The improvement of constituent dissolution and mechanical properties of 7055 aluminum alloy by stepped heat treatments

The effects of stepped heat treatments on the dissolution of soluble remnant constituents and the mechanical properties of a 7055 aluminum alloy were investigated. Optical and electron microscopy observations and electron probe microanalysis were used to analyze the microstructures and tensile fracture surfaces of 7055 alloy processed via various treatment schedules. The observations revealed that by using a lower temperature pretreatment, the individual constituent phases in multi-phase eutectics grew into separated constituent particles and the η phase dissolved completely. Consequently the initial melting temperature of 7055 alloy ingots was increased. It was shown that a suitable pretreatment could enable subsequent high temperature solution treatment and complete dissolution of the constituent particles in 7055 alloys without overheating. This in turn increased the tensile strength and fracture toughness of the 7055 alloy to 805 MPa and 41.5 MPa m1/2, respectively, with approximately 9% tensile elongation. The improvements were attributed to a combination of reduced fracture initiation by fracture of constituent particles near crack tips and an improved age hardening response resulting from the increased concentration of elements dissolved in the matrix.