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The Effect of Processing Parameters and Alloy Composition on the Microstructure Formation and Quality of DC Cast Aluminium Alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties.

The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys.

Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness.

Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting.

The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting.

The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , v, 41 p.
Keyword [en]
Aluminium wrought alloys, AA3xxx and 6xxx, Direct Chill Casting, Unidirectional Solidification, Bridgman technique, Process parameters, Microstructure, Composition modification, Thermal analysis, Inclusions, Metallographical investigations
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4205ISBN: 91-7178-496-9 (print)OAI: oai:DiVA.org:kth-4205DiVA: diva2:11229
Public defence
2006-12-14, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-09-01Bibliographically approved
List of papers
1. Analysis of solidification in a Bridgman furnace as a simulation of DC casting of aluminium alloy slabs
Open this publication in new window or tab >>Analysis of solidification in a Bridgman furnace as a simulation of DC casting of aluminium alloy slabs
2007 (English)In: Materials Science and Technology, ISSN 0267-0836, Vol. 23, no 4, 475-482 p.Article in journal (Refereed) Published
Abstract [en]

The vertical Bridgman directional solidification equipment has been used in several investigations to simulate direct chill casting of wrought aluminium alloys. As a basis for such investigations and alloy developments, it is important to have an understanding of the performance of the furnace used during simulation of the casting conditions. In this investigation the thermal conditions in the furnace have been analysed in detail, both by measurements and by mathematical modelling. The growth characteristics of the furnace, such as gradient, growth rates and cooling rates have been compared to conditions in large ingots. The direct chill casting conditions, which the simulations have been compared to, are casting of slabs of 330 and 600 mm thickness of an aluminium AA3003 type alloy. The results show that the experiments are able to simulate the cooling conditions in the ingots except from the surface zone. Comparisons of the microstructures have been made and a good agreement has been obtained for structure parameters such as grain size and DAS.

Keyword
microstructure, bridgman technique, aluminium alloys, directional solidification, dc casting
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6494 (URN)10.1179/174328407X168892 (DOI)000246106600016 ()
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-09-01Bibliographically approved
2. Solidification studies of 6xxx alloys with different Mg and Si contents
Open this publication in new window or tab >>Solidification studies of 6xxx alloys with different Mg and Si contents
2005 (English)In: LIGHT METALS 2005, 2005, 1039-1044 p.Conference paper, Published paper (Refereed)
Abstract [en]

Directional solidification studies, simulating billet casting, have been made off alloys with balanced and not balanced Mg and Si additions, and with different alloying levels ranging from 6063 to 6082.

The effects of growth rate, grain refinement, temperature gradient and composition on structure formation have been investigated. Increased alloy content strongly influence the solidification structure, but during cooling the coarsening process decrease the differences, and relatively similar structures are obtained at room temperature. Segregation to the surface zone is proposed to have an effect on surface defect formation.

Series
LIGHT METALS, ISSN 0147-0809
Keyword
aluminum; AA 6000 alloys, directional solidification, microstructure coarsening, surface defects
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6495 (URN)000228234700182 ()0-87339-580-8 (ISBN)
Conference
Technical Session on Light Metals 2005 held at the 134th TMS Annual Meeting San Francisco, CA, FEB 13-17, 2005 Minerals, Met & Mat Soc; TMS Aluminum Comm
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-12-06Bibliographically approved
3. Directional solidification of type AA3003 alloys with addition of Zn and Si
Open this publication in new window or tab >>Directional solidification of type AA3003 alloys with addition of Zn and Si
2004 (English)In: SOLIDIFICATION OF ALUMINUM ALLOYS, 2004, 53-62 p.Conference paper, Published paper (Refereed)
Abstract [en]

Directional solidification has been used to study effects on the structure and on materials properties from variations in the alloy contents in type AA 3003 alloys, Structure parameters such as grain size, DAS and secondary phase precipitations have been evaluated, as well as micro segregation, micro hardness and coarsening after solidification.

Here the first part of the project is reported, and additions of Zn and Si have been studied. Zn additions improve the strength of the matrix of the material through solid solution hardening. At a level of 5% the hardness is improved by about 50%, but the solidification structure becomes less promising. The DAS and the secondary phases become coarse, and the solidification range increases significantly as the Zn content increases from 2.5 to 5%.

Additions of Si, which have a significantly lower solubility than Zn, quickly gives larger amounts of secondary phase precipitations and a longer solidification interval.

Keyword
aluminum, AA 3003 alloys, directional solidification, microstructure coarsening
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6496 (URN)000189464200005 ()0-87339-569-7 (ISBN)
Conference
Symposium on Solidification of Aluminum Alloys held at the TMS 2004 Annual Meeting Charlotte, NC, MAR 14-18, 2004 TMS, Mat Proc & Mfg Div, Solidificat Comm
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-12-06Bibliographically approved
4. DTA and DSC Studies of aluminium 3003 alloys with Zn and Cu additions
Open this publication in new window or tab >>DTA and DSC Studies of aluminium 3003 alloys with Zn and Cu additions
2007 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38, no 9, 2138-2147 p.Article in journal (Refereed) Published
Abstract [en]

The effect of Zn additions in the range of 0 to 5 wt pct and Cu contents of 0.15 to 1.0 wt pct on the solidification and precipitation behavior of aluminum alloys of AA 3003 type was investigated with differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX). The results show that the additions of varying Zn and Cu have significant effects on the crystallization process, phase precipitation, and microstructure formation of the alloys. Phases formed have been identified and morphological differences have been discussed.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6497 (URN)10.1007/s11661-007-9251-1 (DOI)000249538900030 ()
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2017-12-14Bibliographically approved
5. Effect of titanium additions on the microstructure of DC-cast aluminium alloys
Open this publication in new window or tab >>Effect of titanium additions on the microstructure of DC-cast aluminium alloys
2005 (English)In: Materials Science and Engineering A, ISSN 0921-5093, Vol. 413-414, 277-282 p.Article in journal (Refereed) Published
Abstract [en]

The effect of the Ti content on the solidification structures of aluminum alloys of AA3003 type was investigated. Some improved corrosion properties can be obtained from increasing the Ti contents in aluminium alloys to a level above the normal practice for grain refinement. However, increasing the Ti content above the peritectic point, 0.15%, can influence the grain refinement and cause casting difficulties. The investigation was made by both characterizing the grain structure over the width of DC-cast rolling ingots of different alloys, and by studying the solidification microstructure of Bridgman directional solidified samples, grown to simulate ingot solidification. Structure properties such as grain size, distances between secondary phases and microstructure coarsening were studied. It was found that with normal Ti contents in the range of 0.015%, the grain refinement is effective. However, upon larger Ti additions to levels around 0.15% the grain structure becomes coarser.

Keyword
aluminium, titanium, wrought alloys, directional solidification, grain size, microstructure
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6498 (URN)10.1016/j.msea.2005.09.006 (DOI)000234202900045 ()
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-12-06Bibliographically approved
6. Method Developed for Quantitative Assessment of Inclusions in Aluminium Billets
Open this publication in new window or tab >>Method Developed for Quantitative Assessment of Inclusions in Aluminium Billets
2007 (English)In: LIGHT METALS 2007, 2007, 679-684 p.Conference paper, Published paper (Refereed)
Abstract [en]

A technique, based on sodium hydroxide hot macro etching, has been developed to deep etch non-metallic inclusions in a way that makes them visible on a macro scale. The distribution of inclusions in transverse sections along DC-cast aluminium billets could thus be studied. The technique has obvious advantages over other common analysis methods in giving a macroscopic spatial distribution of inclusions in a relatively simple way.

The paper describes the development of the new technique and some applications. An analysis was made of how different inclusions behave during prolonged strong etching. Light optical microscopy, TEM and SEM/EDX have been used to identify specific inclusions.

Series
LIGHT METALS, ISSN 0147-0809
Keyword
aluminium, wrought alloys, etching, inclusions, size distribution
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6499 (URN)000247423900113 ()978-0-87339-659-2 (ISBN)
Conference
Symposium on Light Metals held at the 2007 TMS Annual Meeting and Exhibition Orlando, FL, FEB 25-MAR 01, 2007 Minerals Met & Mat Soc
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-12-06Bibliographically approved
7. Solidification Studies of Automotive Heat Exchanger Materials
Open this publication in new window or tab >>Solidification Studies of Automotive Heat Exchanger Materials
2006 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 58, no 11, 56-61 p.Article in journal (Refereed) Published
Abstract [en]

Modifications of the aluminum alloy AA 3003 have been studied to improve and tailor its properties for applications in automotive heat exchangers. Laboratory techniques have been applied to simulate industrial direct-chill casting, and some basic solidification studies have been conducted. The results are coupled to structures observed in industrial-size ingots and discussed in terms of structure-property relations.

Keyword
Aluminum alloys, Ingots, Materials science, Solidification
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6500 (URN)10.1007/s11837-006-0228-0 (DOI)000241928400006 ()
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2017-12-14Bibliographically approved

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