Numerical Simulation And Experimental Study On Pulsed Laser Processing Fracture Splitting Notch Of Con-rod/Crankcase Bearing Block

Compared with the traditional splitting processing technology, fracture splitting processing has the advantages as follows:simplifying process flow, saving finishing equipment, better assembly quality and strong bearing capacity, etc., and it also has important significance to lower manufacturing costs and improve engine performance which has leaded to the development of processing technology of fracture splitting parts. Engine connecting rod and crankcase bearing block is typical splitting part. The structure of the anterior/posterior of the crankcase or that of bearing block on each partition is similar to the big end of connecting rod, but multiple bearings (four-cylinder engine has five and six-cylinder engine has seven ones) are arranged in a straight line with an interval and connected with base into a whole.C70S6 middle carbon microalloy pearlitic steel which belongs to ductile material is widely used in producing connecting rod for its excellent splitting performance, which is used as connecting rod material in 60 kinds of engines only in Germany, and it is the earliest forged-steel connecting rod material which is processed by fracture splitting machining at room temperature. C70S6 is also a common material used in fracture splitting machining connecting rod in China. Engine crankcase is made of gray cast iron, compacted graphite cast iron or other brittle materials. The engine crankcase of Mercedes and Fiat cars is made of gray cast iron.The core and the most important procedure in the fracture splitting processing is the design and processing of the fracture splitting notch. The purpose of pre-fracturing splitting notch is to produce slot gap effect and raise the stress concentration factor which can significantly reduce the fracturing load, so that the part could fracture along the pre-fracture surface. At present, the method of processing fracture splitting notch mainly includes mechanical broaching, wire cutting, powder forging and laser processing.Laser processing fracture splitting notch is a complex process containing laser beam, the work piece, and the surrounding environment. This paper, the target of which is to research pulsed laser processing of fracture splitting notch technology, expand the application range of fracture splitting technology and improve the quality of fracture splitting processing, has a research on the numerical simulation and experimental study on pulsed laser processing fracture splitting notch of C70S6 con-rod and gray iron crankcase bearing block. The paper focuses on the following aspects:1. According to the behaviour of the laser processing fracture splitting notch, a 3-dimensional transient temperature field mode was built for the YAG pulsed laser processing fracture splitting notch by the finite element method. Using APDL language formed a function as follows:QC*exp(-2*(({X}-x_center)^2+({Z}-z_center)^2)/(rb^2)) which was to achieve mobile loading of pulsed laser heat source; using if statement with a conditional branch and DO cycle statement achieved fixed-point and repeated loading of gaussian heat source during reaction stage and intermittent stage; and using a "Life and death cell technology " achieved analysis from the temperature field to morphology of fracture notch. The technical issues such as thermal parameters of materials, parameters of notching simulation, boundary conditions, latent heat and time step were analyzed. The pre-treatment process of temperature field simulation was completed.2. Temperature fields of the laser cutting fracture splitting notch with typical materials C70S6 and gray iron was simulated numerically, and temperature fields under different laser parameters and processing parameters were obtained. The change laws of the depth, width, Continuity, sharpness of notch root of fracture splitting notch were given. Combined with cutting experiment, simulation results were verified. The effects of positive and negative defocus amount, angle of the laser notching and feeding mode of laser cutting head on the processing quality of fracture splitting were discussed.The results of numerical simulation and the laser notching tests showed that:Among the factors which affect the depth of notch, the effect of the pulse power was the largest, spot size and pulse width was the second, and the minimum was scan speed. Among factors which affect the slot opening angle, the effect of the spot size was the largest, pulse power and pulse width took second place, and scanning speed almost had no effects. Among factors affecting sharpness of the roots, the effect of the pulse width was the largest, spot size took second place, and the other processing parameters had little effect. Ablation phenomena was caused mainly by the pulse power, pulse width and scan speed. The incidence angle of laser notching was smaller, notch depth was bigger, but the influence of eliminate slagging was adverse. Promoting laser cutting head was better than dragging when cutting.3. When the depth of the notch is a fixed value, enlarging the sharpness of root can increase the stress concentration factor, which is extremely important to improve the quality of the fracturing processing. According to the material status, energy balance and the change of hole shape, the process was divided into three stages, initial state, quasi-steady destruction and tail state; the effects of pulse width, waveform, pulse frequency, cutting speed on the notch root radius were analyzed; a new pulse waveform was designed to satisfy the request of notch root sharpness.Analysis and experimental results showed that a long pulse width combined with a reasonable cutting speed was the key to obtain a sharp fracture splitting notch. When the pulse width was set to be 1 ms and cutting speed is to be10 mm/s, the sharp fracture splitting notch could be obtained. Furthermore, for a deeper and sharper notch, it could be transformed into much long pulse processing by increasing the frequency. When the frequency was increased to 40 Hz and the pulse width was to 0.5 ms, the fracture splitting notch with a larger aspect ratio and a sharp root could be obtained. By comparison of the morphologies for the fracture splitting notch, it indicated that proposed process could achieve the sharp fracture splitting notch with higher quality by injecting energy reasonably into each stage in laser processing.4. Micro structure of heat affected zone of laser processing fracture notch with two typical materials was studied. The results showed that:①four regions were formed in the laser beam affected zone:fracture notch, regelation zone, solid phase transformation zone and matrix. The change from gray cast iron to small dendritic primary crystal Calais ledeburite in regelation zone occurred, C70S6 steel formed columnar grain and the gap of columnar grain boundaries and MnS particles. In the solid phase transformation zone the changes from cast iron to cryptocrystalline martensite and residual austenite were obtained, a part of C70S6 steel transformed into martensite and the others existed in the form of residual austenite. Pulled crack of martensitic in the heat affected zone during phase transformation partially occurred, and hardened micro-cracks in two materials were formed. Escaping from the gray cast iron surface, CO2 produces a lot of holes on the surface of the groove, and that is pores.②Laser heat affected zone was refined, the hardness of two materials after laser treatment was significantly higher than that of the base metal, and hardening effect was remarkable.③Groove depth and the existence of the thickness of hardened layer of specimens proposed the requirements of the choice of the follow-up allowances. With different laser parameters, the sum of gray iron groove depth and thickness of hardened layer should be between 0.524～0.885mm, the sum of C70S6 depth should be between 0.372～0.761mm. Machining allowances should be greater than the depth of notch depth 0.1mm or so.5. Common defects of cracking and pulsed laser processing fracture splitting notch were discussed and some control measures were raised. Common defects of fracture splitting notch processing are as follows:the depth of cracking groove is too large/too small, the local parts are too deep, length is less than/over ordered value, poor continuity, location accuracy, inconsistence of the depth of two sides groove, scaling loss phenomenon, sharp degree of the root is insufficient, etc., which would cause the occurrence of different kinds of cracking defects. Typical fracture surface morphology were analyzed and the results showed that:pyrolysis quenching treatment during laser processing cracking groove could effectively improve the fracture properties of ductile materials C70S6 and significantly improve the notch sensitivity of gray cast iron which is used in bearing block; the formation of hardened layer with certain thickness ensured the occurrence of brittle fracture of C70S6 steel.