Research On Laser Cladding Integrated Forming Technology Of Compacted Graphite Iron

As compacted graphite iron(CGI)has excellent thermal conductivity,shock absorption,thermal fatigue resistance and casting performance,it has become an ideal material for manufacturing new high-power-density engine blocks,cylinder head and other major equipment.Limited by traditional sand casting technology,the CGI forming parts are inaccurate and cycle is long.Moreover,the casting technology cannot solidify functional structures with material gradients and functional structures for specific properties in different parts of the same casting.So that fatigue damage of affected CGI parts would lead to equipment failure as oxidative corrosion,impact wear,high mechanical load,high temperature load and other extreme cases.Though metal 3d forming by laser cladding provides a new technology for the rapid development of high-performance metal prats,the overall adding manufacturing of large-scale and complex CGI parts could not balance efficiency and cost.In view of the above problem,this paper presents a laser cladding integration forming process of high-performance CGI.A high-precision and low-cost CGI substrate casting is formed in digital pattenless casting technology,and additive manufacturing of high-performance alloy structures or functional features is then performed on the substrate casting by 3d laser cladding forming technology,which achieves the integration of "isomaterial-additive" forming and provides a new idea of high-efficiency and low-cost manufacturing of high-performance CGI parts.Aiming at the key problems of coupling and forming property control in the integration forming process,the functional structure of Ni-Co alloy formed by laser cladding on the substrate of RuT450 castings was studied as follows:The basic theory and mathematical characterization of high-performance CGI laser cladding integration forming were studied,and their geometric model,thermodynamic model and RuT450 casting substrate constitutive model were established.Numerical simulation research on coupling interface temperature field,stress field distribution and their influence laws of integration forming were carried out,and the matching of nickel-based and cobalt-based alloys for RuT450 substrate surface was analyzed.It was optimized that the theoretical range of laser specific energy for cladding was(4.44～7.11)×107 J/m2,which provided basic theoretical support for subsequent process tests and research on key issues.The key factors and theories of the integration forming were studied,and the influence of RuT450 casting microstructure,laser process parameters,and the thermal properties of cladding powder on the metallurgical bonding performance of the coupling interface were analyzed.The form of graphite in the casting structure was found to be the main factor causing metallurgical defects,and the ideal substrate microstructure and graphite were obtained when material was removed by 1.1 mm from casting surface,which was conducive to interfacial metallurgical bonding.The metallurgical bonding performance of the nickel-based alloy powder is better than that of the cobalt-based one with RuT450.The design of nickel-cobalt gradient structure could reduce the thermal stress of the cladding layer and increase the surface hardness of RuT450 integrate forming components to 410-420HV.Aiming at controlling forming defects at the coupling interface in the integration forming process,the main forms and distribution of interface defects were analyzed,the formation theories of typical defects such as pores and cracks were explored,the method to suppress crack defects was proposed by controlling the Invar alloy formed by the reaction of Ni-based alloy powder component and RuT450 substrate.By adding Al,Ti,Nb,and other elements with high affinity to C and O into the nickel-based alloy powder,the pore defects were suppressed,the Fe content in the powder was controlled at around 37.95%and the Ni content at around 45%.With the laser specific energy at 6.19×107 J/m2,a standard Invar phase(Fe0.64Ni0.36)with low expansibility was formed in the substrate phase of the cladding layer,which significantly reduced the thermal linear expansion coefficient and the thermal stress of the cladding layer,defects such as pores and cracks were effectively suppressed.Aiming at the regulation of the microstructure and properties of the coupling interface,the influences of the laser specific energy on the diffusion of interface elements,microstructure evolution and the mechanical properties of the bonding interface were analyzed,and the mass transfer theory of the fused deposition at the interface was clarified.The method to improve the performance of the interface metallurgical bonding was present by regulating the composition of the casting surface of pattenless process.The alloy layer obtained without graphite isolated the white iron produced by metallurgy reaction of graphite in casting matrix,which improved metallurgical bonding quality and mechanical properties of the coupling interface without preheating.Based on the above theoretical and experimental research on highperformance CGI laser cladding integration forming,the applicating verification of the integration forming process for high-performance engine cylinder head was carried out.On the cylinder head casting substrate of RuT450,the valve seat forming of Ni-Co alloy was realized,and qualified samples were obtained.The average surface hardness of the integration forming valve seat could be stably controlled at about 425 HV,increased by 14.9%compared with sintered one,which verified that the engine cylinder head prepared by laser cladding integration forming met the technical requirements.In conclusion,this paper has carried out in-depth researches on the laser cladding integration forming technology of high-performance CGI parts from the aspects of process theory,influencing factors and theories,defect control,structure and performance control,etc.It has been verified on the cylinder head of highperformance engine,which fully proves that this innovative method has great engineering significance for the rapid manufacturing method of high-performance CGI parts and solving the application bottleneck problem,which is the balance of low cost with high efficiency of the additive manufacturing of large components.