Hot Tearing Sensitivity of Al-Mg-Si Alloys Evaluated by X-Ray Microtomography After Constrained Solidification at High Cooling Rate

Abstract

The mechanical behavior of alloys resulting from a mixture of a AA6061 base alloy with different alloys from the Al-Si and Al-Mg series used as filler alloys during welding has been investigated under conditions close to welding. The specimens have been subjected to constrained solidification carried out at a high cooling rate of 80 K/s. In the test developed in this study, the central part of the specimen is initially melted. During solidification, this zone suffers strains only generated by thermal contraction and solidification shrinkage, which can possibly lead to the formation of hot cracks. The hot cracking sensitivity of the various alloys has been determined thanks to X-ray microtomography observations which have allowed imaging the zone solidified under constrained conditions and measuring the volume fraction and the number of open cracks as a function of Si and Mg contents. The variation of stress induced by deformation of the solid during constrained solidification has been also measured as a function of solid fraction. The results show that: (1) stress developed within the solidifying alloy exhibits the same variation whatever the Si and Mg contents. Stress starts increasing at the coherency of the solid skeleton and increases much more sharply when coalescence occurs. (2) the alloys with Si and Mg contents exceeding 3 and 2 wt% respectively are less sensitive to hot tearing in comparison with the base alloy; (3)Al-Mg filler alloys seem to be the best solution to reduce hot cracking in 6061 welded joints.