Wear and Rolling Contact Fatigue Behavior of Locally Laser Cladded Rail Material Using Self-fusible Alloy
With an increase in the axle load of trains,damage to the rails becomes more severe,decreasing their service life.Thus,local repair could be a solution for rail surface damage.Laser cladding is a relatively new additive manufacturing technology that can be used for local damage repair.However,wear and rolling contact fatigue(RCF)damage behaviors,particularly the damage at the boundary between the clad and unclad zones,have not been thoroughly explored.Thus,a pothole is cut off from the U75V rail sample(a roller sample)to simulate the local damage on the rail.Ni-,Fe-,and Co-based self-fluxing alloy powders(F103,Fe-Cr,Fe-58,Stellite 21,Stellite 22,and Stellite 23)are laser cladded at the pothole using a CO2 laser with a rectangular spot size of 7 mm × 1 mm,a laser power of 1.9 kW,a scan speed of 200 mm/min,and a powder feed rate of 15 g/min.The microstructure and hardness of the locally repaired rail materials are analyzed.The wear and RCF behaviors of the laser-repaired rail samples are studied using the twin-disc rolling test with a maximum contact pressure of 1.1 GPa,a slip ratio of 0.75%,and a rotational speed of 500 r/min.The number of cycles for each rolling test is 105.The friction coefficient,wear rate,depth,plastic deformation,and damage morphology are analyzed.The results showed that the cladded sample could be divided into three regions in the depth direction on the cross section or in the rolling direction on the surface:the clad zone,heat-affected zone,and substrate.Fine eutectic and dendritic structures are formed in the laser cladding.The hardness is higher than that of the substrate.The microstructure of the Ni-based clad is coarse,and its hardness is low.The microstructural sizes of the Fe-and Co-based clads are small.The hardness of the Fe-based clads is high and that of the Co-based clads has an intermediate value.During the rolling test,the friction coefficient exhibits an increasing trend during the running-in period and then remains stable.The stable friction coefficient is approximately 0.4 and shows no evident difference for samples with different clads.After the rolling test,the wear rates of the samples with Ni-and Fe-based clads are high,and those of the samples with Co-based clads are low.The wear depth in the clad zone is smaller than that in the unclad zone.The surface hardness of the samples is increased after testing,and plastic deformation of the microstructure is observed in the cross section.Compared with the uncoated zone,the thickness of the plastic deformation layer in the laser cladded zone(that is,clads)is smaller.With an increase in the original hardness of the cladding,the hardness after testing is increased;however,the hardening ratio and plastic deformation layer thickness are decreased.The damage mode of the laser-repaired rail is predominated by fatigue wear.In the unclad zone,the RCF crack length is large whereas the crack angle is small.In the clad zone,the crack length is decreased whereas the crack angle is increased.The RCF damage at the boundary between the clad and unclad zones is the most severe.The crack angle and depth at the boundary are greater than those in the clad and unclad zones.Comparing the six studied clads,notably,the Stellite 21(Co-based)cladded sample presents a lower friction coefficient,smaller wear depth difference between the cladded and uncladded zones,and better RCF resistance,making it more suitable for laser repair of local rail damage.The research results can provide theoretical and technical guidance for the application and optimization of laser cladding technology for local rail repair.