Current analytical thermal models for friction stir welding (FSW) are mainly focused on the steady-state condition. To better understand the FSW process, we propose a transient thermal model for FSW, which considers all the periods of FSW. A temperature-dependent apparent friction coefficient solved by the inverse solution method (ISM) is used to estimate the heat generation rate. The physical reasonableness, self-consistency, and relative achievements of this model are discussed, and the relationships between the heat generation, friction coefficient, and temperature are established. The negative feedback mechanism and positive feedback mechanism are proposed for the first time and found to be the dominant factors in controlling the friction coefficient, heat generation, and in turn the temperature. Furthermore, the negative feedback mechanism is found to be the controller of the steady-state level of FSW. The validity of the proposed model is proved by applying it to FSW of the 6061-T651 and 6063-T5 aluminum alloys.

The effect of the Alclad layer on material flow and defect formation during friction-stir welding (FSW) of 6.5-mm-thick 2024Al-T351 alloy plates was investigated. To characterize the material flow during FSW, different cross sections of the keyhole and ''stop-action weld'' were made for metallographic observations. It was found that the top Alclad assembled at the shoulder/ workpiece interface, thereby weakening the material flow in the shoulder-driven zone and favoring the formation of void defect at high traveling speeds. The bottom Alclad layer extended into the weld at excess material flow state, which could be avoided at balanced material flow state. A conceptual model of material flow was proposed to describe the formation of the weld. It was indicated that a perfect FSW joint of Alclad 2024Al alloy without defect could be obtained at an optimum FSW condition.

In Part II of this series of articles, the transient thermal model, which was introduced in Part I, is used to explore the effects of welding conditions on the heat generation and temperature. FSW of the 6061-T651 aluminum alloy is modeled to demonstrate the model. The following two steps are adopted to study the influence of welding conditions on the heat generation and temperature. First, the thermal model is used to compute the heat generation and temperature for different welding conditions, the calculated results are compared with the reported experimental temperature, and a good agreement is observed. Second, the analytical method is used to explore the approximate functions describing the effect of welding conditions on the heat generation and temperature. Based on the computed results, we discuss the relationship between the welding conditions, heat generation, temperature, and friction coefficient, and propose a relationship map between them for the first time at the end.

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