Due to its low melting point, aluminum requires forging temperatures significantly lower than those of other common metals. The low density of aluminum also results in a reduced ability to retain heat. As a result, heat treatment is essential to prevent rapid cooling of the workpiece. The forging temperatures of aluminum alloys can typically be equal to or close to the billet temperature, a process known as isothermal forging. Fortunately, most die steels do not undergo tempering when used for isothermal forging of aluminum.
The forging temperature range for aluminum alloys is 775-875°F, covering the temperature ranges used for forging, heat treatment, and aluminum alloy processing. The specific forging temperature varies depending on the alloy. Flow behavior includes possible softening under high strain, leading to a tendency for localized flow. Forging very complex shapes can be achieved using isothermal conditions and low stamping speeds. This type of forging is sometimes referred to as precision forging.
For most forging applications, the blank must undergo some deformation processes (cutting, rolling, or extrusion) to break the casting grain structure. For aluminum, continuous casting blanks are more common. Blanks can be homogenized when a more uniform microstructure is required. Blanks have some pores that must be "repaired" through forging. Of course, extruded and forged blanks are also used. Since aluminum has high ductility and a low melting point, grinding saws are not used.
Aluminum alloys are typically forged in gas or electric furnaces, as the melting point of aluminum is significantly lower than that of other common metals. Therefore, due to the relatively lower forging temperatures, convection is the primary mode of heat transfer. Above 1500°F, radiation becomes the dominant form of heat transfer in metals. Since aluminum is not ferromagnetic, induction heating is not feasible.
Forging aluminum alloys with high strain rate sensitivity is typically done on hydraulic presses. The use of hammers and mechanical presses is the exception rather than the rule. The forging process is usually slow, often at 1 inch per second or less. Hot molds, including isothermal molds, are common. In most cases, the tool temperature is within 250°F of the workpiece temperature. Isothermal molds and low strain rates are standard for net shaping/premium forgings.
Forging aluminum alloys requires caution. As the hammer head deforms rapidly, workers may notice the workpiece's heat brittleness. Currently, organic lubricants are being replaced by water-based and synthetic lubricants. Lubrication is a crucial aspect when forging aluminum, as the metal is prone to wear. Aluminum alloys, due to their low flow stress and high ductility, can be forged into very complex shapes compared to other metals. Potential defects include grinding, wear, flow location, and unfilled/absorbed areas. Flow locations may appear as circles or cracks. Large production aluminum parts are precision forged with no defects. Small batch parts are forged in finished molds to remove defects, then cast using the same mold. This is not feasible for other alloys as it would lead to oxidation skin or grain growth.
