7xxx Aluminum Alloy Datasheets Free

COMMERCIAL 7XXX SERIES ALLOYS are capable of being heat treated to exceptionally high strength levels (>600 MPa, or 87 ksi, yield strength). They are commonly produced in the form of sheet, plate, extrusions, rod, bar, and forgings. Zinc, in amounts of 1 to 8%, is the major alloying element in 7xxx series alloys and, when coupled with a smaller percentages of magnesium and copper, results in heat treatable alloys of moderate to very high strength. Other elements, such as zirconium and chromium, are usually also added in small quantities. Wrought heat treatable 7xxx alloys are even more responsive to precipitation hardening than the 2xxx series alloys and can achieve higher strength levels, approaching tensile strengths of 690 MPa (100 ksi). The alloys are based on the Al-Zn-Mg(-Cu) system. The 7xxx alloys can be naturally aged (e.g., Fig. 1), but this is usually not performed because they are not stable if aged at room temperature; that is, their strength gradually increases with increasing time and can continue to do so for years. Therefore, all 7xxx alloys are artificially aged to produce a stable temper.

Figure 2 illustrates relationships among commonly used alloys in the 7xxx series. Various intermetallic phases occur in the ternary Al-Zn-Mg system, the η phase (also known as “M” or Zn₂Mg) being the most important one for age hardening. Alloys with low magnesium contents are situated in the phase field involving the Z-phase (Mg₂Zn₁₁), alloys lower in zinc the T-phase (Mg₃₂(Al,Zn)₄₉), and eventually the β phase (Al₈Mg₅) on the magnesium side (Fig. 3). Many alloys are located in the ternary field α + η + T or even in the α + T field, but still develop more phases related to η instead of T. The T-phase tends to be observable only at temperatures above 200 °C (390 °F) in common alloys.

The Al-Zn-Mg-Cu alloys attain the highest strength levels when precipitation hardened. Because these alloys contain up to 2.5 wt% Cu, they are the least corrosion resistant of the series. However, copper additions reduce the tendency for stress corrosion cracking (SCC) because they enable precipitation hardening at higher temperatures. As a class, these alloys are not weldable and, therefore, are joined using mechanical fasteners. The best known of these alloys are alloys 7075, 7475, and 7050. Some of the newer alloys were developed to optimize fracture toughness and resistance to corrosion, primarily stress corrosion cracking and exfoliation corrosion. This was accomplished through a combination of composition control and processing, primarily through the development of new overaging heat treatments. For some of the newer 2xxx alloys, iron and silicon impurity levels are reduced to maximize fracture toughness. Fracture toughness values of select 7xxx and 2xxx alloys are given in Tables 1 and 2.

Aluminum 7xxx alloys with more than 0.25% Cu are not considered weldable by fusion welding. Although the Al-Zn-Mg alloys cannot attain as high a strength level as alloys containing copper, they have the advantage of being weldable. In addition, heat from the welding process can serve as the solution heat treatment, and they will age at room temperature to tensile strengths of approximately 310 MPa (45 ksi). Alloy developers found that Cu-free alloys containing about 4.5% Zn and 1.5% Mg with small amounts of Cr, Mn, and Zr were both weldable and resistant to stress corrosion cracking. They develop strength intermediate to that of 6061-T6 and 2219-T8, even when press quenched. The alloys can age harden in the weld heat-affected zone up to about 80% of the parent metal by natural aging and can recover 100% of the strength if artificially aged after welding. Consequently, the alloys are often used for welded structures. At relatively high Zn/Mg ratios, they are resistant to SCC in thin sections. Yield strengths can be as much as twice that of the commonly welded alloys of the 5xxx and 6xxx series alloys. To reduce the chance of stress corrosion cracking, the alloys are air quenched from the solution heat treating temperature and then overaged. Air quenching reduces residual stresses and reduces the electrode potential in the microstructure. The aging treatment is often a duplex aging treatment of the T73 type. The commonly welded alloys in this series, such as 7005, are predominantly welded using the 5xxx-series filler alloys.

The solvus temperatures and quench sensitivity of the low-Cu alloys are also much lower than for the higher strength alloys. Alloy 7075 is a widely used alloy with high strength in the T6 (peak aged) condition, but it is more quench sensitive than other alloys like 7050 (Fig. 4). Another limitation of 7075 and similar alloys when they are heat treated to the peak-aged (T6 temper) condition is the occurrence of stress corrosion cracking. Thick plate, forgings, and extrusions of these alloys are particularly vulnerable when stressed in the through-the thickness (short-transverse) direction. In response to these in-service failures, a number of overaged T7 tempers were developed, but with some sacrifice in strength properties.

Alloy 7050 offers strength comparable to that of 7075, but higher strength in thicker sections (Fig. 5). Alloy 7050 (and other newer alloys such as 7150, 7055, 7040, 7085, and 7140,) replaces Cr with Zr additions to prevent recrystallization of hot-worked products during solution heat treatment. The addition of 0.1% Zr is nearly as effective as 0.25% Cr in preventing recrystallization, and Zr does not affect quench sensitivity to the extent that Cr additions do, where Cr-bearing dispersoids function as nucleation sites for precipitation during a slow quench.

Aging practices of alloys 7050, 7075, and 7475 vary with product form, size, equipment, loading procedures, and furnace-control capabilities. The optimum practice for a specific item can be ascertained only by actual trial treatment of the item under specific conditions. Aging of aluminum alloys 7050, 7075, and 7475 from any temper to the T73 and T76 temper requires tighter control of aging variables (e.g., time, temperature, heat-up rate, etc.) than typical for any given item. In addition, when material in a T6-type temper is re-aged to a T73- or T76-type temper, the specific condition of the T6 material (such as property levels and other effects of processing variables) is extremely important and affect the capability of the re-aged material to conform to the requirements specified for the applicable T73- or T76-type temper.

The editors thank Gary Bray of Arconic, Tim Fargo of Kaiser Aluminum, Michael Niedzinski of Constellium, and Robert Sanders of Novelis for their assistance in collecting information for these datasheets.