Development of tools for integrated optimisation and use of aluminium alloys

Abstract: 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.