Through variable temperature retrogression to enhance mechanical properties and corrosion resistance of extruded 7055 aluminum alloy

Abstract

A variable temperature retrogression and re-aging (VT-RRA) was exploited to extruded 7055 aluminum alloy, which can make the comprehensive performance of the alloy exceeds that of traditional constant temperature retrogression RRA (CT-RRA). In the variable temperature retrogression stage (VTRT-stage), the 7055 alloy undergone pre-aged at 120 °C/24 h is heated to 200 °C at a rapid rate of 8 °C/min, then cooled to 120 °C at three different rates: 10 °C/h, 15 °C/h and 20 °C/h. GP zones and small-sized η′ phases dissolve back into the matrix, while the η′ phases with larger size are inclined to coarse in the VTRT-stage. As the cooling rate increases, the average size of precipitates in the finished alloy undergoing re-aging at 120 °C/24 h is smaller (9.6→6.9→6.7 nm), which can effectively prevent excessive degradation of mechanical properties. In the VTRT-stage, the grain boundary precipitates (GBPs) change from continuous to discontinuous, forming the precipitate free zone (PFZ), and the larger the cooling rate, the less the contents of Zn, Mg and Cu elements in the GBPs, the narrower the PFZ (101.0→77.5→51.2 nm), the lower the electrical conductivity (41.9→39.3→36.7 %IACS), and the corrosion resistance achieves good results at around 15 °C/h. Uncoarsened precipitates with higher proportion (41.6%) of η′ phase and less of η phase (2.1%) can be obtained through VT-RRA with cooling rate of 15 °C/h, making the mechanical properties exceed those of traditional CT-RRA. And the appropriate width of PFZ and the discontinuous distribution of GBPs are the reasons why VT-RRA alloy has better corrosion resistance than CT-RRA.

Keywords

• Retrogression and re-aging
• 7055 aluminum alloy
• Precipitates
• Corrosion resistance
• Mechanical properties