The use of aluminum castings in the automotive industry has dramatically increased due to the combined benefits aluminum offers in saving weight and mechanical properties.
Standard aluminum Al-Si alloys are now reaching the limits of thermal and mechanical loads found in higher density power engines. Gasoline and diesel engines put a high level of thermo-mechanical stress on their cylinder heads. One of the challenges facing designers is to counter the effect of thermal fatigue in the combustion chambers.
New generations of engines will require aluminum alloys that sustain higher mechanical properties especially at temperatures above 250°C. Nemak’s research and development team has developed the AlCu range of alloys to meet the need.
Two groups of casting alloys may offer better mechanical properties at high temperatures. The first one seeks to improve the already applied AlSi series of hypoeutectic and eutectic alloys by adding elements such as Ti, Zr, V and Cr. According to past findings these alloys provide adequate tensile and yield strength at temperatures close to 250°C, but have insufficient fracture toughness to be considered for further applications.
The second group is AlCu alloys that aim to achieve the performance requirements for high powered gasoline and diesel engines. Nemak’s continuing focus is on the following topics:
• Evaluate the optimum chemical composition for the AlCu based cast alloy
• Develop the liquid metal processing route to guarantee optimum castability
• Develop the casting process parameters for the new alloy
• Develop the heat treatment processing parameters for this alloy
• Evaluate the potential corrosion predisposition and find solutions to fulfill engine requirements
The main advantage of the AlCu alloys is that they provide significantly improved mechanical properties at higher temperatures. The challenge for Nemak is that the difference of the AlCu alloy system compared to the AlSi/ AlSiCu is its higher eutectic concentration (33.2% Cu compared to 12.5% Si) which negatively affects the castability.
It is well known that coarse grain size leads to poor fluidity and is prone to porosity and hot tearing formation in AlCu alloys. The reduction of the grain size was the key achievement in the research work to solve these drawbacks. A newly-developed liquid metal processing method by Nemak lead to an as-cast structure of the AlCu7 alloy with globular crystalized grains of less than 100 μm diameter.
Another concern around the use of AlCu alloys is their preceived predisposition to corrosion. Nemak is currently running a comprehensive analysis jointly with a widely recognized German university to find out how much this new alloy is sensitive to corrosion under working engine conditions.
Potential counter actions have been defined and could be used if necessary. Further important properties of the new AlCu7 alloy are:
• High conductivity, improving the thermal management of higher power density engines
• Low thermal expansion, which significantly increases the dimensional stability of the component within the engine
• High resistance to thermal fatigue allowing greater engine safety design limits
Nemak has been closely collaborating with leading OEMs to successfully develop and optimize this new alloy according to
the customer’s specific requests. After comprehensive testing the new AlCu7 alloy shows the potential to be successfully used in future engine cylinder heads. This is due to its excellent mechanical properties at high temperatures.
Furthermore, the metallurgical, physical and mechanical properties of the alloy have been fine-tuned by an appropriate melt treatment, heat treatment and alloying elements addition. By developing a specific melt treatment procedure the integrity of the as-cast parts was improved resulting in a low tendency to hot tearing and porosity formation. Nemak’s Rotacast® process has proven to be the preferred casting technique for this alloy. This is due to the rate of solidification inherent to this particular process.
Heat treatment plays an important role to achieve and adjust the mechanical properties as well. The T6 treatment, which comprises solution heat treatment, quenching and artificial aging, is most commonly employed for aluminum casting alloys to achieve the best compromise between strength and ductility. The addition of alloying elements strongly affects the mechanical properties in as-cast and/or age-hardening conditions.
This also improves strength and ductility at room and elevated temperatures through the formation of dispersoids. The resulting mechanical properties, as well as further quality assessments of cast cylinder heads, confirmed Nemak’s expectation that the AlCu7 alloy has high potential to be used in passenger car engines, and in particular for cylinder heads. Further complementary testing of mechanical and thermo-physical properties as well as investigations on corrosion behavior will support the results obtained so far to establish a new high performance AlCu7 alloy.
Driven by the synergy of the high performance Rotacast® process, the availability of the AlCu7 alloy and innovative employees, Nemak says it will continue to be a leading supplier and partner for the automotive industry in the future.