Treatment of Cylinder Blocks of Combustion Engines
Hardening of the Working Surface of the Aluminum Cylinder Block for Internal-Combustion Engines by the Microarc Oxidation Method
Tech Area / Field
- SAT-SUF/Surface Transportation/Space, Aircraft and Surface Transportation
- MAT-SYN/Materials Synthesis and Processing/Materials
3 Approved without Funding
MISIS (Steel and Alloys), Russia, Moscow
- State Enterprise Krasnaya Zvezda, Russia, Moscow
- Keronite PLC, UK, Cambridge
Project summaryThe engine of a modern motor car is about 10–12% of its weight, and the very large and heavy part of the engine is the cylinder block. Substitution of aluminium for iron in cylinder-block manufacture leads to a decrease of weight by 50%. Besides, a decrease of engine weight enables decreasing the weight of other parts: front or rear suspension (depending on the position of the engine), brakes, frame etc. A better thermal conductivity of aluminium cylinder heads and blocks entails a decrease of the weight of the radiator. The engine warmup becomes faster, which also contributes to a decrease of fuel consumption and exhaust emission. Along with this, the same thermal expansion coefficient of the materials of the aluminium monoblock of cylinders and pistons makes it possible to reduce the gap between the piston and the cylinder and, thus, to decrease the consumption of oil, to reduce toxicity, to increase compression.
Up to half of all internal-combustion engines in the world are already fabricated from aluminium alloys, although only 15% of them are yet aluminium blocks without iron sleeves. This is due to the fact that a sufficiently efficient solution for the development of a wear-resistant working surface of the aluminium block without using iron sleeves has not been found. At the same time, iron sleeves do not enable making complete use of the positive effect of the transition to aluminium blocks. Therefore, the leading automotive and engine-constructing companies develop other methods of reinforcing the working surfaces by aluminium cylinders.
One of the ways to strengthen the surfaces of the aluminium cylinder block is microarc oxidation. The method of microarc oxidation (MAO) was initially developed and has been intensively elaborated in Russia since late 1970s. A similar method is known in the USA and Japan as anodic-spark oxidation. MAO occurs as the result of chaotic electric microarcs on the surface of oxidized material, which initially emerge between the electrolyte and conducting regions of oxidized material. The arc discharge leads to a local fusion of metal and its intensive oxidation. In the end, a region of a nonconducting oxide layer is formed, which interrupts the electric microarc. The next arc emerges in another conducting region or, in the further growth of the oxide layer, in the place of its lowest resistance by an electrical breakdown. As the result, an oxide layer (or an MAO coating) with a high hardness and wear resistance is formed.
The main restrictions for the wide use of the MAO method were up to now determined by the difficulties in maintaining the stable process in the treatment of a large number of articles (first and foremost, due to the change of the properties of electrolyte in the process) and in the treatment of large-sized parts (inhomogeneity of the coating and instability of the process). Most works on overcoming these limitations and further elaboration of the MAO method are associated with the search for new electrolytes and special modes of electrolysis. To date, these problems have been mainly solved by experts of the State Scientific and Industrial Enterprise “Splav”.
To use the MAO process for strengthening the surface of aluminium cylinder blocks, it is necessary to solve the problem of oxidizability of aluminium alloys with a high (more than 3%) silicon content, which are traditionally considered to be poorly yielding to MAO.
Aluminium–silicon alloys make 90% of the volume of all casting aluminium alloys. Therefore, the development of wear-resistant surfaces of articles from these alloys is especially important for machine engineering, particularly for the conditions when a significant role is played by the wear resistance of mating parts – counterbodies (for instance, such friction pairs of internal-combustion engines as “piston ring – aluminium piston”, “piston ring – aluminium cylinder block”). It should be emphasized that the phase composition of a layer strengthened by MAO, its mechanical and tribotechnical characteristics had been associated until recently with the oxidation regime and the chemical composition of oxidized material, but not with its structure. In this connection, of undoubted interest was to study the effect of the structure of aluminium–silicon alloys without changing their chemical composition on the quality of oxidation, which is what was done at the preliminary stage of the present work .
Current state of research on the project
To date, the authors of the project have successfully tested the MAO process under laboratory conditions for commercial alloys of hypoeutectic (AK6M2, AK10M2N), eutectic (AK12MMgN) and hypereutectic (AK18, AK21, 01379) compositions. Large experience has been accumulated on the optimization of the structure of oxidized material for subsequent MAO, taking into account the requirements to the material of the base and the MAO coating. The following patent applications were filed: an international application PCT/RU 00/00511 for “A method of producing a coating on articles from silicon-containing aluminium alloys” on 19/12/2000; an application for a German patent DE 100 85 495 T1 “Verfahren zur Beschichtung von Erzeugnissen aus siliziumhaltigen Aluminiumlegierungen” on 23/10/2003; an application for a Russian patent [1–2]. International examination confirmed the novelty and non-infringement quality of the inventions.
In the course of work, the phenomenon of oxide-layer growth inhibition in microarc oxidation by silicon particles was first found, explained and studied. The inhibition leads to a decrease of the thickness of the oxide layer, an increase of its thickness variation and porosity, a decrease of the efficient value of hardness and adhesion . The method developed makes it possible to reduce the negative effect of this phenomenon on the oxidizability of aluminium–silicon alloys.
The MAO process adapted for aluminium–silicon alloys was examined on a test specimen of the iron block with aluminium sleeves, which modelled an aluminium cylinder block of an internal-combustion engine. The test specimen block successfully passed the technological (machinability) and running tests [3–5].
Technical specifications of the unique equipment used
There is all required equipment for friction and wear tests, mechanical tests, chemical and spectral analysis; metallographic and fractographic analyses and X-ray electron probe microanalysis, including the newest equipment for structure studies (electron scanning microscope LEO 1455 VP with X-ray spectrum microanalyzers of the wave and energy types; Axiotech reflected-light microscope with digital videocamera AxioCam (Zeiss, Germany) with a variable magnification up to 2500Ч; stereoscopic microscope Axiotech (Zeiss, Germany); spectrometer SPECTRUMA GDA-750 (SPECTRO) with the glow-discharge tube with the possibility of layer-by-layer analysis.
A specialized stand was set up for tests of materials of the cylinder-and-piston parts. Laboratory techniques for friction and wear tests were worked out; the techniques showed a good correlation with the stand tests on an engine. There is a modern research facility for registration and analysis of acoustic emission signals; the use of the facility in tribotechnical tests gives information on the intensity of wear.
There are possibilities for casting test cylinder blocks from aluminium–silicon alloys, their machining, for stand tests and running trials. The State Research and Industrial Enterprise “Splav” has a universal installation for carrying out the MAO process; the installation is efficient for microarc oxidation of test specimens.
Besides, the authors accumulated a comparison base required for carrying out the work.
Aims and objectives
The aim of the present work: development of an industrial process and of a setup for microarc oxidation of aluminium–silicon alloys for treatment of the working surfaces of aluminium cylinder blocks of internal-combustion engines.
- Development of technical documentation for fabrication of a power source.
- Development of technical documentation for fabrication of an MAO setup (including the system of electrolyte circulation, flow electrolytes and cooler in the circulation contour, accessories for oxidation of the cylinder block).
- Acquisition of materials and component parts; fabrication of the power supply source and the MAO setup, including the fabrication of the required accessories for oxidation of the cylinder block.
- Development of the design and process documentation for fabrication of a sleeve-free aluminium cylinder block for a VAZ-type engine.
- Casting of aluminium blocks and sleeves from alloy AK6M2.
- Heat treatment of blocks and sleeves to produce the required structure before MAO.
- Elaboration of the oxidation process on sleeves from alloy AK6M2.
- Elaboration of the oxidation process on blocks from alloy AK6M2.
- Determination of the initial data for the cost analysis of the MAO process depending on the size of the block and volume of production.
- Release of technical documentation and reports.
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