Which process could be described by heating metals to fuse them together?

The process that involves heating metals to fuse them together is welding. Welding is a fabrication technique that joins materials, usually metals or thermoplastics, by melting the workpieces. The molten material from the surfaces being joined cools and solidifies to form a strong bond. This process can involve various techniques, such as arc welding, gas welding, and laser welding, all of which rely on the principle of applying heat to create a liquid state in the materials being fused.

In contrast, forging involves shaping metal through compressive forces, usually without melting it, and casting involves pouring molten metal into a mold to form a specific shape. Milling is a machining process that uses rotary cutters to remove material from a workpiece, but does not involve the fusion of materials. Thus, welding is uniquely characterized by the fusion of metals through the application of heat, making it the correct answer to the question.