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Turbocharger - from ductile iron to steel

To help reduce automotive CO2 emissions there is a continued trend towards lower weight, higher strength components. Here, Norbert Schütze of Foseco Germany details the transition of the manufacture of manifolds and turbocharger castings in an automotive foundry from ductile iron to steel, focussing on low emission coldbox binders, sand additives and high performance coatings.

Introduction
In the automotive industry the use of turbochargers has been state-of-the-art for many years. However, performance improvement with the same or an even lower displacement is a big challenge. The smaller wall thickness and higher exhaust gas temperatures are a problem when using ductile iron, which is why a steel casting is preferred. This solution also offers higher thermal strength, thinner walls and a more compact construction.
However, this is not as easy because the demands with regard to production processes and moulding materials are higher and the quality has to be redefined.
Hereafter we will focus on the differences occurring when changing from ductile iron to steel from the mould and core point of view and we will take a closer look on the moulding material, the additives and the coatings. A motor with integrated turbocharger can be seen in fig.1.


Initial Situation
When using austenitic cast iron grades D5 or D5S the use of silica sand with cold box binders, organic additives and coatings based on alumina silicate is common practice.
An example of one recipe is 100 per cent silica sand with:
• 0.60 per cent POLITEC* E 6010
• 0.60 per cent POLITEC E 9040
• 1.5 per cent NORACEL* MO 12
This gives a bending strength of approximately 250N/cm2 - this is ideal for ductile iron because casting defects like veining can be avoided.
High compression leads to a high density and because of the silica sand expansion, veining defects occur, which cannot be removed by fettling. Therefore, the use of additives and coatings should help to prevent such veining defects (fig.2).
In addition, the anti-veining SEMCO Sil 4451D coating guarantees a casting free of defects and of the highest quality.
When looking at the coated core we can see the excellent dipping qualities - the coating thickness is consistent and free of runs and drips.
Take for example a cut through turbocharger (fig.3), in which the veining defects are clearly visible, it shows that subsequent cleaning is not possible.
The origin of those veining defects has been extensively described in the past, for example by Stephan Hasse in his casting defects atlas(1).
The plate-like texture and highly insulating nature of the coating filler (fig.4) effectively suppresses the formation of veins. In addition, the partial penetration of the coating particles into the core prevents penetration with liquid metal at these particular areas of the core.

New Situation
Due to the described transition from ductile iron to steel, the moulding material, additives and coatings have to meet higher requirements.
Increased strength levels are necessary as the only way to properly mould thinner walls. The higher pouring temperatures require greater thermal resistance of the mould and core materials to withstand the increased thermo-physical forces applied.
These requirements can be met with the help of converting the core moulding material to a mix of silica and special sand in combination with an inorganic additive.
Composition:
• 75 per cent silica sand
• 25 per cent FOSBEADS with:
0.8 per cent POLITEC XP 1024
0.8 per cent POLITEC XP 1080
2.0 per cent NORACEL M 75
Using a mixture of silica sand and FOSBEADS as a moulding material in combination with sand additives, shown in fig.5, significantly contributes to enabling a reduced wall thickness.
This objective can only be reached by this kind of recipe in combination with an inorganic additive and an efficient cold-box system.
Bending strengths of more than 400N/cm2 can be obtained without any problems and water-based coatings can be used without issue.
The moulding material is coated with an aluminium oxide coating. The coated core is shown in fig.6.
SEMCO COATING 3800 A has been developed to meet special requirements with regards to refractoriness, gas permeability, abrasion resistance and stability. The carrier liquid, of course, is water.
The application and penetration performance was designed in a way that the alumina oxide particles can penetrate up to 1mm deep into the moulding material and at the same time form a consistent layer on the moulding material. These properties are shown in fig.7.
The figures illustrate the coated surface and the penetration by the coating. The selected filler combination supplies a uniform layer thickness and ensures high gas permeability. This will lead - compared to conventional combinations - to a better core gas release. The relevance to the steel area therefore needs no further explanation.
The high gas permeability of the whole system with different coating layer thicknesses is shown in fig.8.
Solidification, thin walls, the special moulding material, the coating, the pouring temperatures and more aspects must of course be seen in an overall context. Thomas Linke has already described the special relationship between wall thickness and solidification when using different size fillers(2). Taking into account all these parameters, the basic requirements for the design and production of good castings are given.

Summary
The objective of this article is to demonstrate the different requirements from a mould and core point of view when producing a complex casting such as a turbocharger, using ductile iron compared to steel.
The results show that the production process has to be reviewed and altered. To do this, the moulding materials, cold-box binders, additives and coatings have to be compared and the differences must be highlighted.

References
1) Hasse Stephan, Casting and Structural Defects, Guss & Gefügefehler 
2) Linke Thomas, Gießerei Erfahrungsaustausch 9/10, 2011. www.giesserei-verlag.de

Contact: Paul Jeffs, UK technical manager, Vesuvius UK Limited - Foseco Foundry Division, Tamworth, Staffordshire B78 3TL UK. Tel: +44 (0) 1827 289999, email: paul.jeffs@foseco.com web: www.foseco.com

* POLITEC and NORACEL are Trade Marks of the Vesuvius Group, registered in certain countries, used under licence.

For copies of the figures refer to the printed version in the June issue of Foundry Trade Journal