Arc welding of high strength aluminium alloys for armour systems applications

Abstract

The ternary Al-Cu-Mg system 2xxx series aluminium alloys were examined as construction materials for armour system applications based upon comparable ballistic properties to the currently employed Al-7xxx series alloys. Utilising MIG welding solidification cracking was evident when welding constrained Al-2024 candidate base material using Al-2319 filler, the only available consumable wire for this series. A previously developed thermodynamic model suggested that an incompatible weld chemistry resulted when welding with this filler which would result in hot cracking due to a wide weld pool freezing range and a low volume fraction of eutectic liquid. As this filler wire was the only commercially available Al-2xxx filler this was seen as the principal limiting factor for exploiting this alloy series. The solution was to vary and control weld chemistry. Two approaches were taken. Firstly advanced arc welding was used to control weld dilution with the base material. A clad layer exhibiting a less crack susceptible composition was deposited using the Cold Metal Transfer process and the binary Al-2319 filler wire. Onto this layer the same filler could then be deposited to provide a structural joint. Although not fully validated, by limiting weld dilution with the base material this technique showed potential as an alternative method for suppressing solidification cracking. The second approach, which forms the core of this work, adapted the conventional tandem MIG welding process to mix different series consumable fillers in a single weld pool to control weld composition. A range of ternary weld mixtures were produced which resulted in the development of a robust thermodynamic model. Validation using this system resulted in weld cracking being eradicated. The concept was then further developed to weld using three filler wires; this expanded the mixing range and allowed further model validation. A range of crack free compositions were produced with differing mechanical properties. An optimum weld composition was determined that was then used for characterisation of the weldment. By varying heat input, base material HAZ softening was controlled with joint failure confined to the weld / base material interface. This was attributed to grain boundary liquation due to the welding temperatures involved resulting in solute rich grain boundaries. These areas did not deform easily under tensile loading initiating fracture of the joint. Acceptable joint strengths were realised however ductility was reduced due to the identified failure mode. Although not tested to military specifications, acceptable mechanical test values were recorded which were closely compliant with the minimum requirements for armour system specifications. As a consequence a filler wire composition was recommended for future prototype development.