Investigations on in-process control of penetration depth for high-power laser welding of thick steel-aluminum joints

Over the last few decades, the demand for lightweight constructions has been increased continuously for several industrial applications, like automotive and ship building, to reduce the weight of vessels in order to minimize the CO₂ emissions as a result of a lower fuel consumption. Lightweight construction is almost applied for ship applications, especially for yachts, which are designed by using aluminum for the deck constructions and steel for the ship hull. For joining these parts, a high-power laser welding process shall be developed. However, the welding of these dissimilar materials is associated with great challenges, due to the different physical properties and the formation of hard and brittle intermetallic phases, which may influence negatively the properties of the weld seam. The quality of dissimilar joints depends strongly on the mixture ratio between the molten amount of steel and aluminum. However, the mixture ratio varies over the weld seam length due to a high dynamic of the keyhole resulted by welding of this material combination. Furthermore, different batches of materials and varied sheet thicknesses t may influence the mixture ratio. In this study, a high-power laser welding process is developed with in-process control of the penetration depth t_P by analyzing the spectral process emissions for dissimilar lap joints of aluminum alloy EN AW-6082 (t = 8 mm) and steel S355 (t = 5–7 mm). In the context of these investigations, an increase of occurring cracks within the weld seam and ejections of molten material with increasing penetration depth t_P can be observed. To achieve a relative high joint strength, the penetration depth t_P must be kept constant at a value of 1.4 mm. In case of varied batch of material, thickness t of the used sheets, welding speed v_S, and leap of the steel sheet thickness t, the penetration depth t_P requested cannot be achieved. Using the in-process control of the penetration depth t_P, the weld seam quality remains almost constantly over the weld seam length, as shown in visual inspections, metallographic analyses, profiles of the penetration depth t_P, and tensile shear testing. Among other things, the appearance of ejections of molten material can be avoided by using the in-process control of the penetration depth t_P.