Weldability of Non-Ferrous Metal Alloys Group

Aluminium

Restraint intensity. For aluminum alloys, weldability is often defined in terms of resistance to solidification cracking. One topic examined at BAM has been the effect of restraint intensity on cracking susceptibility. For these studies, an Al-Mg-Si alloy 6060 was selected for evaluation because of its exceptional poor weldability. Here it was demonstrated that high restraint tends to inhibit cracking [1].

Variable restraint “box frame” welding fixture with interchangeable side plates for adjustable height (i.e. low height gives higher restraint)

Phase analysis. Addition of an Al-Si filler alloy (e.g. Al 4043) is known to improve the weldability of Al 6060. The effect of increasing silicon on the solidification path and resulting microstructure has been examined for both castings and welds using thermal analysis. It was found that with higher Si, the solidification range increases as the solidus drops from 578 to 511 °C, involving lower melting ternary and quaternary eutectics [2].

Comparison of cast and weld microstructures for Al 6060, both with and without Si additions (enlarged picture)

Weldability test. A new weldability test has been developed to evaluate susceptibility to solidification cracking, measured in terms of the critical strain rate to initiate cracking. The controlled tensile weldability (CTW) test applies strain transverse to the weld, at a fixed strain rate, during welding. Welds are made full penetration, bead-on-plate. Local strain is measured across the bead root with an extensometer. It has been found that for higher dilutions of Al 4043 filler, a higher strain rate can be tolerated before cracking [3].

CTW test results defining critical values of local strain rate needed to initiate cracking in Al 6060 welds, depending upon Al 4043 dilution

Cracking model. A model for solidification crack initiation has been developed based upon hydrogen micro-pore formation. This assumes that a stable pore will form from pre-existing nuclei when the internal pressure exceeds the external pressure, which can occur when the partial pressure hydrogen P_g exceeds the surface tension pressure P_γ by approximately 1 atmosphere. The location where a pore becomes activated within the interdendritic space, is controlled by i) the initial concentration of hydrogen in the weld pool and the amount of hydrogen partitioned during solidification and ii) the size of the interdendritic space. If a pore forms with a stable radius that is bigger than the interdendritic space, the pore will be expelled to the weld pool (i.e. maro-pore). If a micro-pore forms below the coherent temperature, it is possible to initiate a solidification crack [4].

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