| Abrasive slurry jet polishing is a novel non-traditional machining method developed from abrasive waterjet technique.Having the advantages of cold processing,non-contact,energy conservation,environment protection and easy-to-control just like the abrasive waterjet technique,the abrasive slurry jet polishing can be more concentrated,even and stable in jet,which soon makes it an effective way of polishing the hard-to-machine material.However,it's not quite easy to use this technique widely for the immature theory and craft of the usage of non-Newtonian fluid on polishing under low pressure.The quantitative control and high quality polishing are still hard to realize.A multiple of fundamental questions related to the material and mechanism remain to be solved.The paper focus on the study of manufacture of high-quality polishing slurry of abrasive slurry jet polishing process,mechanism of material removal,the theory and craft of die steel polishing as well as the application of this technique on the pretreatment and aftertreatment of coated tool.The experiments and results are summarized as follows:(1)To get a high-performance polishing slurry,characteristics of slurry including dispersibility,homogeneity,stability,flow resistance and processing feasibility have been researched.The results show that the alumina solution with SAM1111 have an excellent stability.The RSH value decreases with the increase of setting time.Dispersing agent with its concentration value ranging from 1%to 3%shows a better dispersive behavior.Slurry added with PAM solution doesn't show a well behavior on suspension stability,but shows a obvious improvement after adding the PVA solution,especially when the concentration value ranges from 0.4%to 2%.The result are applied on the contrast experiments of the two methods on polishing the SKD12 die steel,acquiring a surface with roughness being 0.178?m.(2)In order to analyse the rheological behavior and jet characteristics of abrasive slurry jet polishing,the paper reports on the experiments using computer-assisted simulation method to compare the results.Constitutive equations to analyse the flow field both inside and outside the nozzle have been established based on rheological property and the measurement principle of slurry.The results show that the Herschel-Bulkley model fits well with the constitutive equation of abrasive slurry jet polishing.With the dispersing agent being added,alcoholysis degree of PVA1788 decreasing,and the testing area being high and narrow,slurry is localized in the second shearing thinning zone and the viscosity decreases with the increasement of shearing rate.The nozzle exit velocity of the slurry is trapeziform through the Fluent numerical simulation.It's also noticed that a 4.22atm impact occurs at the place where slurry and workpiece collide.The slurry becomes a free jet when it goes out of the nozzle exit.But the flow direction rotates 90° at the moment when the slurry hits the workpiece,turning to the direction parallel to the workpiece's surface.The speed of slurry shows a very tiny drop during the free jet stage and acute drop when it's approaching the workpiece's surface.(3)To study the erosion mechanism of the abrasive slurry jet polishing the ductile/plastic material,curve of single-point erosion pit section and surface morphology under different jet angle have been analysed.The results show that with the jet angle increasing,the shape of the pit section of SKD12 die steel first become flat,and then unimodal channel,and finally bimodal channel,and the curve of section of coated tool become increasingly wide,the shape of which is similar to a bowl.The shearing force along the radial direction is the main factor of the material removal of the SKD12 die steel.The material removal of coated tool is mainly completed by the hitting force in the center and the shearing force along the radial direction.The erosive surface morphology of SKD12 die steel includes scratch,turned edge,plough,concave and upheaval,while that of coated tool includes scratch,concave,brittle fracture,embedment and microcracks.(4)To establish precise polishing theory and craft of die steel,the paper adopted the low-pressure abrasive slurry jet of non-Newtonian fluid.It turns out that the surface quality are positively related to the diameter of the jet nozzle.The Ra of the workpiece is in direct proportion to the diameter of the abrasive grit,unrelated to the initial Ra value.The optimal combination of polishing parameters of SKD12 die steel has been acquired by orthogonal experiments.It's been found that the concentration of abrasive in slurry has a very powerful effect on surface Ra,and the standoff also has a strong influence over the working pressure.The influence extent of polishing parameters over the surface quality of die steel from large to small can be listed as:abrasive concentration,standoff,working pressure,feed rate,diameter of nozzle,polishing spacing,abrasive types and times of polishing.The Ra model has been established through the dimensional method where the average deviation and the standard deviation of the relative error is 3.94%and 3.16%,respectively.Based on the application of the abrasive slurry jet polishing on pre-treatment and post-treatment on coated tool,the microstructure and variation of binding force of coating during the abrasive slurry jet polishing process were studied.The influence of failure mechanisms such as tool wear and breakage was analyzed.The results indicate that,the surface roughness of the coated tool reaches the minimum value at the abrasive concentration of 75g/L,the diameter of abrasive grit of 3?m and the jet angle of 60°.The surface quality become worse when the injection pressure increases.It's been found that the coating without treatment on substrate of the tool shows a rather big flaw compared to those that get a polishing treatment on different extent.Coating with rough polishing has a unsmooth surface.Coating with semi-polishing has a few raised particles.Coating with fine-polishing has a rather even surface morphology.The surface roughness of thetool substrate is in proportion to the binding force of coating,which makes the tool which has a fine-polishing treatment show a best performance.The surface quality of coated tool is the best at the injection pressure of 0.4MPa,the particle diameter of 3?m,the jet angle of 30° and the abrasive concentration of 100g/L.The removal thickness of coating increases with the growing of grit diameter and concentration of abrasive.But with the increasement of jet angle,the removal thickness first grows up and then goes down.In the wearing experiment of coated tool,it's been found that under the same condition,tools with only one-time treatment perform the best,and those with double-times treatment perform the worst,which indicates that the low surface roughness of coated tool doesn't necessarily represent a good cutting performance. |
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