Microstructure Evolution And Performance Control Of Laser Additive Remanufacturing Ductile Iron Component

Ductile cast iron illustrates high strength and toughness,excellent impact resistance,shock absorption,wear resistance and corrosion resistance,and it is widely used in many important components in industry and engineering fields such as large naval engine and crankshaft.However,during the service of ductile iron,various defects such as sand holes,cracking,wear,corrosion and "block dropping" always occur inevitably,resulting in equipment failure.The remanufacturing of ductile cast iron has always been a hard problem in the industry field.Traditional welding repair process is very complex,high cost,environment unfriendly and difficult to meet the practical application of remanufacturing quality.Especially for the remanufacturing of high-precision ductile cast iron and high-performance ductile cast iron,traditional welding repair methods can hardly meet the remanufacturing requirements.Laser remanufacturing technology,as a new technology appeared in recent years,owns a lot of technical advantages that traditional welding repair methods can not match in controlling dilution rate,thermal damage and realizing precision remanufacturing.Therefore,laser remanufacturing provides obvious technical feasibility for highquality remanufacturing of high-performance ductile cast iron.In this paper,considering laser additive remanufacturing high-performance ductile cast iron,firstly,alloy powders which can match ductile iron in many aspects such as physical properties and mechanical properties suitable for the remanufacturing of ductile iron were designed.Microstructure evolution and thermal cycle characteristics during the remanufacturing process were determined.Interface optimization was successfully completed using component control.Chilled structure was eliminated by using powder pre-placed and powder pre-placed + powder feeding method.Mechanical properties were studied and the practical application of laser additive remanufacturing for the cylinder block of large marine ductile cast iron engine was carried out.In order to find out the complex phase evolution mechanism of ductile cast iron during laser remanufacturing process,phase evolution behavior of ductile cast iron during laser remanufacturing process was studied through single cladding,multilayer and the repairing of various grooves.Using ANSYS finite element method,the characteristics of thermal cycles during laser remanufacturing process were systematically studied,and the corresponding relationship between thermal cycles and structure evolution was established,and the evolution mechanism of partly melted zone and heat affected zone during laser remanufacturing process of ductile cast iron was also explored.Considering the remanufacturing process of large volume damage of ductile cast iron,the cracking behavior and mechanism of different groove types during laser remanufacturing process of ductile cast iron were systematically studied by combining the finite element method,and the process strategy to obtain minimum residual stress and strain was obtained.At the same time,powders with low expansion properties were designed and prepared,and the composite layers of NiCu alloy and FeNi36 low expansion alloy were obtained,which greatly reduced the crack tendency of ductile iron produced during laser remanufacturing process,and realized the laser remanufacturing of the pit with the depth of nearly 7 mm.To determine the behavior characteristics of interface region during laser remanufacturing of ductile cast iron,the distribution characteristics and diffusion behavior of interface elements were studied,and several shell structure models based on carbon diffusion of graphite nodule were established.Carbon diffusion equation of graphite spheres and shell growth equation of typical double-shell and single-shell structures during laser remanufacturing process were constructed.It provides a theoretical basis for the microstructure control of the interface.At the same time,combined with the finite element method,the thermal damage behavior in the interface zone was studied,and the influence factors of interface thermal damage were clarified.The corresponding relationship between thermal damage and structure was also established.A method of element control in the interface was proposed to further control the chilled structure of the interface.The interface structure was changed by C,Al and C/Al composite element control methods.The results showed that the chilled structure of the interface was significantly reduced,and the average thickness of the ledeburite shell around the graphite sphere was reduced from 30-50?m to about 10?m.The interface characteristic has been significantly improved.In order to further eliminate the chilled structure of the interface and control the formation of high-hardness martensite phase in the interface,normal laser cladding,powder pre-placed cladding and powder pre-placed + powder feeding composite methods were adopted in the laser remanufacturing of ductile cast iron.The results showed that the brazing interface and the micro-melting interface can be formed.The chilled structure of the interface can be eliminated completely.The martensite content in the interface decreased obviously,and changed from conventional coarse acicular martensite to fine lath martensite.The average and maximum hardness of the interface decrease obviously,and the maximum hardness of the interface zone of single pass can be controlled below 500 HV.Several typical interfacial characteristics were found during powder pre-placed cladding process,and the formation mechanism of those interfacial characteristics was also explored and explained from thermodynamics and dynamics.In order to study the mechanical properties and practical results of laser remanufacturing of ductile iron,a comprehensive evaluation was made on the hardness,tensile strength,impact toughness,wear resistance and color difference of the remanufactured ductile cast iron.At the same time,the practical application of laser remanufacturing for the cylinder block of large marine ductile iron engine was carried out,and various defects such as sand holes and wear of the cylinder block were successfully remanufactured.The results showed that after laser remanufacturing,no cracks and obvious pores could be found,and the properties of the remanufactured part were not lower than those of the substrate.High quality remanufacturing of high performance ductile cast iron was completely realized.