5026 - MATERIAL AND STRUCTURAL BEHAVIOUR OF MIG BUTT WELDS IN 6000 SERIES ALUMINIUM ALLOY EXTRUSIONS FOR RAIL VEHICLES

The welding of 6000 series aluminium alloys can have potentially adverse effects on the material and structural behaviour of the welded joints. In recent accidents involving welded aluminium rail vehicles, some of the longitudinal welds fractured for some metres beyond the zones of severe damage. MIG welding is the main process employed today in the UK and Europe for joining rail vehicle extrusions. This does not mean that the welds will perform as well as the parent material. An extensive mechanical testing programme has been carried out to investigate quasi-static and dynamic material behaviour for the parent material, weld metal and HAZ of MIG welds. The parent material used was 6005A aluminium alloy in the solution treated and artificially aged condition. The filler wires selected were 4043(aluminium-5%silicon) and 5356(aluminium-5%magnesium). Butt welds were produced using conventional procedures in extruded plates and in rail vehicle floor extrusions, from which the test specimens were machined. The testing programme included hardness surveys, tensile, Charpy impact and fracture resistance tests. Structural behaviour of welded rail vehicle floor extrusions has been investigated under quasi-static crush and drop-weight impact loading conditions. These tests were carried out in dynamic servo-hydraulic testing machines and in a 10m drop-weight tower. The present work has concluded that: The aluminium-silicon weld metal was poorer than the weld metal made using aluminium-magnesium filler metal in terms of strength, ductility and fracture resistance; under quasi-static loading, the aluminium-magnesium weld metal outperformed the parent material in terms of the ultimate strength, ductility and fracture toughness; the HAZ was the worst zone in terms of the ability to sustain uniform plastic deformation. By changing from the aluminium-silicon filler to aluminium-magnesium filler, the weld metal strength was greatly improved. Although there might be some scope for improving weld metal strength by consumable development, it is virtually impossible to eliminate the HAZ in conventional MIG fusion welding processes. A programme of work is currently undertaken to explore other options for improvement of joint strength, including redesign of the joint geometry, the use of friction stir welding and of hybrid laser-arc welding.