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Heat treatment of Al-Si-Cu-Mg casting alloys
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
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 [en]
Cast aluminium alloys, Heat treatment, Solution treatment, Artificial ageing, Tensile properties, Plastic deformation, Microstructure, Modelling
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-15695ISBN: 978-91-7385-529-7 (print)OAI: oai:DiVA.org:hj-15695DiVA: diva2:446044
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
List of papers
1. The heat treatment of Al–Si–Cu–Mg casting alloys
Open this publication in new window or tab >>The heat treatment of Al–Si–Cu–Mg casting alloys
2010 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 210, no 10, 1249-1259 p.Article in journal (Refereed) Published
Abstract [en]

Heat treatment of gravity cast aluminium components normally involve a solution heat treatment followed by quenching and subsequent ageing, either naturally or artificially. The mechanical properties of aluminium castings containing Cu and Mg are clearly improved by heat treatment but there are still some uncertainties and contradictions about the phase dissolution, formation of precipitates and the influence of natural and artificial ageing parameters such as time and temperature, on the peak strength of these materials. The present paper reviews over 60 papers in attempt to clarify and map out the influence of each and every sequence that is involved in the heat treatment process on the evolution and kind of precipitates, which in turn determines the mechanical properties of cast aluminium components.

Place, publisher, year, edition, pages
Elsevier, 2010
Keyword
Cast aluminium alloys; Heat treatment; Mechanical properties.
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:hj:diva-11308 (URN)10.1016/j.jmatprotec.2010.03.020 (DOI)
Available from: 2010-01-14 Created: 2010-01-14 Last updated: 2017-12-12Bibliographically approved
2. Optimisation of solution treatment of cast Al–Si–Cu alloys
Open this publication in new window or tab >>Optimisation of solution treatment of cast Al–Si–Cu alloys
2010 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 31, no suppl. 1, 44-49 p.Article in journal (Refereed) Published
Abstract [en]

The influence of solidification rate on the solution treatment response for an Al–8Si–3.1Cu alloy has been investigated. The alloy was cast using the gradient solidification technique to produce samples with three different solidification rates. The samples were solution treated at 495 C for various times between 10 min and 10 h. The concentration of copper in the matrix was measured using the wavelength dispersive spectroscopy technique, WDS.

The results show that the coarseness of the microstructure clearly affects the solution treatment time needed to dissolve particles and obtain a homogenous distribution of copper in the matrix. A short solution treatment time of 10 min is enough to achieve a high and homogenous copper concentration for a material with a fine microstructure (secondary dendrite arm spacing, SDAS of 10 μm), while more than 10 h is needed for a coarse microstructure (SDAS of 50 μm). A model was developed to describe the dissolution and homogenisation process. The model shows good agreement with the experimental results.

Place, publisher, year, edition, pages
Elsevier, 2010
Keyword
Cast aluminium alloys, Solution treatment, Microstructure, Diffusion, WDS, Dissolution, Homogenisation
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:hj:diva-11307 (URN)10.1016/j.matdes.2009.10.035 (DOI)
Available from: 2010-01-14 Created: 2010-01-14 Last updated: 2017-12-12Bibliographically approved
3. Optimization of solution treatment of cast Al-7Si-0.3Mg and Al-8Si-3Cu-0.5Mg alloys
Open this publication in new window or tab >>Optimization of solution treatment of cast Al-7Si-0.3Mg and Al-8Si-3Cu-0.5Mg alloys
2014 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45, no 4, 1916-1927 p.Article in journal (Refereed) Published
Abstract [en]

The influence of solidification rate on the solution-treatment response has been investigated for an Al-7Si-0.3Mg alloy and an Al-8Si-3Cu-0.5Mg alloy. The concentrations of Mg, Cu, and Si in the matrix after different solution-treatment times were measured using a wavelength dispersive spectrometer. All Mg dissolves into the matrix for the Al-Si-Mg alloy when solution treated at 803 K (530 C) because the π-Fe phase is unstable and transforms into short β-Fe plates which release Mg. The Q-Al5Mg8Cu2Si6 phase do not dissolve completely at 768 K (495 C) in the Al-Si-Cu-Mg alloy and the concentration in the matrix reached 0.22 to 0.25 wt pct Mg. The distance between π-Fe phases and Al2Cu phases was found to determine the solution-treatment time needed for dissolution and homogenization for the Al-Si-Mg alloy and Al-Si-Cu-Mg alloy, respectively. From the distance between the phases, a dimensionless diffusion time was calculated which can be used to estimate the solution-treatment times needed for different coarsenesses of the microstructure. A model was developed to describe the dissolution and homogenization processes.

Keyword
Dissolution, Homogenisation, Cast aluminium, WDS
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-16311 (URN)10.1007/s11661-013-2141-9 (DOI)000333525200030 ()2-s2.0-84898857263 (Scopus ID)
Available from: 2011-10-10 Created: 2011-10-10 Last updated: 2017-12-08Bibliographically approved
4. Artificial ageing of Al–Si–Cu–Mg casting alloys
Open this publication in new window or tab >>Artificial ageing of Al–Si–Cu–Mg casting alloys
2011 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, 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
Keyword
Age hardening; Aluminium alloys; Casting; Mechanical characterization
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-16250 (URN)10.1016/j.msea.2011.06.036 (DOI)
Available from: 2011-10-05 Created: 2011-10-05 Last updated: 2017-12-08Bibliographically approved
5. Evaluating the plastic deformation of Al-Si-Cu-Mg casting alloys using the Hollomon and Ludwigson equations
Open this publication in new window or tab >>Evaluating the plastic deformation of Al-Si-Cu-Mg casting alloys using the Hollomon and Ludwigson equations
2011 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336Article in journal (Other academic) Submitted
Abstract [en]

The plastic part of true stress – true strain curves of three Al-Si casting alloys was evaluated using the Hollomon and Ludwigson equations. Three coarsenesses of the microstructure, two ageing temperatures and a number of ageing times were used. The Hollomon equation was able to describe the true stress – true strain curves for microstructures containing shearable precipitates, while the Ludwigson equation was needed for microstructures containing a supersaturated solid solution. A two bump shape appeared on the double logarithmic plot for the Al-Si-Cu alloy and the overaged Al-Si-Cu-Mg alloy. These curves could not be well described by the Hollomon or the Ludwigson equations.

Place, publisher, year, edition, pages
Maney Publishing, 2011
Keyword
Cast aluminium alloys; Plastic deformation; Artificial ageing; Hollomon; Ludwigson
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-16309 (URN)
Available from: 2011-10-10 Created: 2011-10-10 Last updated: 2017-12-08Bibliographically approved
6. Influence of alloy composition, solidification rate and artificial aging on plastic deformation behaviour of Al-Si-Cu-Mg casting alloys
Open this publication in new window or tab >>Influence of alloy composition, solidification rate and artificial aging on plastic deformation behaviour of Al-Si-Cu-Mg casting alloys
2013 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 26, no 1, 28-36 p.Article in journal (Refereed) Published
Abstract [en]

The plastic deformation behaviour of three Al–Si casting alloys was investigated using the Kocks–Mecking strain hardening theory. Three coarsenesses of the microstructure, two aging temperatures and a number of aging times were used. For Al–Si–Mg and Al–Si–Cu–Mg alloys,the dislocation storage rate decreases while the dislocation recovery rate increases with agingtime during underaging, whereas the concentration of alloying elements in solid solution decreases. The storage rate reaches a minimum at the peak aged condition and increases at overaging. The storage and recovery rates of the Al–Si–Cu alloy increase with aging time in the underaged condition and start to decrease during overaging, which indicates that a mixture of shearable and non-shearable precipitates are present during underaging, whereas all precipitates become non-shearable on overaging.

Place, publisher, year, edition, pages
Maney Publishing, 2013
Keyword
Tensile testing, aluminium alloys, casting, heat treatment, strain hardening.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-16310 (URN)10.1179/1743133612Y.0000000025 (DOI)
Available from: 2011-10-10 Created: 2011-10-10 Last updated: 2017-12-08Bibliographically approved
7. Modelling yield strength of heat treated Al–Si–Mg casting alloys
Open this publication in new window or tab >>Modelling yield strength of heat treated Al–Si–Mg casting alloys
2011 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 24, no 6, 338-346 p.Article in journal (Refereed) Published
Abstract [en]

A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0.2 and 0.6 wt-% and aging temperatures between 150°C and 210°C. Spherical precipitates with the composition Mg5Si6, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.

Place, publisher, year, edition, pages
Maney Publishing, 2011
Keyword
Cast aluminium alloys, Modelling, Artificial aging, Coherency strengthening, Orowan strengthening
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-16251 (URN)10.1179/1743133611Y.0000000021 (DOI)
Available from: 2011-10-05 Created: 2011-10-05 Last updated: 2017-12-08Bibliographically approved

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