It is widely recognised that the fundamental mechanisms associated with the weld formation process and their relationships with welding parameters are complex and remain to be fully understood. The present paper reports a series of general findings based on a set of simplified numerical models that were designed to elucidate various aspects of the complex thermomechanical phenomena associated with friction stir welding. The following phenomena were investigated in separate numerical models: (i) coupled friction heat generation; (ii) plastic flow slip zone development; and (iii) three-dimensional heat and material flow. The friction induced heat generation model was used to quantify the contributions of coupled thermomechanical friction heating, including non-linear interfacial phenomena between the tooling (e.g. stir pin) and material being welded. The plastic work induced heating effects were also examined. The plastic slip formation mechanisms were then investigated by considering contributions from various heating mechanisms. Finally, a simplified three-dimensional heat and material flow model, based on the observations from the coupled friction heat generation model, was used to establish some initial insight regarding the heat and material flow. The results from the three subproblem areas were then generalised in the form of a simple parametric relationship between welding variables (i.e. travel and rotating speeds) and weld formation conditions. A series of assumptions were made in constructing these individual models since there exists little information on actual material behaviour under friction stir welding conditions. However, the findings from the present study not only illuminate some of the important weld formation mechanisms in friction stir welding, but also provide an effective framework for more focused investigations into some of the fundamental phenomena identified in the three subproblem areas: such investigations will be reported separately in a future publication.

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Fiber placement process of thermoplastic material is an in situ consolidation process which can significantly reduce consolidation process time and save costs compared with autoclave consolidation. The heat and crystallization behaviours play very critical role in the quality control during the process. In this work, two dimensional model of finite element is presented to perform heat transfer analysis of fiber placement process. The relationship between heat distribution of lamination and process parameters, including roller velocity, main heating temperature, preheating temperature, preheating length and preheating location, is deeply investigated. The numerical results show that for certain heating temperature there is a safe roller velocity which should not be exceeded, and such parameters as main heating length, preheating length and preheating location could be optimized to obtain better quality of product.