(1) In chemical milling the metal is removed by the etching reaction of chemical solutions on the metal; although usually applied to metals, it can also be used on plastics and glass. (2) Electrochemical machining uses the principle of metal plating in reverse, as the work piece, instead of being built up by the plating process, is eaten away in a controlled manner by the action of the electrical current. (3) Electrical discharge machining and grinding erodes or cuts the metal by high-energy sparks or electrical discharges. And (4) laser machining cuts metallic or refractory materials with an intense beam of light from a laser.
arc welding
arc welding
Another further alteration may be “joining,” the process of permanently, sometimes only temporarily, bonding or attaching materials to each other. The term as used here includes welding, brazing, soldering, and adhesive and chemical bonding. In most joining processes, a bond between two pieces of material is produced by application of one or a combination of three kinds of energy: thermal, chemical, or mechanical. A bonding or filler material, the same as or different from the materials being joined, may or may not be used.
The properties of materials can be further altered by hot or cold treatments, by mechanical operations, and by exposure to some forms of radiation. The property modification is usually brought about by a change in the microscopic structure of the material. Both heat-treating, involving temperatures above room temperature, and cold-treating, involving temperatures below room temperature, are included in this category. Thermal treatment is a process in which the temperature of the material is raised or lowered to alter the properties of the original material. Most thermal-treating processes are based on time-temperature cycles that include three steps: heating, holding at temperature, and cooling. Although some thermal treatments are applicable to most families of materials, they are most widely used on metals.
Finally, “finishing” processes may be employed to modify the surfaces of materials in order to protect the material against deterioration by corrosion, oxidation, mechanical wear, or deformation; to provide special surface characteristics such as reflectivity, electrical conductivity or insulation, or bearing properties; or to give the material special decorative effects. There are two broad groups of finishing processes, those in which a coating, usually of a different material, is applied to the surface and those in which the surface of the material is changed by chemical action, heat, or mechanical force. The first group includes metallic coating, such as electroplating; organic finishing, such as painting; and porcelain enameling.