Product Code: ICA12_1003

New Hot Cracking Criterion for Laser Welding in Close-Edge Position
Authors:
Peter Stritt, GSaME Graduate School of Excellence Advanced Manufacturing Engineering; Stuttgart Germany
Rudolf Weber, IFSW Institut fuer Strahlwerkzeuge; Stuttgart Germany
Thomas Graf, IFSW Institut fuer Strahlwerkzeuge; Stuttgart Germany
Steffen Mueller, AUDI AG; Neckarsulm Germany
Jan-Philipp Weberpals, AUDI AG; Neckarsulm Germany
Presented at ICALEO 2012

Hot cracking is one of the major issues in laser welding aluminium alloys. The considered aluminium, magnesium and silicon based alloys (6xxx series) are highly crack sensitive due to a large solidification interval with little residual liquid between the dendrites. When welding 2.7mm thick alloy sheets, hot cracking mainly occurs in the range of 4 to 6mm from the edge of the work piece, whereas at shorter edge distances no longitudinal hot cracking is seen. The existing strain associated theories are not explaining such experimentally observed phenomena.
In order to obtain a clear scientific understanding of the absence of centerline cracks in close edge laser welding condition, a finite element model was set up. High speed films were used to calibrate this model as well as to identify the location of hot crack formation during the solidification phase. Numerically, the transient temperature distribution, resultant deformations and stresses were calculated for laser welding with varying edge distance. The presented new criterion for hot crack initiation takes into account that two structural conditions are required to initiate hot cracking: positive strain applied at the tailing edge of solidification and local tensile stress. Numerically varying the welding edge distance, the changing strain and stress fields are analyzed as well as observed changes in the melt pool shape in close edge laser welding. Such melt pool analysis leads to the finding of different solidification conditions, in which multidirectional solidification is found to promote hot cracking.