Novel casting alloys that retain their mechanical strength at temperatures up to 300 °C are needed in order to improve automotive engine efficiency. The addition of Zr and V is a promising combination as a means of introducing the thermally stable cubic Al3ZrxV1-x nano-precipitates. In this project, the evolution of the Al3ZrxV1-x precipitates in multi-component cast aluminium alloys was characterised in a statistically significant way using small-angle X-ray scattering (SAXS). Unlike established metallurgical techniques, such as transmission electron microscopy (TEM), SAXS measures billions of precipitates in a single experiment. Two Al-6.8Si (wt.%) alloys were employed. One contained 0.30 wt.% Zr and 0.30 wt.% V and the other 0.37 wt.% Zr and 0.34 wt.% V. The alloys were cast and isothermally aged at 400 °C for times ranging from 3 to 50 h. Synchrotron SAXS was undertaken at the Diamond Light Source (DLS). Direct evaluation methods of SAXS spectrum were employed in order to extract information about the average precipitate size, precipitate size distribution and their evolution with ageing time. These results were compared with the results provided via TEM, both validating the methodology, and demonstrating the benefits and drawbacks of SAXS.From the SAXS analysis, the Guinier radius, Rg, was found to increase with ageing time from around 2 nm at 3 h to approximately 9 nm at 50 h. Precipitate volume fraction also increased from 0.11 vol.% to 0.24 vol.% for the lower solute alloy and from 0.14 vol.% to 0.33 vol.% for the higher solute one. In both alloys, the precipitate number density was observed to continuously decrease with ageing time. For the higher solute alloy, the average precipitate radius from the TEM measurements was found to be consistently smaller than Rg. Such a study is important because it extends the experimental SAXS methodology previously used only in simplified alloys to multi-component cast alloys of industrial importance. Moreover, the findings from this investigation suggest that both alloys quickly enter a stage where the precipitate growth and coarsening overlap, and the precipitate number density continuously decreases with time. Finally, this study underlines the importance of preliminary SAXS data treatment and provides an analytical guide in order to successfully characterize low volume fraction of precipitates in multi-component cast alloys.

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.

The objective of this work was to use LEFM in order to assess the detrimental influence of surrounding chloride-containing environments for stainless steels, hardened steel as well as for a cast aluminium alloy. An additional aim was also to use LEFM to assess the influence of porosity on the fatigue properties for different commercial cast aluminium alloys and manufacturing methods. The environmental influence on fatigue performance was mainly evaluated from fatigue crack growth measurements using compact tension (CT) specimens. In addition, fatigue performance in the high cycle regime was studied using spot welded specimens and smooth specimens. Corrosion fatigue tests for stainless steels were performed in different chloride-containing aqueous solutions and compared to the behaviour in air. Variables, which have been investigated, included temperature, redox potential and fatigue test frequency. The environmental influence on fatigue performance has also been compared to localised corrosion properties. Fatigue crack propagation rates were found to be higher in 3% NaCl than in air for all stainless steels investigated. The highest alloyed austenitic steel, 654SMO, showed the least influence of the environment. For duplex stainless steels the environment enhanced fatigue crack propagation rate to a higher degree than for austenitic stainless steels. This is explained by a material-dependent corrosion fatigue mechanism. In the high cycle regime, fatigue properties for spot welded stainless steels specimens were found to be decreased between 30%-40% due to the presence of 3% NaCl. For the hardened steel 100CrMnMo8 a fracture mechanics approach was employed for prediction of corrosion fatigue properties. In this model corrosion pit growth rate and the threshold stress intensity factor for fatigue crack propagation are needed as input parameters. For the high pressure die cast aluminium alloy the environmental influence of fatigue initiation through pre-exposure of smooth specimens was studied. Depending on environment used for pre-exposure, fatigue strength was found to be reduced by up to 50 % compared to the fatigue strength in air. Fatigue strength reduction was clearly associated to corrosion pits in the aluminium material. A fracture mechanics model was further successfully used to predict the environmental influence. The influence of porosity on the fatigue strength for the cast aluminium alloys tested has been described by a Kitagawa diagram. In design, the Kitagawa diagram can be used to predict the largest allowable pore size if the load situation in the component is known. The size of the porosity could either be evaluated directly from x-ray images or from metallographic prepared cross-sections using a method of extreme value analysis / QC 20100907

In the automobile industry, there is a big push for the automotive car manufacturers to base engineering decisions on the results of Computer Aided Engineering (CAE) solutions, and to transform the prototyping and testing, from a costly iterative process to a final verification and validation step. The variability in components material properties and environmental conditions together with the lack of knowledge about the underlying physics of complex systems often make it impractical to make reliable predictions based on only deterministic CAE models. One such area is the CAE modelling of cast aluminium components. These cast aluminium components have gained a huge relevance in the automobile industries due to their commendable mechanical properties. The advantage of the cast aluminium alloys are being a well-established alloy system in manufacturing processes, their functional integrity and relatively low weight. However, the presence of pores and micro-voids obtained during the manufacturing process constitutes a specific material behaviour and establishes a challenge in modelling of the cast materials. Furthermore, the low ductility of the materialdemands for the advanced numerical model to predict the failure. The main focus of this master thesis work is to investigate modelling technique of a cast aluminium alloy component, a spring tower, for a drop tower test and validate the predicted behaviour with the physical test results. Volvo Car Corporation currently uses a material model provided by MATFEM for cast aluminium parts which are explored in this thesis work, to validate the material model for component level testing. The methodology used to achieve this objective was to develop a boundary condition to perform component level tests in the drop tower and to correlate these with the obtained results found by using various modelling techniques in the explicit solver LS-DYNA. Therefore, precise and realistic modelling of the drop tower is crucial because the simulation results can be influenced by major design changes. A detailed finite element model for the spring tower has been developed from the observations made during the physical testing. The refined model showed good agreement with the existing model for the spring tower and observations from physical tests.

Ο χαρακτηρισμός της ποιότητας των χυτών κραμάτων αλουμινίου , γίνεται μέχρι σήμερα μέσω του χαρακτηρισμού της ποιότητας της μικροδομής, μετρήσεων σκληρότητας και πειραμάτων κρούσης και σε μικρότερο βαθμό, δοκιμών εκφυλισμού. Στην παρούσα διατριβή, προτείνεται ένας νέος εμπειρικός δείκτης για τον ποσοτικοποιημένο χαρακτηρισμό της ποιότητας χυτών κραμάτων αλουμινίου. Ο προτεινόμενος δείκτης αξιολογεί την ποιότητα ενός υλικού από την πλευρά του μηχανικού που σχεδιάζει ένα κατασκευαστικό στοιχείο και επομένως την αξιολογεί ως την ικανότητα του υλικού για μηχανικές επιδόσεις. Για την αξιολόγηση αυτή ο προτεινόμενος δείκτης συνεκτιμά την αντοχή και την ολκιμότητα του υλικού σε εκφυλισμό. Παράλληλα, για το χαρακτηρισμό της ποιότητας, ο δείκτης παίρνει υπόψη τη δυσθραυσότητα του υλικού καθώς και τη διασπορά των μηχανικών ιδιοτήτων του υλικού. Η διατύπωση του δείκτη στηρίχτηκε σε έναν ευρείας έκτασης πειραματικό χαρακτηρισμό της μηχανικής συμπεριφοράς σε εφελκυσμό καθώς και της μικροδομής των κυριότερων αεροπορικών χυτών κραμάτων αλουμινίου σε συνάρτηση με τη μεταβολή α) της χημικής σύστασης, β) του ρυθμού στερεοποίησης και γ) της θερμικής κατεργασίας αυτών καθώς και στη διατύπωση εμπειρικών συναρτήσεων για την εξάρτηση των μηχανικών ιδιοτήτων των κραμάτων που εξετάστηκαν από τις παραπάνω μεταβολές των παραμέτρων χύτευσης. Προκειμένου να διευκολυνθεί η αξιοποίηση του δείκτη, διατυπώθηκαν απλουστευμένες προσεγγιστικές εκφράσεις που επιτρέπουν τον υπολογισμό του από δεδομένα των απλών πειραμάτων της σκληρομέτρησης και της κρούσης. Τέλος προτάθηκε μεθοδολογία δημιουργίας χαρτών ποιότητας με βάση τον προταθέντα δείκτη για την υποστήριξη της επιλογής υλικού όταν είναι γνωστές οι απαιτήσεις σε μηχανικές επιδόσεις συγκεκριμένων κατασκευαστικών στοιχείων. / Quality evaluation of cast aluminum alloys is currently made mainly by means of the met- allographic characterization of the alloy’s niicrostructure, hardness measurements, impact tests and, to a lesser extend, tensile tests, are involved, as well. Yet, the overall decision is not a straight forward procedure, relies heavily on the experience of the quality engineer and involves an appreciable amount of subjective judgment. In the present Thesis, a new empirical quality index for the quantitative evaluation of the quality of cast aluminum alloys is introduced. The proposed index evaluates quality which is regarded as the ability of a material for mechanical performance. The index evaluates the quality of a cast alloy on the basis of a balance between the material’s tensile strength and ductility with regard also to the material’s toughness. In the proposed index the scatter in mechanical properties is also accounted. The formulation of the index has been based on an extensive experimental characterization of the tensile behavior and the microstructural features of the main aircraft aluminum cast alloys by varying chemical composition, solidification rate and artificial aging treat- ment. To facilitate the wide spread use of the index, simplified approximate expressions of the index have been formulated as well. These expressions allow for the calculation of the proposed quality index based on hardness measurements and impact test results. The index has been also exploited to devise quality maps, which may be used to support material selection with regard to the mechanical properties required by the design office for a certain application.