SEARCH RESULT

The use of aluminum alloys in automotive industry has greatly increased in recent years. This has been attributed not only to the issues of fuel economy, but also to those of safety, resource conservation and environmental concerns [1]. High-strength age-hardened 7xxx series Al−Zn−Mg−Cu alloys are widely used for aircraft structures, where they are subjected to demanding operating conditions [2]. In this study, retrogression and reaging (RRA) heat treatment was applied to commercial 7075-T6 aluminum alloy (Al–Zn–Mg–Cu), whose chemical composition is shown in Table 1 RRA heat treatment is capable of producing a material with high strength and high corrosion resistance than presented by the T6 temper. In the retrogression treatment, salt bath furnace was used for heating the samples to 250°C and 280°C for 1 min. After that, the samples taking out of furnace and quenched in cold water of 5-8°C temperature. Subsequently intermediate deformations were applied to the samples in 2%, 4% and 6% and one sample was left undeformed for comparison. Following intermediate deformations, the samples were reaged at 120°C for 24 and then cooled to room temperature in air. In order to investigate effect of intermediate deformation on the strength and strain hardening exponent as the indicators of formability characteristics of RRA’ed 7075 alloys, tensile tests were carried out to determine yield strength, ultimate tensile strength and tensile strain hardening exponent (n-value). Hardness and electrical conductivity measurements were also performed on the samples. Variation of strength and elongation at fracture of the samples retrogressed at 250°C (a) and 280°C (b) and then reaged as a function of intermediate deformation is shown in Figure 1 It is obviously seen that strength values decrease whereas ductility (elongation at fracture) generally increases with increasing retrogression temperature. On the other hand, intermediate deformation has not any significant effect on strength and ductility for both retrogression temperatures. Figure 2 shows variation of strain hardening exponent of the samples retrogressed at 250°C and 280°C and then reaged as a function of intermediate deformation. It is very apparent that retrogression temperature has a strong effect on strain hardening exponent, whose value increases with increasing retrogression temperature. Figure 3 indicates variation of hardness and electrical conductivity of the samples retrogressed at 250°C and 280°C and then reaged as a function of intermediate deformation. It is obvious that hardness decreases whereas electrical conductivity increases with increasing retrogression temperature. It is well known that electrical conductivity is a good indicator of corrosion resistance of the samples [3]. Therefore, this can be said that corrosion resistance increases with increasing retrogression temperature, as expected. Overall results demonstrated that higher retrogression temperature is beneficial in the point of formability because it increases ultimate tensile strength to yield strength ratio and strain hardening exponent as well. In the view point of intermediate deformation, it was shown that it does not significantly effect of strength, hardness and electrical conductivity of the samples after RRA, but lead to some decrement in strain hardening exponent, which adversely affects the formability characteristics of 7075 RRA alloy.

International Symposium on Light Alloys and Composite Materials
UHAKS

Armin Rashidi Yakup Yurekturk Murat Baydogan