The challenges of weight reduction and good strength in automotive industry have drawn considerable interest in thixocasting technologies. Joining of such components with conventional fusion welding creates voids, hot cracking, distortion in shape, and more importantly evolution of dendritic microstructure that ultimately would lead to inferior mechanical properties of the weld region. Thus, the purpose of making thixocast component is lost. The friction welding which is a solid state joining process can avoid defects associated with melting and solidification in a typical fusion weld and can be a promising alternative. This process produces a weld under compressive force at the contact of workpieces rotating or moving relative to one another to produce heat and plastically displacing material from the faying surfaces. Research on semisolid processing has its origin in the early 1970s. However, from the literature survey on semisolid processing it is clear that, till date, not much work has been done in field of joining of semisolid processed components. In the area of solid state welding, in particular, it is not at all explored. In view of this, the present work is focused on exploration of joining of Thixocast A356 Aluminium alloy component by friction welding and comparison of its performance with friction weld of conventionally cast sample of the same alloy. The study is carried out experimentally as well as numerically. Moreover, the material behaviour of thixocast component at elevated temperature in solid state is also described with the help of processing maps and constitutive modelling. The hot workability of thixocast and conventionally cast A356 alloy is evaluated with the help of processing maps developed on the basis of the dynamic materials model approach using the flow stress data obtained from the isothermal compression test in wide range of temperature (300-500℃) and strain rates (0.001s-1-10s-1). The domains of the processing map are interpreted in terms of the associated microstructural mechanism. On comparing the flow stress at elevated temperature of thixocast and conventionally cast A356 alloy samples, it is observed that the flow stress of the latter showed higher value at different strain level, temperature and strain rates. This indicates that the flow property of the thixocast alloy sample is better than that of the conventionally cast one (i.e. response to plastic flow is better for the former); while at room temperature thixocast sample has higher strength. Moreover to investigate the general nature of the influence of strain, strain rate and temperature on the compressive deformation characteristics of thixocast A356 and conventionally cast A356 aluminium alloy, a comprehensive model describing the relationship of the flow stress, strain rate and temperature of the alloys at elevated temperatures is proposed by hyperbolic-sine Arrhenius-type equation and Johnson-Cook model. The validity of descriptive results based on the proposed constitutive equation is also investigated and a comparison between two constitutive models is also made. In order to numerically model the friction welding process of a thixocast A356 aluminium alloy and conventionally cast alloy of same material using a finite element method (FEM), temperature dependent physical properties, mechanical properties as well as viscoplastic constitutive equations were used in the model. A two- dimensional axisymmetric finite element model has been developed. The modelling is based on a coupled thermomechanical approach. First, a nonlinear, transient two-dimensional heat transfer model is developed to determine the temperature fields. Later, the temperature fields are used as input for a nonlinear, two-dimensional structural model in order to predict the distortions and von Mises stress. The finite element models are parametrically built using APDL (ANSYS Parametric Design Language) provided by ANSYS. The validation of the model is carried out by comparing with the experiment. Once validated, the thermomechanical model was used to perform parametric studies in order to investigate effects of various process parameters on temperature and stress distribution in the workpieces. This helps in deciding the range of parameters for friction welding experiments in order to get good weld. Both thixocast and conventionally cast samples exhibited similar temperature distribution during the friction welding process, because of identical thermophysical properties. The magnitude of von Mises stress distribution during friction welding of thixocast A356 sample is found to be lower than that of the conventionally cast sample. It is because of their different constitutive behaviour at elevated temperature. Moreover, the developed FEM model can be successfully used to predict the residual stress at various locations for different set of parameters and geometry for friction welding of thixocast and conventionally cast A356 alloy. This helps in reducing time consuming and expensive experiments on residual stress measurement. The chosen experiments based on Taguchi L27 orthogonal array were conducted on the friction welding machine which works on the principles of continuous drive-mechanism. The experimental specimens were machined from thixocast A356 aluminium alloy connecting rods as well as conventionally cast A356 aluminium alloy ingot in the form of cylindrical bars of dimensions 85mm length and 20mm diameter. The parameters used for welding were friction pressure, rpm, forge pressure, burn-off, and upset pressure. The effects of welding parameters on performance characteristics (i.e. tensile strength and weld efficiency) were evaluated. Taguchi method was applied to investigate the influence of each parameter on strength of joints and evaluate the combination of parameters that leads to the highest weld strength. Accordingly optimum process parameters was identified which helps in achieving the tensile strength of more than parent material. The optimized process parameters for friction welding of thixocast A356 aluminium alloy are rpm = 500, friction pressure = 60, upset time = 5, upset pressure = 100 and burn off = 5. The empirical relationships were also developed to predict the tensile strength. The developed relationship can be effectively used to predict the tensile strength of welded joint with a correlation coefficient of 0.86, which indicates the strong positive relationship between predicted and experimental data. Friction welding of thixocast A356 aluminium alloy helps to achieve very fine eutectic silicon particles of the order of 0.4 at the interface due to severe plastic deformation taking place during welding. Obtaining such fine eutectic silicon particles is difficult to be achieved within few seconds of processing by any other method. The hardness variation of friction welded thixocast alloy shows higher value as compared to that of a conventionally cast sample in the heat affected zone, which indicates better weld strength of the former. This was also confirmed by the tensile strength studied and fatigue test. This indicates that weldability of cast alloys will get improved if the microstructure is modified to globular type.

As of 21st September 2017, industrial use of hexavalent chromium (Cr(VI)) will, due to its environmental toxicity and carcinogenicity, be restricted by REACH regulations. Saab Aeronautics is therefore shifting anticorrosion surface treatment of aluminum alloys from hexavalent chromium conversion to trivalent chromium (Cr(III)) passivation. The purpose of this thesis is to investigate the characteristics of conversion coatings formed with the passivation chemical SurTec 650V, in order to facilitate transfer to the more environmental friendly alternative. Process parameters, such as pH and immersion time in SurTec 650V passivation baths, have been investigated for passivation of three different aluminum alloys; 2024, 6061 and 7075. The characteristics of the Cr(III) conversion coatings achieved at laboratory scale in the thesis work have been compared with SurTec 650V process in production scale and with Cr(VI) containing Alodine 1200 and Alodine 1500 processes. The impact of varying process parameters have been investigated with goniometry, x-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to analyse characteristics of the conversion coatings. Differences in chemical composition on the surface and in depth profile was detected with XPS and AES, respectively and topology of coatings was found to differ for different aluminium alloys and parameter combinations. With TEM, the thickness of the conversion coating was found to be approximately 30-50 nm, which is thinner than the coatings formed with Cr(VI) passivation. Characteristics of coatings formed with Cr(III) passivation is concluded to be very dependent on parameter variation, especially for alloy 2024. Differences also occur between passivation at laboratory and production scale. Further evaluation of the production scale SurTec 650V process and corrosion testing should be performed, and an elaboration of a process control is required before the shift to an environmental friendly passivation process can be completely successful at Saab.

Doctor of Philosophy (PhD) / The evolution of atomistic-level nanostructure during the early stages of both standard, high-temperature T6 heat treatment, and low-temperature secondary ageing after interruption of the former (T6I4), has been investigated in rapid hardening Al-Cu-Mg alloys using a variety of microscopy and microanalytical techniques, including transmission electron microscopy (TEM), positron annihilation spectroscopy (PAS) and atom probe tomography (APT). In order to carry out this objective, quantitative data-analysis methods were developed with respect to new cluster-finding algorithms, specifically designed for use with three-dimensional APT data. Prior to this detailed characterisation work, the actual thermal impact from both heat treatment and quenching of small, lab-scale specimens was determined through correlation of both experimental results and calculations that modelled the heat transfer conditions using the lumped capacitance method. Subsequently, the maximum diffusion distance by random walk of the solute atoms was calculated for these periods, bearing significance on the propensity for these atoms to have the ability to cluster together, rather than segregate to the dislocation loops in the microstructure, which have a relatively larger interspacing distance. Age-hardening curves for the Al-1.1Cu-xMg (x = 0, 0.2, 0.5, 0.75, 1.0, 1.7 at.%) alloys at 150ºC show that the rapid hardening phenomenon (RHP) exists for Mg compositions ≥ 0.5Mg. Given that zone-like precipitate structures were unable to be detected by TEM or APT during the early stages of ageing at 150ºC, and that statistically significant dispersions of clusters were found in the APT data after ageing for 60 s, the RHP is attributed to these clustering reactions. Identification of clusters in the APT data has been achieved using the core-linkage algorithm and they have been found to be quite small, containing only a few atoms up to a couple of tens of atoms. The RHP is governed by some critical number density of both Mg clusters and Cu-Mg co-clusters of a critical size, whereas Cu clusters do not contribute significantly to the hardening mechanism. Significance testing indicates that Mg clusters are more significant at smaller clusters sizes and Cu-Mg co-clusters more important at larger cluster sizes. Hardness results also confirm the existence of rapid early hardening during secondary ageing at 65ºC in Al-1.1Cu-1.7Mg. The mechanism of secondary rapid hardening involves a combination of both secondary clustering from solute (mainly Mg atoms) residual in solution, and pre-existing amorphous primary clusters that have slower growth kinetics at the lower secondary ageing temperature. The latter occurs mainly by vacancy-assisted diffusion of Mg atoms as evidenced by the gradual increase of the Mg:Cu ratio of co-clusters. From an alloy design point of view it is important to fully understand the solute distribution in the microstructure to be able to subsequently optimise the configuration for enhanced material properties. The change in dispersion of solute atoms during ageing was determined by combining calculations of % vacancy-solute associations with detailed measurements of the dislocation loops to estimate the solute distribution within the microstructure. The implication of the balance of solute atoms segregated to the loops compared with that in the matrix is then discussed in the context of hardnening mechanisms.

Friction stir welding (FSW) is a low heat input solid state welding technology. It is often used for fabrication of aluminium alloys in transportation applications including railway, shipbuilding, bridge structures and automotive components. In these applications the material is frequently subject to varying load conditions and fatigue failure is a critical issue. In most cases standard codes and fatigue guidelines for aluminium welded joints address only welded structures with conventional welding methods but not those with FSW procedure. In the scope of this thesis fatigue life assessment of friction stir welded components was performed using theoretical approaches along with finite element method (FEM). The further aim of this study was to generate a basis for standardization of fatigue assessment of friction stir welded joints. Friction stir welded hollow aluminium panels of alloy 6005A are investigated. The panels are used for train wall sides, train floors, deck and bridges. Each panel is made of several profiles that are joined with the friction stir welding method. Fatigue bending tests were performed for profiles in these panels. Fatigue cracks and failure appeared at notches in the profiles. With FEM simulations critical positions for crack initiation and failure were identified. The method of critical distance was used to analyse and estimate the fatigue life. It was shown that the failure location and fatigue limit could be predicted for both base metal and weld location. Choice of welding procedure (clamping condition) can significantly influence the fatigue life. It was shown that for some panels the critical distance method was not able to explain the failure in the weld. In this case fracture mechanics together with residual stress analysis were used successfully to predict the failure. Assuming homogeneous material properties throughout the weld and the base material, FEM analysis for T and overlap joints as well can provide a reasonable fatigue prediction. This suggests that the same assumption can be extended to complex components for failure analysis of the friction stir welded joints when using the critical distance method. Fatigue assessment of friction stir welded joints was also performed using standard codes Eurocode 9 and IIW. Fatigue curves of traditional fusion welded joints were used. The results are in reasonable agreement with experimental data and FEM predictions. / QC 20120330

In some applications the stress across the entire cross-section of a component is not uniform but varies with position. For example, maximum shear stress is highest at the inner surface of a thick-walled cylinder subjected to uniform internal pressure and it decreases continuously towards the outer surface. In such applications it would be more appropriate for the component, too, to have varying strength across the cross-section matching with the stress profile it is subjected to. The present work deals with obtaining such a functionally graded material (FGM), characterizing it and testing its mechanical properties in compression. Differential aging heat treatment was used to produce the functionally graded material in a precipitation hardenable Al-4.6%Cu alloy by changing the microstructure. Temperature gradient furnace was used to achieve the gradation in microstructure from one end of the sample to the other end by differential aging of the solution treated sample. Mechanical properties can be varied in any precipitation hardenable alloy by means of producing various precipitates, which will form during the aging sequence. In Al-4.6%Cu alloy one end of the solution treated sample was aged for 38 hours at 170°C and the other end at 70°C by means of a temperature gradient furnace in which the coil density varies along the axis of the furnace. Thus we achieved a difference in mechanical properties from 70°C side to 170°C side as the precipitation during differential aging varied from GP zones at one end to θ' precipitate at the other end. Characterization was done on isothermally aged samples and in FGM using XRD (X-ray diffraction) and TEM (Transmission Electron Microscopy). XRD result showed that the final equilibrium precipitate θ was not formed in any of the heat-treated samples. TEM result showed the various precipitation sequences from GP zones to θ' in the isothermally aged samples and the same was confirmed in the gradient sample by cutting the samples form 70°C side towards the 170°C side and doing TEM on each sample. The properties of FGM in compression were studied using a 9mmx9mmxl8mm-compression sample using DARTEC machine and it was compared with those of isothermally aged samples. For 70°C the 0.2% proof stress was 141MPa and for 170°C it was 226MPa. The corresponding ductility values at the point of inflection on the engineering stress-strain curve for 70°C sample was higher (33%) than the 170°C (22%) sample. For the gradient sample it gave a proof stress of 163MPa and a ductility value of 30%.

Aluminium alloys, including both foundry and wrought alloys, have been extensively used for light-weight structural and functional applications. A grain refined as-cast microstructure is generally highly desirable for either subsequent processing ability or mechanical properties of the finished components. In this thesis, the grain refined microstructures in Al alloys have been achieved by intensive melt shearing using the melt conditioning by advanced shearing technology (MCAST) without deliberate grain refiner additions. Such grain refinement has been attributed to the enhanced heterogeneous nucleation on the dispersed oxide particles. It has been established that the naturally occurring oxides in molten Al alloys normally have a good crystallographic match with the a-Al phase, indicating the high potency of oxide particles as the nucleation sites of the a-Al phase. The governing factors for these oxide particles to be effective grain refiners in Al alloys have been proposed, including the achievement of good wetting between oxide particles and liquid aluminium, a sufficient number density and uniform spatial distribution of the dispersed oxide particles, and near equilibrium kinetic conditions in liquid alloys. In the present study, near equilibrium kinetic conditions can be achieved by intensive melt shearing using a twin screw mechanism, which has been confirmed by the observed equilibrium a-AlFeSi phase in a cast Al alloy and the transformation from g- to a-Al2O3 at 740±20oC under intensive shearing. For different alloy systems, depending on the alloy system, and melting conditions, due to the particular types of oxide formed and its crystallographic and chemical characteristics, the nucleation site of the nucleated phase is different. Specifically, MgAl2O4 relative to MgO, and a-Al2O3 relative to g-Al2O3, have higher potency as heterogeneous nucleation sites of a-Al phase in Al alloys. In future, the modification of the crystallographic match, and of the other surface characteristics related to the interfacial energy between the specific oxides and nucleated phase by trace alloying addition through segregation to the interface between oxides and nucleated phases combined with physical melt processing (such as intensive shearing in the present study) should be investigated in more detail.

The work presented aims to find alternatives for the prototyping of components by sand and plaster mold manufacturing processes that deliver properties similar to high pressure die casting (HPDC). Sand and plaster test samples have been casted. The Al-7Si-0.35Mg alloy has been tested in as cast condition and in a heat treated condition; T5 for sand cast samples and T6 for those plaster cast. The Al-7Si-2Cu-0.35Mg alloy was also tested in as cast condition. Tensile, fatigue and hardness tests have been performed. Microstructural investigation comprising secondary dendrite arm spacing, defects, Fe-rich β-phases and Si size measurements has been performed on the different conditions. The results have been compared to available data for Al-9Si-3Cu-(Fe) alloy used in HPDC. The T5 heat treated sand cast condition has shown to have properties similar to HPDC. All other sand cast conditions, including the previously tested Al-9Si-3Cu-(Fe) alloy, have been shown to be reasonable alternatives.

Semi-solid manufacturing is a near net shape forming process that takes advantage of an alloy's thixotropic behaviour. However, in order to obtain the desired thixotropic properties from an alloy in the semi -solid state, the microstructure of the as-cast feedstock metal needs to display a fine grained, equiaxed primary phase prior to reheating for the forming operation. Various methods are currently in use to obtain the required microstructure of which the MagnetoHydroDynamic (MHD) process is predominant. Two fundamental factors, namely shear rate and cooling rate, influence the formation of the fine grained, equiaxed primary phase during the MHD process. The aim of this research was to produce semi solid billets and in so doing, determine how the influence of the combination of the two fundamental factors contribute towards the final formation of the primary phase and to determine an optimal level ofthese factors' settings to deliver the desired microstructure. An MHD apparatus was constructed and the Taguchi method was used to design an experiment to investigate the influence ofthe fundamental factors involved in casting semi solid feedstock of aluminium A356.2. The issues ofthe formation of a fine eutectic phase and solidification shrinkage were also investigated. An experimental method was designed to investigate the significance ofthe fundamental factors' influence towards the appearance of the primary phase; the latter was evaluated using an image analysis system. The shear rate was controlled by varying the line frequency and the base frequency supplied to the electromagnetic stirrer and the cooling rate was controlled by initiation of a fixed, fast cooling rate at a certain melt temperature (TJ Results showed that a fine grained, equiaxed primary phase, with an average grain size of 55 /lm, was achieved after casting, prior to reheating for forming. The contribution of the base frequency and the line frequency were 8 % and 3.5 % respectively and the contribution ofTi was 86.5 % towards the outcome ofthe result. The cooling rate changed from approximately 0.3 QC/sec to 4.5 QC/sec at Ti. A fine textured eutectic phase was achieved with the fast cooling rate. The solidification shrinkage was accounted for by incorporating a riser on the mould. The feedstock produced in this research was compared, on a microstructural basis, to commercially available Semi Solid Metal (SSM) feedstock from Pechiney and SAG. The research feedstock had a larger, average primary grain size, however, it was more discrete and round grained than the commercial alloys which were finer and more rosette grained. Upon reheating to the semi solid state, ready for forming, the final, evolved grain sizes and shapes were almost identical between the research and commercial feedstock, despite the initial differences in grain sizes and shapes. However, the commercial alloys showed primary grains with trapped eutectic whereas in the research alloy, the primary grains were largely free of trapped eutectic. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.

Commercial alloys are continuously developed to improve their performance. Therefore it is useful to establish new optimisation software, which could be used in development of new materials or in materials selection. In the first part of the thesis, mechanical and technological properties, which are of importance in materials selection in mechanical design, are investigated. Two types of materials are analysed for the mechanical properties, aluminium alloys and stainless steels but only aluminium alloys for the technological properties. Thermodynamic analysis has been used to evaluate the effect of the microstructure. Solid solution hardening has been successfully modelled for both aluminium alloys and stainless steels following the theories by Labusch and Nabarro. The precipitation hardening is most dominant for the hardenable aluminium alloys, but the non-hardenable alloys also increase their strength from precipitation hardening. The non-hardenable alloys are divided into different tempers, which differ in the amount of strain hardening. This has also been modelled successfully. Combining these fundamental results with multiple regressions, models for mechanical and technological properties have been created. Separate models are developed for wrought aluminium alloys and stainless steels. For the aluminium alloys these include the solid solution hardening and the precipitation hardening. For the stainless steels, the thickness, nitrogen content and ferrite content are included together with the solid solution hardening. The second part of the thesis concerns materials selection and materials optimisation. Traditionally materials optimisation includes a preliminary sifting due to the vast number of engineering materials. Then there is a discriminating search followed by an optimisation. In the optimisation part the concept merit indices could be used to rank the materials. A merit index only includes material properties, as for example the characteristic strength, the density or the Young’s modulus. A concept related to the merit indices are the merit exponents, which can be used when no explicit functions for the merit indices are available. The merit exponents can also be used when creating a control area diagram (CAD). These diagrams are used as a design tool, where both the geometry and materials are taken into account. For a situation with several geometrical variables the merit exponents can give information of how much the target function will be influenced by a given property change. This technique can be used for a variety of situations, when there is more than one property limiting the final sizes of a component. Principles for setting up a CAD are given together with how the merit indices and exponents relate to the final CAD. / QC 20110817

Environmental savings can be made by increasing the use of aluminium alloys in the automotive industry as the vehicles can be made lighter. Increasing the knowledge about the heat treatment process is one task in the direction towards this goal. The aim of this work is to investigate and model the heat treatment process for Al-Si casting alloys. Three alloys containing Mg and/or Cu were cast using the gradient solidification technique to achieve three different coarsenesses of the microstructure and a low amount of defects. Solution treatment was studied by measuring the concentration of Mg, Cu and Si in the α-Al matrix using wavelength dispersive spectroscopy (WDS) after various times at a solution treatment temperature. A diffusion based model was developed which estimates the time needed to obtain a high and homogenous concentration of alloying elements for different alloys, temperatures and coarsenesses of the microstructure. It was shown that the yield strength after artificial ageing is weakly dependent on the coarseness of the microstructure when the solution treatment time is adjusted to achieve complete dissolution and homogenisation. The shape and position of ageing curves (yield strength versus ageing time) was investigated empirically in this work and by studying the literature in order to differentiate the mechanisms involved. A diffusion based model for prediction of the yield strength after different ageing times was developed for Al-Si-Mg alloys. The model was validated using data available in the literature. For Al-Si-Cu-Mg alloys further studies regarding the mechanisms involved need to be performed. Changes in the microstructure during a heat treatment process influence the plastic deformation behaviour. The Hollomon equation describes the plastic deformation of alloys containing shearable precipitates well, while the Ludwigson equation is needed when a supersaturated solid solution is present. When non-coherent precipitates are present, none of the equations describe the plastic deformation well. The evolution of the storage rate and recovery rate of dislocations was studied and coupled to the evolution of the microstructure using the Kocks-Mecking strain hardening theory.