*2.3. Material Flow Model*

A welding material is defined as a viscoplastic material. The mechanical response of viscoplastic materials can be defined by the constitutive relationship, hardening law, yield criterion, and flow rule. This paper has some reasonable assumptions without affecting the research results. First, the welding stage studied is a steady state—that is, the heat production and material flow reach a stable state. Second, the plastic softening material is a non-Newtonian, incompressible viscoplastic fluid. Third, elastic strains are negligible.
