Study On Machining Mechanism Of Stainless-steel Tube Inner Wall With Magnetic Compound Fluid

With the development of science and technology,the demand of stainless-steel tubes with ultra-smooth internal surfaces is growing.Stainless steel tubes have a wide range of applications for aerospace,petroleum engineering,biomedical and other fields by virtue of the low thermal conductivity,good insulation performance and safety and hygiene.It is difficult to meet the needs of the use of the industry by the traditional mechanical polishing.Afterwards,the magnetic compound fluid(MCF)polishing technology is regarded as a flexible polishing method to obtain the ultra-smooth surface,and it has attracted extensive attention due to its advantages of controlled material removal,no sub-surface damage layer and green environmental protection.However,the application of this technology in the processing of the inner wall was still flawed and limited by the geometry of the tube.The polishing efficiency of MCF polishing tools in the inner wall of tubes was significantly reduced due to the low abrasive utilization during the polishing process.To improve the processing efficiency,the following aspects of the research were done in this paper.(1)To investigate the influence of the type of magnetic field distribution on the distribution of MCF slurry in the inner wall of the tube,the industrial grade N52 neodymium magnets were selected and the distribution of magnetic lines in the inner surface of the tube under the different combinations of permanent magnets were analyzed using Maxwell software.At the same time,the morphological characteristics of the MCF slurry in the inner wall of the tube under the given experimental conditions was observed using optical camera.The results show that the optimal distribution of magnetic field and the corresponding MCF shape can be obtained when the permanent magnets are distributed at an angle of 120° with the magnetic poles N-S-N or S-NS.In addition,the distribution of MCF slurry in the tube varied with the reciprocating motion of any N pole of the three permanent magnets(or any S pole of the S-N-S distribution).Based on the results of the magnetic field analysis,a polishing apparatus was constructed to realize ultra-precision polishing of the inner surface of tube,meanwhile,a model of material removal under a static magnetic field was developed according to the polishing apparatus.(2)The optimal processing parameters were investigated by the designing four-level orthogonal test using MCF slurry under a static magnetic field,afterwards,the material removal model was established and validated by polishing experiments.The results show that under the static magnetic field,an excellent processing performance of MCF slurry containing the abrasive particle with a diameter of 3 μm and the iron powder with a diameter of 5 μm can be obtained when the rotation speed of workpiece is 1500 r/min.After polishing,the surface roughness of workpiece was reduced by 47.9%.In addition,the depth of material removal was1.47 μm after MCF polishing.Then,the theoretical depth of the material removal calculated according to the "chain length" model and is 5.76μm and 1.54 μm,respectively.It is obviously found that the error between the experimental and theoretical with "single-layer BTC structure" model is 4.55%,which is lower than 5%.(3)The effects of dynamic magnetic field on the machining performance of MCF polishing tools were investigated using the optimum processing parameters obtained from static magnetic field.Then,the influences of dynamic magnetic field generated by the different movements of permanent magnets on surface roughness Ra on the inner wall of tube were investigated.The polishing performance of MCF slurry with a dynamic permanent magnet decrease compared with that with a static magnetic field,which is due to the polishing time of abrasive particles decreased.When the frequency of reciprocating motion f is 0.050 Hz,the smoothest surface with can be obtained,furthermore,the surface roughness Ra decreases from 0.277 μm to 0.158μm after 90 min MCF polishing.The decrease rate of surface roughness Ra is 37.4%.However,the polishing performance of the MCF slurry was significantly improved compared to the static magnetic field,when a pause of dynamic permanent magnet near the workpiece side.After 90 min MCF polishing,the surface roughness Ra decreases from 0.279 μm to 0.131 μm,and the decrease rate of surface roughness was 53.1%.In summary,the smooth surface of inner wall of the stainless-steel tube was successfully obtained using MCF slurry with an asymmetric magnetic field caused by three permanent magnets evenly distributed around the workpiece.The material removal model of "single layer BTC structure" can be used to predict the depth of material removal during non-uniform magnetic field assisted polishing of the inner wall of a tube.The MCF polishing technology for with a dynamic magnetic field provides a solution method to overcome the low utilization rate of abrasive particles in the polishing of the inner wall of a tube.