7xxx series aluminum alloys

The strongest aluminum alloys

7xxx series alloys are the strongest among all aluminum alloys (drawing 1). However, they have a big disadvantage – they are prone to stress corrosion. The main alloying elements - from 1 to 9 % zinc (most often from 4 to 6 %), from 1 to 3 % magnesium, and, for some alloys – to 3,0 % copper, aluminum – all the rest. These alloys are strengthened by thermal treatment.

Important applications of these alloys is related to their high strength. It - aerospace engineering, military equipment and equipment of nuclear power. Moreover, they are used in construction, as well as for the manufacture of sports equipment, for example, ski sticks and tennis rackets.

Figure 1 – Rating popular strength aluminum alloys

7xxx series aluminum alloys

The solubility of zinc in the aluminum decreases from 31,6 % at 275 ° C to 5,6 % at 125 ºС (figure 2). Industrial 7xxx series alloys contain zinc, magnesium and copper with small additions of manganese and chromium, and zirconium to control grain growth and recrystallization.

Figure 2 – “Aluminum” section of the phase diagram aluminum-zinc [2]

7xxx series aluminum alloys are used to manufacture aircraft carrier elements, such as the upper shell wings, stringers, horizontal and vertical stabilizers. Horizontal and vertical stabilizers have the same design criteria as the wings. The upper and lower surface of the horizontal stabilizer and the bending test is critical for their compressive loading. Therefore, the modulus of elasticity in compression is the most important property. Critical design parameters components upper wing parts are compressive strength and fatigue resistance.

Most durable alloys 7xxx series

All Al-Zn-Mg-Cu aluminum system alloys exhibit the highest strength. addition 2 % Copper in combination with magnesium and zinc significantly increases the strength properties of the 7xxx series alloys.

The highest values of tensile strength, which can only be that of aluminum alloys, have the following alloys:

7075: 5,5 % zinc - 2,5 % magnesium - 1,5 % copper;
7079: 4,3 % zinc - 3,3 % magnesium - 0,6 % copper;
7178: 6,8 % zinc - 2,7 % magnesium - 2,0 % copper.

Alloy 7075-T6 has a very high strength-to-weight ratio, low production cost and good machinability. Therefore, it is preferable when selecting a material for structural members of aircraft. At the same time, alloy 7075 It has a fairly low resistance to corrosion. The tendency of the alloy to stress corrosion cracking can be controlled by the proper heat treatment of some materials and additives, such as chromium.

Figure 3 – The level of strength of aluminum alloy 7075-T6 among other structural materials

Aluminium alloy 7075

T6 temper

Alloy 7075 - an alloy of Al-Zn-Mg-Cu-Cr - has the most extensive and prolonged use of the 7xxx series of alloys. It was introduced in Japan in 1943 year, It was a big secret, and used for the manufacture of Japanese military aircraft. Alloy 7075 originally used for parts and components with a thin cross-section, mainly in the form of plates and extrusions. For these products the hardening rate is generally very high and the tensile stresses do not occur in the short transverse direction. Therefore, stress corrosion cracking is not a problem for such high strength materials. a position to T6.

However, when the alloy 7075 used in the products and details of large size and high thickness, it becomes clear, Such products and parts,, thermally hardened to T6 states, often do not meet the specified requirements. products, are prepared by a large machining of large forgings, extruded profiles or plates, then subjected to sustained tensile stress in an unfavorable orientation. Under these conditions, stress corrosion cracking (stress corrosion cracking) is quite common in service..

condition T73

The solution to this problem was the introduction T73 condition for thick and massive articles from alloy 7075. heat treatment, which is used for this condition, It requires a two-step artificial aging. The second step is performed at a higher temperature, something and, which is used to achieve the T6 condition. This additional heat treatment reduces the strength to a level below, which alloy 7075 It reaches a state T6.

T7 overaging state is achieved, which means, alloy that aging continues after reaching its peak hardness and strength values, unlike T6 states.

Numerous experiments and long operating experience confirmed, alloy 7075-T73 has a considerably higher resistance to corrosion under stress, compared with the alloy 7075-T6.

Interesting, that the wheels of the famous Curiosity rover are made of 7075-T7351 alloy by machining from a one-piece forged ring (picture 8).

(and)

Figure 8 - Wheel of the Curiosity rover made of 7075-T7351 aluminum alloy:
a – on the ground; b - under operating conditions on Mars

Magnesium in aluminum alloys 7xxx

Most of the 7xxx series alloys includes magnesium to improve the effectiveness of the mechanism of hardening due to aging. The main phase strengthening particles act MgZn₂. Such alloys have a high strength, but relatively easy to be welded, for example, alloy 7005 (analogue 1915). Alloys of Al-Zn-Mg have a higher susceptibility to heat treatment, than Al-Zn binary alloys, which gives them higher strength.

Copper Aluminum Alloys 7xxx

Adding copper alloys Al-Zn-Mg together with small amounts of chromium and manganese these alloys gives the highest strength of all known aluminum alloys. Alloys of the Al-Zn-Mg-Cu quaternary system have the highest potential for aging hardening of all aluminum alloys: in some alloys, the tensile strength reaches 600 MPa, as, for example, the alloy V95, and even 700 MPa, as in the alloy B96. However, excessive increase in the content of zinc and magnesium reduces the viscous properties and corrosion resistance under stress. In these alloys, zinc and magnesium control the aging process, whereas the role of copper is to increase the rate of aging and increase susceptibility to hardening. Though copper decreases the overall resistance to corrosion, it improves corrosion resistance under stress.

Sources:

Aluminum and Aluminum Alloys /J. R. Davis, ASM, 1993
Trends in aluminium alloy development /R. Rajan at al – Rev. Adv. Mater. Sci. 44 (2016) 383-397

Details about the states of aluminum alloys:

Introduction to Aluminum Alloys and Tempers – J. Gilbert Kaufman