The effect of the inner diameter of the tools on the mechanical properties of studs formed by friction protrusion forming was investigated. The outer diameter of the tool was fixed at 8 mm, whereas the inner diameter was varied among 3,4 and 5 mm. Studs of 5052 aluminum alloy was formed on 5052 aluminum alloy plates by friction protrusion forming using a friction welding machine. Microstructures and Knoop hardness distribution of the formed studs were analyzed. On the surface of the studs, traces of contact with the tool were observed. The backsides of the base plates show discoloration. The weld interfaces were unclear. The use of tools with inner diameter of 3 or 4 mm resulted in the formation of hollow root. The top of the studs had coarser grained structures than the base metal.
