Hot cracking is one of the major issues in laserwelding of high-strength aluminium alloys. Theconsidered aluminium, magnesium and silicon basedalloys (6xxx series) are highly crack sensitive due to alarge solidification interval and little residual liquidbetween the dendrites.When welding of 2.7 mm thick alloy sheets, hotcracking mainly occurs for seams placed at a distanceof about 3 to 6 mm from the edge of the work piece.The existing theories based on strain considerations donot explain why no longitudinal hot cracking isobserved at shorter edge distances.In order to obtain a better scientific understanding ofthis experimental evidence, we performed a theoreticalanalysis based on a finite element model. High speedvideos from experimental welding processes were usedto calibrate this simulation model as well as to identifythe location of hot crack formation during thesolidification phase. The simulations were used tocalculate the transient temperature distributions, theresulting deformations, and the stresses during weldingwith varying distances from the edge of the workpiece.From this we derived changing shapes of the melt poolin close edge condition, indicating differentsolidification paths. Such analysis together with thecommon structural condition of positive strain at thetrailing edge of solidification led to the finding of anew hot criterion for the formation of hot cracks.The criterion implies that positive strain combinedwith multidirectional solidification conditions isresponsible for hot crack formation.
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