Which of the following processes involves joining metal pieces together?

Welding is a process that specifically involves joining metal pieces together through the application of heat and sometimes pressure. In welding, the edges of the metal pieces are melted to form a strong bond upon cooling, effectively creating a single unified piece. This process is critical in various applications, including construction, manufacturing, and repair work, where structural integrity is essential.

On the other hand, shearing, milling, and grinding are primarily processes used for cutting or shaping metal rather than joining. Shearing involves cutting metal sheets or materials without the formation of chips, milling is focused on removing material through rotary cutters to achieve specific shapes or surfaces, and grinding utilizes an abrasive wheel to smooth surfaces or shape parts. Each of these processes has its distinct purpose but does not contribute to the joining of metal pieces as welding does.