Investigation of conduction to keyhole mode transition

There are two very distinct welding modes in laser welding, keyhole and conduction mode. The characteristics of keyhole laser welding; mainly high penetration, high productivity and high aspect ratio have led the industry to focus more on this mode. On the other hand, conduction mode does not have high productivity and has a low aspect ratio, but deep penetration depths can also be achieved using this mode. Despite these disadvantages conduction mode has advantages such as stability of the welding process, high quality, better control of the heat input and no spatter. These characteristics are not normally associated with laser welding, mainly due to keyhole welding being the usual operational mode. Conduction laser welding is a more complicated process than might be expected. This is because of the conflicting requirements of maximising penetration depth whilst maintaining very high quality. This means that thermal transfer needs to be maximised but vaporisation needs to be zero. The aim of this research was to fully understand conduction laser welding mode and therefore, achieve maximum penetration whilst maintaining high quality. The approach was to use power density, interaction time and beam diameter as process parameters in order to study and understand the transition between conduction and keyhole welding modes. The study included the differences between using a continuous wave laser system and a pulsed wave laser system. Most of the study was made in mild steel and was also extended to stainless steel and aluminium to include the effects of the material properties on the transition. For process optimisation the effect of system parameters, power and welding speed, on optimum beam diameter in conduction mode was also examined for aluminium and mild steel. This included a comparison between the use of a statistical empirical model and a finite element model for optimisation of the process. Finally a comparison of residual stress development in conduction and keyhole welding modes was made.