Alloys 771.0 and 772.0 are high-strength, shock-resistant aluminum sand casting alloys that develop a high combination of physical and mechanical properties in the as-cast and room-temperature-aged conditions. Without heat treatment, mechanical properties for both alloys are superior to many conventional aluminum casting alloys after solution heat treatment.
For castings that may require subsequent straightening, 772.0 is very suitable. The day a casting is made, immediately after a 700F (371C) stress-relieving heat treatment, or following a brazing treatment at 1125F (607C), the low yield strength of about 16,000 psi readily facilitates the straightening of a part.
These alloys are completely dimensionally stable to 0.00001 in./in. after stress relieving. Those parts requiring a high degree of dimensional stability may be stress relieved by heat treaiment at 700F (371C) for 4 hours, followed by cooling in still air. No subsequent treatment is required.
Castability
The castability of Alloys 771.0 and 772.0 is considered good. Fluidity and resistance to solidification shrinkage are above average for the 7XX system.
Machinability
The machinability of both alloys is classed as excellent. Both alloys are dimensionally stable and machinable on the day they are cast or stress relieved. Some self-aging alloys require an aging period of up to 21 days before being dimensionally stable or hard enough for machining.
Because no silicon is present, the life of cutting tools is substantially extended, and machining operations can be performed at maximum speeds. Although standard tools may be used, high rake and clearance angles will aid in machining operations.
Brazing
The alloys, in contrast to most others, are not permanently affected by high temperatures (1125F, 607C) of dip or furnace brazing. After brazing, the castings undergo a room temperature aging cycle similar to the regular stress-relieving treatment at 700F (371C) to restore them to their full physical and mechanical properties.
When fully aged after brazing, castings made from these alloys are dimensionally stable and completely stress relieved and have an excellent combination of toughness and high strength to withstand heavy shock, high static, and severe fatigue or endurance loading.
Finishing
Polishing castings of these alloys produces a fine chrome-white finish without the blue tint of many conventional alloys containing copper or silicon. They have unusually low drag resistance and will not smear on a polishing wheel.
The alloys anodize to a satin white finish when the sulfuric acid process is used and dye brilliantly in pastel shades. Because of such qualities, these alloys are used widely for ornamental or decorative purposes.
Corrosion Resistance
Resistance to most forms of corrosion is excellent. In accelerated salt spray tests, the alloys proved to have a resistance equal to that of high-purity 443.0. In addition, 771.0 and 772.0 retain the chrome-white color under corrosive conditions, an advantage for architectural and ornamental castings that are exposed to the aimosphere.
