Welding of high-strength aluminium alloy extrusions

A new class of aluminium alloy extrusions (Constellium HSA6 TM) based on the 6xxx- series has been developed with Cu content higher than in most conventional 6xxx series alloys commonly used for extrusion, which has resulted in excellent mechanical properties and are extremely attractive to the automotive sector. Welding is the most versatile technique for manufacturing of automotive structural components and body-in- white; e.g. a crash management system (CMS), wherein an extruded bumper beam is welded to an extruded crush can. However, fusion welding of highly alloyed, heat- treatable, aluminium alloys may result in solidification cracking in the fusion zone (FZ) and/or liquation cracking in the partially melted zone (PMZ)/heat-affected zone (HAZ). Also, degradation of properties in the HAZ, due to the weld thermal-cycle, may affect the joint integrity of these structural components.

This research focuses on investigating the effect of Metal Inert Gas (MIG), Cold Metal Transfer (CMT) and Laser welding processes to fabricate stub coupons of a prototype CMS using HSA6 as the bumper-beam and AA6008 as the crush-can. A post-weld heat treatment (PWHT) of structural components involving solution-treating, fast quench and ageing is not economical; hence a post-weld ageing (PWA) condition, (a typical T6 ageing treatment) has been determined and its effect has been investigated for the weldments fabricated using the different welding processes. In addition, as the body-in- white goes through a paint-bake (PB) cycle, the effect of PB-cycle has also been investigated. Metallurgical assessment of the weldment has been performed using micro- hardness mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in conjunction with chemical mapping using energy dispersive x-ray spectroscopy (EDS) and grain structure characterisation using electron backscatter diffraction (EBSD). Finally, the effect of welding processes (CMT, Laser and Friction Stir Welding (FSW)) on the mechanical property data (tensile and hardness) of the HSA6 has been investigated for welding in the T4 (T4+PWA) and T6 (T6+PB) conditions.

The elevated Cu content in the HSA6 compared to conventional 6xxx series alloys had no detrimental effects on liquation and solidification cracking, when welded with the filler wires 4943 and 5183. The strength in the weakest region of the HAZ in the HSA6 was improved by a post weld heat treatment irrespective of welding in T4 or T6 temper

condition; this again shows that addition of Cu had no detrimental effects on completely over-ageing the HAZ after welding with the defined welding process parameters. The best processing route to weld HSA6 is to weld in the T4 condition and then do a PWA treatment. The formation of the coarse Q-type phase, growth of the plate-shaped type C precipitates (with Cu after PWA) and nucleation of coarse Q-type phases over the Fe, Mn, and Zr containing dispersoids are responsible for the loss of strength in the weakest area of the HAZ of the HSA6 in the T4+PWA condition. In terms of mechanical performance in the T4+PWA condition, FSW offered the highest joint efficiency of 94%, followed by 85% for CMT and 83% for Laser in the T4+PWA condition.