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Artificial ageing of Al–Si–Cu–Mg casting alloys
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0001-6481-5530
2011 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, Vol. 528, no 24, 7402-7409 p.Article in journal (Refereed) Published
Abstract [en]

The T6 heat treatment is commonly used for gravity cast Al-Si-Cu-Mg alloys. The influence of the alloying elements Cu and Mg and the artificial ageing temperature on the age hardening response were investigated. Artificial ageing was conducted at 170°C and 210°C for various times for three alloys, Al-7Si-0.3Mg, Al-8Si-3Cu and Al-8Si-3Cu-0.5Mg, cast with three different solidification rates (secondary dendrite arm spacing of about 10, 25 and 50μm). The coarseness of the microstructure has a small influence on the yield strength, as long as the solution treatment is adjusted to obtain complete dissolution and homogenisation. The peak yield strength of the Al-Si-Mg alloy is not as sensitive to the ageing temperature as the Al-Si-Cu and Al-Si-Cu-Mg alloys are. The ageing response of the Al-Si-Cu alloy is low and very slow. When 0.5 wt% Mg is added the ageing response increases drastically and a peak yield strength of 380 MPa is obtained after 20 h of ageing at 170°C for the finest microstructure, but the elongation to fracture is decreased to 3%. The elongation to fracture decreases with ageing time in the underaged condition as the yield strength increases for all three alloys. A recovery in elongation to fracture of the Al-Si-Cu-Mg alloy on overageing is obtained for the finest microstructure, while the elongation remains low for the coarser microstructures. The quality index, Q= YS +Kε, can be used to compare the quality of different Al-Si-Mg alloys. This is not true for Al-Si-Cu-Mg alloys, as K depends on the alloy composition. Overageing of the Al-Si-Mg alloy results in a decrease in quality compared to the underaged condition

Place, publisher, year, edition, pages
Elsevier , 2011. Vol. 528, no 24, 7402-7409 p.
Keyword [en]
Age hardening; Aluminium alloys; Casting; Mechanical characterization
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-16250DOI: 10.1016/j.msea.2011.06.036OAI: oai:DiVA.org:hj-16250DiVA: diva2:446040
Available from: 2011-10-05 Created: 2011-10-05 Last updated: 2015-10-02Bibliographically approved
In thesis
1. Heat treatment of Al-Si-Cu-Mg casting alloys
Open this publication in new window or tab >>Heat treatment of Al-Si-Cu-Mg casting alloys
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Göteborg: Chalmers Reproservice, 2011. 45 p.
Series
Doktorsavhandlingar vid Chalmers tekniska högskola, ISSN 0346-718X ; 3210
Keyword
Cast aluminium alloys, Heat treatment, Solution treatment, Artificial ageing, Tensile properties, Plastic deformation, Microstructure, Modelling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-15695 (URN)978-91-7385-529-7 (ISBN)
Public defence
2011-05-20, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2011-10-11 Created: 2011-07-13 Last updated: 2011-12-14Bibliographically approved

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