In this lecture on Welding Metallurgy of Solid–Liquid and Solid-State Processes, we explore brazing, soldering, and solid-state joining, focusing on intermetallic formation, joint strength, and failure modes. The session also covers metallurgical changes in work- and precipitation-hardened metals during solid-state joining, and explains the role of carbon equivalent in weldability, preheat requirements, and crack prevention in ferrous metals.