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Processing effects on hot tearing tendencies of die-cast magnesium components
Dept, Mechanical Engineering / Component Technology, Jönköping University, Jönköping, Sweden.
Dept, Mechanical Engineering / Component Technology, Jönköping University, Jönköping, Sweden.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

A commercial AM50 Mg component was die cast with various process parameter settings. It was found that many of the process parameter settings investigated had a significant effect on the tendency for hot tears to form in critical sections of the component. A fast cavity filling helped to reduce the hot tearing tendency. This was explained by smaller temperature variations inside the casting immediately after cavity filling, which would cause less thermal stresses to form during solidification. Shorter mould opening time was also found to decrease the hot tearing tendency as a result of reduced tensile stresses inside the casting before mould opening. Severe hot tears formed when using an increased intensification pressure. In an attempt to explain this observation, numerical calculations were performed to investigate how the intensification pressure affects the solidification rate and the fraction solid gradients across the casting. It was found that a higher intensification pressure gives rise to steeper temperature gradients and fraction solid gradients during solidification between the surface and the centre of the casting. This would facilitate the initiation of hot tears but also flow of segregated liquid into the opening tears. It was also observed that hot tears in many cases have been filled, partially or even completely, by strongly segregated liquid, which in the Mg-Al system solidifies into a very brittle material.

Keyword [en]
Mg alloy, die casting, hot tearing, process parameters, numerical calculation
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-85351OAI: oai:DiVA.org:liu-85351DiVA: diva2:570354
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19
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

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