Isothermal fusion and solidification of bonding interfaces occur by the self-induced fusion solidification bonding. Reaction diffusion between base metal and insert metal obtains the phase transformation at the bonding interface. Aluminum oxide film between aluminum base metal and copper insert metal obstacles the reaction diffusion. The collision between the oxide film and the metals produces the fracture of the oxide film and the reaction diffusion of each metal. Higher bonding pressure increases the joint strength and the bonding area. The deformation of the bonding interface under the static loading condition and the compression depth of the periodic motion under the dynamic loading condition are increased also with the increase of the bonding pressure. Both of compression deformation and depth promote fracture of aluminum oxide film. Increase of bonding pressure decrease bonding temperature to improve the deformation resistance of aluminum base metal. Bonding temperature affects also the joint strength and the bonding area by the change of the coefficients of reaction diffusion. Suitable bonding pressure and temperature condition obtains high joint strength and bonding area.
This paper describes the joint strength of low carbon steel joints and the selection guide of friction welding conditions for low force requirements, which was made by friction stud welding. When joints were made at a friction pressure of 30MPa with a forge pressure of 30MPa, all joints did not have the fracture on the base metal. All joints, which were made with a friction time of 1.5s (just after the initial peak) and a forge pressure of 60MPa, had the fracture on the base metal. However, all joints with a long friction time such as 5.0s did not have the fracture on the base metal. Furthermore, when joints were made with a friction pressure of 10MPa, the joint efficiency of 100% was not successfully achieved regardless of increasing forge pressure. The cause of the joint with the fracture between the initial weld interface and the base metal was that the peripheral portion of that interface of the stud side (small diameter side) was not completely joined with low friction pressure such as 10MPa. On the other hand, the fracture on the base metal and the joint efficiency of 100% were successfully achieved when joints were made at a friction pressure of 60MPa, a friction time of 0.6s (just after the initial peak), and a forge pressure of 60MPa. All those joints with flash at the initial weld interface had the fracture on the base metal, and it also had the bend ductility of over 15 degrees with no cracking at the initial weld interface by impact shock bending test. However, all joints with a long friction time such as 5.0s in this friction pressure did not also have the fracture on the base metal. Hence, to obtain the joint possessing the fracture on the base metal with no cracking at the initial weld interface, the joint should be made with a friction pressure of 60MPa and a friction time of 0.6s, i.e. an optimum friction pressure and friction time such as the friction torque reached to just after the initial peak. By setting to this condition, the forge pressure will be able to set up the identical friction pressure.
Fatigue crack propagation stage is considered to be the dominant in fatigue fracture of welded joints and some solutions regarding stress issue have been developed. In order to ensure reliability of large scale welded structures, multilateral countermeasure for various factors including microstructure in the HAZ (Heat affected zone) which can affect crack initiation should also be considered. However, the effect of HAZ microstructure has not been fully evaluated due to the difficulty and effort required for a systematic experimental procedure. In the present study, elastic-plastic behaviors under static and cyclic loading in HAZ microstructures made by simulated thermal cycle of a welded joint were investigated. By inputting cyclic tensile properties of these microstructures to finite element analysis of welded joint, damage caused by cyclic loading was calculated. Regardless of consideration of HAZ distribution, fatigue crack initiation site was predicted around welded toe of cruciform welded joint due to large degree of stress concentration. On the other hand, it was confirmed that the predicted fatigue crack initiation life the joint with the HAZ strength was longer than that of the joint with homogeneous base metal property. It is suggested that fatigue life of welded joint can be improved by controlling fatigue crack initiation and initial propagation process even in joints which fatigue crack propagation stage is considered to be dominant.