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Effect of alloying elements on weld properties in CO2 laser welding of aluminium alloys

To cope with requirements arising from environmental problems, particularly in the reduction of carbon dioxide from exhausts, the necessity to reduce the weight of vehicles is growing and has induced a large quantity of research and development in using aluminium alloys for vehicles. With this move, more and more investigations have been made on applying laser welding to the joints in the car body. Aluminium alloys have higher heat conductivity and greater linear expansion coefficient than steels and hence welding deformation tends to become a problem. For this reason, laser welding, whose heat source has a high energy density, has great potential as an alternative joining method to conventional MIG welding processes. However, it has not had many instances of applications, because it does not have high energy efficiency owing to the degree of laser beam reflection on aluminium alloys being higher than that of steels. To obtain the weld bead of keyhole type characteristic of the laser welding, the power density needed in the welding of aluminium alloys is said to be twice as high as that needed in the welding of steel materials. To improve mainly the strength, a variety of elements are often added to aluminium alloys. Structural alloys have elements such as Si, Mg and Zn added to them. These elements affect not only the dynamic properties of the alloys but also the thermodynamic characteristics and the thermal properties, and hence the difference in alloying elements is reported to affect laser weldability as well. The content of Mg and Zn, which have lower boiling points than aluminium, is known empirically to make a greater contribution to the formation of keyhole, but there has never been any quantitative evaluation of these effects. In this study, using a CO 2 laser, representative structural aluminium and its alloys were welded by varying the power density to find the power density (threshold) required to enable keyhole welding and its determining factors. By paying attention to the differences in thermodynamic properties and thermal conductivity which are induced by the alloying elements in various aluminium alloys, a further investigation was made of keyhole formation behaviour and its relationship with them.