Al 6061-T6 is an alloy of Al-Mg-Si which is artificially aged under T6 condition. In the present study, Al 6061-T6 plates of 6 mm thickness welded by a gas metal arc welding(GMAW) technique using an ER-5356 as filler wire. The microhardness profile was obtained through Vickers microhardness test. The pre-qualification tests such as X-ray radiography, tensile, and bending tests were carried out on base metal (BM), heat-affected zone (HAZ), and weld zone (WZ)along rolling direction to study the mechanical behavior. The fatigue crack growth rate (FCGR) tests on the three zones were carried out as per ASTM E-647 standards under stress ratios of 0.1, 0.3, and 0.5. The WZ possessed highest FCGR compared to BM and HAZ. The lowest fracture toughness (K c ) was found for WZ, which is due to fusion of filler wire with BM. The microstructural transformation in HAZ from very fine needle shape β'' precipitates to coarse bar shape β' precipitates produced by the thermal effect during the welding process resulting in moderate FCGR compared to WZ and BM. Mechanical strength and microstructure regeneration during the welding thermal cycle are the two important factors that affect the FCGR in all zones. Fractographic analysis was carried out using a scanning electron microscope (SEM) to study the crack initiation, final fracture, nature of the fracture, and causes of failure under various stress ratios.