Method of laser spot welding coated steels

A method of laser welding overlapping steel workpieces, the method comprising:

• providing a workpiece stack-up that includes overlapping steel workpieces, the workpiece stack-up comprising at least a first steel workpiece and a second steel workpiece, the first steel workpiece providing a top surface of the workpiece stack-up and the second steel workpiece providing a bottom surface of the workpiece stack-up, wherein a faying interface is established between each pair of adjacent overlapping steel workpieces within the workpiece stack-up, and wherein at least one of the steel workpieces in the workpiece stack-up includes a surface coating comprised of a zinc-based material or an aluminum-based material;
• positioning a laser optic welding head to direct a solid-state laser beam at the top surface of the workpiece stack-up such that the laser beam impinges the top surface at a weld site and creates a molten steel weld pool that penetrates into the workpiece stack-up, the laser optic welding head including a focusing lens from which the solid-state laser beam propagates forward towards the top surface of the workpiece stack-up along a longitudinal beam axis, the focusing lens converging the laser beam to a focal point on the longitudinal beam axis that has a focal length between 50 mm and 350 mm, and wherein a lens axis of the focusing lens is coaxial with the longitudinal beam axis of the laser beam;
• increasing an irradiance of the solid-state laser beam at the top surface of the workpiece stack-up from an initial level to an elevated level by reducing a projected sectional area of the solid-state laser beam at a plane of the top surface of the workpiece stack-up, the increase in the irradiance from the initial level to the elevated level causing the molten steel weld pool to grow and to further penetrate into the workpiece stack-up towards the bottom surface;
• and
• ceasing transmission of the solid-state laser beam to allow the molten steel weld pool to solidify into a laser spot weld joint comprised of resolidified composite steel material derived from each of the steel workpieces in the workpiece stack-up, the resolidified composite steel material of the laser spot weld joint intersecting each of the faying interfaces within the workpiece stack-up to fusion weld the steel workpieces together.