Microalloying of Advanced Al-Zn-Mg-Cu Alloy

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The characteristics that make aluminum a widely used structural material are cost effectiveness, high strength, low density, recyclability, and workability. Moreover, aluminum has been used in aerospace structures for more than 70 years. Recent advances in aluminum alloy/temper development are maintaining aluminum alloys as the materials of choice for future commercial aircraft structures, where there is a need to meet cost and weight-requirement objectives.

Combinations of magnesium and zinc in aluminum provide a class of heat treatable alloys, some of which develop the highest strengths presently known for commercial aluminum-base alloys. This is the result of a combination of elements that have a high mutual solid solubility in aluminum and also develop unusually high precipitation-hardening characteristics. Supplementary alloying additions to such alloys include chromium, copper, and manganese.

Al-Zn-Mg-Cu Alloys of complex specified properties, such as strength, ductility, fracture resistance, stress corrosion cracking resistance are required for the application in structures operating under heavy service loadings (aircrafts and rocket industry) with prescribed high level of safety margin.

Aluminum alloys, due to convenient strength-to-density ratio, have found extensive application in heavy-duty structures. This is the case with AlZnMgCu alloys exhibiting the highest strength level, but in the same time high level of susceptibility to fracture and stress corrosion.

Mechanical properties of AlZnMgCu alloys are depended on chemical composition, primarily on total Zn+Mg content and Zn/Mg ratio, on the presence of Mn, Cr, Zr and impurities Fe and Si, on the micro structure as a result of fabrication thermo mechanical and heat treatment conditions, e.g. on morphology, distribution and volume portion of intermetallic (IM) phases and texture.

Based on requirement to develop Al alloy with ultimate tensile strength above 650 MPa (Rm > 650 MPa) and satisfactory level of ductility and fracture toughness, several alloys had been designed with Zn content between 6 and 8.8% and Mg content of 2.15 to 3.5%, saving the same amount of other alloying elements (Cu, Mn, Cr, Zr).

One of the main advantages of AlZnMg alloys in comparison with other aluminum-base alloys is their high strength combined with high ductility. For example, typical 0.2 pct proof strength for alloy AA 7108.70 is approximately 400 MPa and elongation to fracture (A5) is approximately 12% in the T6 condition. This alloy contains typically 5.4% Zn, 1.2% Mg, and 0.15% Zr.

The high strength of these alloys makes them very suitable for structural applications where there are strong demands for high strength combined with minimum weight. One example is bumper beams in cars, which can be made of hollow or semi hollow hot extruded profiles.

One disadvantage of the AlZnMg alloys is that their high room-temperature strength is accompanied with a high deformation resistance at hot working temperatures. The high deformation resistance is mainly attributed to the presence of magnesium, copper, chromium, and zirconium. High deformation resistance at hot working temperatures may result in a low extrusion speed due to limitations in available ram pressure and due to heat generation during extrusion. A high ram pressure causes large stresses in the extrusion tool, and the tool life may therefore be reduced.

Aluminum alloys of the 7075 type (Al-Zn-Mg-Cu) are widely used in airframe structures; they provide very high strength and stiffness, but are prone to stress corrosion cracking (SCC), particularly when aged to the near-peak-strength T6 condition. Their resistance to SCC can be increased by over aging to the T73 temper, but with a loss of strength. For example, the yield strength of 7075-T73 is about 10 to 15 % lower than that of 7075- T6.

To replace the traditional 7xxx Al alloys, such as the US 7075, Russian B95, and the Chinese LC4 and LC9 alloys (similar to the B95 and 7075 alloys, respectively), which were widely used in Chinese airplanes, the Beijing Institute of Aeronautical Materials has recently developed a new super-high-strength IM/Al-Zn-Mg-Cu alloy (C912: Zn–8.6%, Mg-2.6%, Cu-2.4%; Version C912C: +0.1Ce%; Version C912N: +0.1%Ni; Version C912S: +0.2%Sc). The tensile strength and compressive strength of the C912 alloy are higher than those of the traditional 7xxx aluminum alloys, such as the 7075 and 7178 alloys, and are similar to the new Alcoa aluminum alloy 7055 and the Russian alloy B96, which have the highest strengths of the commercial IM/ Al- Zn-Mg-Cu alloys.

To achieve the objective of obtaining a superior alloy, micro alloying elements were added to improve the mechanical and corrosion properties of the C912 alloy. Micro alloying technology was originally developed for micro alloyed steels. Although the amount of micro alloying elements is usually less than 1%, they lead to improved combinations of strength and ductility, weldability, toughness, and corrosion resistance.

Micro alloying element additions such as Zr, Mn, Cr, Ag, and Sc can enhance many critical properties in aluminum alloys. Experiments were conducted to determine the general effect of microalloying elements (such as Sc, Ni, and Ce) on the properties of the C912 alloy.

In particular, it was examined whether Sc has a positive effect on retarding recrystallization and improving the strength and corrosion resistance of the C912 alloy, just as it does in other aluminum alloys, also to examine whether Ni has an effect on improving the strength and corrosion resistance, even though it has limited solubility in aluminum, and to examine whether Ce can improve the alloy is corrosion resistance just as it does in some other wrought aluminum alloys.

Experiments proved the following:

• With appropriate microalloying and heat treatment, the C912 alloy class can exhibit very high strength (>640 MPa) and good SCC resistance. The Sc-modified alloy possessed the highest strength and SCC resistance of the C912 alloys studied.
• The addition of 0.2% Sc results in the formation of A13(Scl-xZrx) phase. This phase is highly effective in refining the microstructure, retarding recrystallization, and strengthening the Al-Zn-Mg-Cu alloy. With its refined and unrecrystallized grain structure, the C912 alloy containing Sc has a significantly improved SCC resistance.
• The addition of 0.1% Ni enhances the tensile strength of the C912 alloy. It also improves the SCC resistance of the alloy.
• In the wrought Al-Zn-Mg-Cu alloy, Ce has little strengthening effect.
• The improvement in SCC resistance of the C912 alloys studied correlates very well with the grain-boundary precipitate size, grain shape, and the volume fraction of matrix precipitates.