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| Home > Publications database > The Effect of Solution Treatment on Cast AlCuMn Alloys for Cylinder Head Production |
| Dissertation / PhD Thesis | PUBDB-2015-00370 |
2014
Vienna
Abstract: New high temperature resistant materials are necessary to increase the efficiency of combustion engines and thus fulfil current environmental regulations. Al-Cu cast alloys are potential candidates for cylinder head production. They show higher tensile and low cycle fatigue strength than the widespread Al-Si alloys, approaching even some grades of ductile iron. However, they have been rarely used for serial production owing to castability problems.In this work, the evolution of the microstructure of the cast B206 (AlCu4.8Mn0.24Mg0.33), AlCu7 (AlCu6.5Mn0.4Zr0.22) and AF52 (AlCu4.7Mn0.4Ti0.17) alloys is investigated by two dimensional (2D) and three dimensional (3D) methods as a function of solution treatment (ST) time at 530 °C. Their tensile, compressive and thermomechanical fatigue (TMF) behaviour is correlated with the 3D microstructural changes provoked by ST. Furthermore, the damage accumulation during RT tensile tests is investigated three dimensionally by means of synchrotron tomography.Part of the highly interconnected aluminides segregated during casting dissolves during the first 4 h of ST. The dissolution of Cu into the -Al matrix provokes a strength increase in all the alloys owing to precipitation hardening. The strengthening of the alloys after ST is approximated as an increase proportional to the capability of the -Al matrix to dissolve Cu atoms. The B206 alloy shows the largest strength and strengthening potential after ST. The volume fraction (Vf) of aluminides remains constant for longer ST times, as well as the strength of the alloys.All the alloys experience an increase in ductility after the first 4 h of ST. The AF52 alloy is in any ST condition the most ductile alloy at RT and at 250 °C followed by B206 and AlCu7. This is due to two different factors: i) the different kind of aluminides present and ii) the effect of the solution treatment. On one hand, needle / platelet-like Al7CuFe (-Fe) aluminides are only present in B206 and AlCu7. These aluminides are more detrimental in terms of ductility than the pseudo-chinese script Al6(CuMnFe) (-Fe) aluminides present in the AF52 alloy. Moreover aluminides in the B206 and AlCu7 alloys present a larger fraction of concave regions with small curvature radii than the AF52 alloy. These regions act as stress concentrators and lead to a crack formation during tensile deformation. On the other hand, the ST reduces the aluminides volume fraction and partially dissolves the 3D aluminides network. Since cracks tend to initiate and propagate through the aluminides network, the ST provokes a reduction of crack nucleation sites and propagation paths.Damage mainly occurs in the form of cracks generated at large aluminides oriented perpendicularly to the loading direction. These cracks propagate through the aluminides network in alloys with large aluminides Vf, which implies aluminides networks with high interconnectiviy and connectedness. Shrinkage pores do not play a significant role in the damage generation process, particularly for the AlCu7 alloy. Porosity growth with further crack propagation through the aluminides network represents the other damage accumulation mechanism, proper of alloys with lower aluminides Vf. This occurs partially in the B206 alloy and particularly in the AF52 alloy. Void generation in the -Al matrix takes place only at elevated temperature, i.e. 250 °C.Recommendations for further alloy design are given based on the analysis of the results obtained.
Keyword(s): Dissertation
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