Microstructure and properties of friction welded joint of aluminum alloy to alloy steel

On account of the urgent demand of lightweight drilling pipe, a composite structure made by 7075-T6 aluminum alloy and 37CrMnMo alloy steel was welded by continuous-drive friction welding method so as to offer the optimized manufacturing process and theoretical support for manufacturing lightweight Al/steel composite drilling pipe. The effects of welding parameters on the appearance, microstructure and mechanical properties of the joint were investigated. The results indicated that the fine grain zone with texture <111> in radius direction appeared at the weld interface due to the fully dynamic recrystallization in the Al side under sufficient heat and pressure, which results in the grain-boundary strengthening under the coupling effect of force and heat. In the heat affected zone, the original fiber texture <111> and <110> in the base metal disappeared with random orientations appeared as a result. The inhomogeneous distribution of heat and force along the radius direction led to intermetallic compounds (IMCs) free zone existing at central zone of weld interface, while a sub-micron sized IMCs layer formed at 1/2 radius zone. Following by the dissolution of second phase particles, the micro-hardness along longitudinal section tended to reach a dynamic equilibrium trend after experiencing a reduction and ascending process at weld interface. The bonding mechanism was in a combined form of mechanical and metallurgical joining, with the highest strength achieved 240 MPa. The relatively smooth fracture appearance at central zone manifested itself a weaker bonding, which revealed that a metallurgical reaction at such zone is highly desired by controlling the friction process in order to further enhance the joint property.