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Characteristics of cast magnesium alloys: microstructures, defects and mechanical properties
Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
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 [en]
Mg alloys, microstructure, mechanical properties, defects, modelling, segregation bands, hot tear, mould constraint, semi-solid casting.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-24808Local ID: 7075ISBN: 91-85299-66-9 (print)OAI: oai:DiVA.org:liu-24808DiVA: diva2:245131
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
List of papers
1. Effect of microstructure on mechanical properties of as-cast Mg-Al alloys
Open this publication in new window or tab >>Effect of microstructure on mechanical properties of as-cast Mg-Al alloys
2004 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 35, no 1, 309-319 p.Article in journal (Refereed) Published
Abstract [en]

The mechanical properties of Mg-Al alloys are mainly determined by the microstructure, i.e., the amount and morphology of the phases, but also by the presence of defects arising from the melt handling and casting process. In order to obtain information about the isolated effect of the microstructure, it is, therefore, necessary to minimize the amounts of defects. In this study, this has been achieved by remelting and solidifying the alloys in a gradient furnace. The drawing rate was varied from 0.3 to 6 mm/s, which yielded a wide variety of microstructures. Three samples were produced for each parameter set, in order to have a statistical basis for the evaluation. The results showed that homogeneous and reproducible samples could be produced, and that the tensile properties obtained showed a very small scatter. The effects of microstructural parameters such as grain size, secondary dendrite arm spacing (SDAS), eutectic fraction, and eutectic morphology on the yield strength, ultimate tensile strength (UTS), fracture elongation, and hardness has been investigated.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85320 (URN)10.1007/s11661-004-0132-6 (DOI)
Available from: 2012-11-16 Created: 2012-11-16 Last updated: 2017-12-07
2. Modeling of microstructure: mechanical property relations in cast Mg-Al alloys
Open this publication in new window or tab >>Modeling of microstructure: mechanical property relations in cast Mg-Al alloys
2003 (English)In: International Conference on Modeliing of Casting Welding and Advanced Solidification Processes X, 2003, 165-172 p.Conference paper, Published paper (Refereed)
Abstract [en]

Binary MG-Al alloys ranging from 3-15 wt% Al have been investigated by gradient solidification techniques, which gives homogeneous and nearly defect-free tensile test samples. Based on the experimental results, models have been developed which describe the effect of microstructiral parameters and cooling conditions, such as grain size of α-pohase, volume fraction of the eutectic phase and cooling rate, on the tensile properties, such as hardness, yield strenght and fracture elongation.

Keyword
Mg-Al alloys, gradient solidification, modelling, microstructure, mechanical properties, hardness, elongation, yield strength, tensile strenght, divorced euectic
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30038 (URN)15491 (Local ID)15491 (Archive number)15491 (OAI)
Conference
International Conference on Modelling of Casting Welding and Advanced Solidification Provesses X, Sandestin, Florida, May 25-30, 2003
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-04
3. On the reliability of yield strength data evaluated from as-cast tensile test bars of Mg-alloys
Open this publication in new window or tab >>On the reliability of yield strength data evaluated from as-cast tensile test bars of Mg-alloys
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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85345 (URN)10.1002/3527603565.ch79 (DOI)
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
4. The effect of mould constraints on the 0.2% proof stress of as-cast Mg-Al alloys
Open this publication in new window or tab >>The effect of mould constraints on the 0.2% proof stress of as-cast Mg-Al alloys
2005 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 488-489, 165-168 p.Article in journal (Refereed) Published
Abstract [en]

When a casting solidifies and cools down in a metallic mould, stresses and strains develop as a result of uneven temperature distributions, metal shrinkage and mould constraints. At ejection from the mould, the casting usually springs back slightly when the elastic part of the strain is released. However, due to the low flow stress at high temperatures it is likely that the casting also has experienced some plastic deformations, meaning that the dislocation density has increased. This paper discusses how the existence of remaining plastic deformations affects the initial flow behavior and the yield strength during tensile testing of as cast Mg-Al alloys with different degrees of mould constraints.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85346 (URN)10.4028/www.scientific.net/MSF.488-489.165 (DOI)
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2017-12-07
5. Characteristics of microstructure and banded defects in die cast AM50 magnesium components
Open this publication in new window or tab >>Characteristics of microstructure and banded defects in die cast AM50 magnesium components
2005 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 18, no 6, 377-384 p.Article in journal (Refereed) Published
Abstract [en]

A number of commercial AM50 magnesium alloy components have been die cast under various casting conditions. The influence of process parameters on the microstructural features as well as on the defect known as the 'segregation band defect' were investigated. It was found that pouring temperature and injection speed have obvious effects on the morphology of presolidified crystals inside the casting. More presolidified crystals were formed under the condition of lower pouring temperature, and more broken dendrites and spherical crystals were formed under higher injection speed. A variety of segregation band morphologies and the influence of various casting conditions on the appearance of band type defects were examined. It was discovered that intensification pressure has the strongest influence on the appearance of segregation bands inside the castings. By applying high intensification pressure, the risk of tearing along the bands and the segregation levels inside the band were both dramatically decreased. The influence of band defects on the mechanical properties of the components was established.

Keyword
Small and medium sized enterprises, decision criteria, manufacturing characteristics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85348 (URN)10.1179/136404605225023216 (DOI)
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2017-12-07
6. Mechanisms of segregation band defect formation in pressure die-cast magnesium components
Open this publication in new window or tab >>Mechanisms of segregation band defect formation in pressure die-cast magnesium components
2005 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 488-489, 381-384 p.Article in journal (Refereed) Published
Abstract [en]

Segregation bands which normally follow the outer contours of a casting are common in commercial magnesium alloy pressure die castings. Several models have been proposed in the literatures which attempt to explain mechanisms behind the formation of this type of segregation bands. However, it is difficult to explain some phenomena which occur in real die cast components. In this paper, a new theory concerning the formation of one common and detrimental segregation band defect (Type I) has been proposed, which is based on a coupled analysis of heat flow and volume changes during solidification. The formation of this type bands was related to a pressure drop in the liquid and resulting flow of segregated liquid from the surrounding two-phase regions. Mechanism on the formation of the other type segregation band (Type II or under surface band) is also proposed. The sudden increase of cooling rate at the moment of applying intensification pressure is believe to has main contribution to the formation of this type of bands.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85349 (URN)10.4028/www.scientific.net/MSF.488-489.381 (DOI)
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2017-12-07
7. Processing effects on hot tearing tendencies of die-cast magnesium components
Open this publication in new window or tab >>Processing effects on hot tearing tendencies of die-cast magnesium components
(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
Mg alloy, die casting, hot tearing, process parameters, numerical calculation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85351 (URN)
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19
8. RSF: an innovative semi-solid forming process for manufacturing of MG components
Open this publication in new window or tab >>RSF: an innovative semi-solid forming process for manufacturing of MG components
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Semi-solid forming is a promising process for manufacturing metal components. The characteristics of semi-solid forming processes are described in this paper. Some commercial semi-solid forming processes as well as its advantages and disadvantages are also discussed. A new developed semi-solid forming process called Rapid Slurry Forming (RSF) process is introduced. The RSF process is based on the enthalpy exchange between at least two alloy systems having different enthalpies, with the aim of forming semi-solid slurry with a desired solid fraction. By using this process large amounts of high quality metal slurry can be produced in a short time. Experimental work has been done to produce AM60 and AZ91 slurries. Satisfied results were achieved, which demonstrated that the RSF process is a suitable and promising process for semi-solid manufacturing ofMg components.

Keyword
Semi-solid, RSF, enthalpy exchange, Mg alloy, process, high integrity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85352 (URN)
Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19

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