Development of Metal Matrix Composite for Cylinder Block

Transcription

1 Seoul 2000 FISITA World Automotive Congress F2000A065 June 12-15, 2000, Seoul, Korea Development of Metal Matrix Composite for Cylinder Block Manabu Fujine 1) Shinji Kato 1) Toshihiro Takami 1) Shigeru Hotta 2) 1) Toyota Motor Corporation,1,Toyota-cho,Toyota,Aichi, ,Japan 2) Toyota Central R&D Labs.,Inc.,Nagakute,Aichi, ,Japan A new Metal Matrix Composite (hereafter MMC) material for cylinder block bores has been developed, which makes it possible to reduce engine weight and size. To decide type, size and volume fractions of the MMC reinforcements, wear and scuffing characteristics as well as tensile strength were evaluated. Based on wear test results, crystallized alumina-silica fiber and mullite particles were selected as MMC reinforcements. In the case of counter-part (piston ring material) wear, the smaller particle size, the lesser ring material wear. For scuffing characteristics, the smaller particle size and the higher volume fraction, the higher the scuffing resistance. However, it is necessary to limit the volume fraction of reinforcements and particle size, because too high volume fraction and too small particle size causes a degradation in the tensile strength of the MMC. This is caused by an increase in micro porosity of MMC. Therefore, MMC reinforced crystallized alumina-silica fiber (Vf5%) and 12 µm mullite particle (Vf10%) are selected as optimal material, because this MMC material supported the desired wear characteristic and tensile strength. Electrochemical machining (ECM) was adopted to ensure the scuffing resistance. Relationships between scuffing characteristics and ECM-processed bore surface characteristics were clarified. This MMC cylinder block was commercialized in August Keywords: Metal matrix composite, Cylinder block, Tribology, Wear INTRODUCTION To improve the fuel economy of automobiles, R&D efforts aiming at reducing the weight of automobile parts are currently strong. For engine parts, cylinder block material is rapidly changing from cast iron to aluminum, and aluminum die-cast cylinder blocks with cast iron liners are now widely used. The all-aluminum cylinder block (Fig. 1) without cast iron liners can be reduced in weight of liner portions by simple replacement of material. In addition, since its bore-tobore distance can be set smaller than that of the aluminum die-cast cylinder block with cast iron liners, the all-aluminum cylinder block can be made smaller in size and lower in weight. This all-aluminum cylinder block can also be effectively used in making a highperformance engine by increasing the bore diameter without changing the bore pitch. For the all-aluminum cylinder block, the primary challenge is to meet tribological characteristics required for the cylinder bore surface. Hypereutectic aluminum alloy (A390), fiber-reinforced metal (FRM), powder-reinforced metal (PMC), Ni-SiC-plating, etc. [1],[2] have so far been used as materials for all-aluminum cylinder bores. However, all these materials have the disadvantage of greater wear in the bore or the piston ring. With the aim of overcoming this problem, a new metal matrix composite (MMC) for cylinder bores with superior tribological characteristics and sufficient strength to allow a narrow bore-to-bore distance was developed. Steps of this development are as follows: (1) Wear state of aluminum bore surface after engine durability tests was studied, and candidate reinforcement materials were selected. (2) The relation between tribological characteristics (wear resistance and scuffing resistance of bore materials and wear resistance of the piston ring material) and MMC composition / bore surface characteristics were studied by screening tests using test 1 specimens. (3) The relation between MMC composition and material tensile strength was studied. Fig.1 All aluminum cylinder block with MMC bores ENGINE OPERATING ENVIRONMENT AND WEAR STATE OF CYLINDER BORE SURFACE When an engine is operated under low-watertemperature conditions, the cylinder bore wear increases [3] due to the following mechanism: {1} When water temperature is low, bore surface temperature is also low, which means that the vapor in combustion gas condenses into water-drops on the bore surface. {2} Combustion products dissolve in water-drops on the bore surface. {3} Acids, such as hydrochloric acid, nitric acid and sulfuric acid are produced from these dissolved combustion products, gasoline components etc. {4} Oil film disappears. Since piston rings slide against the bore surface without an oil film, the bore surface or the piston ring wear

2 increase due to abrasion. The abrasive wear varies with the type of fuel used, increasing with fuel with a high sulfur content. Fig. 2 shows the wear state of the bore surface for the bore material of A390 and FRM after an engine durability test. Judging from the wear states of the aluminum matrix and the reinforcement, the wear mechanism applying to the cylinder bore surface or piston ring is assumed to be the following: {1} Portions of aluminum matrix are worn and become concave because the hardness of aluminum matrix is low. {2} Reinforcements become convex because the hardness of reinforcement is high. {3} Reinforcements repeatedly come in sliding contact with the piston ring material. {4} Reinforcements break into pieces and the pieces detach at each sliding contact. {5} Abrasive wear occurs. Therefore, when using MMC as cylinder bore material, it is essential to select optimum reinforcement material. material. (4) Fe plating is adapted as a surface treatment for piston skirt. WEAR TEST METHOD AND CONDITIONS Wear depths of MMC materials and the piston ring material were evaluated using the wear tester shown in Fig. 3, under the conditions specified in Table 1. As MMC reinforcement material, two different kinds of fibers and three different kinds of particles (see Table 2) were selected. Fig.3 Wear tester Table 1 Wear test conditions (1) A390 bore Sliding speed Load Testing time Lubricant Piston ring material 0.3m/s 1800N 30min 5w-30 engine oil 0.85C-17Cr(SUS440B) gas nitrided Table 2 MMC Reinforcements Reinforcement Hardness Fiber Alumina HV1300 Crystallized alumina-silica HV1000 Particle Alumina : 3 micro metre HV2000 Mullite : 3,12,25 micro metre HV1000 Si : 25 micro metre HV700 Cost: Alumina fiber > crystallized alumina-silica fibers mullite particle (3 µm) > mullite particle (12 µm) (2) FRM bore Fig.2 SEM micrographs of bore surface after engine durability testing MATERIAL DEVELOPMENT The MMC material for the cylinder bore was developed under the following assumptions to get good wear properties and productivity: (1) Typical aluminum die-cast alloy containing 11% silicon and 2.5% copper (JIS ADC12) is used for aluminum matrix. (2) Since conventional aspirating process is used to produce the preform, the preform consists of either fibers, or fibers and particles. (3) Stainless steel with nitride is used as piston ring 2 WEAR TEST RESULTS Fig. 4 shows the wear test results. The lower the hardness of fibers and particles, and the smaller the particle size, the lesser the ring wears. Among various MMC materials, the MMC reinforced by crystallized alumina-silica fibers and mullite particle diameter of 12 µm provides the best wear property and cost performance. Fig. 5 shows the relation between the volume fraction of 12 µm mullite particle and wear properties. When the volume fraction of mullite particle is in the range between 5% to 20%, variance of wear properties is small.

3 Alumina fiber Alumina fiber +Si Alumina fiber +mullite(25) Alumina fiber +mullite(12) Alumina fiber +mullite(3) Bore wear Ringwear materials and surface characteristics of the bore must be optimized to ensure high scuffing resistance. A scuffing test was carried out using the scuffing tester shown in Fig. 6, under the conditions specified in Table 3. The period from test start-up until the time friction coefficient drastically increases (hereinafter referred to as scuffing time ) were evaluated. Alumina fiber +alumina Alumina-silica +mullite(12) Alumina-silica +mullite(3) Wear depth (miron metre) Fig.4 Wear test results Fig 6 Scuffing tester Table 3 Scuffing test conditions Bore wear depth (micro metre) Ring wear depth (micro metre) Volume fraction of mullite particle (%) Volume fraction of mullite particle (%) Fig.5 Relation between volume fraction of particle (12 µm) and wear properties SCUFFING TEST METHOD AND CONDITIONS Scuffing occurs at low temperature, immediately after engine start-up when oil lubrication is not yet sufficient. The scuffing resistance depends on bore material, piston ring material and surface characteristics of the bore. Honing marks on the bore surface work as oil depositories to avoid scuffing. On the MMC bore surface, honing marks disappear early since aluminum matrix tends to subject to plastically deformation [4]. In addition, aluminum matrix can easily adhere to the piston ring surface. Consequently, reinforcement 3 Reciprocating speed 500cycles/min Reciprocating stroke 40mm Load 30N Lubricate 10W-30 engine oil Lubrication Marginally lubricated condition Piston ring material 0.85C-17Cr(SUS440B) gas nitrided SCUFFING TEST RESULTS AND DISCUSSION Fig. 7 shows the relations between scuffing time and volume fraction of particle for various particle sizes. The smaller the particle size and the higher the volume fraction, the longer the scuffing time becomes. This is presumably due to the following: The smaller the particle size, and the higher the volume fraction, the higher the particle distribution density becomes (meaning that the clearance between particles decreases), so that the growth of adhesive is restrained. When the particle size of the MMC reinforcement is minimized and the reinforcement volume fraction is maximized, high scuffing resistance can be attained only by honing the surface. Scuffing time (min) Volume fraction of particle (%) Fig.7 Relations between scuffing time and volume fraction of particle for each sizes

4 TENSILE STRENGTH TEST RESULTS Fig. 8 shows the tensile strength of the MMC which 12 µm mullite particles were used. The degradation of tensile strength is observed when the volume fraction of the reinforcement is higher. This is because a higher volume fraction results in a smaller clearance between the reinforcements and therefore an increase of the microporosity in the MMC. The degradation of tensile strength is also observed when 3 µm particles are used. In order to secure satisfactory tensile strength and castability, the reinforcement volume fraction should be smaller, and the particle size should be larger. Tensile strength (Mpa) Volume fraction of reinforcements (%) Fig.8 Relation between MMC strength and volume fraction of reinforcements FE-PLATING FOR PISTON The piston skirt surface must be treated to prevent streak shapes of piston skirt from wearing out in an MMC cylinder bore. For this purpose, high-hardness Fe-P plating was developed and applied to the piston skirt. ENGINE TESTS WITH MASS-PRODUCTION SPECIFICATIONS Table 4 gives the specifications of the developed MMC cylinder block for mass production. Fig. 10 shows the microstructure of the developed MMC. Various engine durability tests were carried out, and the performance and reliability of this MMC cylinder block were evaluated. The tests revealed that there was no reliability problem associated with bore wear, ring wear and head gasket sealing with a bore-to-bore distance of 5.5 mm. Table 4 Specifications of the MMC cylinder block Cylinder bore material Crystallized alumina-silica fiber 5Vf% +12micron metre mullite particle 10Vf% Surface treatment for bore ECM process Piston ring material 0.85C-17Cr(SUS440B) gas nitrided Piston skart finishing Fe-P plating SURFACE CHARACTERISTICS OF BORE WITH ECM A decrease in volume fraction of reinforcement in order to secure high tensile strength and good castability results in low scuffing resistance. Electrochemical machining (ECM) was studied to secure good scuffing resistance. ECM creates micro depressions on the aluminum matrix portion of the MMC bore surface. Fig. 9 shows the relation between scuffing time and the difference between surface roughness before ECM (after honing) and after ECM. This difference represents the degree of depressions created by ECM. Scuffing resistance increases with the difference value. Presumably, a higher degree of depressions results in a higher lubricant-retaining performance and a higher scuffing resistance. Scuffing time (min) Diffrence between surface roughness before ECM and after ECM(Rz micro metre) Fig.9 Relation between scuffing time and the difference between surface roughness before ECM and after ECM Fig.10 Microstructure of developed MMC CONCLUSION (1) A new MMC material for cylinder bores with wellbalanced tribological characteristics, tensile strength and castability was developed. Optimal reinforcement composition is 5Vf% crystallized alumina-silica fiber + 10Vf% 12µm mullite particles. (2) High scuffing resistance of the bore surface can be secured by oil depositories, introduced as micro depressions by ECM. (3) The developed MMC cylinder block has been applied to the new high performance 2ZZ-GE engine of the new Celica model and contributed largely to its improved dynamic performance. 4

5 ACKNOWLEDGMENT This development is the result of close cooperation by many people in and outside the company. We would like to express our gratitude to all these people for their kind assistance in our development study. REFERENCES [1] Funatani, 1995, Weight Reduction of Engine and Surface Treatment of Aluminum Alloy, Metals&Technology, Vol.65, NO.4, [2] E.Ohgami, 1991, Nissan s New V8 and L4 Aluminum Cylinder Block, SAE Paper [3] Murakami, 1995, Diesel Engine Sulfuric Acid Corrosion Wear Analysis, Automotive Engineering Society Theses, Vol.26 NO [4] F.Bin, and G.S.Cole, 1992, Scuffing Resistance of Selected Materials as protection for Bores in Aluminum Engine Blocks, SAE Paper