Papers by Keyword: AA2024 Aluminium Alloy

Abstract: Friction stir welding (FSW) is a relatively new solid-state joining technology for metals. It shows no solidification-related joint imperfections which makes it utmost suitable for hard-to-weld highly alloyed aerospace aluminium grades, like AA 2xxx and AA 7xxx. These alloys are often cladded with a thin layer of pure aluminium for corrosion protection. Friction stir welding of such materials requires removal of the clad layer prior to welding to prevent weakening of the joint by the soft clad material. This leaves the welded region vulnerable to corrosion after the joining process. Post-weld restoration of the clad layer is required to restore the protective action of the clad layer and as such to enhance the life expectancy of the welded construction. In this work the deposition of thin layers of pure aluminium on AA 2xxx and AA 7xxx alloys is studied employing an innovative FSW tool. The tool shoulder is equipped with strategically placed internal channels that allow delivery of filler type of material into the weld zone. Depending on the channel architecture used, filler material can be deposited on top of the work piece surface and/or mixed with the work piece surface region. The cladding is done in the solid state avoiding many problems with solidification and interface reactivity often observed with other surface modification techniques, such as laser surface engineering, plasma spraying or casting. Here, the filler material is deposited on top of the work piece; the modified tool is not equipped with a tool pin. The work comprises an in depth study of the influence of process conditions on the microstructural changes in the underlying work piece and on the quality of the bonding of the clad material (99.5 % aluminium) to the work piece material. Apart from the usual process conditions, such as tool rotation speed, translation speed, down force and tool angle also the delivery pressure and rate of the filler supply system can be varied. The influence of the usual process conditions on the microstructure of the underlying work piece is similar to that observed with “traditional” FSW. Changes in hardness can be related to the amount of heat generated by the welding process. Shape and dimensions of the microstructural zones found are typical for welds made without a tool pin. The effect of the small amount of clad material deposited on top of the work piece on the temperature distribution is small. The amount of heat required to heat it up is negligible to the heat required to heat up the work piece and the tool. The quality of the bonded clad layer is dependent on the amount of heat and plastic deformation generated at the interfaces between the tool, the filler material and the work piece. Tool angle, tool shape and supply rate of the filler supply system determine the layer thickness.

Abstract: Materials which form the surface and subcutaneous layers of an extrudate experience large deformations when they traverse the die land. This, when added to the inhomogeneity caused by the dead metal zone, leads to considerable modifications to the deformation parameters when compared to the remainder of the extrusion. The distribution of structure is therefore greatly inhomogeneous. Reference to both empirical and physical models of the recrystallisation process indicates that nucleation and growth will differ at these locations in those aluminium alloys that are usually solution treated and aged subsequent to the deformation process. Since static recrystallisation has a significant influence on many of the properties of the extrudate, it is therefore essential to provide the methodology to predict these variations. In the work presented, a physical model, for AA2024, based on dislocation density, subgrain size and misorientation is modified and integrated into the commercial finite element method (FEM) code, FORGE, to study the microstructure changes. Axi-symmetrical and shape extrusion are presented as examples. The evolution of the substructure influencing static recrystallisation is studied. The predicted results show an agreement with the experimental measurement. The distribution of equivalent strain, temperature compensated strain rate and temperatures are also presented to aid interpretation. Importantly the properties of hard alloys improve as the temperature of the extrusion is raised. This phenomenon is discussed and theoretically justified. This paper also presents some innovative work where the physically based models, and the Cellular Automata (CA) method, are combined to simulate the static recrystallisation process. The FEM is adopted to provide the initial morphology and state variables for the structure models, such as the equivalent strain, the temperature and the equivalent strain rate. The subgrain size, and dislocation densities are calculated from physically based models and are transferred to CA models to construct the data required to define the initial state for recrystallisation. Simulation results are compared with experimental measurements. It is demonstrated that CA integrated with the physically based models is effective in predicting the structural changes by selecting a suitable neighbourhood and reasonable transition rules.

Abstract: Liquation cracking in the partially melted zone (PMZ) of aluminum welds was studied. The PMZ is the region immediately outside the fusion zone where the material is heated above the eutectic temperature. Highly crack-susceptible alloys 2024 and 7075 were welded using gas-metal arc welding (GMAW) with filler metals 1100 and 4043, respectively. Circular-patch welds were made on 3.2 mm thick workpiece with full penetration, and single-pass welds were made on 9.5 mm thick workpiece with partial penetration. Liquation cracking was observed in all welds. Dualpass welds were also made on 9.5 mm thick workpiece, with overlapping between the penetration tips of the two partial-penetration passes made on the opposite sides of the workpiece. Liquation cracking was found in the first pass but not in the second pass. The results were explained using TfS (temperature vs. fraction solid) curves of the weld metal (WM) and the PMZ based on the following criterion proposed recently: liquation cracking can occur if WM fS > PMZ fS during PMZ solidification.

Abstract: Although Friction Stir Welding (FSW) avoids many of the problems encountered when fusion welding high strength Al-alloys, it can still result in substantial residual stresses that have a detrimental impact on service life. An FE model has been developed to investigate the effectives of the mechanical tensioning technique for controlling residual stresses in FSWs. The model purely considered the heat input and the mechanical effects of the tool were ignored. Variables, such as tensioning level, heat input, and plate geometry, have been studied. Good general agreement was found between modelling results and residual stress measurements, justifying the assumption that the stress development is dominated by the thermal field. The results showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~ 50% of the room temperature yield stress, tensile were replaced by compressive residual stresses within the weld.

Abstract: To predict strength evolution of precipitation hardening alloys, a wide range of modelling approaches have been proposed. The most accurate published models are physics-based approaches which use both nanoscale processes with their related constants and parameters, as well as parameters calibrated to one or more macroscale measurements of yield strength of one or more samples. Recent developments in submodels including analytical expressions for volume fraction and size evolution including impingement and coarsening are reviewed. It is also shown that Kampmann-Wagner and JMAK models are generally not consistent with data on the progress of precipitations in the main precipitation hardening Al alloys systems, and improved model formulations are described.

Abstract: Carbon Fiber (CF)/Metallic Alloy (MA) laminar structures, also known as Fiber Metal Laminates (FML) allow diminishing the airship weight. Because of that the use of these materials is growing continuously in the aerospace industry. These composites materials need to be drilled because of the assembly requirements in the different airship elements. The most common problems that can appeared when those structures are machined are related with the interaction of the tool with dissimilar materials, which need different cutting parameters for the optimized machining process. This work reports on the results about a study of the dry drilling processes of hybrid composites Carbon Fiber/aluminum alloy, and especially CF/AA2024.

Abstract: Finite element modelling has proved to be an effective tool for the investigation of trends effected by changing welding conditions. This is especially important in mechanical tensioning of friction stir welds because of the large number of parameters involved. In this paper, an FE model is used to examine the effectiveness of the mechanical tensioning technique for controlling residual stresses in FSWs by the investigation of trends caused by changes to the welding parameters. Comparisons between different geometries, traverse speeds, and welding off-axis angle all produced consistent results, and showed that the peak stresses are most strongly influenced by both the local tensioning and heat input, and not by the more global welding conditions. The results also showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~50% of the room temperature yield stress, tensile stresses were replaced by compressive residual stresses within the weld.

Abstract: Sheet materials of the alloys 6056 and 2024A were joined using the friction stir welding technique. Similar and dissimilar butt welds were produced. Strength and corrosion behaviour of the joints were investigated in different heat treatment conditions. Hardness profiles across the weld revealed minima in the heat affected zones, being very pronounced at the advancing side of the joints. The hardness of the nugget zone increased after post-weld artificial aging. Strengths of postweld heat treated similar 6056 and dissimilar 2024A/6056 joints approached 90 and 85 %, respectively, of the ultimate tensile strength of 6056-T7X. As found by electrochemical measurements, alloy 2024A-T3 exhibited the most noble corrosion potential and artificially aged alloy 6056 the most active potential. Besides microstructural changes caused by the welding process, galvanic coupling affected the corrosion behaviour of friction stir welds, in particular with joints of dissimilar aluminium alloys, as indicated by accelerated corrosion of alloy 6056 in the nugget region. Intensified corrosion attack was also observed in the heat affected zones.

Abstract: Numerical analysis of radial extrusion process combined with backward extrusion has been performed to investigate the forming characteristics of an aluminum alloy in a combined extrusion process. Various variables such as gap size, die corner radius and frictional conditions are adopted as design or process parameters for analysis in this paper. The main investigation is focused on the analysis of forming characteristics of AA 2024 aluminum alloy in terms of material flow into backward can and radial flange sections. Due to various die geometries and process conditions such as frictional conditions, the material flow into a can and flange shows different patterns during the combined extrusion process and its characteristics are well summarized quantitatively in this paper in terms of forming load, volume ratio etc. Extensive simulation work leads to quantitative relationships between process conditions and the forming characteristics such as volume ratio of flange to can and the size of can and flange in terms of the can height extruded backward. It is easily seen from the simulation results that the volume ratio, which is defined as the ratio of flange volume to can volume, increases as the gap size and/or die corner radius increase. However, it is interesting to note that the frictional condition has little influence on the forming load and the deformation patterns. Usually, the frictional condition is a greatest process variable in normal forging operation. It might be believed from the simulation results that the frictional conditions are not a major process parameter in case of combined extrusion processes. It is also found that the can size, which is defined as the height of billet after forming, turns out to be even smaller than that of initial billet under a certain condition of die geometry.

Abstract: This paper is concerned with the analysis of the forming load characteristics of a forward-backward can extrusion in both combined and sequence operation. A commercially available finite element program, which is coded in the rigid-plastic finite element method, has been employed to investigate the forming load characteristics. AA 2024 aluminum alloy is selected as a model material. The analysis in the present study is extended to the selection of press frame capacity for producing efficiently final product at low cost. The possible extrusion processes to shape a forward-backward can component with different outer diameters are categorized to estimate quantitatively the force requirement for forming forward-backward can part, forming energy, and maximum pressure exerted on the die-material interfaces, respectively. The categorized processes are composed of combined and/or some basic extrusion processes such as sequence operation. Based on the simulation results about forming load characteristics, the frame capacity of a mechanical press of crank-drive type suitable for a selected process could be determined along with securing the load capacity and with considering productivity. In addition, it is suggested that different load capacities be selected for different dimensions of a part such as wall thickness in forward direction and etc. It is concluded quantitatively from the simulation results that the combined operation is superior to sequence operation in terms of relatively low forming load and thus it leads to low cost for forming equipments. However, it is also known from the simulation results that the precise control of dimensional accuracy is not so easy in combined operation. The results in this paper could be a good reference for analysis of forming process for complex parts and selection of proper frame capacity of a mechanical press to achieve low production cost and thus high productivity.