The parts that are joined are known as a parent material. A number are considered ' unweldable,' a term not usually found in dictionaries but useful and descriptive in engineering. Some materials require the use of specific processes and techniques. Welding is a fabrication process whereby two or more parts are fused together by means of heat, pressure or both forming a join as the parts cool. Welding is usually used on metals and thermoplastics but can also be used on wood. The completed welded joint may be referred to as a weldment. National Structural Integrity Research Centre.Structural Integrity Research Foundation.