Based in the foothills of the Sierra Madres, Nemak is part of the Alfa group and is fast developing a global reputation, read on for an insight into the foundry and how it is overcoming its challenges.
Challenge
The automotive industry is one of the most dynamic sectors, the continuous search for efficiency and desire to create the best car is always challenging OEM’s and their suppliers to invest in the development of new technologies. The current situation with regards the cost of fuels has propelled the effort to develop better engines which can improve the fuel consumption while reducing their effect on our environment. Aluminium has been a key player in this effort, due to its lower weight and better thermal properties allowing the engine to perform better.
Since OEMs are also changing the mix of engines as they adapt to market conditions, emigrating from larger V8’s to smaller V6’s and I4’s, it will be especially important for them to consider in their design high-tech aluminium heads and blocks to achieve greater horsepower while significantly reducing fuel consumption and emissions. Consequently the casting industry will evolve, making more complex components in which auto parts will not only need to be lightweight and robust, but also keep up with strict market demands in terms of environmental protection and fuel efficiency. Commercial vehicles powertrain migration to alternate technologies will gradually take place
Passenger vehicles powertrain is shifting to newer technologies such as electric, hydrogen and fuel cell. However hybrid vehicles - a combination of electric motors and internal combustion engines - will be the trends in the automotive industry in the next decades. This translates into smaller internal combustion engines without sacrificing net power, in other words - higher power density.
Nemak’s challenge, as a high technology aluminium cylinder heads and blocks producer for internal combustion engines, is to develop new processes and light alloys which withstand the higher loads in slimmer geometries to meet the power demands of hybrid vehicles.
A deep understanding of alloys, raw materials for core production and casting processes has allowed Nemak to meet the ever increasing demands of the automotive industry.
Physical properties (such as density, thermal conductivity, monotonic and dynamic mechanical properties) and castability are two technical factors which determine the selection of a particular alloy for a cylinder head or engine block.
As aluminium density is about one third of iron, weight saving is one of the main advantages of using the light alloy instead of ferrous alloys. Cylinder blocks for petrol applications migrate from cast iron to aluminium to reduce fuel consumption. During the period from 2000 to 2011 the number of engine cylinder blocks that migrated from iron to aluminium is expected to grow from 29 to 60%.
Casting processes
Rapid solidification rates are critical in aluminium alloy to enhance mechanical properties. When this is achieved, part microstructure presents less porosity and less rich Cu and Fe-phases segregation. As a result mechanical properties are maximised. Also improving heat treatment processes had been part of the key to improve mechanical properties.
Nemak embarks on all casting processes available in the market.
Cylinder heads
Currently Nemak utilises two processes to improve mechanical properties for cylinder heads - Rotacast and the Nemak enhanced gravity in semi-permanent mould (enhanced GSPM) process.
In the case of the patented Rotacast process, the mould is tilted during the casting process. This allows the colder metal to reach the critical areas of the product (deck face), resulting in higher mechanical properties. The improvement in mechanical properties occurs because the quicker the alloy solidifies the less porosity is allowed to precipitate. Also less segregation of secondary brittle phases such as Fe-rich intermetallics are allowed to nucleate.
The Enhanced GSPM uses novel mould materials and cooling systems at the critical areas of the mould to achieve similar benefits as those of the Rotacast process. This is usually done at the combustion chambers areas of the cylinder head.
One alternative to further improve mechanical properties for premium applications is moving from the traditional Al-Si systems to the Al-Cu alloys. Recent developments by the foundry’s R&D department had produced Al-Cu alloys for casting applications where the UTS goes as high as 440 MPa and yield strength in the range of 400 MPa with elongations between 3 and 6%. The initial concept development for the production of cylinder heads has shown promising results.
This alloy has excellent mechanical properties at high temperatures as shown in the following table:
Alloy / heat treatment Al-Cu / T6 Al-Cu / T7 Al-Cu / T6 Al-Cu / T7 A356 / T6 A356 / T6
Temp (°C) 200 250 200 250 200 250
UTS (MPa) 280 155 260 150 81 55
YS (MPa) 245 115 225 115 71 50
This alloy presents a thermal conductivity coefficient at 250°C of 190 W/m K.
Currently the production of the first prototypes, for concept validation, is being carried out for three different OEMs.
An attractive option for diesel or high performance cylinder heads is the alloy AlSi7Cu0.5, developed by Nemak. This alloy offers a good compromise between mechanical properties and thermal conductivity at high temperatures.
Another area where the foundry has undergone extensive work is heat treatment. The main objective is to maximise the mechanical properties without inducing residual stresses into the parts. A cost effective alternative to increase mechanical properties in the GSPM process is the Nemak Heat Treatment (NHT), where the cylinder head is rapidly cooled right after casting with a mix of air and water. This increases part hardness up to a level which complies with the client’s specification without the use of long heat treatments, or part redesign.
Blocks
Aluminium fatigue endurance is improved significantly when pore size (and number) is reduced. Intermetallics play a secondary role to porosity for aluminium alloys. The Nemak low pressure precision sand (NLPPS) and core package system (CPS) processes allow rapid solidification rates at the bulk head areas increasing significantly the high cycle mechanical fatigue limit. Three sigma average is 100 MPa at 150°C.
There are several types of porosity; hydrogen pick up from the atmosphere, due to casting contraction (shrinkages) and turbulent metal filling into the mould. The first one is controlled by proper degassing practices (N2 injection into the melt). In the case of NLPPS and CPS processes porosity is addressed during product development. This is one of the big advantages of using sand cores; any change on the design of the part can be implemented almost immediately, either for evaluation or as a permanent change. Turbulence is minimised by the right runner design.
Nemak also uses with the Cosworth Process, which rotates the sand package ensuring superior mechanical properties at the deck face of the blocks.
Recently the foundry has developed a combination of the NLPPS process and roll over to achieve the best mechanical properties provided from both concepts; this is already in production of a V6 block.
Bore plasma coating
Iron sleeves or liners are used in aluminium blocks to withstand friction and wearing from pistons and rings displacement during the combustion cycle. These can be cast-in at the moment of block moulding, or pressed in at the engine plant. Unfortunately the side effects are an increase in engine weight and lower thermal conductivity form the combustion bore to the water jacket. Weight saving and optimum heat transfer is key for engine efficiency. Plasma coating techniques are replacing these liners with a thin coat of an iron m).alloy (~400 Nemak has available both leading bore coating processes. The first mass production high power engine blocks will be produced in Nemak Dillingen in late 2011.
Advanced simulation and modelling
Getting involved in the early stages of an engine development, allows Nemak to participate in the design of the engine component from the stand point of the foundry. This results in an optimised product which allows a smoother launching to production at a lower cost.
The foundry relies on process and solidification simulation and modelling for a successful program launching. CAD/CAM/CAE infrastructure includes state-of-the-art software to simulate not only solidification and defect presence in the casting, but part residual stresses, casting response to heat treatment and sand core optimisation. Tooling design is also a significant aspect of Nemak’s CAD department.
Computer simulations help to predict porosity in the casting allowing the foundry to ‘virtually test’ different runner systems before casting a single actual part. This leads to an optimum design of the gating and risers.
Teams formed by product engineering, advanced manufacturing, R&D, and of course CAD designers, lead to shorter product development times.
Sustainability
The use of secondary alloys is a good environmental friendly alternative. Currently the foundry’s worldwide operations recycle more than the 45% of the metal used for the production of cylinder heads and blocks. R&D projects on secondary alloys optimisation complies with lower cost but achieves all the physical properties specified by customers.
Sand reclamation is recognised as an important process which helps to reduce costs and is used in several plants in the group.
Nemak is currently developing a proprietary sand core binder (Nemak Binder), which is an alternative to current petroleum-based binders. Its attributes are: it is water soluble, has lower recycling costs and is environmentally friendly. This will reduce part production cycle times and save energy which is currently being used for core removal. In parallel an inorganic-binder option is also available that is an environmentally friendly alternative. The lack of fumes generated during casting allows colder moulds resulting in higher mechanical properties in the GSPM process. The first castings using this concept will be produced in 2011.
About Nemak
Founded in 1979, Nemak specialises in the production of aluminium cylinder heads, engine blocks and transmission parts, and other aluminium components for automotive applications.
The company has experienced a steady growth rate thanks to strategic acquisitions, as well as organic growth. With 27 manufacturing facilities located in 12 different countries in Asia, Europe and North and South America, and nearly 15,000 employees worldwide, Nemak is ready to meet the demands of the automotive industry around the world.
Source: foundry-suppliers.com
