Study Of Microstructure And Performances Of Molybdenum Extrusion Die Material

In hot extrusion processing,the extruded metal contacts with extrusion die for a long time,and the temperature of extrusion as well as the extruded metal are always higher than the hammer forging die.The working temperature of extrusion die even reaches 600-800? in extruding nonferrous metals such as copper alloys and aluminum alloys.So in the process of conform extrusion,extrusion die needs forced cooling to avoid damaging the mold by excess temperature.Hot-work die materials for conform extrusion industry mainly include carbon steel,cemented carbide(cobalt cemented carbide and steel bonded carbide)and ceramic material currently.Each of the extrusion die materials are has its characteristic but lots of defects respectively,which always lead to the failure of extrusion die directly.Molybdenum alloys are commonly used for high temperature materials for their good physical and mechanical properties in high temperature.The study of Mo-WC-Co alloy mainly makes use of the self-lubrication of Mo,solution strengthening of Mo-W solid,toughness of Co and other phases(strong combining ability of Mo2 C phase and Mo basis)to ensure basic mechanics performances of the material for extrusion die.The alloy studied in the paper is prepared by powder metallurgy.The effect rule of different ball-milling,vacuum sintering and heat treatment parameters are studied by mechanics performance tests at room and high temperature,tensile tests,OM,SEM,BSE,EDS and XRD tests.In specific aspects of certain mechanics performances,the study mainly adds specific element to alloy(C)and the corresponding oxide(Al2O3)to adjust the alloy phase change so as to improve the mechanical properties of alloy in terms of specific mechanics.The comprehensive research results are as follows.(1)There isn't phase transition or alloying of composite powder in the process of ball milling and the phase transition occurs in when sintering temperature reaches 1420?,the last phase of sintered alloy samples are Mo-W solid,Mo2 C and Co2Mo4 C.Liquid Co element can dissolve W and C element,meanwhile provide the channel for the spread of the WC and accelerate the diffusion and decomposition of WC in the alloy until WC decomposes completely.Decarburization of WC mainly includes oxidation decarburization and liquid Co dissolution decarburization.Oxidation decarburization is mainly caused by the existence of O in composite powder.The high affinity of Co atoms to W and C atoms accelerates diffusion of W and C,which leads to liquid Co dissolution decarburization;The combination between Mo and C is stronger than W and C,so as to grab C atom from W2 C to a certain extent,which leads to form Mo2 C,meanwhile occurs solution between Mo and W.While Co2Mo4 C phase is mainly formed from precipitation of Mo2 C in liquid Co.(2)Active carbon(free)won't change the composition of the alloy and phases,but it can remove impurities of alloy,thus improve performances of alloy.The content of active carbon is in the scope of 0.2% and the strength and hardness of the alloy increases with the increase of active carbon content.The maximum of carbon content is 0.2%,the strength is 1171.7 MPaand hardness(HRC)is 53.4.When the content of active carbon reaches 0.25%,the strength and hardness of the alloy decrease sharply.Precipitation of Mo2 C occurs at grain boundary,blocking grain growth in sintering process and leading to grain refinement.Active carbon can reduce liquid Co to C of dissolved in the WC alloy and reduce the probability of WC decarburization.(3)Grain size and porosity of alloy decreases with the increase of Al2O3.Sintered density of alloy increases first and decreases then with the increase of Al2O3.The density reaches the top when Al2O3 content is 0.2%.The hardness of alloy rises with the increase of Al2O3 and tendency goes gentle when Al2O3 is more than 0.2%.Abrasion Resistance of alloy increases with the increase of Al2O3 content in a certain range.When Al2O3 content is 0.2%,the abrasion resistance of alloy reaches the top.The wear-resisting performance declines sharply when Al2O3 content is more than 0.2%.