In order to improve the understanding of hot tearing during laser welding of aluminium
alloys, the rheology of the alloys in the mushy state must be characterized. The present work
investigates the mechanical behaviour of the aerospace alloy AA6056 using a specially designed
isothermal tensile test in the mushy state. Using a Gleeble thermo-mechanical machine, two
different tests have been performed: i) tests during partial remelting and ii) tests after partial
solidification at a high cooling rate. These tests have been carried out not only on the 6056 alloy but
also on a mix between 6056 and 4047 Al-Si alloy which corresponds to the composition of the
nugget of a laser using a filler wire.
The increase of the solid fraction results in an increase of the maximum stress and a change on the
fracture surface from a smooth dendritic to a more ductile one. Moreover, the alloys exhibit a
typical visco plastic behaviour with an increase of the maximal stress with the strain rate. When the
test is performed at a particular solid fraction of 0.97, the fracture is more erratic and the ductility is
low. The results show the existence of a ductile/brittle/ductile transition with the fraction of solid.
The fracture stress is shown to be higher when testing after partial remelting as compared to partial
solidification for the same solid fraction. This is due to the difference in microstructure of the
mushy zone and more particularly in the connectivity of the solid skeleton. An adapted creep law is
used to describe the mechanical behaviour of alloys during the partial remelting test using the
fraction of grain boundary wetted by the liquid given by Wray. This law is shown to be irrelevant to
the partial solidification tests, as a result of the modified geometry of the liquid phase. From these
tests, we have determined a new law relating the solid fraction to the fraction of grain boundaries
wetted by the liquid. This law is a useful tool to predict the mechanical behaviour when mechanical
loading occurs during solidification.
Mechanical Behaviour of Aluminium Alloys