Effect of Natural Ageing on Subsequent Artificial Ageing of AA7075 Aluminum Alloy

Abstract

The effects of natural ageing treatment prior to artificial ageing treatment on the microstructures and mechanical properties of AA7075 Al-5.7Zn-2.6Mg-1.5Cu-0.18Cr-0.08Mn-0.05Si-0.17Fe (wt.%) aluminum alloy have been investigated. The hardness of solution-treated samples (91.0 HV) profoundly increased to 146.8 HV after 7 days of natural ageing. The purpose of the present work was to examine the kinetic hardening evolution in subsequent artificial ageing treatments of samples naturally aged for 7 days and their counterparts without natural ageing. The former were labelled as NA-7d samples, and the latter, NA-0d samples. After artificial ageing at 120 °C for 2 h, the hardness of NA-0d samples increased rapidly to 148.2 HV, which was approximately the same as that of the specimens with natural ageing for 7 days, compensating for the prior state of lower hardness without natural ageing. After being treated at 120 °C for 16 h, the ultimate tensile strength (UTS) and yield strength (YS) of NA-7d reached the highest value, respectively, 601 MPa and 539 MPa, followed by a slight decrement of UTS when aged to 24 h. On the other hand, NA-0d specimens aged at 120 °C for 16 and 24 h showed nearly the same UTS (598 MPa); the former possessed YS of 538 MPa, and the latter, 545 MPa. The results presumably reveal that the peak ageing condition for NA-0d samples can be achieved under 24 h ageing at 120 °C. Under the same treatment at 120 °C for 24 h, the size of η’ phase in NA-7d sample (with a length of 4.96 nm) coarsened and grew larger than that in NA-0d sample (with a length of 3.46 nm). In addition, some η’ phase in the NA-7d sample was found to be transformed into the η2 phase. The results indicated that the naturally aged specimens (NA-7d) reached the peak ageing condition earlier, but did not significantly enhance the UTS in AA7075 aluminum alloy, as compared to the samples without prior natural ageing (NA-0d).

Influence of Heat Treatments on Microstructure and Hardness of a High-Strength Al-Zn-Mg-Cu-Zr Alloy Processed by Laser Powder Bed Fusion

“…The increase in hardness due to natural aging is not surprising, as 7xxx series alloys are known to 'age' at room temperature after solution treatment [53,54]. However, we have seen in the previous section that solutes are present in the supersaturated solid solution of the matrix (Table 2) and are located at the grain boundaries.…”

Research on Alloy Design and Process Optimization of Al–Mg–Zn-Cu-Based Aluminum Alloy Sheets for Automobiles with Secured Formability and Bake-Hardenability

“…Therefore, currently, Al-Mg-Si alloys are mainly used, which have high formability and relatively low yield strength, even after natural aging, and excellent bakehardening characteristics after paint bake treatment. On the other hand, in the case of Al-Mg-Zn-Cu alloys, it is generally known that maximum strength is obtained through artificial aging at a temperature of around 120 • C [14][15][16], and excellent mechanical properties of over 500 MPa can be secured [17,18]. But, despite their high-strength characteristics, they are not preferred due to increased yield strength due to natural aging, increased sheet-forming pressure, and reduced mechanical properties due to grain coarsening at bake-hardening temperatures [7,[19][20][21].…”

Optimization on the electrical discharge machining (EDM) process parameters of aged AA7075/TiC metal matrix composites

“…The findings indicate that the natural aging process can significantly improves the strength of AA7075 due to the creation of clusters in the AA matrix. This, in turn, expedites the precipitation of the 'phase during subsequent artificial aging treatment (Tai et al, 2022). Enhancements in microhardness and wear resistance were achieved by elevating the heating temperature and prolonging the heating time.…”