Physically short and long-crack growth behavior of MIG welded Al-5.8%Mg alloy

Abstract

In this study, we investigated the physically short and long-crack growth behavior of Al-5.8%Mg alloy, a special hardened alloy used for welding of Al-5083 plates. Microstructural analysis on both longitudinal and cross-sectional plane of welded plates showed no crystallographic texture with nearly equi-axed grains of size 85 μm and gas porosities. Fatigue tests were done at 50 Hz using CT-specimens at different R-ratios (0.1,0.5 and 0.8) to characterize the long-crack growth behavior. Compliance method was used for crack-closure measurements. An increase in crack growth rates and reduction in threshold ΔK was observed with increasing R-ratio. This R-ratio effect could be rationalized after compensating for crack-closure. Special specimens for short-crack growth fatigue tests were cut from pre-cracked CT-specimens. Constant load-range fatigue tests at 50 Hz were conducted at three different R-ratios (-1,0.1 and 0.5) to characterize the effect of mean-stress on short-crack growth behavior. It appeared that the physically short-cracks initially propagated in closure-free environment and then deviated towards their long-crack growth rates. The microstructural effects on short-cracks were more pronounced at lower mean-stresses or R-ratios. The physically short-crack growth effect could not be removed completely using the crack-closure concept alone, suggesting the role of other growth-retarding mechanisms. A modified empirical model including the effect of other possible growth-retarding mechanisms was proposed to capture the behavior of physically short-cracks in this material.