Multi-pass-narrow-gap-laser-beam-welding of hot crack sensitive thick aluminum plates

Heat treatable aluminium alloys are one of the most important materials for light weight constructions of the modern economy. They combine very high specific strength and excellent corrosion resistance. Furthermore, they are perfectly suitable for forging, extrusion and rolling. That provides a wide range of applications in the field for high-loaded light-weight constructions. However, a large variety of semifinished products have to be welded in the final construction. Here an important current process limit for laser beam welding is the thickness of the aluminium sheets. Such limitations have been reduced by the availability of multi-kilowatt lasers; however, they could not be fully eradiated. Physical properties of aluminum that cause this are the high heat conductivity, the large heat capacity and the high thermal expansion coefficient. Especially for deep penetration welding the insufficient dilution of filler wire into the root of the weld seam becomes more prevalent with the material thickness. This point causes an increasing risk of hot cracks and weld seam strength under matching. This paper presents the new multi-pass-narrow- gaplaser beam welding technology (MPNG-LBW) for welding sheet metal of substantial wall thicknesses to overcome the mentioned problems. It uses high brightness laser beam sources with multimode or single mode propagation mode (beam parameter product of 0.4 to 2.2 mm mrad). The advantage of these excellent beam qualities is that the laser beam can be perfectly inserted into a weld groove with a high aspect ratio of an average gap size of 2-3 mm, see Figure 1. In this way a multi-pass-welding for sheet thicknesses up to 50 mm can be realized by using low laser power of less than 5 kW. An improved melting of the wire and a good gap bridging capability could be achieved by a beam scanning technique. This provides high process reliability and ensures the homogeneous mixture of the alloying elements with the base material to obtain multi layer weld seams with high strength and without welding defects.