Weldment mechanical properties of aluminum-copper-lithium alloy, 2090, at ambient and cryogenic temperatures

Abstract

Weldment mechanical properties of Al-Cu-Li alloy, 2090, at ambient and cryogeinc temperatures have been investigated. The primary objective of this work is to develop a mechanistic understanding of the weldment mechanical properties with specific emphasis on their relation to the fusion zone and heat affected zone microstructures.In the fusion zone, prior thermomechanical processing of the base metal is erased during welding. The as-welded and post-weld heat treated fusion zones lack the precipitates, size, volume fraction, and homogeneity of the base metal. The TI precipitates are found either in the vicinity of other intermetallics or at the boundaries. Low weldment elongation is caused by the formation of intermetallics and continous film on the boundaries.Even with alloy addition to the fusion zone, the strength mismatch continues to exist due to the difference in strengthening mechanism: solid solution strengthening in the fusion zone and precipitation strengthening in the base metal. The ultimate tensile strength of the as-welded weldments is only 50% of the base metal yield strength, and localized deformation occurs in the softer fusion zone. A weldment elongation increases substantially with increasing Cu content in the fusion zone at both test temperatures. Magnesium additions to the fusion zone change the dendrite morphology, affecting the distribution of Cu segregation at the boundaries. The Mg welds show higher yield strengths, but the Cu-Mg combination leads to embrittlement.The difference in properties of the heat affected zone and the base metal is primarily caused by the dissolution of strengthening phases. At 578 K, the strength degrades due to dissolution of 0' phase and at 773 K, dissolution of T 1 phase. At 648 K, the growth of the equilibrium phases occurs: Tl at both the matrix and the subgrain boundaries, and T2 at the grain boundaries. The problem region in the weldment will be the partially melted region and the fusion boundary. This is due to dissolution of the strengthening phases in the matrix and the formation of eqUilibrium phases and intermetallics at the boundaries.