Study On The Strength And Toughness Of Water Cooling Jacket Die-casting Die And Its Cracking Behavior

Water cooling jacket plays an important role in the cooling system of automobile engine.It is made by high pressure die casting of liquid aluminum with water jacket die.Its cost and output are closely related to the service life of water jacket die.In the service process of die casting die,the surface of die casting die is not only scoured by high temperature,high pressure and high speed aluminum alloy liquid,but also subjected to periodic alternating stress.Generally,the service life of aluminum alloy die casting parts is about 100000 times,while the water jacket mold regularly forms cracks at the intersection of the two cylindrical surfaces of the mold.The service life is usually 10000 to 20000 mold times,and some of them have early cracking and failure after only using hundreds of mold times,which seriously reduces the product quality and production efficiency.Based on the hot cracking failure of the water cooling jacket die-casting die used in a mold factory,the early failure H11 steel water jacket die was selected and its microstructure was observed by light microscope;The macro mechanical properties were measured by room temperature impact test and hardness test;The crack morphology and crack tip elements were analyzed by SEM and EDS.Based on the microstructure,mechanical properties and crack characteristics,the causes of early cracking of water jacket die were studied.The hot working steel of EAF refining and ESR was selected.The differences of inclusions,segregation and carbides were observed and analyzed by OP,SEM and EDS.combined with room temperature impact toughness,the influence of metallurgical quality on the strength and toughness of hot working steel was compared and analyzed.The effect of hardness on toughness and isotropy of high purity DIEVAR steel was studied.The difference of crack initiation and propagation under different hardness was analyzed by Oscillographic impact test.The results are as follows1.The main reasons for the early cracking of the water jacket die are as follows:the stress concentration at the intersection of the two cylinder surfaces of the die;The toughness of hot work steel is insufficient due to the serious band segregation.During the service,the strength of H11 steel water jacket die is low.The single main crack initiated at the intersection of two cylinder surfaces intensifies the stress concentration.Coupled with the lack of toughness,the main crack expands rapidly.After 1250 times of cyclic use,abnormal cracking occurs and early failure occurs.2.Vacuum refining hot working steel,even H11 steel with low alloying degree,is prone to severe band segregation and massive eutectic carbide,and has more impurity elements,which significantly reduces the impact toughness of the material.The Charpy V-notch impact energy of H11 steel wire is only 15.9J.The results show that the ESR hot work steel has high purity and uniform chemical composition,which reduces the formation of large eutectic carbides.The strength and toughness of H13 steel with high alloying phase is higher than that of H11 steel.3.The results show that when the hardness of hot working steel is 43 HRC,the maximum bending strength,crack formation and propagation energy,toughness and impact energy of hot working steel are 27.9J;When the hardness is 47 HRC,the brittleness of the steel is large,the crack initiation is early,but the bending strength is low,resulting in the decrease of toughness,and the impact energy is about 18.0J;When the hardness is as low as 37 HRC,although the plastic deformation ability of hot work steel is improved,the bending strength is insufficient and the impact energy is the lowest,only about 12.5J.4.The isotropy of hot working steel,i.e.the ratio of transverse to longitudinal impact energy,is closely related to hardness.When the hardness of DIEVAR steel is43 HRC,the isotropy is 0.85;When the hardness is 47 HRC,the isotropy is higher,which is 0.92.When the hardness of hot work steel is high,the influence of segregation and inclusion on the toughness of hot work steel is less than that of hardness increasing.Therefore,although the impact energy of hot work steel is reduced by increasing hardness,the isotropy is higher.5.Better strength and toughness can be obtained by selecting hot work steel H13 or DIEVAR with high purity and high alloying degree and adjusting the tempering hardness to about 43 HHRC.The high strength and toughness of the water jacket die can delay the crack initiation at the intersection stress concentration of the two cylinders in the service process,and tend to produce multiple cracks,which is conducive to dispersing the stress and improving the life of the die.In addition,reducing the Si content of hot work steel and reducing the oxidation of hot crack tip can slow down the propagation of crack tip to a certain extent.