772 aluminium alloy is a high strength alloy that is widely used in aerospace applications due to its high shock and corrosion resistance properties. The main alloying element is zinc, which accounts for 6.5 – 7.5 % of the metal.
Parts made from 772 aluminium alloy can only be created by sand casting. Even without heat treatment, the alloy has superior properties to many other aluminium alloys. It develops these physical properties immediately post-casting and as it settles to room temperature conditions.
What is sand casting?
Before taking a detailed look at the properties of 772 aluminium alloy, it’s useful to get a picture of the process in which it is made.
To put it simply, sand casting is a process whereby a solid 3D pattern is made and is embedded in sand using a gating system. It is then made into a mould using clay or an adhesive. The molten metal mixture is then poured into the mould cavity and then is left to cool. Finally the sand mould is broken away to reveal the finished casting.
If parts needs straightening after casting, 772 is an ideal alloy. Whether the alloy is heat-treated at 371°C or brazed at 607°C, it can be almost immediately straightened due to its relatively low yield strength (16,000 psi).
Following stress relief, 772 alloy has remarkable dimensional stability (coefficient of thermal stability is 0.00001 in/in). In the event that an even higher degree of dimensional stability is needed, the alloy can be heat-treated at 371° C for several hours, after which it is cooled in still air.
Castability of 772 Aluminium Alloy
772 is relatively easy to cast. It has high fluidity which means it is easy to pour, and it has an above-average resistance to solidification shrinkage. It out-performs other alloys in the 7XX series in terms of castability.
Machinability of 772 Aluminium Alloy
This is the property that sets the 772 alloy apart from other alloys. Its machinability is excellent, meaning it can be machined very soon after casting or stress relief. When you compare this to some other alloys that need to be aged for up to 21 days before the dimensional stability or hardness is strong enough to machine, it’s easy to see the advantage of using 772 alloy in manufacturing environments where speed is important.
Another advantage is that it contains no silicon, therefore cutting tools will last longer and the speed of machining can be much greater.
Brazing 772 Aluminium Alloy
772 aluminium alloy is fairly unique in that it is not permanently altered by the extreme temperatures of brazing whether that is using a dip method or furnace. Usually the castings will be aged at room temperature after brazing to optimise their physical properties as well as developing mechanical ones.
After this ageing cycle is completed the alloy is now fully dimensionally stable and stress relieved. This means it now has all of the properties that make it desirable for aerospace engineering, i.e. high level of toughness and strength, an ability to withstand shock, static, fatigue and high loading.
Corrosion resistance of 772 Aluminium Alloy
Another property that makes 772 alloy ideal for both aeronautical and marine engineering is that it is highly resistant to corrosion. Salt spray tests performed on the 772 alloy showed it to have extremely high corrosion resistance, right up there with very pure alloys such as 443. Even when exposed to highly corrosive conditions, 772 keeps its original chrome colour, which makes it popular for decorative architectural casting especially those exposed to the elements.
Finishing processes for 772 Aluminium Alloy
After casting, 772 alloy is normally polished to a reflective sheen that is chrome/white in colour. It lacks the slight blue tinge of wrought alloys that contain higher quantities of silicon or copper. It’s straightforward to bring it to a shiny finish on a polishing wheel, as it doesn’t smear easily and has a low drag-resistance.
If desired, 772 alloy can be brought to a satin white colour by anodising it in sulphuric acid. It can also be dyed to create different shades. For these reasons, 772 is often used decoratively, for alloy wheel trims on cars for instance.
All in all, 772 alloy is a good material choice when you need a tough, durable, corrosion resistant, easily shaped and machined alloy. This makes it particularly useful in the design of aerospace, marine and heavy-duty land vehicles.
The fact that it is easily polished and coloured also makes it a good choice for ornamental reasons, for instance architectural features of buildings that are on an exposed seafront.
The limitations of 772 aluminium alloy are that it can only be made by a process of sand casting which can reduce the accuracy of fabrication.