Friction forge riveting: A new joining method for connecting 40Cr steel and TC4 titanium alloy

Abstract Bolted joints are usually used in connecting titanium alloy and steel structures in the shipbuilding industry, but bolted joints are easy to slip or loosen because of slamming and wave load. A new riveting process of friction forge riveting (FFR) was first proposed to connect 40Cr steel plate and TC4 titanium alloy plate through 40Cr rivets. Furthermore, joint formation, microstructures and mechanical performances of the FFR joints were studied. The results showed that the high loading speed led to the decrease of the fluidity of the rivet material and the increase of the root angle, which resulted in the rivet edge material thinning, stacking into a non-compact lamellar structure, accompanied by defects such as tunnels and flash. The heat generated by friction led to the re-austenitizing of the tempered sorbite. At different temperatures and cooling speeds, coarse and hard martensite structure was formed at the driven rivet head, and a pearlite band was generated in the riveting column. The hardness of the driven rivet head showed a regular circular distribution, and the hardness of riveting column center was obviously reduced. The best FFR joint was produced at the parameter of 2000 rpm rotating speed and 6 mm/min loading speed, of which the maximum cross-tension load and ultimate tensile-shear load were reaching 82% and 90% of the bolted joint respectively.