Rapid modern technological changes and improvements bring great motivations in advanced material designs and fabrications. In this context, metal matrix composites, as an emerging material category, have undergone great developments over the past 50 years. Their primary applications, such as automotive, aerospace and military industries, require materials with increasingly strict specifications, especially high stiffness, lightweight and superior strength. For these advanced applications, carbon fiber reinforced aluminum matrix composites have proven their enormous potential where outstanding machinability, engineering reliability and economy efficiency are vital priorities. To contribute in the understanding and development of carbon fiber reinforced aluminum matrix composites, this study focuses on composite fabrication, mechanical testing and physical property modelling. The composites are fabricated by squeeze casting. Plain weave carbon fiber (AS4 Hexcel) is used as reinforcement, while aluminum alloy 6061 is used as matrix. The improvement of the squeeze casting fabrication process is focused on reducing leakage while combining thermal expansion pressure with post-processing pressing. Three different fiber volume fractions are investigated to achieve optimum mechanical properties. Piston-on-ring (POR) bend tests are used to measure the biaxial flexural stiffness and fracture strength on disc samples. The stress-strain curves and fracture surfaces reveal the effect of fiber-matrix interface bonding on composite bend behaviour. The composites achieved up to 11.6%, 248.3% and 90.1% increase in flexural modulus, strain hardening modulus and yield strength as compared with the unreinforced aluminum alloy control group, respectively. Analytical modelling and finite element modelling are used to comparatively characterise and verify the composite effective flexural modulus and strength. Specifically, they allowed iii evaluating how far the experimental results deviate from idealized assumptions of the models, which provides an insight into the composite sample quality, particularly at fiber-matrix interfaces. Overall, the models agree well with experimental results in identifying an improvement in flexural modulus up to a carbon fiber volume fraction of 4.81vol%. However, beyond a fiber content of 3.74vol%, there is risk of deterioration of mechanical properties, particularly the strength. This is because higher carbon fiber volume fractions restrict the infiltration and wetting of carbon fibre by the liquid, potentially leading to poor fiber-matrix interface bonding. It is shown that higher thermal expansion pressures and subsequent post-processing pressing can overcome this challenge at higher carbon fiber volume contents by reducing fiber-aluminum contact angle, improving infiltration, reducing defects such as porosity, and overall improving fiber-matrix bonding.

The Diploma thesis deals with the verification of Jominy end-quench test when analysis of hardenability of aluminium alloys is considered. The problem was solved experimentally, by developing end-quench curves of selected grades of aluminium alloys. Degree of hardenability of alloy is hardness after complete strengthening heat treatment. Samples of aluminium alloys commonly used for aircraft structures were prepared and then subjected to end-quench tests in various quenchants. On the basis of performed experiments, suitability of Jominy end-quench test for comparison of hardenability of alloys was found. Part of the thesis deals with the creation of equivalent cooling rate diagrams using the developed end-quench curves.

The aim of this thesis is influence of precipitation hardening regime, specifically quench rate, on mechanical properties of aluminium alloy A356 (AlSi7Mg0.3). Samples were after solution treatment quenched into water with different temperatures and age hardened. Tensile strength tests, hardness tests and microstructure observations were done after heat tretment.

Aluminium alloys, which are the most widely used materials in the aircraft and aerospace industries, find their applications due to high strength–to-density ratio, resistance to catastrophic fracture, high degree of toughness, fabricability including good weldability and availability. High strength aluminum alloys are used in different forms like sheets, forgings and extruded rods, welded and machined components in the aerospace industry. One major application of the aluminium alloys in the space sector is in the launch vehicle and satellite sub-systems. The Indian Space Research Organization has met major challenges of indigenization of suitable aluminium alloys, for example, Al-Cu alloys (like AA2219) and Al–Zn-Mg alloys (like AA7075 and AFNOR 7020). Many failures of the metallic sub-systems made of different grades of aluminum alloys have confirmed that high levels of residual stresses and unacceptable microstructures have played a role. Crystallographic texture in these materials has a very significant role to play in the performance of these materials in service. The anisotropy in the mechanical properties caused by crystallographic texture would add to the woes of the existing problems of residual stresses and directionality in the microstructure. In this context, a detailed study of crystallographic texture and residual stresses of high strength aluminium alloys is mandatory. It is also important to study the influence of texture on the anisotropy in mechanical properties. The present research programme aims at addressing some of these aspects. The entire work has been divided in three major sections, namely macro and micro texture analysis, non-destructive measurement of residual stresses using X-ray Diffraction (XRD) and the Ultrasonic Testing (UST) and the study of anisotropy in the mechanical properties arising due to the above two factors. The thesis composition is as follows. In Chapter I, a detailed survey of the literature has been presented wherein basic physical metallurgy for different aluminum alloys of interest has been given. Thereafter, details of texture measurement by the X-ray diffraction and Electron Back Scatter Diffraction (EBSD) are presented. This is followed by a detailed review on the texture studies carried out in aluminium alloys under various conditions. Literature review on the two non-destructive methods, namely the X-ray diffraction and ultrasonic method has been carried out in detail. In order to account for microstructural changes, Differential Scanning Calorimetry (DSC) was carried out. Recent work on the mechanical property anisotropy arising due to high degree of mechanical working in aluminium alloys has been reviewed. Chapter II includes the experimental details involved in the course of the present investigation. The procedural details of cold rolling and associated microstructural changes are given in this chapter. This is followed by the texture measurement methods. Experimental details of the bulk texture measurement using the X-ray diffraction and micro texture measurements by the Electron Back Scatter Diffraction (EBSD) in the SEM are described. Details of the texture computation procedure as well as micro texture analysis methods are also presented. Basic principles of the non-destructive methods of measuring residual stresses, viz., the X-ray diffraction and the Ultrasonic testing, including the theory of measurements, are dealt with. Finally, the details of measurements of anisotropy in mechanical properties, including simulation carried out, for the three alloys are delineated. Chapter III deals with the results of the crystallographic texture measurements carried out on the cold rolled and artificially aged aluminium alloys. Results obtained from the pole figure analysis, Orientation Distribution Function (ODF) method and estimation of the various fibres present in the cold rolled material and the volume fraction of the texture components are discussed in detail for the three aluminium alloys. Results of the micro texture measurements using the EBSD are presented, explained and analyzed in detail. A comparison of the inverse pole figures (IPFs), Image Quality (IQ) maps, Misorientation angle, Grain Orientation Spread (GOS), Kernal Average Misorientation (KAM), CSL boundaries, Grain size and Grain boundary character distribution (GBCD) for materials cold rolled to different reduction for each of the alloys are done and analyzed. Conclusions are drawn regarding the evolution of texture from the above analysis. Deformation texture components Cu, Bs and S increase from the starting material as the rolling percentage increases. On the other hand, recrystallization texture components of Goss and Cube are observed to be weak. AFNOR 7020 developed the strongest texture followed by the AA7075 and AA2219 alloys. The Bs component is stronger in AFNOR 7020 alloy. This is attributed to the shear banding. Average KAM value increases as the cold working in the material increases confirming that the material contains high dislocation density at higher working percentages. Chapter IV deals with residual stresses in the aluminium alloys. Measurement of residual stresses has been carried out on the same sheets and plates, wherever it was possible, using the two methods. The residual stresses have been measured in two mutually perpendicular directions of the aluminium alloy sheets. Residual stress measurements by the ultrasonic method using the Critically Refracted Longitudinal (LCR) wave technique is also used to measure the subsurface stresses non-destructively. Acousto Elastic Coefficients (AEC) is determined for the alloys, in uniaxial tension. Using the AEC for the alloys, the RS at a depth of 3mm are evaluated using a 2MHz probe. Results of the stresses measured by the two methods have been discussed. The trends and anisotropy in the stress values due to texture are discussed and compared with the literature available. Surface residual stresses by the XRD method show compressive stresses at a majority of the locations. Residual stresses measured by the ultrasonic technique, which has a depth of penetration of about 3mm, have shown tensile stresses on many locations. Residual stresses are influenced by the crystallographic texture. Anisotropy in stress values in the longitudinal and transverse directions is demonstrated. In Chapter V, the anisotropy in mechanical properties for the three alloys is discussed in detail. The anisotropy in the three directions, namely the parallel, transverse and 45 deg orientation to the rolling directions is evaluated. The Lankford parameter, otherwise known as Plastic Anisotropy Ratio “r”, has been measured from the tensile tests of the alloy samples in the cold rolled conditions. These have been compared with the computed “r” from the XRD ODF data using the VPSC simulations and found to be qualitatively matching. These trends are discussed with the available literature on the anisotropy of the mechanical properties for aluminium alloys. Samples subjected to high cold rolling show anisotropy of UTS, YS and ‘n’ values. Experimentally measured “r” values in all the deformation conditions match the trend qualitatively with the simulated ones. The maximum anisotropy was observed at 45o orientation to the rolling direction in all the three alloys. Chapter VI gives the summary of the results from the study and the suggestions for future work.

No description available.

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.

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.

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