Preparation Of Fe-Al Based Laser Cladding Coating And Its Toughening And Anti-wearing Mechanism

Owing to the advantages such as low cost,low density,excellent oxidation resistance,outstanding corrosion resistance,and high specific strength,Fe-Al intermetallic compound is a new generation of structural material used in a high temperature environment,which can be applied in chemical,mechanical and energy fields.However,the poor tenacity restricts the manufacturing and processing of bulk Fe-Al intermetallic compounds,which has become a main issue to promote its practical applications.To overcome its drawbacks and to maximum the advantages,in this thesis,a new idea to prepare Fe-Al coating and to enhance its plasticity came out.Hence,a novelty surface technology--laser cladding is explored to prepare a Fe-Al based coating on the surface of 1045 steel in this thesis,together with in-situ grown Al₂O₃ nano-particles reinforcement and Cr alloying to further strengthen and toughen the Fe-Al coating,which have been considered as effective methods to reduce the formation of cracks,and promote the dislocation sliding as well.The novelty surface technology developed in this thesis not only can make good use of the advantages of laser cladding including high efficiency,easy-operation and low deformation,but also can obtain high performance and multi-functionalized alloyed Fe-Al coatings with interfacial compatibility with the substrate,excellent mechanical properties,and outstanding wear resistance.As a result,the use of noble alloyed elements can be avoided and the total cost can be reduced.More importantly,the work developed in this thesis can broaden the application of Fe-Al intermetallic compound,which is significant in terms of practical application and economic effectiveness.According to the Fe-Al phase diagram and the features of laser cladding,the ratio of the metal powder as feedstock was designed as Fe:Al=71:29 at%.Aiming at practical applications,a highly effective and easy-operative mechanical alloying technique was chosen to prepare the additive metal feedstock.The effect of the high energy ball milling time on the phase,morphology and microstructure of the Fe-Al powder and the corresponding laser cladding layers were studied systematically.The mechanism of the structure evolution of the Fe-Al powder during the ball milling process was proposed.In addition,the reaction and diffusion model of the Fe-Al powder under high energy laser cladding was constructed.The results showed that the ball-milling time affects the cold-welding and crush process,as well as the diffusion speed of the atoms,leading to different grain sizes,morphologies and alloying degrees of the Fe-Al metal powders.Subsequently,the obtained powders showed different properties for the absorption of the laser,the mobility of the molten metal and the structure of the solidification coating.With the increase of the ball-milling time,the size of the powder became smaller and the surface energy was also enhanced,promoting the mutual diffusion of the atoms.Finally,Fe?Al?solid solution was formed in the powder.When the ball-milling time reached an ideal value of 20 h,the grain size,the average particle size(D₅₀)and the specific surface area of the obtained metal feedstock are 7.7692 nm,1.13?m and 2016m²/kg,respectively.The distribution of the size was the narrowest,reaching the best dynamic equilibrium between the crushing and cold-welding of the powder.The solid solubility of Al in Fe reached approximate saturation?28.2 at%?.The laser cladding Fe-Al coating fabricated using this powder was composed of a majority of B₂ type and a small amount of DO₃ type Fe₃Al,which exhibited the best quality without defects such as shrinks and cracks.Moreover,the contact between the laser cladding coating and the1045 steel substrate was metallurgical binding.The solidification structures of the laser cladding coating varied from equiaxed dendrites to columnar crystals and then cellular crystals along the cross-sectional direction from the surface to the substrate.In particular,the crystal structure of the laser cladding was mainly composed of equiaxed dendrites with a small size.However,overlong balling time would cause the re-agglomeration of the powder and the regrowth of the grains,leading to the formation of cracks and the coarsening of the microstructure in the corresponding Fe-Al laser cladding coating.Furthermore,the influence of the ball milling time on the mechanical properties and wear resistance of the Fe-Al laser cladding coatings was studied in this thesis.Overall,the Fe-Al laser cladding coating fabricated using the powder under high-energy ball milling for 20 h exhibited excellent comprehensive mechanical performance and good wear resistance at ambient and high temperatures.The average value of the surface micro-hardness was 527.2 HV_0.1,and the energy of resistant plastic deformation is 0.677×10^-9J,indicating that this coating possessed higher hardness and resilience,which is beneficial for the enhancement of its wear resistance at room and elevated?400°C?temperatures.To overcome the brittleness of the Fe-Al coatings at room temperature,an innovative calcination process was performed to in-situ grow Al₂O₃ nanoparticles on the surface of the Fe-Al powder as the reinforced phase to improve the tenacity of the Fe-Al laser cladding coatings.The effects of the calcination temperature on the particle size of the in-situ grown Al₂O₃ nanoparticles and the microstructures of the corresponding Al₂O₃/Fe-Al laser cladding coatings were investigated.Moreover,the intrinsic relationship between the distribution of the reinforced phase and the defects of the coatings were studied.In particular,the mobility mechanism of the reinforced particles in the molten pool during the non-equilibrium solidification process and the heat and mass transport between the reinforced particles and the matrix powders were studied in detail.In this regard,the performance of the laser cladding coating can be controlled by controlling the structure of the metal powder feedstock.The results indicated that the calcination temperature do affect the size and content of the in-situ grown Al₂O₃ nanoparticles,thereby exhibiting different strengthening and toughening properties of the corresponding coatings.When the calcination temperature was increased to 850°C,the in-situ grown Al₂O₃ nanoparticles with nanometer sizes?50¹00 nm?were achieved.The corresponding Al₂O₃/Fe-Al laser cladding coating consisted of uniform distribution of the Al₂O₃nanoparticles,resulting in the confined growth of the crystals and the formation of small microstructures.However,higher calcination temperature led to the over growth of the Al₂O₃ particles to micro sizes?500 nm¹?m?.As a result,severe agglomeration of the precursor powders was observed.In addition,the compatibility of the reinforced particles and the matrix particles in the laser cladding coatings became worse,resulting in defects such as cracks in the interface.Then,the effects of the size and content of the in-situ grown Al₂O₃ particles on the mechanical properties and the wear resistance of the Fe-Al laser cladding coatings were investigated in detail.The experimental results revealed that Al₂O₃ nanoparticles?50¹00 nm?can prevent the generation and propagation of cracks in the laser cladding coatings,effectively enhancing the plasticity and tenacity of the coatings.As a result,the micro-hardness was increased to 651.9 HV_0.1,whereas the energy of the resistant plastic deformation was decreased to 0.677×10^-99 J.Moreover,the reinforced effect of the Al₂O₃ nanoparticles reduced the abrasion of the coatings at room temperature.Owing to the self-lubricating ability of the Al₂O₃ nanoparticles,the wear resistance of the coatings at a high temperature was also increased.In summary,the reinforced mechanism of the Al₂O₃ nanoparticles in the Fe-Al laser clad coatings was a coupling effect of dispersion strengthening,grain refining strengthening,bridging toughening and reducing environmental embrittlement.In addition to nanoparticle reinforcement,a Cr alloying method was also proposed to enhance the tenacity of the Fe-Al laser cladding coatings.The optimized Cr content in the Fe-Al laser cladding coating was 5 at%.According to the comprehensive analysis of the electronic structures,bonding feature,and dislocation sliding via theoretical calculations,the toughening effect of Cr in the Fe-Al laser cladding coatings was proposed.It revealed that Cr changed the density of the electronic cloud,the atomic bonds and the gran boundaries,and reduced the antiphase domain boundary energy,leading to the decomposition and sliding of the dislocation and enhancing the plasticity and tenacity of the coatings.The experimental results revealed that the Fe-Al-Cr laser cladding coatings are mainly composed of Fe₃Al?Cr?,which possesses high intensity and excellent mechanical properties.As a result,the Fe-Al-Cr laser cladding coatings exhibited excellent wear resistance at both room and high temperatures.However,when the Cr content was increased to 7.5 at%that exceeded the saturation solubility of the Fe₃Al,a hard phase of Cr₂Al was formed in the corresponding laser cladding coatings.Consequently,the coarsening and aliquation of the solidification structures were observed,which severely affected the plasticity,tenacity and wear resistance of the Fe-Al laser cladding coatings.