Study On Microstructure And Properties Of Engine Cylinder Head Remanufactured By Plasma Cladding

With the development of green low-carbon circular economy system and the rapid development of automotive industry,the remanufacturing industry of automotive parts is flourishing.As an important component of automobile engine,the cylinder head has complex structure and high manufacturing cost.It is particularly important to use advanced remanufacturing technology to repair the surface of the failed cylinder head so that it can restore or even exceed the original performance,and produce enormous economic and social benefits.Plasma cladding technology is a new and promising technology for surface modification and remanufacturing of metals,which has been widely used in water conservancy,coal mine,metallurgy,ocean,mining and machinery industries.Therefore,the application of plasma cladding remanufacturing technology to the repair of engine cylinder heads,prolonging its service life and maximizing its engineering application value is a new exploration and research idea of green circular economy.In this paper,the failure of engine cylinder head is taken as the research object to understand its working conditions and service environment,and the common failure modes and causes are systematically analyzed.The main failure causes and forms of engine cylinder head are fatigue cracking damage caused by thermal and cold cycles.Based on the service environment of cylinder head and the characteristics of substrate material,Ni45 alloy cladded layer was prepared on HT250 surface by plasma cladding technology.The optimum plasma cladding related process parameters were obtained by orthogonal experiment and multi-index range analysis.The microstructure and phase composition of the cladded layer under the optimum process parameters were analyzed by means of optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD).The nanoindenter,microhardness tester,universal testing machine and impact testing machine were used to test the elastic modulus,microhardness,tensile strength and impact toughness of the cladded layer,and the relationship between microstructure and performance was established.Based on the service environment of the cylinder head,the thermal fatigue properties of the cladded layer and the remanufactured layer were tested,and the mechanism of thermal fatigue failure behavior and thermal fatigue crack initiation and propagation of the coating was studied.The main conclusions are as follows:(1)The optimurn process parametetrs for preparing Ni45 alloy cladded layer on HT250 surface are:cladding current 55 A,scanning rate 1.0 mm·s-1 and powder feeding rate 10 g·min-1.The main and secondary factors affecting the comprehensive quality of cladded layer are:cladding current>powder feeding rate>scanning rate.(2)From the bottom to the top of the cladded layer,the rapid solidification characteristics are plane crystal,cellular crystal to columnar dendrite.The phase of cladded layer is mainly composed of ?-(Ni,Fe)?FeNi3?Ni22Si?(Cr,Fe)7C3 and Fe23(C,B)6.The heterogeneous diffusion of carbon atoms and the difference of thermophysical properties between graphite and matrix lead to the formation of complex structures of martensite,retained austenite,graphite,pearlite and secondary cementite in the bonding zone.The heat affected zone has not undergone the melting process,forming a mixture of martensite,retained austenite and graphite.The elements between the cladding interface and the substrate region are mutually diffused,and the Fe element in the interface is gradually reduced,while the Cr and Ni elements are in opposite distribution,The content of Si and C on both sides of the interface is not significantly changed,and the metallurgical bonding is good.(3)The elastic modul.us,microhardness,tensile strength and impact toughness of the cladded layer are better than those of the substrate.The tensile and impact tests of the remanufactured layer and the measurem,ent of the elastic modulus on both sides of the interface show that the remanufactured cladded layer is closely bound to the substrate,and the mechanical properties of the interface match well.(4)The surface crack growth rate of cladded layer and remanufactured layer is significantly lower than that of the substrate.After 500 cycles of hot and cold cycles,the average main crack lengths of the cladded layer and the rem,anufactured layer are 2.18 mm and 1.84 mm,respectively,and the average main crack length of the gray cast iron substrate is 7.3 1 mm,which is 3.3 times and 3.9 times of the cladded layer and the remanufactured layer respectively,and the number and width of surface cracks of the cladded layer and the remanufactured layer are obviously smaller than that of the substrate,indicating that the thermal fatigue resistance of Ni45 alloy cladded layer p repared by the plasma cladding technique is superior to the substrate and meets the performance requirements of cylinder head remanufacturing.At the same time,the reason for the excellent thermal fatigue resistance of the cladded layer is that the cladded layer has high strength and good oxidation resistance.The crack sources of cladded layer and substrate all originate from V-notch,and the cracks of cladded layer mainly propagate along the grain boundary b etween dendrites.