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Surface structure formation in direct chill (DC) casting of al alloys
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.ORCID iD: 0000-0002-3622-4247
2014 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 66, no 5, p. 700-710Article 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.

Place, publisher, year, edition, pages
2014. Vol. 66, no 5, p. 700-710
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:miun:diva-22036DOI: 10.1007/s11837-014-0950-yISI: 000335503700006Scopus ID: 2-s2.0-84900438439OAI: oai:DiVA.org:miun-22036DiVA, id: diva2:720733
Note

Language of Original Document: English

Available from: 2014-06-02 Created: 2014-05-30 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Investigation of Surface Formation in As-Cast and Homogenized 6xxx Aluminium Billets
Open this publication in new window or tab >>Investigation of Surface Formation in As-Cast and Homogenized 6xxx Aluminium Billets
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. p. 52
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 259
Keywords
DC casting, surface segregation, surface defects, intermetallic phase, phase transformation, homogenization
National Category
Engineering and Technology
Identifiers
urn:nbn:se:miun:diva-30007 (URN)978-91-88527-08-0 (ISBN)
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

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Bayat, NazlinCarlberg, Torbjörn

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