This paper offers an overview of fundamentals and recent global developments of cores and binders used in metalcasting operations. The basic technical characteristics of salt and sand cores are provided along with organic and inorganic binder systems. A particular attention is paid to inorganic binders, receiving recently a renewed interest.
The description of research and development efforts is accompanied by history and present status of commercial solutions used by the casting industry. The new development directions, aimed at meeting the growing complexity of the component design requirements and ever-more stringent environmental regulations, are emphasised.
Introduction In metalcasting, cores are mould parts used to form internal holes and cavities in a cast product. Depending on casting technique, the core can be completely integrated into the casting die/mould or loosely laid therein. After solidification of the metal, the die is taken apart and the cast product is released. Then the core is broken, removed from the product and usually disposed of, although there have been applications for re-usable cores.1
While some product cavities have functional purposes, such as cylinders in engine blocks, others are just for weight reduction. For example, some automotive engine parts such as camshafts or crankshafts have a designed hollow structure for a considerable weight reduction.
Depending on casting methods which include gravity casting, low pressure or high pressure die casting, the cores may require a high strength as the pressure increases with each technique. Examples of components manufactured with casting cores are shown in Fig. 1.
In order to make the casting core structurally strong to withstand operating pressures, binders are used. The core binder refers to the chemical mixture which adheres the filler to provide the core strength. Hence, the core represents a composite comprising the filler and the binder.
Since the typical volume of the latter is of the order of 1–3%, the filler is the predominant component of the core. However, despite such a low volume, conventional core binders emit up to 70% of a foundry volatile organic compound.2 The binder chemistry is, therefore, of key importance.
There are many types of cores in the casting industry; the primary focus of this paper is on sand cores with organic and inorganic binders as well as surface coatings for core strength increase. To provide the full picture, however, salt cores are also included. The objective is to provide an overview of fundamentals and the present status of commercial cores used by the global casting industry.
The casting cores have to withstand tough requirements which depend on casting technique and alloy used. The temperature and its gradient represent one of many challenges. At the moment of casting the maximum temperature experienced by a core may exceed 1500uC on the metal side with a steep gradient through the core body.
A specific challenge occurs during die casting of aluminium alloys, known for having light weight, relatively high strength, high productivity, and high dimensional accuracy. For these reasons, it is widely being used in applications such as the automotive industry. Since more complicated shapes are required for die casting parts, it becomes difficult to manufacture such geometries without breaking the core. In general, in high pressure die casting, the flow velocity used exceeds 30 m s–1 at gates and the hydrostatic pressure is more than 60 MPa.
Such demanding casting conditions tend to cause high mechanical loading on the core.3 Thus, a core that is able to withstand these conditions is required. However, increasing the core strength causes a decrease in its collapsibility which translates to longer time for core removal.
Due to this, the total processing cost tends to increase, as well. To provide a solution, an extensive research has been conducted into producing the core that is able to withstand pressure and temperature, as required in die casting and at the same time it allows for its easy removal. Two major types of cores are explored here, sand and salt cores. Historically, sand cores were first commonly accepted by the general casting industry.
These cores are disposable ones, designed only to be used once. The problem with typical sand cores is that once the casting technique changes to that which requires higher pressure, the core becomes harder to remove from the cast product.4 To address the issue of core removal, salt cores were introduced.
The concept of salt cores is that once casting is completed, the core would not need to be broken down but rather dissolved with water.5 Using salt cores also helped to simplify manufacturing the complex shapes due to the good castability of salt mixtures without reinforcements.3 In applications where parts must be absolutely free of core sand residues, salt cores provide a solution. However, even the use of salt cores has some drawbacks.
The growing demand for shape complexity, strength and easy removal cause the industry to divert its attention between sand and salt cores. Moreover, a hybrid ceramic-sand core technology was developed for highly demanding applications.6 A casting process that combines aerospace ceramic cores with automotive sand cores was recently developed by Southwest Research Institute.7
This hybrid core is specifically suited for the purpose of allowing precision casting of extremely small passages in the automotive cast iron/steel components. It was designed to enable the production of heavy-duty diesel engines with the higher peak cylinder pressure capability than current state of the art engines. The ceramic section in the core is used where coolant passages between the engine gas exchange port walls and the fuel injector or spark plug are formed.
There are different mixtures to prepare a salt core. The most common use sodium carbonate (Na2CO3), potassium chloride (KCl) and sodium chloride (NaCl), melting these components and applying the moulding technique to create a core.8 This helps the core to obtain a high static compressive strength and dimensional stability, required during casting.
The detrimental features of salt mixtures are shrinkage cavities, microporosity and heat creaks. The latter may be created in the salt core when a volume change takes place such as during solidification.
Development directions of salt cores An example of the salt core chemistry includes bromine ions, carbonate ions, and at least one of sodium ions or potassium ions. As ion precursors, sodium bromide NaBr and sodium carbonate Na2CO3 are used.
Since applications often require complex shapes than could be achieved by dry-compaction method, the ingredients are rather melted by heating to obtain a melt and then injected into the metal mould under high pressure. After solidification, the salt core is removed from the mould. An application of this core includes die casting and gravity casting.9,10 Another possible process to make a salt core uses a sodium chloride mixture, which is preheated to a temperature of maximum 30uC above liquidus and then poured into a mould.
The core is formed by solidifying the melt inside the mould.11 A semisolid processing technique was used in a concept to produce salt cores with a mixture of sodium carbonate, potassium carbonate, sodium chloride and potassium chloride.5
Since the salt core is formed by solidification in a metal mould after high pressure injection, having a precursor of semisolid mixture of sodium and potassium salts, instead of full liquid with a liquidus temperature of 700uC, allowed for precise reproduction of the shape required. To generate a semisolid slurry, the molten salt was undercooled into the 595–645uC temperature range, directly before injection.
The pressure applied on the cavity remained constant until mould opening, in order to obtain high strength. The concept was tested with the expendable salt cores used for water jacket in a closed-deck type, 4- cylinder engine block.
To remove the core, the engine block was submerged into a dissolution tank which contains liquids such as hydrochloric acid and hot water. Four binary systems for high pressure die casting cores, tested by Yamaha included NaCl–Na2CO3, KCl–K2CO3, KCl–NaCl, and K2CO3–Na2CO3. 3,12 To determine the core strength, a four point bending test was carried out.
It was found that the strength of NaCl–Na2CO3 was over 20 MPa in the Na2CO3 content region between 20 and 30 mol.-%, as well as 50 and 70 mol.-%. The highest strength for this system was 30 MPa at the composition of NaCl–70 mol.- %Na2CO3. For KCl–K2CO3 the strength was over 10 MPa in the K2CO3 region at 20 mol.-%, and between 50 and 60 mol.-%.
The highest strength for this system exceeded 25 MPa in between the K2CO3 content of 50 and 60 mol.- %. It was noticed that the strength of the systems KCl–NaCl and K2CO3–Na2CO3 was below 6 MPa. The reason was the solidification structure and the solid state phase transformations. It was also found that, in the NaCl–Na2CO3 system, the hardness value of the primary particles was higher than in the KCl–K2CO3 system, explaining why the NaCl– Na2CO3 strength was higher than that of KCl–K2CO3.
The salt cores for cast pistons produced by CeramTec AG and Austrian Foundry Institute have similar advantages as cores mentioned above. Their gas emission during core storage and casting is minimal, making it environmentally friendly as well (Fig. 2).
The cores are manufactured using a fully automated process.13 Another type of salt core that has a soluble and insoluble portion was developed by Brunswick Corporation.14 The aim was to make the soluble portion of the core to dissolve after casting was completed thus allowing removal of the insoluble portion.
Fraunhofer IFAM, is also researching salt cores, focusing attention on pressing, compaction and sintering. The density distribution and a selection of casting parameters are considered to control flow peaks in the salt core.15 The salt core technology was explored to produce a cylinder crankcase.16 To provide cooling, a water jacket usually surrounds a cylinder sleeve and is often open at the top on the cylinder side with crankcases.
The salt core is fixed when it is placed near the end of the cylinder sleeve on the cylinder head side. This increases the stability of the core during casting process. The salt core in Daimler’s patent16 comprises of a crown which is attached to a core shroud with the cylinder head. It is placed in the cylinder sleeve so that the crown covers one end of the sleeve on the cylinder head. As a result the salt core is connected to both the cylinder sleeve and the die casting tool. This helps the core to withstand the high pressure.
Based on the solution proposed, the salt core can be placed in different arrangements during the casting process thus changing some of the characteristics of the core. Besides improving the salt core properties, placement of the salt core can also help the process of casting a cylinder crankcase.
According to Daimler’s research,17 the salt core is able to support the water jacket against casting pressure and it is able to prevent undesired penetration of melt into the cavities at incorrectly sealed casting–mould contact surfaces. Since the salt core remains in the cylinder crankcase during the pressure die casting, this improves the stability of the cylinder liner during the encapsulation, where higher pressure occurs and increases the process safety.
The media connections of the coolant in the water jacket make it easier to dissolve the salt core as well when the casting process is completed.
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