Multi-beam laser spot welding of coated steels

A method of laser spot 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 first laser beam and a second laser beam at the top surface of the workpiece stack-up such that the first and second laser beams impinge the top surface at a weld site and create a molten steel weld pool that penetrates into the workpiece stack-up, the laser optic welding head including a beamsplitter and a focusing element, the beamsplitter functioning to split a collimated laser beam into first and second output collimated laser beams, and the focusing element functioning to focus each of the first and second output collimated laser beams into the first and second laser beams that ultimately impinge the top surface of the workpiece stack-up, the focusing element converging each of the first and second laser beams to a focal point that has a focal length between 50 mm and 350 mm;
• increasing an overall combined irradiance of the first and second laser beams at the top surface of the workpiece stack-up from an initial level to an elevated level by reducing a total projected sectional area of the first and second laser beams at a plane of the top surface of the workpiece stack-up, the increase in the overall combined 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 first and second laser beams to allow the molten steel weld pool to solidify into a laser spot weld joint comprised of resolidified composite steel workpiece material derived from each of the steel workpieces in the workpiece stack-up, the resolidified composite steel workpiece 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.