Introduction to Aluminum – Properties, Processing, and Applications of this Widely Used Metal
Overview
Aluminum, or aluminum, is a ubiquitous lightweight silver-white metal with the atomic symbol Al and atomic number 13. With low density and high strength-to-weight ratio, aluminum and its alloys display excellent malleability, formability, ductility, conductivity, corrosion resistance, and recyclability.
After iron and steel, aluminum is the most abundantly produced and consumed non-ferrous metal, finding uses across transportation, construction, packaging, machinery, and electrical industries. Key attributes like electrical conductivity, reflectivity, and corrosion resistance drive aluminum’s versatility. Understanding aluminum’s capabilities and processing methods enables effective engineering design and manufacturing.
Pure aluminum has low strength compared to alloys, which incorporate elements like copper, silicon, magnesium, manganese, and zinc. Wrought and cast aluminum alloys provide tailored properties. New aluminum grades, composites, and processing innovations continue advancing applications.
Physical Properties
With a density of around 2.7 g/cm3, aluminum is one-third the weight of steel or copper. This lightweight directly enables fuel efficiency in vehicles, range in aircraft, and portability across markets needing lightweight structures.
The strength of pure aluminum is around 90 MPa, but cold working can increase this by 330 MPa and alloys multiply strength considerably. Hardness ranges from 60-150 BHN for wrought alloys. Aluminum retains strength and toughness at cryogenic temperatures, with little low-temperature embrittlement.
The electrical conductivity of aluminum is 62% that of copper by IACS percentage. Its thermal conductivity is 50-60% of copper. These conductive properties team with light weight to make aluminum ideal for power transmission lines, heat exchangers, and electronics cooling.
Aluminum has reflectivity exceeding 90% for visible light and infrared. It also exhibits excellent UV and microwave reflectance. These factors contribute to applications ranging from aluminum foil to telescope mirrors to aircraft laser/radar stealth.
The coefficient of thermal expansion for aluminum is relatively high at 25 μm/(m·K). This must be accounted for in the design to prevent thermal stresses. On the plus side, the high CTE aids aluminum’s pressure tightness and filling ability in cast molds.
In terms of corrosion, aluminum forms a protective oxide layer resisting damage in water, air, and chemical exposures. High-purity alloys display excellent resistance. Alloying elements like copper and iron may reduce corrosion properties. Overall, aluminum is highly corrosion-resistant across environments.
Mechanical Properties
The tensile strength of pure aluminum is 90 MPa, but alloying with elements like magnesium, silicon, and especially copper significantly increases strength while retaining ductility. Heat-treatable alloys also multiply strength. The strongest aluminum alloys approach yields strengths over 500 MPa.
Elongation at break averages 12-25% for wrought alloys, but certain compositions have elongation up to 40% for excellent formability. Aluminum maintains its tensile strength and ductility down to cryogenic temperatures. It has no true fatigue limit, but good fatigue strength.
The fracture toughness of aluminum is lower than steel but generally sufficient for design needs. Alloys like 7075 have toughness values around 30 MPa√m. Impact resistance varies, with some alloys having excellent energy absorption during impact. Overall, aluminum provides a versatile balance of strength, ductility, and toughness.
Processing Techniques
Aluminum’s excellent castability, machinability, and formability enable the cost-efficient processing of diverse products. Key methods include:
Casting: Aluminum readily pours and solidifies into sand, permanent, and die-cast molds. This enables high volume production of complex, net-shape components like engine blocks and housings. Castability is affected by alloy fluidity, shrinkage, hot cracking tendency, and more.
Extrusion: Forcing heated aluminum billets through dies makes versatile sections like bars, rods, tubes, and machined stock. Alloys are selected for extrudability. Feed rate, die friction, and temperature control extrusion quality.
Forging: Applying compressive force to aluminum preforms enables the manufacture of strong, seamless components like automotive control arms and aircraft fittings. Alloy ductility affects enforceability. Die design is critical.
Machining: Aluminum machines easily use conventional or CNC methods at high speeds and feeds. Multi-axis machining centers output aluminum automotive and aerospace components. Chip control and heat are considerations.
Other common processes like rolling, welding, heat treatment, and surface finishing tailor aluminum’s properties and performance. Choosing optimal procedures and parameters ensures cost-effective fabrication.
Major Applications
Major markets for aluminum include transportation, electrical, construction, consumer goods, machinery, and packaging. More specific applications include:
Aerospace: Aluminum alloys comprise over 80% of commercial aircraft structures because low density provides fuel savings. High-strength 2xxx and 7xxx alloys are heavily used. titanium in hot sections.
Automotive: Automakers choose aluminum for engine blocks, wheels, panels, frames, and more to improve fuel economy through light-weighting. 6xxx sheet alloys see high automotive use.
Electrical: Aluminum wiring, high voltage conductors, transformer windings, capacitors, printed circuit boards, and heat sinks utilize aluminum’s conductivity and thermal performance.
Building Construction: Siding, gutters, doors, roofs, and structural framing all leverage aluminum’s strength, corrosion resistance, and lightweight. 5xxx dominates construction alloys.
Packaging: Aluminum foil and containers keep food fresh while minimizing weight. Recyclability and low environmental impact add to the advantages. 1xxx alloys like 1050 and 1100 are common.
Comparison to Alternatives
Compared to steel, aluminum offers one-third the weight for equivalent strength in lightly loaded applications. Compared to titanium or magnesium, aluminum has higher strength and formability. Against polymers, aluminum provides thermal performance, ruggedness, and recyclability.
Cost is higher than steel but lower than titanium or polymer composites. However, aluminum enables performance gains in fuel efficiency and corrosion resistance leading to lower operational costs. Density is slightly higher than composites but superior to steel. With new high-strength alloys and processing, aluminum competes against ever broader materials categories.
Ongoing Developments
Advanced joining techniques like friction stir welding integrate aluminum structures. Additive manufacturing processes are being adapted for aluminum components with complex geometries. New alloy compositions and multi-material structures further increase capabilities.
Automotive and aerospace industries continue driving aluminum research, including technologies like single crystal castings for turbine blades. Designers also develop novel aluminum surface treatments and coatings for enhanced wear and corrosion resistance in demanding applications.
Embedded sensors in aluminum structures enable smart monitoring of strains, cracks, and corrosion. Digital simulation better optimizes aluminum products and manufacturing. These areas highlight exciting innovation potential.
Conclusion
Aluminum’s unique combination of light weight, strength, formability, conductivity, reflectivity, corrosion resistance, and recyclability makes it a multifaceted material upholding advanced industries worldwide. Each year over 65 million metric tons of aluminum are produced as demand grows steadily. Ongoing metallurgical research and processing developments will unlock novel high-performance applications, further demonstrating aluminum’s indispensability in engineering designs.