| With the development of abrasive water jet machining technology, micro-abrasive slurry jet polishing technology is proposed. It is a new type of precision processing technology which is combined the set of fluid mechanics with surface technology. Micro-abrasive slurry jet polishing technology is high flexibile compared with other traditional machining techniques due to its characterization of low cutting force, no heat distortion, non-polluting, high material utilization ratio and polishing workpieces with almost all of the materials and geometry shapes, especially the small complex surface parts. Currently, it is only carried out some exploratory experimental study on the micro-abrasive slurry jet polishing technology in our country and abroad, and there is no theoretical system. In this paper, it is proposed that complex precision metal molds is polisheded partially and cutting amount is controllable by polishing basic theory and machining technology of micro-abrasive slurry jet, which is combined jet polishing technology and low pressure characteristics of non-Newtonian fluid. At the same time, polyacrylamide (PAM) polymer added into slurry can stabilize the jet beam and have effect of cluster. Long-chain polymers are added into micro-abrasive water jet and mixed with water in the fluidized bed to improve the properties of the slurry jet It is also studied the dynamics of slurry and abrasive particles motion, nozzle efficiency and system energy transfer efficiency, material removal mechanism of micro-abrasive particles erosion into the workpiece, effect factors analysis of stress and strain and equivalent plastic strain during the process of polishing mold steel 40CrMnMo7 with micro-abrasive slurry jet. The paper uses the research methods combined theory with experiment and computer simulation. The results are characterized by high-speed photography, impact measurement, scanning electron microscopy and three-dimensional surface morphology. Main researches and conclusions are described as follows:When micro-abrasive particles are mixed with water and the slurry is fluided in the mixing tank as the fluidized bed, they are raised in center position of the fluidized bed and decreased in the annular region near the wall. The rise and fall positions of the reverse zero point velocity are in the range of 0.6-0.7 times of non-dimensional radius, which is increased with the increasing of slurry viscosity and abrasive particle size. The slurry velocity and cross-section concentration distribution is parabolic along the radial direction of the fluidized bed.Established a micro-abrasive slurry jet computational fluid dynamics (CFD) model, The maximum slurry velocity in the nozzle exit is simulated respectively as 34.2,48.4, 59.3m/s under the condition of jet pressure 0.6,1.2 and 1.8MPa based on FLUENT analysis software. Simulation and experimental results and theoretical calculation results are in good agreement. With the increasing of jet distance from nozzle exit to the atmosphere, the dynamic pressure and slurry velocity increases continuously, up to the maximum, then linearly decreases, then is non-linear decline. The machining application region of micro-abrasive slurry jet is the region where the velocity keeps constant and linearly decreases. In the process of micro-abrasive slurry jet, abrasive particles is firstly accelerated until its velocity is close to the slurry velocity, then the slurry maintains a higher acceleration than abrasive particles when they are advancd simultaneously in the concentration tube, finally the slurry begins to slow and abrasive particles continues lower acceleration until the both velocity reaches the same when they are advanced from nozzle exit to the atmosphere. The slip velocity of slurry and abrasive particles is decreasing with farther away from the exit, until they converge to zero.By measuring impact on the workpiece of Cr40MnMo7 materials and nozzle efficiency of different jet parameters, the maxium signal intensity and peak size of impact force is abrasive slurry jet added polymer, next abrasive slurry jet, then pure water jet, and the difference of impact force between micro-abrasive slurry jet and pure water jet is about 20%. Similarly, one can conclude that with jet pressure increasing, the nozzle efficiency will reduce, the ratio of jet impact force and hydraulic energy will reduce, too. The impact force calculated theoretically is 20 percent larger than that measured actually. Therefore, the nozzle efficiency is evaluated by the ratio of jet impact force and hydraulic energy, and the nozzle structure is also optimized by it.As for the research of materials erosion processes and erosion mechanisms, we researched the effect factors like impact velocity, impact angle, shape and number of abrasive paticles, etc. After the workpiece is impacted with abrasive particles, the relationship between maximum residual stress and maximum stress and impact velocity is linear, the erosion rate and impact angle is similar parabolic relationship, erosion stress peaks in a 75°impact angle, strain peaks at 55°, the volume removal rate remains stable with three particles, the stress and material removal rate is greater with abrasive particles with sharp corner shape. When the impact angle is less than 15°, erosion mechanism of the workpiece is shallow plowing and particle curling, it is mainly micro-cutting and deep plowing when the impact angle is greater than 15°and less than 75°, there are material pits and uplifts removal work when the impact angle is greater than 75°and less than 90°. The material removal amount is increasing when the incident angle increase from 30°to 60°, until reaches the maximum at 60°～75°, then is reducing with the increasing of the incident angle from 75°～90°.Compared micro-abrasive slurry jet polishing 40CrMnMo7 mold steel surface with micro-abrasive water jet polishing, the former is embedded less abrasive particles into the workpiece. It is embedded more abrasive particles into the workpiece with the larger material removal polishing with fixed point than that of relatively less materials with a slip velocity of nozzle. The abrasive slurry jet polishing added long-chain polymer PAM is different characteristic with different jet distance. The steep cutting depth is deeper in the transition region. The depth of material removal is 1.5-2 times than ordinary abrasive water jet in the range of smaller jet distance, but it is obvious different in the range of larger jet distance (typically greater than 40mm). The surface roughness Ra of the workpiece reaches 0.049μm after it has been polished 45 minuters with #200040CrMnMo7 abrasive jet added polymer. In micro-abrasive slurry jet polishing, micro-abrasive particles can be reused, the suitable recycling is a continuous loop cycle.
|
PDF Full Text Request