Automotive Focus: Heat-Treating Systems Meet Foundry Needs
The reduction of the total weight of an automobile leads to a reduction in fuel consumption. This improved fuel economy also reduces CO2 emissions released into the atmosphere. To decrease the overall weight of vehicles as their size has increased, the automotive industry is using more light metal components to reduce fuel consumption and lower emissions. Iron castings, including engine blocks, cylinder heads, brake calipers and discs and suspension components are being replaced by aluminum castings.
Castings represent on average 80% of the total aluminum content (about 260 lb, or 120 kg) in a 2001 model year car. This number has grown rapidly and will grow even more within the next 8 to 10 years. The number of light vehicles having both aluminum engine blocks and cylinder heads will soon reach a market penetration of about 95%.
The traditional production of aluminum castings is costly and energy intensive. The casting process flow consists of several independent steps, carried out without any connection to previous process steps. The U.S. Department of Energy in a forward-looking industrial study cited that "?nergy savings in heat processing industries are to be achieved by integrating heat processing steps into manufacturing operations."
The traditional casting process involves picking up the casting from a die, a permanent mold or precision sand mold (PSM) followed by one or more operations (Fig. 1). These subsequent operations typically involve intensive cooling to reduce the casting temperature for further processes like de-gating, trimming, mechanical sand mold and sand core removal and manual cleaning. The castings are then reheated for the heat treating process.
Stand-alone equipment and processes require handling operations, and transports can cause damage to the castings. This additional equipment and handling also requires additional personnel and manufacturing floor space and more energy.
New patented technologies are available for semipermanent mold castings and castings made in precision sand molds using organic binders, where the casting is placed into the heat-treating furnace immediately after solidification and without the operations mentioned above. This new technology, known as the 3-in-1 process (Fig. 2), processes hot castings and can carry out three foundry processes simultaneously in one automated machine including:
- De-coring and sand removal
- Thermal sand reclamation
- Heat treatment of castings
The core and mold sand binders contain substantial energy when exposed to the oxygen-rich furnace atmosphere. As the binder burns and sand grains are released from the molds and cores, high velocity roof-mounted axial fans blow down through the castings and carry the sand to the bottom of the furnace. The high velocity fans create pressure drops as the fast moving air passes the irregularly shaped castings. This pressure drop causes turbulence inside and outside the castings, which removes the loose sand. The high velocity fans also provide excellent air distribution within the furnace and allow very tight temperature control tolerances. Castings emerge from the solution heat treatment process free of sand molds and sand cores.
Sand collected in the hopper-type bottom of the furnace is automatically discharged as thoroughly reclaimed sand for return to be used for molds and cores.
Placing the casting directly into the heat treatment furnace saves a substantial amount of energy compared to the traditional process where castings are cooled after solidification and then heated up again to the solution temperature. The heat energy used for melting the metal is conserved and used for the three processes inside the treatment furnace.
After completion of either a full cycle T4 or T6 heat treatment, castings are cooled to ambient temperature and can then be directly transported to further processes such as de-gating, riser removal and machining.
Combining heat treating together with the foundry process and using the advantages of the system also means that the furnace can be used for reclaiming additional sand coming from the core shop (cured sand) or reclaiming of rejected cores.
Gases collected from pouring and solidification areas, as well as from the core shop, can be used as combustion air in the furnace, where the harmful fumes are incinerated.
The combination of several main aluminum casting processes is reflected in a significant reduction in production costs, and the process is environmentally friendly and efficient (Fig.3). Audits in foundries using the 3-in-1 process show an average reduction of production costs of more than 30%.
Fluidized-bed technology emerging
Fluidized bed technology also can be used for simultaneous heat treatment, thermal mold and core sand removal, and thermal sand reclamation. The process involves submerging semipermanent mold or just-cast precision sand molds into a bed of hot fluidized sand. Similarly to the 3-in-1 process, castings and sand are processed in an atmosphere using oxygen and heat. Once again, transferring the castings to this process while they are still hot saves energy and processing time. This process has the same positive energy and environmental attributes of the 3-in-1 process technology and provides a very efficient process for some castings for secondary processing operations including heat treatment.
Quenching and aging operations use traditional technologies. Air quenching has been improved as a complementary technology to the 3-in-1 heat treating process. Many installations use air quenching for automotive castings, particularly cylinder heads and engine blocks. Engine blocks often have cylinder liners made of cast iron, so air quenching is the preferred process, because the slow cooling accommodates the different cooling characteristics of the iron and the aluminum. Uniformity of the airflow around the casting is important because variation in heat transfer rates will provide corresponding temperature gradients, which can cause stresses and distortion.
Air quenching has many benefits over quenching in cold water. Traditional water quenching can lead to stresses, as well as to deformities in the castings if the stresses are high enough. Stress can tend to be compressive in the exterior areas of the casting, which are subject to the most severe quench. The internal portions tend to achieve tensile stresses due to the slower cooling rates. Larger castings such as aluminum engine blocks are subject to significant nonuniformity and consequently higher quenching stress. By quenching the casting at a slower rate, the temperatures can equalize throughout the casting.
The effectiveness of air quenching depends on the quench airflow, quench air temperature, casting configuration and arrangement of the castings in the load. Castings have a Brinell hardness of 95 to 105 and higher. A typical quench rate is at 90F (50C) or higher per minute to a target temperature of about 465F (240C). To provide the necessary airflow distribution, the quench chambers are specially designed to provide a high uniform air velocity.
Another viable technology developed by CEC to process aluminum castings is air-mist quenching. Using experience from numerous air-quench applications, water in the form of fog or mist is now being applied. The process has advantages of cooling faster than air quenching while reducing the amount of air handling equipment required.