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Known for the Silitec material is widely used in cylinder bore lining for engine blocks material.Transferred to aluminium, the monolithic concept only works with hypereutectic AlSi alloys which, by means of primary silicon particles, provide a comparable wear resistant surface. The disadvantage is that engine blocks made from hypereutectic AlSi alloys are relatively expensive in comparison with grey cast iron,but have the competitive advantages.This spray-compacted, hypereutectic aluminum-silicon alloy is cast into aluminum cylinder blocks using the high pressure die casting (HPDC) process. Silitec™ is well suited to the HPDC process because the thermal energy of the molten aluminum is removed rapidly through the metal die.
Monolithic aluminium concept (ALUSIL®)
Hypereutectic AlSi alloys (AlSi17Cu4Mg, e.g. ALUSIL®) can be used to produce monolithic aluminium engine blocks. During solidification, primary Si particles are precipitated which – after an appropriate machining and honing procedure – protect the cylinder bore surfaces in the form of small wear resistant grains and directly provide the required tribological surface characteristics for a proper operation of the engine.
The advantages of this solution are:
Low weight (no grey cast iron cylinder liners necessary)
Compact cylinder block design (no liners, minimum spacing between bores: 4 mm)
High thermal conductivity (no gap)
Low distortion (minimum residual stresses)
Small assembly tolerances for pistons (similar thermal expansion coefficient as piston material)
Easy recycling (no extraneous material).
However, according to the present state-of-the-art, this alloy can only be cast using the LPDC (Low Pressure Die Casting) process due to the following limitations:
High pouring temperature
Large solidification interval
Segregation and inhomogeneous distribution of primary silicon particles.
Heterogeneous concept: Cast-in aluminium cylinder liners (SILITEC® and ALBOND®)
The SILITEC® hypereutectic liner materials are produced by spray compaction; the spray-compacted ingots are subsequently extruded. The high solidification rate of the spray compaction process leads to significantly smaller primary silicon particles than in standard casting processes and ensures excellent tribological properties of the liner surface after the special honing process. Since the same base material is used, metallic bonding between the liner and the engine block is achieved over more than 50% of the contact surface. The result is an engine block showing low distortion and high dimensional stability.
Aluminium-based cylinder liners are most suitable for high pressure die casting where melting-through is avoided because the thermal energy of the molten aluminum is removed rapidly through the metal die. Other casting processes such as sand, semi-permanent mold and low-pressure die casting have longer solidification times, leading to the transfer of additional thermal energy to the aluminium liners. This additional energy may result in localized melting and deformation of the liner. Thus, in order to use aluminium cylinder liners in casting processes other than HPDC, it becomes necessary to develop alloys that are more resistant to residual heat in the casting process.
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Cast-in aluminum cylinder linings by ultilizing Silitec material
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