Cast vs. Forged vs. Sheet Aluminum: What Are the Differences?

Aluminum alloys combine high specific strength, excellent corrosion resistance, and good processability. They are the automotive industry’s second most widely used material after steel. As vehicle lightweighting requirements increase, aluminum usage continues to rise. Automotive components increasingly use cast aluminum alloys, forged aluminum alloys, and aluminum sheet.

Cast aluminum alloys are commonly used for supports and housings, including transmission housings, engine blocks and cylinder heads, and steering-knuckle brackets. Production processes include vacuum die casting, oxygen-flushed die casting, and squeeze casting.

Cast Aluminum Alloys

In high-vacuum die casting, the metal cavity is evacuated before filling, theoretically preventing gas from mixing with the molten metal. The resulting castings have low gas content, high density, and significantly improved mechanical properties. To support subsequent heat treatment and welding, the effective vacuum pressure in the die cavity must be below 5 kPa. The following figure compares high-vacuum and conventional vacuum die castings.

Oxygen-flushed die casting also reduces porosity. The cavity is first filled with oxygen. As molten aluminum enters, it reacts with the oxygen to form uniformly distributed alumina particles. These particles are smaller than 1 μm and account for only 0.1% to 0.2% of the total weight, so they have almost no effect on subsequent processing. The following diagram shows the principle.

In squeeze casting, molten metal is poured directly into an open die. The die is then closed and static pressure is applied, producing rheological forming, high-pressure solidification, and a small amount of plastic deformation to create a high-quality metal component. The process is divided mainly into direct and indirect squeeze casting. In direct squeeze casting, pressure acts directly on the molten metal, producing a pronounced high-pressure solidification effect; it is generally suitable for simply shaped castings. In indirect squeeze casting, punch pressure is transmitted through the shot sleeve and biscuit to the casting. The applied pressure is limited and the process is relatively slow, making it suitable for high-volume production of complex parts. The following figure compares the two processes.

Forged Aluminum Alloys

Forged aluminum alloys are generally used for parts that require good mechanical performance, including critical chassis components such as suspension control arms and steering knuckles. Current forging methods include liquid die forging, semi-solid die forging, isothermal forging, and powder forging.

Liquid die forging is often used to produce one-piece wheels. The finished products have good mechanical properties and surface quality, but expensive equipment and a complex process keep manufacturing costs high.

Semi-solid die forging resembles liquid die forging. A billet is heated to a semi-solid state, injected rapidly into the die, and subjected to static pressure so that liquid flow and plastic deformation form the part. Compared with liquid die forging, it uses a lower forming temperature and produces less volumetric shrinkage. Shrinkage cavities and collapse are less likely, die life is extended, gas entrainment is reduced, and finished-part quality is substantially improved.

In isothermal forging, a heated billet is placed in a temperature-controlled die and forged slowly at constant temperature. Constant temperature and low forging speed allow more complete dynamic recrystallization, a more uniform microstructure, and improved ductility. A recently studied advanced process for 6082 aluminum alloy combines semi-solid and isothermal forging: the billet is preforged at the solution-treatment temperature, pre-aged, and finally isothermally forged at 200°C. The process produced acceptable parts with higher hardness and strength by improving precipitation and dislocation strengthening.

Aluminum Sheet

After 2012, stricter pedestrian-protection regulations in Europe and North America drove extensive use of aluminum sheet for automotive outer panels. By 2020, aluminum accounted for 75% of hood applications in North America, while its share in doors, luggage compartments, and roofs exceeded 25%. Some Audi, Jaguar, Ford, and Tesla models use predominantly aluminum body structures. In addition to conventional stamping and drawing, hot forming of aluminum sheet has advanced significantly and is now an important high-performance forming technology.

Hot forming was developed to address the low ductility and difficult deformation of high-strength aluminum alloys. Hot stamping is the principal method. A recent study combined a backpropagation neural network with a multi-objective genetic algorithm to optimize key variables such as blank temperature, stamping speed, and die clearance. The resulting components had tensile strengths of 300 to 320 MPa, consistent with the ultimate strength of 6061-T6 aluminum alloy. The optimized parameters produced parts with satisfactory performance, good mechanical properties, and high forming accuracy.

Cold forming of aluminum sheet is used mainly for automotive body panels. Complex geometry, low relative thickness, and high surface-quality requirements make some panels prone to wrinkling, cracking, and springback. During cold stamping, researchers studied how process parameters affect thinning and springback, then compensated the die surface and adjusted the parameters. These measures effectively suppressed springback, wrinkling, and cracking.