when the atoms at the edge of one piece come close enough to the atoms at the edge of another piece for interatomic attraction to develop, thetwo pieces become one. Although this concept is easy todescribe, it is not simple to effect. Surface roughness,impurities, fitting imperfections, and the varied proper-ties of the materials being joined complicate the joiningprocess. Welding processes and procedures have beendeveloped to overcome these difficulties by incorporat-ing the use of heat or pressure, or both. Though por-tions of this description do not apply to brazing,soldering, and adhesive bonding, an explanation will begiven when these processes are described later in thechapter