Hot cracking is a fatal defect occurring in welding of alloys. The heat input during the welding process leads to thermomechanical stress and strain in the material. Hot cracking is mainly caused by these thermomechanical loads, as they affect the solidification zone. The spatially resolved measurement of these strains during laser welding of the AlMgSi-alloy AA6014 close to the edge of sheet metals is of major interest to describe the mechanism of hot-crack formation.
Lateral displacements were measured by digital image correlation. The use of a high-speed-camera enabled a temporal resolution of 1 ms. A spatial resolution of a image scale of 100 px/mm was achieved by high optical magnification and a fine stochastic pattern. Due to the low melting temperature of aluminium alloys, the paint of the applied pattern was not affected by the heat. Therefore, it was possible to determine the deformation in the immediate vicinity of the solidification zone.
To create different strain and stress conditions, welds in different edge distances were performed. Welding in an edge distance of 4 mm resulted in pure tensile strain and reproducible centerline cracking. When welding in a larger edge-distance of 6 mm transversal compressive strain surrounded the weld-pool. However, the compression was followed by a tensile strain, sufficient intense to initiate a crack. In case of welding in 8 mm edge distance there was no tensile strain present to initiate a hot crack.