Liquation cracking in partial penetration aluminium welds: Assessing tendencies to liquate, crack and backfill

Aluminium alloys are susceptible to liquation cracking in the partially melted zone (PMZ), where grain boundary liquation occurs during welding. Alloys 2024, 6061 and 7075 were gas-metal arc welded with fillers 1100 and 4043, and liquation cracking near the weld root was examined. Curves of temperature (T) versus solid fraction (fs), based on the Scheil equation for Al-Cu-Mg-Mn-Si-Zn alloys, were calculated for the solidifying PMZ and weld metal at the fusion boundary. Judging from the freezing temperature range and the liquid fraction, these curves suggested that liquation decreases in the order of 7075, 2024 and 6061, consistent with experiments. They also suggested that 1100 increases the weld metal fs, thus promoting liquation cracking (by strengthening the solidifying and contracting weld metal that pulls the PMZ) and discouraging backfilling (by reducing the inter dendritic liquid), while 4043 does the opposite except during PMZ terminal solidification in 7075 and 2024. These are consistent with experiments.