Turbocharger cast parts for new high-performance engines require a material that is coordinated to the component behavior, not least due to the aim to reduce exhaust gases and the increasing performances. The requirements for these highly stressed casting components rise as the unit size and performance capability increase.
A turbocharger essentially consists of two major assemblies:
Regarding the exhaust gases, the turbine wheel in the turbine housing absorbs the energy from the exhaust gases and mechanically transmits it to the compressor via a shaft. The turbine wheel can turn with a speed of 160,000 to 300,000 rpm. In a spark-ignition engine, exhaust gas temperatures of up to 1,050°C must be endured in the area of the turbine housing. The turbine wheel, the bypass flap and the heat shield also reach correspondingly high temperatures. The heat shield prevents the heat from penetrating into the bearing housing. In general, the bearing housing is water-cooled in order to prevent an impermissible temperature rise after the engine has been switched off. In this component, four different media come together in a confined space:
- Hot exhaust gas
- Cold air
- Water for cooling
- Oil that must not get too hot
In the foundry, three parts are primarily cast for the "exhaust gas turbocharger" component:
- The turbine housing
- In a diesel engine: Usually high-alloyed nodular casting types, such as D2 and D5.
- In a spark-ignition engine, due to very high operating temperatures: austentic cast steel types with high Ni-Cr contents.
- The bearing housing
- Usually made of gray cast iron
- The turbine wheel
- Highly heat-resistant nickel base alloy that is melted and cast in a vacuum
The turbine housing
The turbine housing is of key importance here, as it is placed under extremely high thermal stress and is also the most expensive component due to its size, complexity and material. The caster is confronted with thin housing walls, highly elaborate to fine structures with high temperature gradients within the component, and frequent temperature changes during operation. In addition, the turbocharger is also produced as a combined component in combination with the exhaust manifold nowadays. Under full load, the entire component is exposed to extreme thermal and mechanical stress. The fact that it becomes red-hot underlines this. Years of experience and the use of state-of-the-art design and simulation methods are necessary to meet these requirements.
Casting methods and materials for the turbine housing
The cores for the cast part are manufactured from Croning® (registered trademark of ACTech GmbH) sand or with different cold box formulations, and the associated mold is manufactured from bentonite sand. This is a matter of classic sandcasting.
Nowadays, most turbine housings are produced using nodular casting (quality GJSA-XNiSiCr 35-5-2, i.e. Ni-resist D5 S). According to specialized literature, this austentic cast iron with nodular graphite has operating temperatures of up to 850°C. In exceptional cases, it is even supposed to reach 900°C.
The full load proportion and the heat-flow density increase in the modern downsizing engines. These factors must be taken into account when determining the life span of engines.
Nowadays, the exhaust gas temperatures are also generally increasing to temperatures of up to 1,050°C in current engine concepts. Against this background, turbochargers are designed differently today and new, higher quality materials are required. Heat-resistant cast steel, whose materials and behavior are known from petrochemistry, lends itself to this purpose. However, this experience must be broken down to be applied to a significantly smaller component size and the fine wall thicknesses of the turbochargers. The price-related aspects of the new materials must also be taken into consideration here. Heat-resistant austentic types of cast steel in particular are used for exhaust gas temperatures of up to 1,050°C. The qualities have high proportions of chrome and nickel, which has a particularly positive influence on stability and thermal-shock resistance.
In contrast to D5 S, the process yields casting temperatures that are 200°C to 300°C higher; this is also why it is more difficult to cast the material. This means that high-quality binding agents, highly refractory coatings and possibly also additives or special sands are used for the molds and cores of these turbochargers. The cold box process is also used to produce the molds in steel casting in order to prevent the high temperatures from causing reactions with the bentonite sand.
The concept for the gating and feeder system, which is to be removed from the cast part with as little effort as possible, becomes significantly more complex. This requires high-quality filters for casting the material in as stabilized a way as possible. Due to the small component size and gate area, optimum efficiency of the feeders must be ensured.
However, the greatest challenge for the caster is that production must be implemented with the smallest possible reject rates and with high productivity. The mold plate must be designed in such a way that as many parts as possible can be cast per mold.
For optimum use of the mold plate, it must be covered in as narrow and symmetrical a fashion as possible, so that the components can be blasted after casting and separated with the highest possible efficiency using the gating technology. The position and type of feeders must be determined in such a way that, ideally, the feeder necks are at the same mold height and their contact surfaces with the component are absolutely small. Both in iron and in steel casting, the materials are high-alloyed and require the feeders to be optimally designed in order to keep the effort of removing the feeders and the amount of returns to a minimum for reasons of cost.
The overall melting process, from the application of the alloys to the melting and up to the casting of the molds, must take place with the highest possible consistency. Exact compliance with all specifications is the basic requirement for avoiding structural problems and casting defects such as bubbles, pinholes, microporosity and even shrink holes.
The fact that melting is done with constantly changing charge materials and that the highly sensitive materials are exposed to constantly changing conditions during the melting operation often causes an increase in rejections. In addition to precisely setting the basic structure and the graphite form (especially in GJS), their size and distribution, it must be ensured that selected and tried-and-tested high quality treatment materials are used for the melt.
Another important topic in the production of turbine housings is the process-consistent prevention of veining, which occurs primarily on the inside of the turbine in the mold joint of the cores or in the adjoining area. The cleaning effort would be extremely high in such a case and could only be performed with special tools.
The fact that silica sand is used in general means that besides the selection of the binder and coating, the additive plays a key role in ensuring veining-free casting. The type of additive and also the amount added have a key influence on the result.
A high-quality coating, nowadays usually water-based, is necessary to create particularly clean and smooth surfaces on the inside of the turbine. As the component tolerances are extremely narrow, the coating must have excellent rheological properties and its formulation must be so sophisticated that structural degenerations at the boundary between the core and the cast part are avoided.
When the cores are cast, casting gases develop from the decomposition products of the binding agent. It must therefore be ensured that as small an amount of binders as possible is used and that gas extraction is ensured as far as possible by means of suitable core venting.
Feeding the cast part is indispensable in both nodular casting and steel casting. Mold filling and solidification simulations should serve as a basis for the selection and positioning of the feeder. In addition to being small, EXACTCAST™ mini breaker core feeders ensure a correspondingly small contact surface with the cast part with optimum mold properties. An unpressurized gating system that works with filters prevents turbulent mold filling and reduces the risk of bubble defects and cold laps. In general, bentonite-bonded mold material is used in mold production. The contents of moisture, fluorine and nitrogen in the bentonite sand in particular can have a highly negative effect on the casting quality. In order to counteract the risk of pinholes, fluorine-free EXACTCAST™ feeders should be used and the proportions of moisture and nitrogen should be small if possible. The material that forms lustrous carbon should also be used as constantly as possible and should by no means be changed frequently. If cast steel is used, the mold is also produced from chemically bonded mold materials in order to ensure constant properties and to cope with the high casting temperatures accordingly.
The turbine wheel
The turbine wheel is the component of the turbocharger that is subjected to most stress. Here, too, temperatures reach up to 1,050°C, and the purely mechanical forces place the component under extreme stress. These turbine wheels are generally melted from high-strength nickel base alloys. The entire process, from melting to casting, takes place in a vacuum.
The bearing housing
The connection of the bearing housing to the turbine housing has the greatest influence on the overall life span of the exhaust gas turbocharger. The largest temperature gradient occurs here. The water cooling requires new development in this area, too, due to the higher exhaust gas temperatures, as the amount of heat that needs to be dissipated is significantly higher. The bearing housing itself is usually made of gray cast iron and cast using the sand casting method. High productivity and little rework must be ensured when producing the cast parts. The cores are produced using different organic core manufacturing methods in this case, too. The mold consists of bentonite-bonded mold material.
System solutions for turbocharger casting
We meet the highest requirements in engine casting, the products sold by ASK Chemicals are also modified and adapted to the special base materials and raw materials that predominate in the foundries of the individual countries. The products are consistently aligned to the suitability for application and daily use in the respective country in close cooperation with the foundries.
Casting simulation, used by ASK Chemicals Design Services Group, is a tool for simulating different physical processes that take place in the mold during casting. This refers mainly to the three processes of mold filling, solidification and the formation of stress during cooling. The purpose of simulating these physical processes is to examine the casting and solidification processes quickly and efficiently, to prevent shrink holes and microporosity, to minimize residual stress and distortion, and to reduce the number of prototypes and test castings. ASK Chemicals Design Services Group uses this tool consistently to draw conclusions for further steps in the development from the results together with the customers. The goal of every project is to achieve a casting quality that corresponds to the requirements of the specification. Proceeding in collaboration can yield significant synergy effects that range from mold and core production and melting operation to inoculation, casting, mold filling and filtering. Suggestions for improving the entire production process are developed and implemented in collaboration. Ultimately, this means that production- or cast part-specific products are available.