Chapter 15 - Joining Dissimilar Material Combinations

Abstract

Dissimilar materials must frequently be joined to create hybrid structures in which diverse or even divergent properties, unattainable in individual materials, are required. Hybrid structures enable minimization of weight, improved damage tolerance in the joined structure, optimized matching of properties to design needs, and improved economy in basic material costs as well as the cost of fabricating difficult materials. The challenge posed in joining of dissimilar materials is obtaining sufficient compatibility to permit the materials to join in the first place and the joint to function in service in the second place. Compatibility must be chemical, physical, and mechanical, and the closer the matches, the better. While every combination of the basic material types is conceivable, not every combination is practical or even logical based on drastic differences in fundamental structures and properties. For joining metals to ceramics, the choices include (1) mechanical fastening by interlocking features or fasteners; (2) direct joining by non-fusion or, to a lesser extent, fusion welding; and (3) indirect joining using intermediates that include organic or inorganic adhesives or metallic fillers that are melted to enable brazing or soldering or to remain solid for assisting in diffusion welding. When metals are joined to glass, the principal need is usually hermeticity and mechanical strength being secondary. On the other hand, metals can be joined to polymers simply by using polymeric adhesives, although mechanical fasteners are also used. For joining metals to composites, the material making up the matrix of the composite needs to be considered and then the joining process is selected accordingly.