There has been a rapid growth in the use of the laser welding process due to its higher productivity rate compared to conventional processes. However its development in the field of aluminium alloys has been limited due to the presence of solidification defects (e.g. porosity, hot cracking). This study is focused on the understanding of the problem of hot cracking during welding. Numerous welding tests were done under different conditions, followed by microstructural examinations so as to understand the key parameters controlling the apparition of this defect. These parameters were then studied independently thanks to the development of a testing apparatus that enables tensile tests to be done in the semi-solid
state. The first series of tests were run isothermally, firstly to understand the mechanical behaviour of the semi-solid under simple conditions, and secondly to develop a rheological law for semi-solid deformation under tension. Next, non-isothermal tensile tests were run using high cooling rates to allow the observation of the behaviour of the semi-solid under conditions close to those in which hot cracking is observed. These results have lead to the development of a criterion for the rupture of liquid films that is applicable to welding and a rhelogical law that can be integrated into a welding simulation. Finally we have also shown that the composition of the molten zone of welds influences the global mechanical properties of a weld through the modification of the susceptibility to plastic localisation.