| As a cold cutting technology,the abrasive water suspension jet(AWSJ)is used in many challenging scenarios such as bomb disposal and underground cutting in coal mines.However,sparks produced in the cutting process severely threaten the safety and limit AWSJ applications.This thesis studied the mechanism of spark generation and supression methods of AWSJ cutting of metal.The main content includes two parts.One is the research on spark generation mechanism.Firstly,study of the energy transmission process during abrasive particle abrasion of metal was conducted based on the energy conservation law and deformation energy theory,and the maximum transient temperature model of metal under the impact of abrasive water jet was established.Secondly,numerical simulation of metal abrasion under the impact of multiple abrasive particles was carried out to verify the maximum transient temperature model of metal under the impact of AWSJ using the nonlinear dynamics and finite element theory.Finally,the verification experiments of AWSJ cutting of different metals were carried out.The other is the research on the law of spark generation and its suppression methods in the process of AWSJ cutting of metal.Experimental study on the mechanism of spark generation in AWSJ cutting of metal was carried out,and a model for predicting the spark density grade was established using mathematical method such as linear regression etc and used to predict the influence of impact parameters,say,abrasive hardness,metal hardness,jet pressure,target distance,abrasive particle size,on the regular pattern of spark generation.Additionally,methods of spark suppression were explored by changing the property of abrasive water jet,namely,adding different kinds of additives into the abrasive water jet in cutting experiments respectively,the additives with better spark suppression effect were obtained by comparison of experiment results.The following main conclusions were drawn from the researches that were carryed out:(1)The mathematical maximum transient temperature model of metal under the impact of abrasive water jet can well match the phenomena of sparks observed in the experiments.The mathematical model can reveal the internal relation between the maximum transient temperature of the impacted metal and the jet parameters under the impact of abrasive water jet with different jet parameters,and can be used to predict the critical pressure under which spark will appear during AWSJ cutting of different metals.(2)The dynamic process of multiple abrasive particles impacting different kinds of metal surfaces at high speed and the temperature distribution on the metal surfaces were demonstrated through numerical simulation using the commercial software ANSYS-Auto DYN.The results of numerical simulation show that the maximum transient temperature of different metal surfaces is in good agreement with the results of discrete numerical analysis of the mathematical model,with errors less than 5%,greatly meets theoretical expectation from discrete numerical analysis.(3)Spark density grade is related to impact parameters in the experiments of AWSJ cutting of titanium alloy,among which the influence of jet pressure on spark density grade is much greater than the other four factors,abrasive hardness,mental hardness,target distance,and abrasive particle size.Decreasing abrasive water jet pressure is an effective method to reduce spark density grade.A model for predicting spark density grade of AWSJ cutting of titanium alloy was established,and the model can quantify the effect of impact parameters on spark density grade.(4)The changing trend of spark density grade with jet pressure remains unchanged with the adding of polymer,but its value increased by 15.438 on average,which is not conducive to the supression of spark generation.The changing trend of spark density grade with jet pressure remains unchanged with the adding of extinguishing agent,and the addition of two kinds of alkali metal salts reduced the spark density grade by190.625 and 214.656 on average respectively,both are conducive to the supression of spark generation,and the spark supression effect of easily decomposable extinguishing agent is better than the other extinguishing agent.The changing trend of spark density grade with the jet pressure remains unchanged with the adding of flame retardant,but its value decreased by 184.813 on average,which is conducive to the supression of spark generation.The easily decomposable extinguishing agent is the best additive to supress the spark generation in AWSJ cutting of titanium alloy.The findings of this study can supplement cutting mechanism of metal with AWSJ,provide theoretical basis and experimental support for AWSJ safe cutting of metal in dangerous conditons,and make significant contributions to science and engineering of AWSJ cutting technology.There are 44 figures,21 tables and 130 references in this dissertation. |
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