Research On The Effect Of Ultrasonic Impact On Compound Modified Layers Of Laser Cladding-low Temperature Ion-sulfurizing

Seawater hydraulic pump is the core power component in the hydraulic transmission system,and its working medium is the seawater with high vaporization pressure and low viscosity,which leads to serious corrosion and wear.Making composite solid self-lubricating coating by Ni-based metal-cermet laser cladding and ion-sulfurizing layer is an effective way to solve the above problems.However,the composition of Ni-based metal-ceramic is complex,and the content of Fe in the cladding layer is limited,which leads to sulfurizing time and temperature are obviously higher than that of low temperature ion-sulfurizing layer on the surface of iron,moreover,the uniformity of the sulfur is poor and the thickness is thin,and its self-lubrication performance is greatly reduced.In this paper,in order to obtain a thicker,more uniform,better self-lubricating performance sulfur layer,the surface of Ni-based metal-ceramic layer was treated by ultrasonic impact firstly,then making sulfur layer on the specimen after ultrasonic impact.A plastic deformation zone of about 300-400?m can be formed on the surface of specimen by ultrasonic impact.And the plastic deformation zone is divided into severe plastic deformation zone and second plastic deformation zone.In second plastic deformation zone,the dislocation accumulation near grain boundaries forms dislocation walls,then the coarse dendrites are divided into fine equiaxed grains;In the near surface area due to the strong impact,the fine equiaxed grains continue to evolve into rod-like grains perpendicular to the direction of shock force,then the rod-like grains are divided into nanometer grains,this is the severe plastic deformation zone and the thickness is about 5?m.After ultrasonic impact,the surface roughness of the cladding layer is decreased slightly,the hardness is increased a little,the tribological properties and the corrosion resistance is increased significantly.The ultrasonic impact and non-ultrasonic impact specimens are respectively subjected to low temperature ion sulfurization under the condition of that the temperature is 280?and the holding time is 2.5h,and the surface of the specimen with ultrasonic impact can be formed with 2.97?m thick of sulfur layer,which is about 2.25 times as high as that of the specimen without ultrasonic impact.The main sulfur phases of the sulfur layer are FeS and FeS₂.And the proportion of FeS in the sulfide layer of specimen with ultrasonic impact increases from 24.61%to30.49%,so the ultrasonic impact can promote the formation of FeS.The surface roughness of the specimen after ion-sulfurizing increased slightly due to the enrichment of sulfide particles,especially the the specimen with ultrasonic impact.And the friction coefficient of the specimen under the condition of dry friction is obviously reduced after ion-sulfurizing.In addition,the friction coefficient of specimen with ultrasonic impact is more stable,and has better tribological properties.Sulfide is preferentially concentrated at the boundary or defect where the adsorption energy is higher,and ultrasonic impact increases the number of grain boundaries and defects on the surface of the cladding layer,so the growth rate of sulfide is faster on the specimen with ultrasonic impact.With the development of the process of ion-sulfurizing,the sulfur layer grows from the grain boundary to the eutectic region of low melting point,and then to the dendrite region.When the specimen is covered by sulfide,the sulfide will continue to grow preferentially on the surface of the sulfide layer,but the growth rate becomes slow.In the process of ion-sulfurizing,the first part of the holding time is a period of rapid increase of sulfide content,in the later period,the active iron decreases,the reaction with sulfur weakens,and the sulfur content in the sulfur layer increases slowly,due to the continuous adding H₂S,the sulfur melting furnace is in a sulfur rich environment,and tends to generate FeS₂,which reduces the lubrication effect of the sulfur layer.The holding time of the normal specimen should be controlled at about 2h,while the ultrasonic impact specimen should be controlled at about 1h,it obviously shows that ultrasonic impact can significantly shorten the holding time and improve the efficiency.