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On the reliability of yield strength data evaluated from as-cast tensile test bars of Mg-alloys
Jönköping University, Jönköping, Sweden.
Jönköping University, Jönköping, Sweden.
2003 (English)In: Magnesium: Proceedings of the 6th International Conference Magnesium Alloys and Their Applications / [ed] Karl U. Kainer, 2003, 501-506 p.Conference paper, Published paper (Other academic)
Abstract [en]

Within the automotive sector, energetic efforts are today being made to decrease vehicle weight in order to minimize the emission of combustion gases. In the light of this work it is natural that the use of light metals, and especially magnesium, has increased considerably during the last 5-10 years. Some applications in cars where cast magnesium components are used today include seat frames, instrument panels, transfer cases, wheels, steering wheels and various kinds of housings.

Place, publisher, year, edition, pages
2003. 501-506 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-85345DOI: 10.1002/3527603565.ch79OAI: oai:DiVA.org:liu-85345DiVA: diva2:570331
Conference
6th International Conference on Magnesium Alloys and Their Applications, Wolfsburg, Germany
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2013-11-04
In thesis
1. Characteristics of cast magnesium alloys: microstructures, defects and mechanical properties
Open this publication in new window or tab >>Characteristics of cast magnesium alloys: microstructures, defects and mechanical properties
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnesium is the lightest of all structural metals. As such, and combined  with good mechanical and physical properties, it forms the basis for commercial alloys that have found successful use in a wide variety of applications. The work presented in this thesis has aimed to investigate the characteristics of microstructures and defects in cast magnesium alloys, as well as how they influence the mechanical properties.

The microstructural features of binary Mg-Al alloys with various Al concentrations were investigated under a range of cooling conditions. The experiments were done by using a gradient solidification technique to achieve homogeneous and nearly defectfree magnesium alloy specimens, through which the isolated effect of microstructural parameters on the mechanical properties could be evaluated. Some models were developed to describe the relationships between the microstructure and mechanical properties in the Mg-Al alloys. In this work, it was also found that the mechanical properties of cast Mg components were affected by some non-structural factors. It is proposed that mould constraints in the die during cooling can have a significant effect on the yield strength of die-cast magnesium components.

Investigations were also done concerning the characteristics of the microstructure, especially the so called pre-solidified crystals, as well as two common defects, segregation bands and hot-tears, in die-cast Mg components. The effects of process parameters on the formation of the microstructure and these defects were thoroughly investigated in some high pressure die cast commercial components. Applying a high intensification pressure was found to have a strong effect in minimizing the segregation band formation, however at the same time promoting the formation of hot tears. A new theory describing the mechanisms for segregation band formation is proposed. The theory was confirmed by simulations of solidification process during die casting, and by experimental verification. The tensile stresses built up in the residual liquid due to the solidification (and cooling) shrinkage of the casting, which causes a pressure drop and viscous flow of enriched liquid inside the mushy zone, is believed to be the main reason for segregation band formation.

In order to be able to produce premium quality and cost effective Mg components, a new semi-solid casting process was developed. The process, called Rapid Slurry Forming (RSF), is based on a rapid but precise control of the enthalpy of the metal, thereby permitting the operator to obtain the desired solid fraction in the slurry. By using this process, large amounts of high quality semi-solid slurry can be produced in some minutes. This new technology opens up possibilities for producing Mgcomponents with well described microstructures, low defect contents and good mechanical properties.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2005. 32 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 955
Keyword
Mg alloys, microstructure, mechanical properties, defects, modelling, segregation bands, hot tear, mould constraint, semi-solid casting.
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-24808 (URN)7075 (Local ID)91-85299-66-9 (ISBN)7075 (Archive number)7075 (OAI)
Public defence
2005-06-10, Ingenjörshögskolan i Jönköping i föreläsningssal E167, Ingenjörshögskolan, Jönköping, 10:00 (English)
Opponent
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2012-11-19Bibliographically approved
2. On the microstructure and mechanical properties of Mg-Al alloys
Open this publication in new window or tab >>On the microstructure and mechanical properties of Mg-Al alloys
2004 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The microstructural features and the mechanical properties of binary Mg-Al alloys have been investigated by using a gradient solidification technique. Homogeneous and nearly defect-free samples were produced under well controlled cooling conditions, through which the microstructural characteristics as well as the relationships between microstructure and mechanical properties in Mg-AI alloys were precisely defined.

It was found that the eutectic structure Mg-AI alloys becomes more and more divorced when increasing the cooling rate and by decreasing the AI concentration. A concept called divorced degree is introduced in this work to quantify the divorced level of the alloys, by which other structural parameters can be also quantified, including the eutectic fraction and, solid solution aluminium concentrations in the matrix. Concerning the relation between the microstructure and mechanical properties, it is found that yield strength and hardness are mainly dependent on the structural spacings (grain sizes and dendrite arm spacing, DAS) and solid solution aluminium content in the a-phase. For higher aluminium containing alloys (larger than 10wt% AI) the rigid eutectic networks will also contribute to the strength and hardness. All the microstructural parameters contribute to the ultimate tensile strength (UTS) as well as fracture elongation of the alloys. The premium combination was found in Mg-8AI alloy, where the highest UTS and relatively high elongation can be achieved. Mathematical modeling has been performed to relate the microstructural parameters and casting conditions, such as grain size, volume fraction of different phases, solid solute AI concentration and cooling rate, to the mechanical properties of the alloys, including yield strength, hardness and fracture elongation. The distinguishing differences in the mechanical behaviour between pressure die-cast magnesium alloy components and other casting processes were also investigated. The differences were related to mould constraint in the die during the cooling sequence.

One special and severe defect called segregation band which occurs in die-cast magnesium alloy components has also been investigated. A new theory to describe the segregation band formation mechanism during die casting has been proposed in this work. The tensile stresses built up in the residual liquid due to the solidification (and cooling) shrinkage of the casting, which cause a pressure drop and viscous flow of enriched liquid inside the mushy zone, is believed to be the main reason for segregation band formation.

Place, publisher, year, edition, pages
Jönköping: Jönköping, 2004. 34 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1087
Keyword
Mg-AI, microstructure, mechanical properties, divorced degree, modeling, mould constraint, segregation band
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-23151 (URN)2555 (Local ID)91-7373-934-0 (ISBN)2555 (Archive number)2555 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-11-04

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