What is Superhard aluminium alloy?
Superhard aluminum alloy refers to an aluminum-zinc-magnesium-copper alloy with ultra-high strength, which is the highest strength among existing aluminum alloys, so it is also known as ultra-high-strength aluminum alloy. Its typical alloy grades are 7A04 (LC4) alloy in China, 7075 alloy in the United States and B95 alloy in the former Soviet Union. In order to further improve the strength, on the basis of these alloys, China developed the LC6 alloy, the United States developed the 7178 alloy, and the former Soviet Union developed the B96 alloy. These alloys can be heat treated to strengthen and used in the artificially aged state.
A class of wrought aluminum alloys, mainly aluminum-zinc-magnesium-copper alloys, codenamed LC. The strength limit can reach 500-700MPa, which is the highest type of aluminum alloy at room temperature. In addition to the main alloying elements, small amounts of chromium, manganese, titanium, etc. are often added. Zinc and magnesium are the main strengthening elements, copper plays a supplementary strengthening role and improves stress corrosion resistance, and manganese and chromium can improve artificial aging strengthening effect. The main strengthening phases are MgZn2 and Al2Mg3Zn3, followed by Al2CuMg. All are used in the quenched and artificially aged state. Due to poor corrosion resistance, an aluminum-clad layer containing 1% zinc is generally coated on the surface. Mainly used for structural parts with greater stress. Such as aircraft girders, stringers, reinforcement frame, landing gear, etc. The main grades are LC4 (containing 1.4-2.0% copper, 1.8-2.8% magnesium, 0.2-0.6% manganese, 5-7% zinc, 0.10-0.25% chromium), LC6 (containing 2.2-2.8% copper, 2.5-3.2% Magnesium, 0.2~0.5% manganese, 7.6~8.6% zinc, 0.1~0.25% chromium) and LC9 (containing 1.2~2.0% copper, 2~3% magnesium, 5.1~6.1% zinc, 0.16~0.30% chromium), etc.
Main added elements
Zinc, magnesium and copper are the main added elements of the alloy, and their total content can be as high as 13.5% in some alloys (LC6 and B96 alloys), and the ratio of zinc to magnesium (Zn/Mg) is about 2.7. The main phase composition of the alloy is η (MgZn2), T (Al2Mg3Zn3) and S (Al2CuMg) phase (see the phase of aluminum alloy); a small amount of added manganese, chromium, titanium and other transition group elements are generated in the alloy. Improve plasticity and corrosion resistance, and can inhibit the recrystallization process. Iron and silicon are harmful impurities that not only reduce strength and corrosion resistance, but also significantly reduce fracture toughness.
Features
The characteristics of this type of alloy are that the solid solution strengthening temperature range is relatively wide (460 ~ 475 ° C), the heat treatment strengthening effect is obvious, and the strength is very high (up to 600MPa). The alloy has good plastic workability in the hot and annealed state. The grades and compositions of several typical superhard aluminum alloys in various countries are listed in Table 1, and the corresponding mechanical properties are listed in Table 2.
Table 1: Typical grades and chemical compositions of superhard aluminum alloys in various countries
| Type | Country | Main Element % | Impurities/% | Zinc to magnesium ratio | ||||||
| Zn | Mg | Cu | Cr | Mn | Ti | Fe | Si | Zn/Mg | ||
| 7A04(LC4) | CHINA | 5.0～7.0 | 1.8～2.8 | 1.4～2.0 | 0.1～0.25 | 0.2～0.6 | 0.5 | 0.5 | 2.61 | |
| LC6 | CHINA | 7.6～8.6 | 2.5～3.2 | 2.2～2.8 | 0.1～0.25 | 0.2～0.5 | 0.5 | 0.3 | 2.82 | |
| 7075 | USA | 5.1～6.1 | 2.1～2.9 | 1.2～2.0 | 0.18～0.28 | 0.3 | 0.2 | 0.5 | 0.4 | 2.24 |
| 7178 | USA | 6.3～7.3 | 2.4～3.1 | 1.6～2.4 | 0.18～0.35 | 0.3 | 0.2 | 0.5 | 0.4 | 2.47 |
| B95 | RUSSIA | 5.0～7.0 | 1.8～2.8 | 1.4～2.0 | 0.1～0.25 | 0.2～0.6 | 0.5 | 0.5 | 2.61 | |
| B96 | RUSSIA | 8.0～9.0 | 2.3～3.0 | 2.0～2.6 | ≤0.05 | 0.1 | 0.4 | 0.3 | 3.21 | |
Table 2: Mechanical properties of typical superhard aluminum alloys in various countries
| Alloy | Type | Mechanical properties | Product type | ||
| σb/MPa | σ0.2/MPa | δ/% | |||
| 7A04(LC4) | T6(CS) | 600 | 550 | 8 | Profile |
| LC6 | T6(CS) | 680 | 640 | 7 | Profile |
| 7075 | T6 | 570 | 500 | 11 | Profile |
| 7178 | T6 | 610 | 530 | 9 | profiles and bars |
| B95 | T1 | 600 | 560 | 7 | Extruded strip |
| B96Ц1 | T1 | 740 | 710 | 7 | bar |
Note: T6(CS) – quenched and artificially aged state (peak) in China; T6 – quenched and artificially aged in the United States (peak); T1 – quenched and artificially aged in Russia (peak).
Disadvantage
The disadvantage of this type of alloy is that the stress corrosion resistance and fracture toughness are low, and the heat resistance is not good, and it usually works below 120 °C. In order to improve corrosion resistance, LB1 alloy can be coated on sheet products, and profiles and forgings without aluminum can be anodized. In order to improve stress corrosion resistance and fracture toughness, graded aging treatment can be carried out, but some of the strength will be lost.
Applications
Super-hard aluminum alloy is widely used in aircraft manufacturing industry, suitable for making aircraft beams, frame stringers, joints, skins, landing gear and high-strength compression parts
