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Influence of Heat Treatment on the Surface Structure of 6082 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
2017 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48A, no 10, p. 5085-5094Article in journal (Other academic) Published
Abstract [en]

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. Although the heat treatment of the bulk areas of the 6xxx Al alloys has been the focus of many previous studies, the process of phase transformation at the very surface has not been paid the same attention. In this study, microstructures of a homogenized billet of a 6082 alloy at the area very close to the surface were investigated. By comparing the X-ray diffraction patterns (XRD) of heat-treated samples as a function of different holding times, the gradual phase transformations could be followed, and using GDOES and map analysis by EDX, the alloying elemental redistribution was analyzed. Partial remelting and porosity growth was detected, and transformation rates were faster than in bulk material and from what is known from industrial processes.

Place, publisher, year, edition, pages
2017. Vol. 48A, no 10, p. 5085-5094
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:miun:diva-30009DOI: 10.1007/s11661-017-4207-6ISI: 000408884300058Scopus ID: 2-s2.0-85025633715OAI: oai:DiVA.org:miun-30009DiVA, id: diva2:1072575
Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-12-18Bibliographically 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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