Study On Microstructure And Properties Of Laser Additive Manufactured Co-Mo-Cr-Si Alloy

Co-Mo-Cr-Si superalloy is widely used in various industrial fields with harsh environment because of its high hardness,excellent wear resistance,oxidation resistance and corrosion resistance.Because of the complex process and high cost,the traditional casting method can not produce parts suitable for various working environments,which limits the application range of alloys.In this project,Co-Mo-Cr-Si superalloy is produced by laser additive manufacturing(LAM)technology.This technology can produce complex parts,effectively reduce the production cost of parts and improve production efficiency.In this study,besides analyzing the micro structures and properties of Co-Mo-Cr-Si alloy produced by LAM,different schemes of heat treatment are carried out to study the effect of heat treatment on the micro structures and properties of Co-Mo-Cr-Si superalloy.At the same time,the high temperature oxidation resistance of Co-Mo-Cr-Si superalloy at 800? and 1000? is also studied.The results show that the microstructures of Co-Mo-Cr-Si superalloys fabricated by LAM are mainly composed of cobalt matrix dendrites and eutectic network structure between dendrites.Cobalt matrix dendrites are composed of y-Co which has Face-Centered Cubic(FCC)crystal structure,while the eutectic structure is composed of y-Co and intermetallic compound Laves phase.After solution treatment,the original cobalt dendrites and eutectic network structure between dendrites disappear,and striplike and blocky Laves phases are precipitated on the Co matrix.The volume fraction of Laves phase in solution treated samples are higher than that in deposited sample.At the same time,with the increase of solution temperature,the grain width and volume fraction of Laves phase show an increasing trend.After aging treatment,a large number of granular Laves phases are precipitated on the cobalt matrix dendrites.At the same time,with the increase of aging temperature,the original eutectic network structure changes to block and fine strip.For solution plus aging treatment,both solution temperature and aging temperature have certain effects on the precipitation of acicular Laves phase.The precipitation of acicular Laves phase increases with the increase of aging temperature.No new strengthening phase was found in Co-Mo-Cr-Si alloy in different heat treatment schemes,but the morphology,size and content of Laves phase in Co-Mo-Cr-Si alloy can be effectively changed.The microhardness of Co-Mo-Cr-Si superalloy fabricated by LAM is 506 HV0.3.After solution treatment or aging treatment,the microhardness of Co-Mo-Cr-Si alloy increases significantly,while the microhardness of solution plus aging treated samples decrease slightly compared with that of solution treated samples.The size,content and morphology of Laves phase change with different heat treatment temperatures and methods,which affects the hardness of the samples.In the friction and wear experiments at room temperature,the wear resistance of solution treated samples is the best,and the wear resistance of each sample is also related to its hardness.Laves phase has serious brittleness at room temperature.It is easy to fall off from the matrix in wear experiments.The wear mechanism of Co-Mo-Cr-Si alloy is mainly abrasive wear at room temperature.In the friction and wear experiments at high temperatures(400?,600? and 800?),the wear resistance of each sample has been improved with the increase of temperature,which is mainly related to the oxide film formed on the surface of the sample.At 800?,dense oxide films are formed on the surface of each sample.Because the hardness of Laves phase increases with the increase of temperature,the phenomenon of selective wear is found at 800?.The reason why the wear resistance of aging and solution plus aging samples are worse than that of untreated and solution treated samples at high temperature may be related to the decrease of fracture toughness caused by grid and acicular Laves phases in microstructures.The high temperature oxidation resistance of Co-Mo-Cr-Si superalloy fabricated by LAM is closely related to temperature.The oxidation of the alloy follows subparabolic oxidation kinetics at 800?,and the oxidation resistance is better.The mass gain of Co-Mo-Cr-Si alloy is 0.70 mg/cm2and the mass gain rate is 0.146 g/m2·h under 48 hours of high temperature oxidation.Its oxidation resistance is poor at 1000?,showing a linear kinetics.The mass gain of Co-Mo-Cr-Si alloy is 7.41 mg/cm2 and the mass gain rate is 1.541g/m2-h under 48 hours high temperature oxidation.At 800?,the oxide layer of Co-Mo-Cr-Si alloy is divided into several layers.The outer layer is composed of CoCr2O4 and Cr2O3,the second layer is SiO2 and MoO3,and the inner layer is MoO3 and CoO.The continuous and dense Cr2O3 oxide film on the outer layer can effectively control the diffusion of O element to the inside of the material,and the Si element on the second layer can also reduce the diffusion of O element to the inside to a certain extent.Therefore,Co-Mo-Cr-Si alloy has good high temperature oxidation resistance at 800?.At 1000?,Cr2O3 is converted to CrO3 and volatilized continuously,which prevents the formation of continuous and dense Cr2O3 oxide film on the surface of the material which can prevent the diffusion of O element.At the same time,because of the poor adhesion of the oxide film,oxide peeling occurs,which causes the internal oxidation continuously,resulting in its poor oxidation resistance.