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Analysis of solidification in a Bridgman furnace as a simulation of DC casting of aluminium alloy slabs
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Processing.
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.

Place, publisher, year, edition, pages
2007. Vol. 23, no 4, 475-482 p.
Keyword [en]
microstructure, bridgman technique, aluminium alloys, directional solidification, dc casting
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-6494DOI: 10.1179/174328407X168892ISI: 000246106600016OAI: oai:DiVA.org:kth-6494DiVA: diva2:11222
Note
QC 20100901Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2010-09-01Bibliographically approved
In thesis
1. The Effect of Processing Parameters and Alloy Composition on the Microstructure Formation and Quality of DC Cast Aluminium Alloys
Open this publication in new window or tab >>The Effect of Processing Parameters and Alloy Composition on the Microstructure Formation and Quality of DC Cast Aluminium Alloys
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
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:nbn:se:kth:diva-4205 (URN)91-7178-496-9 (ISBN)
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

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