Study On Cutting Mechanism And Performance Optimization Of Tungsten Steel By Liquid Nitrogen Assisted Abrasive Water Jet

In view of the processing problems of "difficulty and low efficiency" encountered in abrasive water jet(AWJ)cutting cemented carbide,this thesis proposed a combined processing method of "liquid nitrogen assisted AWJ".Taking YG8/YG20 tungsten steel as the research object,a series of studies have been carried out to reveal the failure mechanism and fracture form of tungsten steel under this processing method as well as comprehensively improve the cutting performance of AWJ under this working condition.In this thesis,theoretical derivation,numerical simulation and experiment were combined.The heat transfer process of cooling tungsten steel with liquid nitrogen was studied.The numerical model of temperature field changes inside and outside of tungsten steel under liquid nitrogen impact was established.The method of liquid nitrogen impact to achieve ideal cooling efficiency was explored.On this basis,the micro abrasion damage mechanism of tungsten steel under the impact of AWJ at ultralow temperature was studied.The mathematical model of the micro process of cutting tungsten steel by AWJ at ultra-low temperature as well as the characterization and fracture model of material damage at macro level were established.Then,the structural characteristics of AWJ and its contact form with tungsten steel were explored.The calculation methods and influencing factors of the cutting accuracy and cutting speed of AWJ on tungsten steel assisted by liquid nitrogen were studied quantitatively.Finally,orthogonal cutting experiment was carried out.The micro scale observation and analysis of its kerf surface were carried out.The influence of process parameters and cutting methods on the surface morphology of tungsten steel was explored.The optimization scheme was proposed.The results show that under the cooling of liquid nitrogen jet,the tungsten steel sample can rapidly cool to the lowest central temperature of 153 K within 3s.Then the low temperature area diffuses around and the overall temperature field becomes stable after 20 s,forming a 10 mm wide "low temperature main influence area".Theoretical analysis shows that there is a basic temperature characteristic of "high point and low surface" at the cutting area when tungsten steel is cut by AWJ.Combined with the experimental results,tungsten steel is prone to ductile failure in the area with upper cutting depth and brittle failure in the area with lower cutting depth.AWJ will form smooth area and rough area after being shot from the nozzle.Adding polymer to increase the length of smooth area can improve the cutting accuracy,while reducing the depth of the site where the brittle failure occurs can increase the cutting speed.When the liquid nitrogen cooling temperature is controlled at-80℃,the best cutting performance can be obtained.There are many "peak" bulges on the ductile failure area,and the overall surface roughness is small.There are several "ridges" on the brittle failure area.The brittle failure area is smooth locally,but the overall surface roughness is large.The surface roughness can be reduced by 10.14-29.17% by adopting the optimization scheme of "inverted secondary reverse cutting" proposed in this thesis.In this thesis,the cutting process of tungsten steel with liquid nitrogen assisted AWJ was comprehensively studied from the perspectives of temperature characteristics,cutting and damage mechanism,cutting accuracy and speed,surface quality.The research results of this thesis will give certain guiding significance for the field of AWJ machining of high hardness metals.This thesis has 81 figures,18 tables and 103 references.