New hot cracking criterion for laser welding in close-edge position

Abstract

Hot cracking is one of the major issues in laser welding of high-strength aluminium alloys. The considered aluminium, magnesium and silicon based alloys (6xxx series) are highly crack sensitive due to a large solidification interval and little residual liquid between the dendrites.When welding of 2.7 mm thick alloy sheets, hot cracking mainly occurs for seams placed at a distance of about 3 to 6 mm from the edge of the work piece. The existing theories based on strain considerations do not explain why no longitudinal hot cracking is observed at shorter edge distances.In order to obtain a better scientific understanding of this experimental evidence, we performed a theoretical analysis based on a finite element model. High speed videos from experimental welding processes were used to calibrate this simulation model as well as to identify the location of hot crack formation during the solidification phase. The simulations were used to calculate the transient temperature distributions, the resulting deformations, and the stresses during welding with varying distances from the edge of the work piece.From this we derived changing shapes of the melt pool in close edge condition, indicating different solidification paths. Such analysis together with the common structural condition of positive strain at the trailing edge of solidification led to the finding of a new hot criterion for the formation of hot cracks.The criterion implies that positive strain combined with multidirectional solidification conditions is responsible for hot crack formation.Hot cracking is one of the major issues in laser welding of high-strength aluminium alloys. The considered aluminium, magnesium and silicon based alloys (6xxx series) are highly crack sensitive due to a large solidification interval and little residual liquid between the dendrites.When welding of 2.7 mm thick alloy sheets, hot cracking mainly occurs for seams placed at a distance of about 3 to 6 mm from the edge of the work piece. The existing theories based on strain considerations do not explain why no longitudinal hot cracking is observed at shorter edge distances.In order to obtain a better scientific understanding of this experimental evidence, we performed a theoretical analysis based on a finite element model. High speed videos from experimental welding processes were used to calibrate this simulation model as well as to identify the location of hot crack formation during the solidification phase. The simulations were used to calculate the transient temperature distributions, the resulting deformat...