Study On Tapered Micro Nozzle Fabrication Based On Abrasive Flow Machining

Abrasive Flow Machining technology is a relatively new method of surface finishing process, in which the work-piece is cut microscalely by viscoelastic fluid abrasive under pressure. AFM was originally developed for aerospace industry and was introduced to engine applications, including injector hole finishing. Proper AFM process can improve the surface quality and discharge coefficient of the nozzle. However, an excessive process can also cause enlargement of the nozzle hole as a side effect, which can adversely affect emissions. Based on this, a lot of research work was done by experiment observation or analysis to get qualitative conclusions, which has obtained certain achievement. However, injector nozzle geometry variation due to AFM process parameter changes is not fully understood.Based on full investigation of the micro nozzle hole processing technology in domestic and international research, deep theoretical analysis and experimental research has been carried out, which focus on the equipment configuration of micro hole processing and results test, the numerical simulation of cutting process and processing technology. The main research contents, the methods and conclusions are as follows: (1) Based on the principle of the abrasive flow machining technology, technical composition, characteristics and influence factors, the flow form of the fluid abrasive in a typical circular pipe has been selected to discuss, getting the motion equations of fluid abrasive in the direction of z, r and θ under cylindrical coordinates. And the collision model of a particle in the work-piece surface has been established through mechanical analysis of single grit, obtaining the conclusion that grits are firstly pressed on the surface of micro hole and then plough to cut microscalely is the effective movement pattern, which lays the theoretical foundation for equipment configuration of AFM and simulation of processing.(2) The processing method of drilling holes and AFM subsequently was selected to realize the purpose of chamfer, deburring, and inverted cone shaping through theoretical analysis and experiment of the existing AFM technology and inverted cone shaping technology of micro hole. CNC machine tool and Japan tungsten steel drill was chosen to be drilling equipment, and one-way flow of AFM machine tool was used. Finite element analysis was simulated to the work-piece, getting the conclusion that the deformation was in elastic range. Finally, inlet diameter and outlet diameter of micro holes were measured through scanning electron microscope, and the taper and surface profile of sectioning were measured through digital microscope, which provide experimental verification for the size and shape measuring of the micro holes.(3) The correlate theory of fabricating taper holes on nozzles and the fluid dynamic theory of FLUENT software were deep studied, then Liquid-solid two phase flowing model and Realizable k-ε turbulent model have been used for numerical simulation, achieving the streamline picture of abrasive fluid in the process. In order to describe the internal variables of fluid more intuitively and quantitatively, micro holes have been selected as the research object, getting the pressure and velocity distribution nephogram as well as curve of particle phase flowing on the hole wall along the flow direction, which provided theory support for subsequent calculation of microscale cutting.(4) The relationship equation of micro hole removal by AFM and the characteristics of grits were described, namely the experience formula of PRESTON equation in surface finishing process, and the cutting depth coefficient algorithm was deduced. By comparing the calculated and tested taper degree of micro holes, and the error is in a reasonable scope, then the rationality of the calculation method was verified. Many experiments about different factors affecting the AFM were carried out, then pressure as well as velocity of particle phase changes were analyzed under the condition of different inlet flow and different abrasive viscosity, and then the taper degree of micro holes were calculated. After multiple sets of experiment comparison, the influence of the parameters changes of AFM process on micro hole fabrication was analyzed, which provides reference to the actual micro hole shaping.