Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Welding, in engineering, any process in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. Most of the processes may be grouped into two main categories: pressure welding, in which the weld is achieved by pressure; and heat welding, in which the weld is achieved by heat. Heat welding is the most common welding process used today. Brazing and soldering (see Solder) are other means of joining metals.

With the development of new techniques during the first half of the 20th century, welding replaced bolting and riveting in the construction of many types of structures, including bridges, buildings, and ships. It is also a basic process in the automotive and aircraft industries and in the manufacture of machinery. Along with soldering and brazing, it is essential in the production of virtually every manufactured product involving metals.

The welding process best suited to joining two pieces of metal depends on the physical properties of the metals, the specific use to which they are applied, and the production facilities available. Welding processes are generally classified according to the sources of heat and pressure used.

The original pressure process was forge welding. Forge welding was practiced for centuries by blacksmiths and other artisans. The metals are brought to a suitable temperature in a furnace, and the weld is achieved by hammering or other mechanical pressure. Forge welding is used rarely in modern manufacturing.

The welding processes most commonly employed today include gas welding, arc welding, and resistance welding. Other joining processes include thermite welding, laser welding, and electron-beam welding.