Research On Key Technology Of Engine Crankcase Main Bearing Block Fracture Splitting Processing

In the manufacture of engine, the processing procedures sub-body parts are verycomplex, the precise anastomosis and precise processing of surface are difficult to solve.The cracking processing technology is a kind of manufacturing technology which breaksthrough the traditional processing concept with the development of the automotive industryin recent years. This manufacturing technology makes precise positioning of sub-body partssurface possible. Compared with the traditional sub-body processing technology, crackingprocessing has the properties of process simple, saving equipment, well assembly quality,well loading capacity. It can reduce manufacturing costs and improve engine performance.Therefore, it has become the development direction of subdivision parts manufacturingtechnology. The engine cylinder block main bearing chock is a typical subdivision part.Multiple bearing chocks locate at interval collinearly and are connected by the base to forma whole. The engine cylinder block is cast using gray cast iron, vermicular cast iron andother brittle materials. The Mercedes-Benz, Fiat and other car's engine cylinder block is castfrom gray cast iron. In the cracking process, the core and the primary process is the designand processing of the cracking notch. The aim of prefabricating cracking notch is to produceinitiate cracking points and improve the stress concentration level. When the crackingthreshold is reached, the parts produce crack extension along the pre-fracture surface andthe cracking load significantly reduced.At present, the cracking notch is processed through using mechanical broaching andlaser cutting. By mechanical broaching method, the important cracking notch geometricparameters such as notch depth, notch field angle, curvature radius of notch root caneffectively be controlled; By laser cutting technology, the hardened layer can be formed inthe surface heat-affected zone of the material to increase the brittleness of the material andthe cracking load is reduced. In this paper, based on cracking processing key technologiesstudied previously and combing with the engine cylinder block main bearing chock theirown structural characteristics and material properties, the numerical analysis andexperimental research on the pulse laser machining gray cast iron cylinder block materialcracking notch is done and a certain amount of innovation outcomes are obtained. In thispaper, this work is mainly concentrated in the following areas:1.According to the fracture mechanics theory, the numerical model is constituted inaccordance with the engine cylinder block main bearing chock of Jetta. In this process, theboundary load is imposed according to the actual stress state and the contact state of the expansion bushing in the cracking process to make it accord with actual stress state. Themodel is local refined according to the characteristics of the local stress concentration in thecrack area. So the accuracy of the calculation is increased and the differences betweensimulation state and the actual stress state are reduced. According to the stress state, thecracking state is determined using maximum tensile stress criterion. Finally, the realcracking load is draw from an experimental. The result shows that the differences betweenthe calculation results obtained by simulation and the real values are only2.09%, whichprovide reference for making the process parameters. There are three factors of affectingcracking notch geometry, namely, notch depth, notch field angle and curvature radius ofnotch tip. Simulation analysis of three of the common cylinder block materials whichinclude gray cast iron HT250, vermicular graphite cast iron RUT380and ductile ironQT500-7is respectively carried out to obtain the effect disciplinarian of each factor on thecracking load, finally optimal values of cracking notch geometrical parameters of threekinds of cylinder body materials are determined.There are three factors of affecting cracking notch geometry, namely, notch depth,notch field angle and curvature radius of notch tip. For each factor, the simulation analysisis done to obtain effect disciplinarian of the cracking load. For nodular cast iron(model isQT500-7) engine cylinder block main bearing chock laser cracking processing, theoptimization parameters should be chosen as: when the processing method is broachingprocessing, the optimal value of notch depth is h=0.5mm, the optimal value of fieldangleα=60°, the optimal value of curvature radius of roots r=0.1mm; when the processingmethod is laser processing, the optimal value of notch depth h=0.5mm, the optimal value offield angle range α=0°, the optimal value of curvature radius of roots r=0.05mm. Theoptimization results provide a basis for constituting a reasonable parameters range and theselection of cracking loads.2.By using the interaction theory of laser and metal materials and studying thecharacteristics of laser cutting engine cylinder block cast iron material, comparative analysisof the effects of output mode of the solid Nd:YAG pulse laser and continuous laser on theprocessing effect of the cracking notch is made, which determines that the pulse Nd: YAGlaser is the best light source to process the cracking notch. The laser cutting parameters areanalyzed, which conclude peak power, pulse time, pulse frequency, cutting speed, auxiliarygases, gas pressure, work-piece surface roughness, cutting mode, etc. The effectdisciplinarian of the above laser parameters on notch depth and notch width is obtainedthrough the experiment of laser cutting grey cast iron HT250. As peak power decreases,cracking groove depth is reduced; as time pulse increases, cracking notch depth is increased;the change of pulse frequency has almost no effect on notch depth; the reduction of cuttingspeed can increase cracking groove depth, but the trend is not obvious; using pure oxygenas auxiliary gas cutting can greatly increase the depth of the cracking notch, and when thenitrogen is used as auxiliary gas, the cutting depth is the shallowest; increasing the pressureof auxiliary gas is helpful to deepen the cracking notch; the effect of increasing incidentangles on the notch depth is downtrend; improving the work-piece surface roughness canalso promote the increase of cracking notch depth; push cutting is beneficial to improvenotch depth. In the aspect of notch width, the cracking notch width is decreased with the decrease in peak power; as time pulse increases, notch width is increased; the changes ofpulse frequency and laser incident angle have no effect on the cracking notch width; thedecrease of the cutting speed could increase cracking notch width; using pure oxygen asauxiliary gas to cut can increase notch width, and when the nitrogen is used as auxiliary gas,the cutting width is the narrowest; as auxiliary gas pressure increases, cracking notch widthis increased; improving the work-piece surface roughness can also promote the increase ofcracking notch width; pull cutting can make cracking notch width increased.3.The significant of each laser parameter affecting the notch depth and width isanalyzed applying orthogonal experiment method and the analysis results of range andvariance are obtained. The results show that the pulse peak power is the most importantfactor which affects the notch depth and pulse width is the most important factor that affectsthe notch width. The processing parameters of cast iron cylinder block main bearing chockcracking notch are determined according to the cylinder block main bearing chock crackingtechnology principles and methods, cracking material properties, cracking technologyparameters and their impact law. This provides a reliable technical basis for the research anddevelopment of main bearing chock cracking technology in China.4.The microstructure of heat-affected zone, microscopic hardness and hardening layerdepth are studied experimentally using tool microscope, scanning electron microscopy andmicroscopic-hardness instrument. Properties of a pulse laser processing cracking notch arediscussed and brittle materials pulse laser processing cracking notch affecting the crackingtechnology and its superiority are revealed. The results show that three zones are formednear the laser spot active region: the cracking notch, resolidification zone and solid-statephase transition district; hard and brittle martensite and micro-cracks mainly produce in theheat affected zone. The structure becomes much finer. The hardness is significantly higherthan the hardness of the matrix. The microscopic hardness of material melted zone isapproximately850Hv and phase transition hardening area is approximately550Hv; thecylinder block materials pulse laser processing cracking notch hardened layer depthincreases with the increasing of laser pulse power and pulse width and decreases with theincreasing of scanning speed. The presence of the hardened layer thickness determines thevalue of follow-up process redundancy.5.Loading speed has a great influence on the cracking load and cracking quality. Forthe example of QT500material cylinder block main bearing chock, by means of simulationanalysis, the theory of cracking speed affecting cracking performance is studied. The resultsshow that when the loading speed changes in the range of100kN/S, equivalent strainchanges very small. But when the loading speed increases500kN/S, equivalent strainsuddenly decline. Thereafter with the increasing of the upload speed, the strain changeslittle. Now, the material enters the strain rate sensitive stage. The fracture strain is verysmall and the fracture quality is very well. When the loading speed is in the range of1kN/S~100kN/S, the root displacement of the crack notch and the top of the arcdisplacement bore (deformation) does not change too much, and when the speed increasesto500kN/S~2500kN/S, the root displacement of the crack notch and the top of the arcdisplacement bore (deformation) suddenly decreases, cracking could be greatly improved.