Hr Shercliff

Friction stir welding of aluminium alloys

Abstract The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed. The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects. After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non-heat-treatable and heat-treatable). Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion. It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications. In spite of this, much remains to be learned about the process and opportunities for further research and development are identified.

A process model for age hardening of aluminium alloys—I. The model

Abstract Process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys. A model for the isothermal ageing curve is developed. This is demonstrated for a number of alloys and the success of the approach is assessed. Applications and a new diagram, showing the variation of strength with temperature and time, are described in an accompanying paper.

The mechanical properties of natural materials, II. Microstructures for mechanical efficiency

Many natural materials have exceptionally high values of the mechanical performance indices described in the previous, companion paper. For beams and plates of a given stiffness or strength, or for a column of a given buckling resistance, woods, palms and bamboo are among the most efficient materials available. Their mechanical efficiency arises from their combination of composite and cellular microstructures. In this paper we analyse the microstructures which give rise to exceptional performance, describe the fabrication and testing of model materials with those microstructures and discuss the implications for design of mechanically efficient engineering materials.

Microstructural modelling in metals processing

Abstract Modelling of microstructure evolution has long been part of physical metallurgy, both in the laboratory and in industry. Until recently, however, physically-based modelling was limited to idealised alloys under controlled laboratory conditions, while the complexity of industrial processing of commercial alloys necessitated empirical approaches. The advent of powerful computing facilities, and in particular the rapid growth in application of the finite element method to metals processing, has stimulated new research with the aim of bringing the two areas together. The present review describes progress in this rapidly expanding field. The preliminary review in Section 1 illustrates the potential of integrating microstructural modelling with the very detailed process histories which are available from modern finite element analyses. The ‘internal state variable’ approach is identified as a particularly appropriate method to reach the objective of providing useful predictive capability in industrial processing, within the scope of personal computers. The general method, and its simplification for single parameter models in which the microstructure evolution may be treated as an ‘isokinetic’ reaction, are outlined in Section 2 . The paper then presents internal state variable formulations for a range of thermal problems, as temperature-controlled processes dominate. These are solid-state diffusional transformations (precipitate dissolution, nucleation and growth, and coarsening) in Section 3 ; solidification and subsequent solid-state phase transformations in Section 4 ; and grain growth in Section 5 . Case studies are presented for each problem, illustrating the applicability of the approach to industrially relevant processing of commercial alloys. Examples range from casting, to cooling after hot forming, to heat treatment and welding. Bringing together this range of phenomena and processes in one review makes two specific points: (a) the state variable formulation offers a robust modelling framework, in which real non-isothermal process histories may be readily linked to fundamental isothermal theories of microstructure evolution, (b) depending on the problem, different levels of approximation can be accepted without invalidating the results, but the modeller needs to exercise critical judgement to find the optimum level of complexity. Finally in Section 6 , a number of examples are presented in which state variable models have been fully integrated with finite element analyses, with two examples from welding and one from casting. These case studies demonstrate that significant opportunities now exist for enhancing industrial processing capabilities. At the same time, the underlying microstructural modelling provides a renewed stimulus for improving scientific understanding of some of the classical problems of physical metallurgy.

Modelling of precipitation reactions in industrial processing

Abstract The present investigation is concerned with modelling of diffusion-controlled precipitation reactions in industrial processing, with particular emphasis on heat treatment. In the first part of the paper the components of the model are outlined and constitutive equations presented which allow the fraction transformed to be calculated as a function of time and temperature. The model uses a combination of chemical thermodynamics and kinetic theory to describe the microstructure evolution, with the particular feature of writing the Avrami equation in a differential form. In general, the solution of the differential equation requires stepwise integration in temperature-time space over a predetermined thermal cycle, but the mathematical treatment can largely be simplified if the additivity condition pertaining to an isokinetic reaction is satisfied. The theory is thus generic in the sense that it can be adopted to a wide spectrum of materials and heat treatment conditions, ranging from low and high alloy steels to aluminium alloys. In the second part of the paper this formalism has been applied to describe the quench sensitivity of AA6082 extrusions. The process model takes into account the thermal history of the base material and allows calculation of the peak strength following artificial ageing for a wide range of cooling conditions. The results show that the peak strength is both a function of the alloy composition, the homogenizing conditions and the cooling rate through the critical temperature range for β′-Mg 2 Si precipitation, in agreement with general experience. It is concluded that the existing theoretical framework is sufficiently comprehensive to serve as a tool for alloy design and optimization of cooling schedules for AlMgSi extrusions and an illustration of this is given towards the end of the paper.

Experimental and numerical analysis of aluminium alloy 7075-T7351 friction stir welds

Abstract This paper describes a systematic series of friction stir welding experiments using aluminium alloy 7075, designed to provide validation data for a numerical model of the process. The numerical model used the commercial computational field dynamics package, FLUENT, and the trials focussed on weld temperature and torque measurements. There were several significant findings that have both practical use and are pertinent to future modelling work. First, the temperature profiles and weld quality were affected by the type of tool material. Second, in thick section welds the material reached temperatures very near to the solidus. As a consequence this limited the heat generation, so the weld power was largely independent of the rotation speed. Third, several of the welds experienced the problem of surface scaling which was exacerbated by high rotation speeds and a high plunge depth. Finally, an empirical equation for predicting the weld power was derived from the experimental power input.

Dissimilar friction stir welds in AA5083-AA6082. Part I: Process parameter effects on thermal history and weld properties

The aim of this study was to explore the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082. To that end a systematic set of nine instrumented welds were made using rotation speeds of 280, 560, and 840 rpm and traverse speeds of 100, 200, and 300 mm/min with AA5083 on the advancing side and another nine with the materials reversed. For comparison a smaller series of AA5083-AA5083 and AA6082-AA6082 welds were also made. Thermocouple measurements, tool torque, extent of material mixing, and macrostructural observations all indicate that the temperature under the tool is more strongly dependent on the rotation than the traverse speed. It was found that in the current case, the power (energy/s) and heat input (energy/mm) do not correlate simply with the weld temperature. As a result, such metrics may not be suitable for characterizing the conditions under which welds are produced.

Model for predicting heat generation and temperature in friction stir welding from the material properties

Abstract This paper describes a simple numerical model for predicting the heat generation in friction stir welding (FSW) from the material hot deformation and thermal properties, the process parameters, and the tool and plate dimensions. The model idealises the deformation zone as a two-dimensional axisymmetric problem, but allowance is made for the effect of translation by averaging the three-dimensional temperature distribution around the tool in the real weld. The model successfully predicts the weld temperature field and has been applied with minimal recalibration to aerospace aluminium alloys 2024, 7449 and 6013, which span a wide range of strength. The conditions under the tool are presented as novel maps of flow stress against temperature and strain rate, giving insight into the relationship between material properties and optimum welding conditions. This highlights the need in FSW for experimental high strain rate tests close to the solidus temperature. The model is used to illustrate the optimisation of process conditions such as rotation speed in a given alloy and to demonstrate the sensitivity to key parameters such as contact radius under the shoulder, and the choice of stick or slip conditions. The aim of the model is to provide a predictive capability for FSW temperature fields directly from the material properties and weld conditions, without recourse to complex computational fluid dynamics (CFD) software. This will enable simpler integration with models for prediction of, for example, the weld microstructure and properties.

Neutron and synchrotron measurements of residual strain in TIG welded aluminium alloy 2024

Tungsten inert gas (TIG) welding is one method of joining aluminium alloys with potential application in the aerospace industry. However, for it to be seriously considered as an alternative to mechanical fasteners the interrelated problems of residual stress and distortion need to be addressed. In this paper neutron, laboratory and synchrotron X-ray diffraction methods are used to provide non-destructive information about the residual stress field in TIG-welded 2024 Al alloy. The results compare well despite the differing penetration and sampling volumes associated with each technique. It is found that the magnitudes of the tensile longitudinal stresses decrease along the plate due to progressive heating up of the plate ahead of the arc during welding, so that steady-state conditions are not achieved. Comparison of the data with a finite element model indicates that softening of the heat-affected region must be included to simulate the resulting stress field. The FE model is found to be in good agreement with the data especially in the vicinity of the weld slope-out.

A process model for age hardening of aluminium alloys—II. Applications of the model

The process model for ageing of aluminium alloys, developed in Part I [H. R. Shercliff and M. F. Ashby, Acta metall. mater.38, 1789 (1990)], is applied to a number of heat treatments, establishing a basis for such problems as the prediction of the strength loss in the heat-affected zone of welds. First, the reheating of previously aged material is considered. Heat treatment using a parabolic thermal cycle is then modelled in terms of an equivalent isothermal treatment, and extension to weld thermal cycles is considered. Finally the isothermal models are presented as novel “iso-yield diagrams”, which are useful for evaluating the data from thermal cycles and have potential as process diagrams.