Quality requirements placed on today’s brake discs are more stringent than ever. The primary reason for this rests on increasing demands with the end product. For instance, higher vehicle weights, more powerful engines and more effective brake calipers are all specific contributing factors. Ultimately, these all place significantly high demands on both the thermal and mechanical material properties of the brake disc.
The friction rings for brake discs in rail vehicles with medium and low brake power are made from cast iron with lamellar graphite. As a first approximation, the material can be selected depending on the traveling speed. The possible materials range from GJS brakes to cast aluminum and steel brakes to ceramic (CMC) brakes for high-speed trains. Before the final determination of the material, the brake power, braking energy, collective load and the expected life span must also be considered.
Nowadays, vertical and horizontal molding machines can ensure high dimensional accuracy. Ideally, casting is done using automatic casting equipment and a pouring stream inoculation is carried out during mold filling. The cold box process as a core molding process holds a dominant position on the European market. Coated cores are used to ensure highest surface quality at the casting contours.
Insufficient graphite formation, shrink holes, microporosity, veining and gas defects are the most frequent defects in this product group.
The graphite formation in the material has a significant influence on the behavior of the brake disc. Strong deviations from the desired A-graphite in combination with shorter graphite lamellas can lead to poorer tribological behavior (interaction between the friction ring, brake pad, disc geometry and braking conditions) and to increased thermal load of the brake disc. D-graphite and what is known as undercooled graphite are not acceptable in the microstructure.
Due to the use of higher casting temperatures, the thermomechanical load of the cores and coatings is of key importance here. In addition, high demands are placed on the binder systems due to the short cycle times in core production in combination with an automated technique for extracting the core from the molding tools and the high initial strength the core requires as a result. High resistance of the binder systems to water-based coatings is already a basic requirement. There are high demands placed on core production itself in terms of the quality of the individual core shooting machines, the degree of automation and the interlinking of the downstream processes.
For this group of cast parts in particular, the use of simulation tools is of great importance. The required A-graphite in the part is greatly influenced by the cooling speed and the mold filling, which is confirmed in the evaluation of the mold filling and solidification simulations. Appropriate gating and feeding technologies in combination with ceramic filters and optimum inoculation during mold filling make it possible to push forward toward the limits of casting in order to produce defect-free cast parts.
System solutions for brake disc casting
Due to the continuing adaptations to and tightening of the environmental specifications, the number of restrictions regarding the core production processes used so far keeps rising, and these processes can often only be adapted by means of cost-intensive measures. The use of inorganic core binder systems provides an appropriate alternative for the purpose of reducing emission and odor pollution that was implemented in groundwork carried out by ASK Chemicals together with a foundry. Brake disc cores for casting with cast iron were produced with the modified INOTEC™ inorganic binder system. Adaptations with respect to the INOTEC™ promoters and coatings used were made in several test series in order to adapt them to the respective system properties. The optimization of the process and the production parameters is now to be continued in further test series, using the results that were achieved in iron casting as a basis. The undisputed goal is to find a system solution with process reliability for inorganic binders in order to apply these binders in iron foundries in a process-consistent manner in the future. ASK Chemicals also offers system solutions for conventional organic binders to the effect that all components are coordinated to the respective mold material and the material as well as to the existing equipment and systems and their peripheries. Furthermore, the coating used is of extraordinary importance for brake disc cores in particular, so that it, too, is included in the system solution in combination with highly refractory additives.
In the area of metallurgy, ASK Chemicals and its other partners also offer continuing system developments of complex inoculants in the form of ladle, pouring stream and mold inoculants and inoculation wire as well as combination filters (mold inoculation and filter) for use in iron casting.
The application engineers and sales force employees of ASK Chemicals give the customers advice on how to ensure consistent casting quality in their production processes. This is done by means of competent application consultancy, joint system developments and process optimization.
As 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. Naturally, this also applies to Germany above all.
Casting simulation 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.