Arc welding of high strength aluminium alloys for armour systems applications

摘要

The ternary Al-Cu-Mg system 2xxx series aluminium alloys were examined as constructionmaterials for armour system applications based upon comparable ballistic properties to thecurrently employed Al-7xxx series alloys. Utilising MIG welding solidification cracking wasevident when welding constrained Al-2024 candidate base material using Al-2319 filler, theonly available consumable wire for this series. A previously developed thermodynamicmodel suggested that an incompatible weld chemistry resulted when welding with this fillerwhich would result in hot cracking due to a wide weld pool freezing range and a low volumefraction of eutectic liquid. As this filler wire was the only commercially available Al-2xxxfiller this was seen as the principal limiting factor for exploiting this alloy series. The solutionwas to vary and control weld chemistry. Two approaches were taken. Firstly advanced arcwelding was used to control weld dilution with the base material. A clad layer exhibiting aless crack susceptible composition was deposited using the Cold Metal Transfer process andthe binary Al-2319 filler wire. Onto this layer the same filler could then be deposited toprovide a structural joint. Although not fully validated, by limiting weld dilution with thebase material this technique showed potential as an alternative method for suppressingsolidification cracking. The second approach, which forms the core of this work, adapted theconventional tandem MIG welding process to mix different series consumable fillers in asingle weld pool to control weld composition. A range of ternary weld mixtures wereproduced which resulted in the development of a robust thermodynamic model. Validationusing this system resulted in weld cracking being eradicated. The concept was then furtherdeveloped to weld using three filler wires; this expanded the mixing range and allowedfurther model validation. A range of crack free compositions were produced with differingmechanical properties. An optimum weld composition was determined that was then used forcharacterisation of the weldment. By varying heat input, base material HAZ softening wascontrolled with joint failure confined to the weld / base material interface. This was attributedto grain boundary liquation due to the welding temperatures involved resulting in solute richgrain boundaries. These areas did not deform easily under tensile loading initiating fracture ofthe joint. Acceptable joint strengths were realised however ductility was reduced due to theidentified failure mode. Although not tested to military specifications, acceptable mechanicaltest values were recorded which were closely compliant with the minimum requirements forarmour system specifications. As a consequence a filler wire composition was recommendedfor future prototype development.