Heat Source Evolution And Joint Formation In Rotary Friction Welding Process

Rotary Friction Welding(RFW) possesses outstanding advantages of high quality, high efficiency and energy saving, which, however, has not been well applied in manufacturing of the key components used in aerospace and other high–tech areas because the selection of operation parameters is arbitrary. The basic reason is the lack of systematic technology theory that could guide the process design and the selection of reliable operation window. The establishment of RFW technology theory has to be focused on the joint formation mechanism that further needs deep understanding of the heat generation mechanism, the temperature field and its evolution.This work carries out the experimental study on the joint formation in Rotary Friction Welding. Friction welding exhibits the nature of friction heat, which is resulted from the ’friction impedance’ to do work that releases heat. Thus, the corresponding parameters in RFW were isolated and characterized including friction torque, temperature, burn–off rate and corona bond, from which theoretical models were constructed based on basic friction mechanisms, i.e., slide friction and stick friction. Such work issues the evolution law of the torque(impedance) and the interfacial temperature field and the growth rule of the corona bond, of which the corresponding analytical models were established, thus revealing the interface friction heat generation behavior and mechanism. Finally, the joint forming characteristics of typical homogeneous and heterogeneous pairs were studied by utilizing the above theory. The main research contents and achievements are as follows:Firstly, the experimental study was focused on the evolution of heat generation and temperature field in the first stage of the rotary friction welding, which revealed platforms of the friction torque and the surface temperature. The occurance of torque platform implies unchanged interface impedance, corresponding to the mechanism of slide friction that obeys the Amontons–Coulomb’s law of friction. Such finding was further confirmed by the occurrence of temperature platform. This platform shows a dynamic balance driven by a constant interfacial heat generation. The slide friction corresponds to low temperature platform, whereas the stick friction corresponds to high temperature platform. Both platforms show heat generation mechanisms with different temporal and spatial features, i.e., slide occurs in the initial stage of friction, and experiences sequentially from the interface edge to the center, having a typical feature of a relatively low torque and temperature platform. Stick starts at a circle area near 1/2R(R is radius), and then the stick area grows towards the edge and the center of the interface simultaneously, having the typical feature of a sharp rise in the torque.Secondly, by using means of terminating the welding process and maintaining the as–welded microstructure with water cooling, the corona bond was found to initiate at position between 1/3R and 2/3R, and mostly near the 1/2R. The internal temperature field evolution shows that the corona bond region coincides with the heat concentrated area. The growth rate of the corona bond in width direction is much larger than that in the height direction. There is a critical pressure, over which the nucleation time is negligible so that the growing of corona bond mainly covers the first stage. The evolution of microstructure in the weld zone was studied, in which the thermal–mechanical coupling bi–factor method of temperature and strain rate was proposed. This method was later applied in analyzing the 20–mild steel joint. The result shows that the grains have grown up severely after austenitizing owing to the long time holding at higher temperature for the nugget zone. While cooling, the proeutectoid ferrites were precipitated in acicular or flake morphologies along the certain crystal oriented eutectic austenite, leading to the formation of Widmanstatten structure. The EBSD analysis on nugget zone of the joints formed under different friction time shows that, the microstructure of the metal in the corona bond zone has been fully crystallized in the first stage, which, however, consequently encounter deformation, grain boundary migration and so on with the proceeding of the welding process. Such mechanism eventually leads to incomplete dynamic recrystallization.Thirdly, based on the experiments, the linear hypothesis for heat intensity was proposed, accordingly an analytical model for the temperature field was established. This model was definitely derived by introducing the maximum entropy generation principle of the nonlinear nonequilibrium thermodynamics, which issued the criterion for the transition of the friction mechanism. Accordingly, the physical model was described for the evolution of the corona bond, of which the corresponding analytical model of the growth rate was built by using the energy conservation principle of heat generation and heat consumption. The model was verified the accuracy and reliability as the calculated results were consistent well with the experiments. Therefore, the established model can be chosen as the principle for the selection of the optimized parameters, under which the integrated joint can be got.Finally, the joint formation characteristics of typical homogeneous and heterogeneous pairs were investigated, from which the effect of heat patterns on the mechanical properties was studied. The phenomenological model between the heat pattern and mechanical property was found, meanwhile, the optimization criterion of the heat pattern with the comprehensive factor as the target parameter was proposed. The uneven distribution of the heat generation and temperature field along the radius direction causes the cambered heat pattern. However, owing to its special thermal–mechanical coupling effect, the inevitable brittle Fe–Ti intermetallic compound presents the features of size reduction and dispersed distribution, which enhanced joint efficiency. By comparing with the diffusion bonding method, it showed that the joint efficiency of the diffusion bonded joint is far less than that of the rotary friction welded joint even though the complex process and multi–interlayer were designed. It indicated that the unique thermal–mechanical coupled heat generation is the intrinsic reason for the high efficiency and high strength of the rotary friction welded dissimilar joint.