Thesis defense of Raquel Hidalgo

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Thesis defense of Raquel Hidalgo

Thesis

Thesis defense of Raquel Hidalgo

Title of the thesis: Desarrollo de aleaciones de aluminio de elevadas prestaciones mecánicas y método de predicción de vida a fatiga orientados a componentes de automoción. Obtained the SOBRESALIENTE CUM LAUDE qualification.

2019·05·21

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  • Title of the thesis: Desarrollo de aleaciones de aluminio  de elevadas prestaciones mecánicas y método de predicción  de vida a fatiga orientados a componentes de automoción.
  • PhD programme: DOCTORATE PROGRAMME IN MECHANICAL ENGINEERING AND ELECTRICAL ENERGY
  • Thesis directors: Jon Ander Esnaola Ramos and Nuria Herrero Dorca
  • Court:
    • President: Dr. D. Juan José De Damborenea González (Consejo Superior de Investigaciones Científicas)
    • Vocal: Dr. D. Javier Belzunce Varela (Universidad de Oviedo)
    • Vocal: Dra. Dña. Ana Isabel Fernández Calvo (IK4-Azterlan)
    • Vocal: Dr. D. Iñaki Hurtado Hurtado (Mondragon Unibertsitatea)
    • Secretary: Dr. D. Iñigo Llavori Osa (Mondragon Unibertsitatea)

Abstract

The restrictions on CO2 emissions into the atmosphere promoted by the European Union have obliged manufacturers to reduce the weight of their vehicles and, in turn, their individual components. This has led to a transition of materials, from cast iron to aluminium alloys, thanks to their lower density. However, aluminium alloys present inferior mechanical properties, and difficulties, therefore, in terms of complying with the requirements required of the parts, especially in those areas subjected to greater stresses. Hence, the present thesis is centred on the improvement of behaviour in service of components manufactured in low-pressure die-casting and subjected to strict loads, as could be the case with wheel rings and knuckle arms. 

Firstly, aluminium alloy alternatives to A356 (Al-Si-Mg), the most used currently, have been developed with the objective of increasing its mechanical properties. To this end, two routes were chosen: adding Cu to the A356 composition resulting in Al-Si-Cu-Mg alloys, in which it is necessary to combine high strength and elongation with good resistance to corrosion; and adapting 7xxx wrought alloys compositions series to be used in foundry processes. In both cases, the study was focused on the design of the composition and the optimum heat treatment to be applied. As a result, two alloys were obtained, one from each family which, once heat treated, reach up to 357 MPa of yield strength, 398 MPa of UTS and 11.1% of elongation, representing an improvement of 90, 53 y 120% respectively in the tensile properties of the A356 alloy. 

Additionally, due to their position in the vehicle, these components are subjected to high stresses over time, which can cause fatigue failure. The fatigue behaviour of a part depends on different factors, such as surface quality, stress concentrators, residual stresses... Furthermore, in casting components, defects introduced during this process are determining. Among these, porosity is the one that has a greater impact. The specific models developed thus far do not consider sufficiently the influence of this factor on fatigue life, with the result that they may provide errors in the estimation. As an alternative, in the present thesis, a new prediction model for fatigue life estimation in casting aluminium alloys is presented. It considers the effect of porosity in both initiation and propagation stages, giving predictions with logarithmic errors no greater than 2%.