| Aim at to develop the automobile materials for piston and cylinder core, JilinUniversity has developed Al2O3 and carbon (C) short fiber reinforced ZL109 alloyhybrid composites (Al2O3f+Cf/ZL109). The wear resistance and mechanicalperformance of the composites were systematically and deeply studied. Some newresults have been obtained as following. The content and tropism of Al2O3f as well asCf fibers determine the wear resistance of the composites. The interface andmicrostructure influence the mechanical properties, etc. Otherwise, the machinabilityof the composites has not been studied. In order to promote the application of thiscomposites, the studies on cutting properties and cutting mechanisms ofAl2O3f+Cf/ZL109 were systematically carried out in this paper. On the one hand, theresearches can promote the widely application of Al2O3f+Cf/ZL109 hybridcomposites at automobile industry in our country, improve the quality and servicelife of engines and the whole properties of the automobile, reduce the consumptionof the gasoline, and mitigate the pollution of atmosphere. On the other hand, becauseAl2O3f+Cf/ZL109 is a new type of short fibers reinforced hybrid composites, itscutting properties are ambiguous and need to be discovered. At the same time, as thecomposites has complicated and heterogeneous multiphase structure, its machiningprinciple is different from the other materials. So its plastic-elastic deformation andtool wear as well as surface quality during cutting process are still unbeknown andneed to be deeply investigated. Based on the former experiences, hereby, someresearches listed below were deeply and systematically carried out by author.1. The samples of Al2O3f+Cf/ZL109 hybrid composites have been prepared bymeans of squeeze casting method. To improve the distribution status of short fibersand the stability of the properties of the composites, the technical parameters, such asthe volume fraction of reinforcing fibers, technical parameters of squeeze casting andheat treatment parameters, were optimized. The results are as following: the volumefraction of Al2O3 short fibers is 12%, the volume fraction of carbon short fibers is 4%,the squeezing pressure is 110 MPa, the preheated temperature of the fiber preform is400℃and the pouring temperature of molten Al alloy is 720 ℃. The optimal heattreatment parameters are solution treatment for 4 hours at 510 ℃and ageing at 170℃for 4 to 8 hours.2. The cutting properties have been investigated comprehensively andsystematically. The influences of the volume fraction and tropism of short Cf andAl2O3f fibers on tool wear, cutting force as well as surface finish were studied andanalyzed. The research results are as following:(1) The style of tool wear is mainly abrasive wear on the back. The conditionsof tool wear get worse when the volume fraction of Al2O3f and Cf fibers increase. Theeffect of Al2O3f fiber on tool wear is stronger than Cf fiber. The degree of tool wearunder hybrid reinforcement exceeds that of single reinforcement, but the interactionbetween two fibers is not notable. During cutting process, the tool wear get worsewhen the feed motion is perpendicular to the axis of fibers compared to parallelmode. And the same conditions occur when the cutting speed grows. The tool weardecreases with the increase of the feed or cutting depth when the feed or cuttingdepth is smaller.(2) Cutting depth is the major influential factor to cutting force, feed is theminor factor and cutting speed is the least one. Under such conditions that thevolume fraction of fibers is lower, the cutting force increases with the increase of thevolume fraction of Al2O3f fibers, but decreases with the increase of the volumefraction of the Cf fibers. Otherwise, when the volume fraction of fibers is higher, thecutting force decreases with the increase in the content of two kind of fibers, amongwhich the main force fall off rapidly, even smaller than the branch force of cuttingdepth component.(3) The surface roughness decreases with the increase of cutting speed andincreases with the increase of the cutting depth and feed. When the volume fractionof fibers increases, the surface roughness decreases. When the feed motion isperpendicular to the axis of fiber, the surface roughness is smaller than that whenparallel to the axis of fiber. (4) PCD cutter is better one for Al2O3f+Cf/ZL109 hybrid composites because thecutting force, the amounts of tool wear and the surface roughness are small and thesurface integrality is high. 3. Based on the analyses on the adaptability of M.E.Merchant's cutting model ofsingle shear-plane to the Al2O3f+Cf/ZL109 hybrid composites by dislocation theory,the principles of cutting deformation, chip forming and breaking, and tool wear areall deeply investigated for the first time. Above all, the mathematical model betweentool wear and volume fraction of Al2O3f fibers is established. The research results areas following: (1) During cutting process, the behavior of fibers within shear-planecharacterizes fracture sheared by stress concentration effects because of dislocationblock and axial stress effects because of matrix distortion. The critical condition is: τ≤nGb s λSo the slippage deformation region is narrow. At this rate the M.E.Merchant's singleshear-plane cutting model is still suitable for Al2O3f+Cf/ZL109 hybrid composites. (2) The major factor that determines the degree of plastic deformation and theshape of slippage plane is total volume fraction of the two kinds of fibers. When thetotal volume fraction of fibers is higher, the fibers within shear-plane are mainlysheared to break by dislocation. The shape of slippage plane is regular and thedeformation occurs in a narrow scale. When the volume fraction of fibers is lower,only part of fibers could be sheared to fracture by the dislocation, therefore thedeformation occurs in a larger scale. (3) The forming principle of the chip of Al2O3f+Cf/ZL109 hybrid composites isshearing and slippage deformation. The effects of fibers on chip fracturing lie in theirinducing cracks and augmenting shear angle. Two of the effects decrease the lengthof chips. The volume fraction of fibers, cutting depth, and feed are the mainly factorsto control the length of chips. (4) The main reason for tool wear is micro-machine effect of Al2O3f fibersduring the cutting progress of Al2O3f+Cf/ZL109 hybrid composites. Cf fibers havedouble effects on the tool wear. In addition, the adhesive wear of ZL109 alloy on toolalso occurs when the cutting speed is very high. (5) By using the mathematically analytic solution, it has been established thatmathematical model between the volume of tool wear and the average width VB ofwearing zone on the back of tools when the front angle γ0 of tool is taken intoaccounted. W = h 1 VB2 2 ctgα0 ? tgγ0 (6) According to the wear mechanisms of Al2O3f fibers on tool and the dominantactions of Al2O3f fibers, the mathematical model between rear surface wear VB andthe content of Al2O3f fibers has been established as below. VB = 2 2ζδsin κr (ctgα0 ? tgγ0) πd2ap vtfVAl2O3 fIt has been proved that the consilient degrees between theoretical curves andexperimental curves are improved with the increase of the volume fraction of Al2O3ffiber. The theoretical value of tool wear VB conformed well to the experimentalresults when the volume fraction of fiber is high, and the error is smaller than 6%.Therefore, this model is suitable for tool wear prediction of Al2O3f/ZL109composites while the volume fraction of fibers is higher. 4. The drilling force and surface roughness increase with the increase of thevolume fraction of Al2O3f fibers because of the wear of drill bit. Moreover theinfluence of the wear of drill bit on drilling force and surface roughness is more thanthat of the volume fraction of fibers. Compared to the parallel manner, the tool wearand drilling force are both higher while the surface roughness is lower when feedmotion is perpendicular to the axes of fibers. 5. For the first time, Al2O3f+Cf/ZL109 hybrid composites have been machinedadopted compound vibration drilling method under low frequency. The cuttingparameters have been optimized. The feasibility by this method to improve themachining property is also discussed. The results show that the swing of 20 μm andthe frequency of 60~100 Hz are the optimal vibration parameters in drilling process.Compared to ordinary drilling manners, the compound vibration manner has some...
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