Improved Properties of Aluminium Cast Alloys due to a Boron Grain Refinement

2 IMPROVED PROPERT I ES OF ALUMIN IUM CA ST ALLOYS DUE TO A BORON GR A IN REF INEMENT

IMPROVED PROPERTIES OF ALUMINIUM CAST ALLOYS DUE TO A BORON GRAIN REFINEMENT

Dr. Andreas Kleine, Dr. Macel Rosefort, Christina Matthies, Dr. Hubert KochTRIMET ALUMINIUM AG, Aluminiumallee 1, 45356 Essen, Germany

A. PithanHonsel AG, Germany

Keywords: Aluminium, cast alloys, grain refinement, bo-ron

AbstractThe TRIMET-Group provides the aluminium processing in-dustry with primary metal, alloys, semi-finished products, castings and recycling. It produces and supplies the most diverse forms and alloys of aluminium –customized to the users’ special needs. The approach of a strategic link of stock-related know-how combined with long-standing experience with the material aluminium has a prolonged effect. Research and development are a very specific chal-lenge for TRIMET due to the pressure to innovate to which the automobile industry in particular is subject worldwide. For this reason the development of new materials with improved properties and process-engineering advances were the focus of this project.

The targets of this project are to improve properties of aluminium cast alloys. The basic concepts for improving the mechanical properties were to refine the grain size and to increase the heat conductivity. The latter gives bet-ter material performance for all applications with local heat maxima. Due to better heat conductivity the maxi-mal temperature will be lower. This yields better local me-chanical properties.

Therefore a melt treatment with boron was tested, which provides two main effects:- The removal of elements like titanium, zirconium etc.

yields a higher electrical conductivity and heat conduc-tivity.

- Furthermore the boron can be used as a grain refiner instead of TiB2 to get a better grain structure for im-proved mechanical properties.

This paper describes the development process, the experi-ments and the results from the laboratory examinations. It gives an outlook on the potential of such aluminium

alloys including the microstructure advantages, the me-chanical properties and the possible applications.IntroductionImproving component properties is the main aspect of al-loy development. The focus of this work is improving the mechanical properties by influencing the grain size and increasing the heat conductivity. This gives better material performance for all applications with local heat maxima. Due to better heat conductivity the maximal temperature will be lower. This yields better local mechanical proper-ties. As a result of these facts the fatigue behaviour should be increased.

Usually a boron melt treatment is used for increasing the electrical and heat conductivity, [1]. This effect is caused by the removal of elements like titanium, zirconium etc. Additionally boron should work as a grain refiner instead of TiB2, [1], [2]. Hence a better grain structure with im-proved mechanical properties was expected.

ResultsAll castings, which were carried out for this investigation, are separately casted test bars with the following param-eters:- Alloy AlSi7Mg- Mould material: gray iron- Mould temperature 370°C- Melt temperature 700-720°C- Cooling conditions air cooling- Heat treatment T6Stated below the outstanding results of this investigation can be seen. The grain refinement of the boron addition can be seen in FIG. 1. In comparison to the TiB2 refined alloy the boron refined alloy yields a much finer grain structure.

The optimized microstructure results in significantly better mechanical properties. While the yield strength is nearly the same, the UTS is increased. But the greatest effect

Figure 1. Examples for the grain structure of an GK-AlSi7Mg T6 a) with TiB2 grain refinement; b) with boron. All specimens are separately cast-ed test bars.

IMPROVED PROPERT I ES OF ALUMIN IUM CA ST ALLOYS DUE TO A BORON GR A IN REF INEMENT 3

could be observed for the ductility. The doubling of the elongation in comparison to the TiB2 refined material is a great benefit for multiple applications.

TAB. 1. Mechanical properties and grain sizes of the inves-tigated specimen. All data are average values.

The second main aspect in this investigation was to im-prove the conductivity. The well-known advantages of the boron melt treatment could be confirmed, [1]. As well as the electrical conductivity, the heat conductivity could also be increased significantly, FIG. 2. For example the heat conductivity is increased at RT from 178W/Km to 203W/Km. This means an overall increase of more than 14%.

ConclusionsAs predicted, the boron melt treatment yields great im-provements in the conductivity. Additionally the boron in the melt is an excellent grain refiner for example for Al-Si7Mg. This results in an optimized microstructure there-fore in improved mechanical properties. Especially the re-markable improvement of the elongation in combination with the higher heat conductivity shows great promise for a higher thermal fatigue resistance. Hence many appli-cations in combustions engines, brakes and many other components in different areas could be optimized by this new material.

Grain refinement Yield strength UTS Elongation Grain size

MPa MPa % µm

TiB2 252 292 4,0 500

AlB2 250 315 8,4 250

 

References[1]: G.K.Sigworth, M.M.Guzowski, „Kornfeinung untereutektischer Al-Si-Gußlegierungen“, Gießerei-Praxis, Nr.5/1987, Fachverlag Schiele & Schön GmbH, Berlin, 1987

[2]: E.Nachtigall, “Kornfeinung bei Aluminiumguß”, ALUMINIUM, 42.Jahrg. 1966 . 11, Fachzeitschrift der deutschen Aluminiumindustrie. Hrsg. von der Aluminium-Zentrale, Düsseldorf, 1966

Figure 2. Improvement of the heat and electrical conductivity of the bo-ron grain refined AlSi7Mg in comparison to the AlSi7Mg with TiB2.

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