| The new type of compacted graphite iron(CGI)material can obtain higher strength under higher carbon equivalent,making the casting thin-walled lightweight,which plays a key role in achieving high emission standards and improving the reliability of the engine.However,when intermittently cutting CGI,thermal cracks caused by thermal stress will lead to tool chipping failure,which greatly reduces tool life.In view of this,this paper proposes to fabricate a new type of oblique gradient cemented carbide tool specially designed for milling CGI,which can inhibit the initiation of thermal cracks and improve the tool life.Based on the three-dimensional transient temperature field of the tool for high-speed milling CGI,the tool materials with high hardness and high strength and toughness were selected.Based on the three-dimensional transient thermal stress field of the tool for high-speed milling CGI,the thermal crack initiation area of the tool was predicted.According to the thermal crack initiation area,the gradient direction angle was accurately designed,and the WC-Al2O3 oblique gradient tool with good cutting performance was successfully fabricated.The successful fabrication of oblique gradient cutting tools to inhibit thermal crack initiation has important theoretical and practical significance for the efficient cutting of CGI,and provides theoretical guidance for the fabrication of other high-performance intermittent cutting tools.The three-dimensional transient temperature field model for high-speed milling CGI of the tool was established.Based on the heat source method and Fluent software,the heat flow time and the cutting ratio were set according to the actual milling experimental conditions.The UDF file was written to realize the change of boundary conditions in the cutting stage and the noncutting stage,as well as the change of heat flux density,loading area and cutting thickness with time in milling process.The non-uniformity of heat flux distribution on the rake face of the tool was considered,and the transient temperature field model for high-speed milling CGI of the tool was established.The temperature field model of the tool was experimentally verified by high-speed orthogonal cutting experiments.The error was less than 6%,indicating that the model had high accuracy.The three-dimensional transient thermal stress field of the tool during high-speed milling of CGI was obtained,and a new method for predicting the thermal crack initiation area of the tool based on the three-dimensional transient thermal stress field of the tool for high-speed milling CGI was proposed.The three-dimensional transient temperature field model of the tool for high-speed milling CGI was loaded by Ansys software,and the displacement boundary conditions of the tool were set according to the actual milling experimental conditions,and the three-dimensional transient thermal stress field for high-speed milling CGI of the tool was obtained.The temperature change law of cemented carbide tool during symmetrical end milling was simulated by using the intense laser simulation experiment.According to the actual cutting process,the heating time and cooling time of the laser heated tool were determined,which were respectively the same as the cutting time and the non-cutting time during the actual milling.The number of thermal shocks during the heating was the same as the number of thermal shocks when the tool failed during the actual milling.Through the thermal fatigue experiment,the thermal stress value of the tool when the thermal crack was initiated was determined.Combined with the three-dimensional thermal stress field of the tool,the thermal crack initiation area on the tool was determined.The cutting experiment showed that the predicted thermal crack initiation area was consistent with the thermal crack initiation range in the actual tool milling process.In order to fabricate oblique gradient tool material which can inhibit thermal crack,WCAl2O3 composite carbide homogeneous tool material with high hardness,high strength and toughness was optimized.In high-speed milling CGI,according to the temperature field model for high-speed milling CGI of the tool,the maximum temperature of the tool reached 909.4℃,and it was not suitable to only use WC-Co cemented carbide tool.Compared with WC-Co,Al2O3 had higher oxidation resistance,high temperature mechanical properties,excellent chemical stability,wear resistance and heat resistance.Using WC-Co cemented carbide as the matrix material,and optimizing Al2O3 component content,sintering temperature and holding time,WC-Al2O3 composite tool materials with high strength,toughness and hardness were fabricated.The WA5(90 wt%WC-5 wt%Al2O3-4 wt%Co-1 wt%Y2O3)with 5 wt%Al2O3 content had the best comprehensive mechanical properties.Its fracture toughness was 12.3±0.1 MPa·m1/2,flexure strength was 1385±65 MPa,and Vickers hardness was 22.8±1.6 GPa.It was found that in order to ensure the excellent comprehensive mechanical properties of WC-Al2O3 composite cemented carbide tools,the Al2O3 content was less than 10 wt%.This study provided guidance for the component design of WC-Al2O3 multilayer gradient carbide tool materials in the next step.In order to reduce the thermal stress of the tool and inhibit the initiation of thermal cracks,combined with the feasibility of the design of the gradient tool structure,the design criterion of the oblique gradient function tool material for milling CGI was proposed.Based on the orientation of the thermal crack initiation area of the tool,the gradient direction angle was accurately designed to be 71.6°.In order to improve the comprehensive mechanical properties of tool materials,the gradient layers and sintering process of gradient tool were optimized by vacuum hot pressing sintering process.The sintering temperature was 1450℃,the holding time was 30 min,and the gradient layers were 5 layers.According to the designed gradient direction angle,a new type of WC-Al2O3 oblique gradient cemented carbide tool(code name TDA-X)was successfully fabricated.The thermal fatigue resistance and thermal shock resistance of homogeneous tool material,non-oblique gradient tool material and oblique gradient tool material were compared and studied.The results showed that compared with the thermal fatigue performance and thermal shock performance of the three tools,the oblique gradient tool was the best,the gradient tool was the second,and the homogeneous tool was the worst.The friction and wear characteristics of homogeneous tool material YG6 and gradient carbide tool material TDA were compared and studied.The results showed that compared with homogeneous tool material YG6,the friction coefficient of gradient carbide tool material TDA and compacted graphite iron GJV450 was lower and the adhesive wear was lighter.It was the most suitable tool material for processing CGI.The cutting performance of the new oblique gradient tool for milling CGI was studied and the effect of the new tool on inhibiting thermal crack initiation was verified.Firstly,the optimal cutting parameters of the new gradient tool for milling compacted graphite iron GJV450 were determined by orthogonal experiment,aiming at optimizing the removal of materials while simultaneously meeting the quality requirements of semi finished surfaces.Then,under this cutting parameter,the wear and breakage life of nonoblique gradient tool TDA(gradient direction angle of 0°),oblique gradient tool TDA-X(gradient direction angle of 71.6°),TDA-XD(gradient direction angle of 45°)with different angles and commercial tool HNGX090520ML(SY)were compared and the tool failure mechanisms were analyzed.The tool life of TDA-X was the highest,and there was no thermal crack on the tool.The failure modes of TDA-X,TDA-XD and TDA tools were crater wear on the rake face,and the wear mechanisms were adhesive wear,diffusion wear and oxidation wear.The main failure modes of the commercial blade S Y were the large area coating of the rake face and the serious chipping of the tool tip,and the occurrence of thermal and mechanical cracks.By comparing the wear morphology of the new oblique gradient tool TDA-X and the commercial tool for milling CGI at higher cutting speed,it was verified that TDA-X not only played a significant role in inhibiting the initiation of thermal cracks in the tool,making the tool life longer than that of commercial tools,but also had good surface quality. |
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