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Investigation of Surface Formation in As-Cast and Homogenized 6xxx Aluminium Billets
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. Mid Sweden University. (Materials Science)ORCID iD: 0000-0002-0401-4384
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The direct chill (DC) casting technique to produce billets for extrusion and ingots for rollingwas developed in the 1930s. The principle, which is still valid, is a two-stage cooling with a primary cooling at a mould surface followed by water spraying directly on the surface. Improvements of this technique have mainly focused on changes to the primary cooling, where a water-cooled metal mould has been replaced by different techniques to minimize cooling at this stage. The drive for development comes from the extrusion industry, which can increase the productivity and quality of extruded profiles by improving the billet surface appearance and structure. Hot top casting supported by airflow against the casting surface during the primary cooling is currently the standard procedure to achieve acceptable billet surfaces. The goal is to minimize the depth of the surface segregation zone, which is the governing factor for the appearance of different phases in the surface region. Billet surface quality is evaluated by quantifying surface appearance, segregation zone thickness, and  occurrence of large Mg2Si and β-particles near the surface. The β-Al5FeSi intermetallic phase and coarse Mg2Si particles have negative effects on extrudability and workability of 6xxx Al alloys billets. To achieve extruded products with a high surface quality the as-cast billets are  heat-treated before extrusion. During heat treatment the undesired intermetallic particles, i.e., β-AlFeSi platelets are transformed to rounded α-Al(FeMn)Si intermetallic phases.

In this  research the formation of the surface segregation for smooth defect-free surfaces in both as-cast and homogenized billets was studied. In addition, the surfaces with defects such as wavy, spot and vertical drag defects were investigated and possible mechanisms for initiation of those defects were explained. Moreover, for a better understanding of the homogenization process in-situ studies of the heat treatment of 6082, 6005, 6060 and 6063 Al alloys were carried out by using a transmission electron microscope (TEM). Based on the observations, an explanation of the probable mechanisms taking place during transformation from β-to α-phase was presented.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2017. , 52 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 259
Keyword [en]
DC casting, surface segregation, surface defects, intermetallic phase, phase transformation, homogenization
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:miun:diva-30007ISBN: 978-91-88527-08-0 (print)OAI: oai:DiVA.org:miun-30007DiVA: diva2:1072505
Public defence
2017-03-14, O102, Holmgatan 10, 851 70, Sundsvall, 13:15 (English)
Opponent
Supervisors
Note

Vid tidpunkten för disputationen var följande delarbeten opublicerade: delarbete 5 manuskript, delarbete 6 inskickat och delarbete 7 inskickat.

At the time of the doctoral defence the following papers were unpublished: paper 5 manuscript, paper 6 submitted, paper 7 submitted.

Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-02-09Bibliographically approved
List of papers
1. Surface structure formation in direct chill (DC) casting of al alloys
Open this publication in new window or tab >>Surface structure formation in direct chill (DC) casting of al alloys
2014 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 66, no 5, 700-710 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study is to increase the understanding of the surface zone formation during direct chill (DC) casting of aluminum billets produced by the air slip technology. The depth of the shell zone, with compositions deviating from the bulk, is of large importance for the subsequent extrusion productivity and quality of final products. The surface microstructures of 6060 and 6005 aluminum alloys in three different surface appearances - defect free, wavy surface, and spot defects - were studied. The surface microstructures and outer appearance, segregation depth, and phase formation were investigated for the mentioned cases. The results were discussed and explained based on the exudation of liquid metal through the mushy zone and the fact that the exudated liquid is contained within a surface oxide skin. Outward solidification in the surface layer was quantitatively analyzed, and the oxide skin movements explained meniscus line formation. Phases forming at different positions in the segregation zone were analyzed and coupled to a cellular solidification in the exudated layer.

National Category
Materials Engineering
Identifiers
urn:nbn:se:miun:diva-22036 (URN)10.1007/s11837-014-0950-y (DOI)000335503700006 ()2-s2.0-84900438439 (Scopus ID)
Note

Language of Original Document: English

Available from: 2014-06-02 Created: 2014-05-30 Last updated: 2017-02-08Bibliographically approved
2. Surface formation in direct chill (DC) casting of 6082 aluminium alloys
Open this publication in new window or tab >>Surface formation in direct chill (DC) casting of 6082 aluminium alloys
2016 (English)In: IOP Conference Series: Materials Science and Engineering, Institute of Physics Publishing (IOPP), 2016, Vol. 117, no 1, 012038Conference paper, (Refereed)
Abstract [en]

Surface defects in aluminium billet production are a real problem for the subsequent extrusion procedure. Extrusion productivity can be influenced by the surface properties, which is defined as surface appearance, surface segregation zone depth and large Mg2Si and β-particles (Al5FeSi). In this research the surface formation during DC casting of 6082 aluminium billets produced by the air slip technology is studied. The surface microstructures of 6082 aluminium alloys with smooth and wavy surface appearances were investigated, including segregation zone depths and phase formation. The results were discussed based on the exudation of liquid metal through the mushy zone. The specific appearance of the wavy surface of 6082 alloys was correlated to how the oxide skin adheres to the underlying mushy zone and coupled to the dendritic coherency and surface tension of the skin. The occurrence of different phases at the very surface and in the layer just below was explained by variations in solidification directions and subsequent segregation patterns.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-29685 (URN)10.1088/1757-899X/117/1/012038 (DOI)2-s2.0-84968853364 (Scopus ID)
Conference
4th International Conference on Advances in Solidification Processes, ICASP 2014, 8 July 2014 through 11 July 2014
Note

Conference Paper

Available from: 2016-12-19 Created: 2016-12-19 Last updated: 2017-02-08Bibliographically approved
3. Surface Segregation and Surface Defect Formation During Aluminum Billet Casting
Open this publication in new window or tab >>Surface Segregation and Surface Defect Formation During Aluminum Billet Casting
2015 (English)In: Transactions of the Indian Institute of Metals, ISSN 0972-2815, E-ISSN 0975-1645, Vol. 68, no 6, 1065-1069 p.Article in journal (Refereed) Published
Abstract [en]

In this paper the surface zone formation during direct chill casting of aluminum billets, produced by the air slip technology, is discussed. The shell zone depth and compositions have been quantitatively studied, and the surface microstructures of 6060, 6005 and 6082 alloys are compared and coupled to surface appearances. The understanding of the results is based on the exudation of liquid metal through the mushy zone and the fact that the exudate liquid is contained within a surface oxide skin, and the oxide skin movements are coupled to various surface appearances. The major defects that occur during billet castings are different kinds of vertical drags or horizontal bandings, also called lapping. The structures coupled to these defects and theories for their formation are discussed.

Keyword
Aluminum alloys, Direct chill (DC) casting, Surface segregation, Surface defects
National Category
Materials Engineering
Identifiers
urn:nbn:se:miun:diva-26522 (URN)10.1007/s12666-015-0647-0 (DOI)000364029400013 ()2-s2.0-84946488252 (Scopus ID)
Available from: 2015-12-16 Created: 2015-12-16 Last updated: 2017-02-08Bibliographically approved
4. On Liquid Metal Wetting of Casting Rings for DC Casting
Open this publication in new window or tab >>On Liquid Metal Wetting of Casting Rings for DC Casting
2015 (English)In: TMS Light Metals / [ed] Margaret Hyland, Warrendale: Minerals, Metals & Materials Society, 2015, Vol. 2015-January, 859-864 p.Conference paper, (Refereed)
Abstract [en]

During air–slip direct chill casting of aluminum alloys, uneven or low air pressure areas at the surface of the porous graphite ring mold, can cause the metal to stick to the graphite ring and produce traces along the billets called vertical drags (VD). To prevent this type of defect, it would be an advantage to decrease the wettability of the graphite ring.  It has been shown that nanostructured surfaces can have hydrophobic properties, and a question is weather the same effect can be achieved for molten metals. A TiO2 nanowire structure is proposed to cover the casting ring, and the wetting behavior of such a structure has been studied, and compared to the wetting at a regular graphite ring surface by the dispensed sessile drop method. The results showed that the nanostructured surface very strongly dewetting metal droplets.

Place, publisher, year, edition, pages
Warrendale: Minerals, Metals & Materials Society, 2015
Series
TMS Light Metals, ISSN 0147-0809
Keyword
DC Casting, Wetting, Casting ring, Aluminium
National Category
Engineering and Technology
Identifiers
urn:nbn:se:miun:diva-26338 (URN)2-s2.0-84942925150 (Scopus ID)9781119082446 (ISBN)
Conference
Light Metals 2015 - TMS 2015 Annual Meeting and Exhibition; Walt Disney WorldOrlando; United States; 15 March 2015 through 19 March 2015; Code 113955
Funder
Knowledge Foundation
Available from: 2015-12-01 Created: 2015-12-01 Last updated: 2017-02-08Bibliographically approved
5. Influence of Heat Treatment on Surface Structure of 6082 Al Alloys
Open this publication in new window or tab >>Influence of Heat Treatment on Surface Structure of 6082 Al Alloys
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:miun:diva-30009 (URN)
Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-02-08Bibliographically approved
6. In-Situ Study of Phase Transformations during Homogenization of 6005 and 6082 Al Alloys
Open this publication in new window or tab >>In-Situ Study of Phase Transformations during Homogenization of 6005 and 6082 Al Alloys
(English)Article in journal (Refereed) Submitted
National Category
Materials Engineering
Identifiers
urn:nbn:se:miun:diva-30010 (URN)
Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-02-08Bibliographically approved
7. In-Situ Study of Phase Transformations during Homogenization of 6060 and 6063 Al Alloys
Open this publication in new window or tab >>In-Situ Study of Phase Transformations during Homogenization of 6060 and 6063 Al Alloys
(English)Article in journal (Refereed) Submitted
National Category
Materials Engineering
Identifiers
urn:nbn:se:miun:diva-30011 (URN)
Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-02-08Bibliographically approved

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