Laser welding of aluminium alloys
Abstract: This thesis treats laser welding of aluminium alloys from a practical perspective with elements of mathematical analysis. The theoretical work has in all cases been verified experimentally. The aluminium alloys studied are from the 5xxx and 6xxx groups which are common for example in the automotive industry. Aluminium has many unique physical properties. The properties which more than others have been shown to influence the welding process is its high reflection, high thermal conductivity, low melting point, low viscosity and the alloying elements used. The most important physical properties have been described and studied experimentally as well as theoretically. The high surface reflectivity was shown to be of little importance when welding has initiated because the deep and narrow gas/plasma filled 'keyhole' captures the incident light which leads to considerably higher total absorption. The high thermal conductivity was shown to lead to a large weld width compared to the condition of other common metals. In addition the thermal conductivity together with low melting point and high boiling point was shown to yield a relatively large weld melt pool. A large melt pool together with the low viscosity has been shown to result in instability of the melt during welding. A number of defects can occur when laser welding aluminium alloys. The defects considered to be most important to try to avoid include porosity, edge effects and instability problems. Porosity has been shown to consist of spherical (gas filled) and cylindrical voids with different origins. Edge effects were found to be of different types when welding thin and thick material respectively. Common for both is that the heat distribution in the workpiece determines the result and that it can be theoretically predicted. Instability problems include random blowholes, smoke, spatter and weld depth variations. Natural variations exist but they can be controlled by the process parameters. The one most important cause of spatter from the melt is the amount of magnesium as an alloying element. The strength of the welded joints has been tested in static as well as dynamic loading. In normal tensile tests the best joints reached 90% of the strength of the parent material. Using wire as a filler a strength close to 100% was realised but this was due to a larger weld cross section. In fatigue tests the laser welds have performed better than other melt welding processes such as TIG and MIG. A fast and user friendly model predicting the weld dimensions was developed to cover laser welding with CO2 and Nd:YAG lasers of a range of common materials including aluminium alloys. The conclusion of this work is that welding of aluminium alloys is a challenge. It is predicted that methods such as riveting and clinching in the future in many cases will be replaced by the more cost effective laser welding. Laser welding of aluminium alloys will be increasingly used in automated industry as knowledge of the process continues to grow. In cases where thin aluminium alloys are to be joined at high speed lasers are expected to dominate.
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