What Are Metals?

Metals are a class of elemental and compound materials characterized by metallic bonding, in which valence electrons are delocalized across the entire solid rather than localized between specific atom pairs, producing high electrical and thermal conductivity, optical reflectivity, mechanical ductility, and the capacity for plastic deformation without fracture. Approximately 80 of the 118 confirmed chemical elements are metals, ranging from abundant structural metals such as iron, aluminum, and copper to rare precious metals such as gold, platinum, and palladium. The study of metals spans materials science, solid-state physics, chemistry, and multiple branches of engineering.

Metals adopt ordered crystalline structures in the solid state. The three most common lattice arrangements are face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). FCC metals such as copper, aluminum, and nickel are highly ductile because the geometry of their close-packed planes provides many slip systems for dislocation movement. BCC metals such as iron and tungsten are stronger and harder at room temperature. The grain structure that results from solidification, and its subsequent modification by deformation and heat treatment, governs mechanical performance in engineering applications, as documented in the University of Maryland Materials Science and Engineering metals overview.

Alloys and Composite Metal Forms

Pure metals are rarely used as structural materials because alloying consistently improves relevant properties. Steel, the most widely produced alloy in the world, is an iron-carbon system with properties tunable from soft, weldable mild steel (0.1 weight percent carbon) to ultra-hard tool steel (1.0 percent carbon). Nickel-titanium (Nitinol) exhibits shape-memory behavior, recovering its original form after large deformations, which is exploited in medical stents and actuators. Aluminum alloys of the 2000 and 7000 series combine low density with high specific strength for aerospace and automotive uses.

Filler metals are a specialized category used in welding and brazing to join base metal components. A filler metal must wet and bond to the base metal, have a melting point compatible with the joining process, and produce a joint with adequate strength and corrosion resistance. Silver-based brazing alloys, copper-zinc brasses, and nickel-based filler metals each address specific temperature and environmental requirements in different joining applications.

Metal Foam

Metal foam is a cellular metallic material in which voids occupy 70 to 95 percent of the total volume, yielding densities as low as 5 to 25 percent of the solid metal. Open-cell foams have interconnected porosity that allows fluid flow; closed-cell foams seal the voids and provide flotation and superior impact energy absorption. Research published in PMC on aluminum foam from recycled chips demonstrates that open-cell aluminum foam can be produced from machining scrap using salt as a space holder and compaction below 300 degrees Celsius, a significantly more energy-efficient route than remelting. Fraunhofer IFAM in Germany has led industrialization of metal foam for structural acoustic panels, crash absorbers, and heat exchangers.

Processing and Manufacturing

Industrial transformation of metals into useful products involves several forming categories. Die casting injects molten metal under high pressure into precision steel dies, producing near-net-shape components for automotive and consumer electronics in aluminum, zinc, and magnesium alloys at high production rates. Blanking is a sheet metal shearing process in which a press tool cuts flat shapes from coil or sheet stock; it is fundamental to the production of electrical contacts, chassis parts, and stamped brackets. Both processes depend on metal flow and formability, which are determined by the crystallographic texture and strain hardening characteristics of the input material. The Fraunhofer Institute for Manufacturing Technology and Advanced Materials conducts applied research spanning these and other metal forming and joining processes.

Applications

Metals have applications across virtually every engineering sector, including:

• Structural components in civil infrastructure, transportation, and aerospace systems
• Electrical conductors, transformer cores, and electromagnetic shielding in power systems
• Medical implants, surgical instruments, and diagnostic device components
• Packaging and household goods fabricated by stamping, drawing, and casting
• Electronic contacts, heat sinks, and interconnects in semiconductor packaging