Numerical modelling and experimental validation of the effect of laser beam defocusing on process optimization during fiber laser welding of automotive press-hardened steels

•This work uses numerical modelling and experimental validation to study the effect of laser beam defocusing on the weld pool geomtery during the laser welding of 22MnB5 grade press-hardened steel.•The study presents the effect of beam defocusing on the weld pool geometry while holding other parameters constant, as the welding mode transitions from the full penetration mode when the beam is fully focused, to the partial penetration mode as the beam is defocused.•It has been shown through a detailed experimental study that by defocusing the laser beam, the welding process can be stabilized which allows the weld pool geometry to be controlled and leads to the elimination of defects such as severe concavity without needing to decrease the laser power or welding speed.•A numerical solution is presented that uses three-dimensional transient finite element analysis to calculate the temperature fields and predict the molten weld pool geometry in both the full and partial penetration welding modes to show how the change in beam defocusing distance impacts the shape and size of the weld.•The findings of this work have a high impact potential as the simple, yet highly accurate numerical model presented in this work allows for the proposed method of defocusing the laser beam to improve the stability of the laser welding process, so it can be readily used in streamlining the manufacturing of laser-welded blanks used in the automotive industry.