Structure, Strength, Ductility in Aluminum Alloys: Constitutive Relations Analysis for Performance Evaluation

Abstract

Aluminum alloys AA5754 and AA6063 were thermomechanically processed (TMP) to investigate the relationship between structure, strength, and ductility, and effect on failure. Model parameters from the Saimoto-Van Houtte (SVH) constitutive equation were determined from experimentally measured stress-strain curves and correlated to the evolving microstructure of the two alloys at various stages of TMP. Using the SVH equation and the concept of equivalent plastic work, the yield loci for AA5754 after four different treatment conditions were modeled, allowing for the balanced biaxial and plane strain loading ratios to be found. Interfacial decohesion was observed in both alloys by analysis of the fracture surface and is attributed to iron atom segregation during cooling. This quench sensitivity effect can ideally be removed through TMP. Nano-void nucleation and growth ductile failure models were developed for both undeformed control and prestrained AA6063 conditions to determine if the source of failure is present within the initial microstructure, as proposed by Lloyd. The evolving fracture strains, yield stress and inter-obstacle spacing at yield were all found to evolve with a temporal exponent of 0.3 and 0.2 for the control and prestrained conditions, respectively. This evidence indicates that both TMP routes result in failure initiating at precipitates and that introduction of dislocations changes the diffusion kinetics of artificial ageing. If the quench-sensitivity effect remained after TMP, higher temperature annealing at 230°C was found to improve ductility, and is believed to be due to precipitation of Al6Fe into the grain boundaries. The balanced biaxial conditions in AA5754 were determined from the SVH-based yield locus model to be all located off the 1:1 applied stress ratio. The plane strain conditions were determined to change due to the differences in work hardening in the two directions. The plane strain condition is redefined as a net zero increase in minor strain, rather than imposing a zero strain in the minor direction.