“…The most common typical characteristic features like surface appearance and flash formation, shape and size of the nugget zone, onion ring-like pattern, banded structure formation, and the various defects (most common voids, cavities, tunnel, and pinholes) are visually examined at different magnifications (low or high) [59,66,[78][79][80][81]. The conditions like the addition of reinforcement particles, workpiece material property (including thickness, type, temperature, and thermal conductivity), process parameters range, and tool geometries are the most important that play a major role in the origin of all the above-mentioned features [53,54,[82][83][84][85].…”