The Surface Rolling Densification, Friction And Wear Properties And Rolling Fatigue Of The Iron-based Sintered Material

Fe-based powder metallurgy material and parts are widely used in auto industry. But the strength of PM material manufactured by traditional methods is reduced due to the residual pores. Rolling processing could improve the density and properties of material with low cost. Accoding to the development trend of the high density PM parts, aiming at the shortage of fundmental research on the surface densification technology, this work systematically study the surface densification technology, and investigate on the process, mechanism, friction and wear properties and rolling contact fatigue of surface densification of Iron-based powder metallurgy sintered material by using self-densigned rolling tool and equipment. Our research provides the basic knowledge and technology support of PM surface densification technology. The main results show that:Newly rolling tool and rolling equipment used to densify the material’s surface were invented. The magnitude of the rolling force when rolling is started and stoped can be measured accurately due to the facts that the rolling tool is fixedly arranged on the lathe. Rolling tool is longitudinally traveled to provide a load to the material surface. A density gradient layer is formed from the surface to the core of material. The rolling force stops decreasing in a short time. The plastic in the rolling process is occurred at the initial stage.The work investigated on the effects of rolling force(longitudinal feed), spindle speed and material density on the surface performance of Fe-2Cu-0.6C powder metallurgy material. Results indicate that the rolling force is the main parameter. The densification depth and surface hardness increase significantly as the rolling force increases. The spindle speed is the secondary parameter. The depth and hardness vary insignificantly as the spindle speed increases. The effect of material density on the surface densification decreases with an increase in the rolling force. The hardening layer depth is proportional to the rolling pressure, inversely proportional to the yield strength of material, and associated with the relative density and poisson’s ratio of material. A densified depth of 335 μm with a surface hardness of 315 HV0.1 is obtained at a rolling force of 2800 N and a spindle speed of 360 r/min. The tensile strength approaches 444 MPa which is improved by 75%. The elongation achieves at 4.5%. After rolling, more equiaxed dimples can be observed in the facture, and a white bright layer with a depth of about 10μm can be observed which is a long-strip tear ridge layerThe pores of rolled material are changed from sphere to line in shape with an increase in depth. The lamellar spacing of surface pearlite becomes narrow obviously. Pearlite is oriented along the rolling direction and the orientation of pearlite lamellar has been changed to parallel to the sample surface. Ferrite is stretched along the direction of rolling, and the grain is refined. The morphology of pearlite on the rolled surface is related to the angle between the orientation of the lamellar and the rolling direction. At a large angle, the pearlite is wavy, even is broken and granular pearlite is formed. Pearlite is network at a low angle.The sliding friction and wear properties of material are enhanced after rolling. The friction coefficient and wear volume of the rolled material are lower significantly than that of the unrolled material under dry friction. The shere stress of the rolled sample on the contact surface and in the subsurface is reduced significantly. Wear loss is caused by flake spalling and grooving. And the adhesive wear is reduced. Under lubricated friction, the friction coefficient and wear volume are lower than that of the unrolled material at a heavy load. Under lubricated friction with a heavy load and dry friction, when the friction direction is opposite to the rolling direction, the friction coefficient and wear volume of the rolled samples are higher comparing to that of those rolled samples that the direction of friction and rolling are same.The rolling contact properties of material are enhanced also after rolling. The rolling friction coefficient and wear volume of the rolled material are decreased significantly by 75% and 50%, respectively. Under light load, the wear of the rolled material is caused by delamination and ploughing. Under heavy load, the wear is caused by delamination and pits. According to the crack propogation model, the shere stress that is loaded on a crack to grow to a certain length in a given time is proportional to the elastic modulus. Comparing to the unrolled sample, spalling is occured at heavy load after rolling. The wear rate of the rolled sample becomes lower, and trend to stabilize at a low cycle. The wear is caused by delamination and ploughing at a low cycle, and delamination, pits and spalling at a high cycle. According to the crack propogation model, the crack propogation life is proportional to the elastic modulus square, and inversely proportional to the shere stress square. Saplling generates at a high cycle after rolling. The rolling contact fatigue of rolled sample is improved. Its characteristic life is 2.82 times of that of the unrolled sample. The failure modes of rolled sample are spalling and pits. But the spalling is tiner than that of the unrolled sample. And the delamination is not severe.