Modeling And Simulation Of Fluid Field And Temperature Field Coupling System Of Laser Grooving

New connecting rod fracture splitting is highly valued and widely used indomestic and foreign markets with its advantages such as high precision andefficiency. In the first stage of connecting rod fracture splitting, two notches should beprocessed symmetrically with same shape at predetermined fracture locations. Then, anormal stress perpendicular to the predetermined fracture surface is applied. Stressconcentration forms near the notch, which leads to the brittle fracture of the big end ofconnecting rod under low stress. The three-dimensional natural fracture surface can beused as positioning and the joint surface to complete high precision assembly. As thefracture splitting notch is one of the essential conditions to realize brittle fracture inconnecting rod fracture splitting production, the sizes and processing method of thenotch have important effects on fracture splitting quality. Pulse laser cutting iscurrently the most advanced notch processing method.Laser cutting is a method which makes the high energy cutting converge on theworkpiece surface to melt and vaporize the metal and forms the notch by removingthe molten metal and metal vapor with assistant gases. The laser cutting mechanism iscomplex, and the pulse laser grooving belongs to blind notch processing, so themachining accuracy and quality can be influenced by many factors such as the natureof material, laser, processing and assistant gases parameters.Considering the interaction of assistant gas fluid field and temperature field, themechanism of a keyhole forming process by pulse laser was discussed and analyzed inthis paper. A numerical model was built to realize the simulation of keyhole formingprocess. Then, the effects of laser and assistant gas parameters on the sizes and qualityof the keyhole were studied, which can improve the quality of the notch and even thefracture splitting process and provide the basis for reducing product defects. Inaddition, pulse laser is also used in precision machining field such as laser punch andindentation, so the analysis has the leading meaning to the related precision machining field.The main research work and achievements of this paper are as follows:(1) A numerical model was established for a single keyhole forming process,and the depth adaptive heat source model was built with the help of secondarydevelopment program. The interaction between assistant gas fluid field andtemperature field during the keyhole forming process was considered. The simulationrealized the forming process of the keyhole under the joint action of the two factors,and its validity was proved by experiment.(2) The structure characteristics and functions of assistant gas fluid field duringthe process were researched. The results show that assistant gas expands rapidly afterflowing out of the nozzle, and most gas escapes form the both sides of the nozzle,while only a little gas hits the workpiece surface and some of it flows into the keyholeto promote the keyhole formation. In the flow process, strong turbulence forms onboth sides of the nozzle and around the keyhole, affecting the flow and deslaggingfunction of gas. When the gas escaping from the keyhole interacts with the comingflow gas, a Mach disk may form. The existence of Mach disk hinders the gas fromflowing into the keyhole and reduces the effective function and utilization of assistantgas.(3) Using the single factor variable method, the distribution characteristics ofassistant gas fluid field and its effects on the sizes and quality of the keyhole underdifferent inlet pressure and back pressure were studied. The results show that theincrease of inlet pressure within a certain range can lead gas pressure and velocity torise, which can enhance the deslagging function of assistant gas and increase the sizesof the keyhole, but if the inlet pressure is too high, the sizes of the keyhole maydecrease and there may be slags because of eddy current and other factors. Increasingthe back pressure within a small range can improve the utilization rate of assistant gasand the forming status at the bottom of the keyhole, but reduce the keyhole diameterat the same time. When the back pressure becomes larger, the gas velocity decreasesobviously and the keyhole forms with poor quality.(4)The dynamic change of the temperature field and the updated situation ofheat source model over simulation time were analyzed. The energy concentration andsmall heat affected zone of laser processing was verified by the simulation.(5) The effects of laser power and focus radius on temperature and the sizes and quality of keyhole were analyzed, as well as the effects of assistant gas parameters onthe temperature field. The results show that the increase of the laser power cansignificantly increase the keyhole sizes, but if the power is too high, the amount ofslag increases slightly. The decrease of focus radius makes the power moreconcentrated, which leads the keyhole depth to increase and the keyhole diameter todecrease at the same time. The small diameter of the keyhole influences the functionof assistant gas in the keyhole and results in the slight reduction of keyhole depth.