The search for increased longevity and product reliability is driving changes in casting and forging design, materials and processes. In recent years manufacturers have presented metallurgists with ever more challenging requirements, especially in the area of powertrain components. To meet these challenges, designers and metallurgists have had to come together at the very earliest stage of the design process to optimise and identify the most appropriate foundry techniques.
Andrea Weiss from ACTech GmbH is well aware of the increasingly complex castings that automotive manufacturers wish to explore and develop in the search for high performance and reduced weight.
ACTech develops casting prototypes and undertakes small batch production as part of development work alongside many OEMs and their suppliers. As one of the leading global rapid prototyping companies, it sees its task is to produce prototypes that, from the very first casting, are comparable to the items which will emerge in series production. Using simulation techniques it is able to construct prototype castings in small batches and provide accurate stress and porosity forecasts for series production components. Working with foundries, it is able to smooth the process from design to final series production, with quality as the overriding concern.
Complex castings, with ever more demand to minimize wall thickness, appear first in high-end vehicles but quickly the objective becomes to achieve similar levels of performance in higher volumes and against stringent cost constraints. Increasing competitiveness and a febrile market has limited the development phase of new technologies. ACTech hopes that by assisting its customers through the development of rapid prototyping technologies it can influence the final product in a beneficial way.
An examination of the range of prototype components ACTech is involved with quickly highlights two themes. Firstly, the increasing tendency to integrate components within a singe casting. Examples of integrated manifolding and turbine housings – or even cylinder heads with integral exhaust collectors – seem a significant part of its work. Secondly, all these developments demand precise control of wall thickness and surface finish. Similarly, the search for higher reliability and longer life span encourages designers to explore the replacement of traditional external oil and coolant hoses with more complex integrated cast housings for water and oil pumps and their associated distribution channels.
Increasing yield and reducing waste
Increasing the metal yield of casting technologies remains a key concern. This becomes especially critical as cast components become more complex in shape and customers more demanding of tolerances. With magnesium castings becoming increasingly important for powertrain developments, attention has inevitable fallen on the relatively poor yield performance of traditional casting technologies. The metal yield of some traditional magnesium casting is typically around 50%, with unused material not easily recyclable as remelting the residual metal creates oxides and compounds that contaminate the alloy.
CSIRO – the Australian National Science Agency – working with commercial partners, has developed an alternative casting process called T-Mag, which it claims has significant advantages in both enhanced yield and, combined with all the best features of permanent mould casting, T-Mag allows the incorporation of sand cores and thus makes possible the casting of complex shapes. The issue of yield is just one of the considerations in casting processes. Low pressure die casting is a long established process used for chassis components but is now increasingly being used in powertrain component production as it offers a good compromise between quality and cost, as well as allowing a wide range of complex shapes to be produced. Using gas under modest pressure to fill the die cavity ensures the residue returns to the furnace. Crucial to the success of the process is the establishment and control of proper cooling circuits to affect the solidification path of the alloy, with the major mass of the casting being the last to solidify. Low injection velocity and high cycle time make possible the accurate control of the fluid dynamics, reducing defects. The result is a high yield and a reduction in machining costs due to the elimination of feeders. A high degree of automation is possible and the resulting castings have good tolerances and technical characteristics.
Magnesium and aluminium in the powertrain
Magnesium remains a material with a particular potential for powertrain components, especially for high-end vehicles where cost is of less importance than ultimate performance. It is also especially relevant to new developments in vehicles with alternative power sources, where reducing driveline weight can have significant impact on range and performance capabilities.
Research in the North American Die Casting Association’s (NADCA) HyperCast project, has explored the potential of self-propagating high temperature synthesis as a way of introducing particulates into aluminium or magnesium castings. Such additions can alter and enhance their mechanical and thermal properties. The injection moulder can control the percentage of particulates in the material in order to offer resultant castings that can have application specific controlled coefficients of thermal expansion. One project goal for the NADCA project is the promotion of magnesium for use in engine blocks. Poor creep performance at high temperatures and the potential for distortion and deformation has restricted the use of the materials in these applications. The HyperCast process offers the possibility of overcoming these difficulties through the introduction of appropriate ceramic particles in a metal matrix.
Low pressure lost foam casting is another technique which offers the possibility of increased used of magnesium. With this process, unlike a gravity lost foam casting, a significant degree of control can be exerted on the filling speed. Throughout the casting process the mould is subjected to low pressure which allows the removal of casting gases and helps stabilise the mould. Complex casting for water pump components and automatic transmission housings can be created in this way.
Simulation and design
Perhaps some of the most important developments relating to casting and forging – as in many other areas of production – lie not in the physical production processes themselves but in the software and analytical tools that allow designers to optimise the production process. With crankshaft production, for example, balancing simulation software can assist in the development of designs. The effect of the forging process on balance can be analysed along with changes to counterweight geometry. Forging simulation technology is used by the Kalyani Group, for example, to facilitate process design and establish optimum forming performance. Use of this technology avoids the cost of sample forging runs and avoids defects and non-fills. Fully automated forging lines, along with the use of micro-alloyed steels to obviate the need for heat treatments, offer customers new options for product design.
Other innovative techniques include powder metal (PM) technologies. A wide range of automotive powertrain components use sintered metal techniques. Forged powder metal technology has a long record of use in con rod manufacture, offering high strength and performance consistency as a result of a homogenous microstructure. With consistent mass control and a limited range of finishing operations required, such material has already long proved its usefulness. It is, after all, a technique originally developed as long ago as the 1930s – though extensively refined since. More recently, companies such as GKN Powdered Metals have developed new PM aluminium materials which offer engineers a new option when weight reduction and performance improvement are sought. Surface densified PM is another varient of the process enabling lightweight, complex, highly stressed gear clusters to be manufactured without the weight penalty attached to fully dense products. Such PM products can often be delivered requiring only the very minimum of machining in application. In other applications the ability to provide and carefully tailored oil retention capabilities can be very helpful.