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

Abstract

International audienceThe 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

Determination of Solidification of Rigidity Point Temperature Using a New Method

“…By the study of student t results, we can observe that the method with the highest values is obtained for Method 5. The results obtained show a result that is consistent with previous results in the literature, since it has been determined that magnesium is the main element that influences the RPT values and is the element with the highest student t coefficient [20]. This method is the only one that indicates that silicon also has a statistical effect on the RPT values.…”

“…The increase of percentage of alloying elements in an Al alloy decrease the solidification process related temperature until the minimum solidification temperature, which is reached in the eutectic composition. Giraud and his team [20] observed that the coalescence solid fraction is independent of Si content and depends on Mg content for AlSi 10 Mg alloys, because a solid skeleton is developed earlier in the Mg-rich alloys, like π-Al 8 Mg 3 FeSi 6 . The Fe and Ni as alloying elements can form β-Al 5 SiFe, Al 3 Ni, and Al 7 Cu 2 Fe needle like structures, that they can reduce the solid particles moving to pass each other, as shown in Figure 9.…”

Solid Fraction Determination at the Rigidity Point by Advanced Thermal Analysis

“…It promotes faster rigidity, because it reduces the solidification time and because it hinders the movement of the remaining eutectic liquid. It has also been observed that Mg has an effect on increasing the coalescence of solid fraction, because a solid skeleton can develop earlier in Mg-rich alloys, such as the π-Al 8 Mg 3 FeSi 6 phase [51], preventing free movement of the remaining liquid. Mg 2 Si has a typical bone structure that restricts the movement of the remaining liquid.…”

Influence of the solidification path of AlMgSi aluminium alloys on the critical strain rate during remote laser beam welding

“…Figure 2 shows the A-values of the considered alloys as a function of their amount of silicon. It shows the typical -shaped curve [10] describing the susceptibility to hot cracking with a peak where the concentration of silicon is close to 1% as known from experiments with ring casting specimens [24,25] as well as from experiments on laser beam welding [26].…”