Study On Microstructure And Fracture Mechanism Of Motor Shaft By Laser Repair

The shaft part play a decisive role in the modern industry, and its work environment is becoming increasingly complex with the development of modern industry. It also has more strict requirements to the surface property such as wearing resistance, corrosion resistance, strength and so on. As a result, the rejection rate of shaft part is rising sharply. Laser repair has the following characteristics:high energy density, excellent controllability of heat input and accurate positioning of process. It also can be combined with computer numeric control technology to achieve the efficient repairing of shaft part. And the proper selection and arrangement of alloy powders can achieve the surface modification. Using laser to repair shaft part is becoming a trend. It has a large market potential and economic benefit.This project is to research and solve the fracture failure of motor shaft after laser repaired. The main research content is to analyze microstructure organization, phase composition and fracture mechanism of the repair layer to improve the process and guide the actual product.The base material of motor shaft is hot-rolled quality carbon structural steel (45steel), and the alloy powder used in the course of laser repairing is Fe26. Optical microscopy and micro hardness tester are used to observe and analyze the microstructure organization and the distribution of micro hardness in the repair layer. The laser repairing layer is divided into the cladding zone, heat affected zone and transition zone according to the microstructure characteristic. The cladding zone is showed as quasi eutectic structure. Plannar crystal, cellular crystal and dendrites are formed sequentially. The dentrite has thick grains and long trunk, its secondary nucleation appear frequently. These characteristics weaken the toughness of the cladding zone. The complete recrystal in heat affected zone has fine ferrite and pearlite, but the proeutectoid ferrite distribute in the partial recrystal zone. The micro hardness decrease from centre of molten poll to the base metal, and its numeral value in the cladding zone is much higher than in the heat affected zone. The highest numeral value is in the dentrite.X-ray diffractometer is used to analyze the phase composition in the cladding layer. There are main (Fe, Cr) solid solution (a-phase) and Cr-Fe solid solution (8-phase), and there are (Fe, Cr) more than the Cr-Fe. Secondly the cladding layer also contain small amount of solid solution CrFe4, Fe-Cr-Ni, (Fe, Ni), intermetallic compound Cr7C3 and so on. The solid solution Cr-Fe and CrFe4 are so brittle that can accelerated the brittlement of alloy.Scanning electronic microscope is used to analyze the micro structure, crack initiation mechanism and fracture morphology. There is main single phase (Fe, Cr) solid solution (a-phase) in the Plannar crystal of cladding layer. The basal of cellular crystal and dendrites are (Fe, Cr) solid solution and the relief phase is Cr-Fe solid solution.The micro crack belong to polygonization crack. It initiated in the cladding layer or base metal, located in the two-phase interface of solid solution (Fe, Cr) and Cr-Fe or in the solid solution (Fe, Cr). There are two propagated direction, and one is parallel with the interface of the transition zone, the other one is parallel with the cladding thickness-direction. Macroscopic morphology of the fracture zone is divided into mirror, mist and serrated zone. Its morphology is mainly made up of quasi-cleavage (QC),intergranular ereaks (IG) and dimple (DR). There is mainly intergranular and dimple in the mist zone, and serrated zone is similar with mist zone, but the dimple in the serrated zone is the subject of the fracture mechanism.