November 2025 Volume 7

MATERIALS

The general steps for forging aluminum alloys are like other alloys and consist of the following: 1. The aluminum alloy input stock is cut to the appropriate size for the forging. 2. The cut stock is heated along with the forging equipment to the appropriate temperature for the specific alloy and end use of the finished part 8 . 3. The forging equipment and workpiece are lubricated to ensure that the part will readily release from the forging equipment. 4. The forging process takes place with the specifics such as strain rate and deformation mode depending on the forging technique used 8 . 5. The part is finished by the removal of flash generated in the forging process, finish machining, if necessary, along with cleaning, heat treating, and final part inspection 8 . The forging of aluminum alloys is a common and precise method to produce high-quality aluminum alloy parts for many industries (see Figure 8).

238°C) for a set duration. Natural aging can also occur at room temperature, but it's a much slower process.

Figure 9: Aluminum Heat Processes and Resulting Tempers

Forging Surface Considerations Although aluminum is generally resistant to corrosion, some applications may require a surface treatment to enhance that resistance and/or protect against stress corrosion cracking and galvanic couplings. For example, NASA has published guidelines that list some of the high strength forging alloys (2219, 7075) which are considered to have “low resistance” to stress corrosion cracking (see Figure 10).

Figure 8: Cut to Weight Input and Finished Die Forged Parts

Heat Treatment of Aluminum Forgings Post-forging, the heat treatable alloys are typically processed through a solution and aging treatment (see Figure 9). Heat treating takes the solid, alloyed metal and heats it to a specific point. The alloying elements, called solute, are homogeneously distributed with aluminum putting them in a solid solution. The metal is subsequently quenched, or rapidly cooled, which freezes the solute atoms in place. The solute atoms consequently combine into a finely distributed precipitate. This occurs at room temperature, which is called natural aging or in a low temperature furnace operation which is called artificial aging. The thermal treatments are typically done in specially designed furnaces with integral quench tanks to facilitate a fast transfer to quenching media and rapid cooling. 1. Solution Heat Treatment (SHT) - This process involves heating the aluminum alloy to a high temperature (typically between 840°F and 1020°F or 450°C and 550°C). The purpose of SHT is to dissolve alloying elements into the aluminum matrix, creating a solid solution. The alloy is held at this temperature for a specific time to ensure complete dissolution. 2. Quenching - After solution heat treatment, the aluminum is rapidly cooled, usually in water or glycol. This rapid cooling "locks" the alloying elements in solution, preventing them from precipitating out. Quenching is a critical step, and the cooling rate must be carefully controlled to avoid distortion or cracking. 3. Aging (Artificial Aging) - Aging involves heating the quenched aluminum to a lower temperature for a specific time. This allows the alloying elements to precipitate out of the solid solution, further enhancing strength and hardness. Artificial aging is done at elevated temperatures (e.g., 240°F to 460°F or 116°C to

Figure 10: SCC Table for High Strength Aluminum Forging Alloys 14 Aluminum and aluminum alloys are positioned at the top (most anodic or active) of the galvanic series and when coupled with most other structural alloys (SS, Ni, can create the driving force for galvanic corrosion. Standards like MIL-STD-889 give guidance on common surface treatments for aluminum alloys to protect them from exposure to electrolyte (galvanic) and/or damaging (SCC) environments that enable those mechanisms 15 . Per MIL-STD-889, in order of preference for best enhancement: 1. Anodic coating (MIL-A-8625) post treated with a paint or resin coating system. 2. Chromate conversion coating (MIL-DTL-5541) post treated with a paint or resin coating system; or anodic coating, sealed, with resin seal (when porous castings are used, impregnated with resin prior to surface treating and finishing). 3. Chromate conversion coating without paint or resin coating system, for electrical, thermal conducting purposes in mild atmospheres in absence of saline, alkaline or acidic conditions.

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