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Losses of life and property due to catastrophic failure of structures are often traced to defective welds. However, major advances have taken place in welding science and technology in the last few decades. With the development of new methodologies at the crossroad of basic and applied sciences, the promise of science-based tailoring of composition, structure, and properties of the weldments may be fulfilled. This will require resolution of several contemporary issues and problems concerning the structure and properties of the weldments as well as intelligent control and automation of the welding processes.

Microstructure evolutions and degradations of aluminum-metal-matrix composites during fusion welding were studied and compared with thermodynamic calculations. In fusion welds of Al 2 O 3 -reinforced composites, the decomposition of Al 2 O 3 was observed. In fusion welds of SiC whisker-reinforced composites, the decomposition of SiC to Al 4 C 3 ϩ Si by reaction with molten aluminum occurred. These phenomena led to unacceptable fusion welds in aluminum metal-matrix composites. Successful welds were produced in the same composites by friction stir welding (FSW). Significant reorientation of SiC whiskers close to the boundary of the dynamically recrystallized and thermomechanically affected zone (TMAZ) was observed. The small hardening in the dynamically recrystallized region was attributed to the presence of dislocation tangles in between SiC whiskers.