Final answer:
A swaging tool is used to form a solid material insert in a cast iron lead joint by flaring or expanding the joint.
Step-by-step explanation:
The tool used to form a solid material insert in a cast iron lead joint is called a swaging tool. Swaging is a process that involves shaping or reducing the diameter of a material by using a tool, such as a swaging die, to compress and reshape it. In the context of a cast iron lead joint, a swaging tool is commonly used to flare or expand the lead joint to securely hold the solid material insert.