Study Of Mechanical Properties And Oxidation Properties Of Laser Shock Peening And Heat Treatment Composite Reinforced IN718 Alloy

To accommodate the increasingly severe service environment in aerospace,the future development of high-temperature alloys is aimed at higher thrust-to-weight ratios and service at higher temperatures.The key to achieving this goal depends on the comprehensive mechanical properties and resistance to high temperature oxidation of aero-engine hot side components.Severe plastic deformation techniques and aging heat treatments are effective in improving the overall mechanical properties of precipitation-reinforced alloys,however,often at the sacrifice of ductility.Composite treatment processes to modulate the alloy structure to obtain a good combination of strength and ductility have received a lot of attention from researchers.Laser shock peening（LSP）,as an advanced surface strengthening technique,has incomparable advantages over conventional surface strengthening processes due to its high strain rate,high peak pressure,and high repetition frequency.In this paper,we take IN718 alloy as the research object to investigate the mechanism of gradient microstructure,the mechanism of heterogeneous nucleation of precipitation phase and the mechanism of microstructure evolution on the improvement of mechanical properties and high-temperature oxidation properties of the alloy induced by the combined process of laser shock peening and heat treatment.The details of the study and the main conclusions obtained are as follows:（1）Theoretical model analysis of laser impact strengthening and heat treatment was carried out.Based on the laser shock peening theory,the evolution of microstructure such as dislocation,twinning and grain refinement induced by laser shock waves and the mechanism of residual compressive stress on the surface were analyzed.On this basis,the nucleation kinetics of the precipitated phase and the directional precipitation behavior of the precipitated phase under the effect of residual stress are combined.It is concluded that only when the heterogeneous nucleation of the precipitated phase is dominated by the high dislocation density and the homogeneous nucleation is suppressed,the number of nucleation of the strengthened phase can be effectively increased,the strength of the alloy can be improved and the ductility can be maintained.In addition,the residual compressive stresses promote the formation of bothγ’’[100]andγ’’[010]variants,which provide higher yield strength compared toγ’’[001]variant for uniaxial tension.（2）A comparative study on the effects of mechanical properties of IN718 alloy after untreated,heat treatment,laser shock peening,and combined laser shock and heat treatment strengthening was carried out.The mechanical properties such as microhardness,elastic modulus,nanohardness,yield strength,tensile strength and elongation of each specimen under different processes were analyzed by microhardness test,nanoindentation test and tensile test.The results showed that the samples strengthened by the combination of laser shock and heat treatment had the most excellent mechanical properties,with 124.9%,49.0%,23.2%and 39.5%increase in surface microhardness,nano-hardness,yield strength and tensile strength,respectively.Paradoxically,the modulus of elasticity was increased by only 9.5%,while ductility was maintained.（3）A study was carried out to investigate the mechanism of the influence of microstructure on the mechanical properties of IN718 alloy after untreated,heat treatment,laser shock peening,and combined laser shock and heat treatment peening.The stress state,grain size changes,dislocation structure,precipitation phase characteristics and tensile fracture morphology of each specimen under different processes were observed and analyzed by X-ray diffractometer（XRD）,optical microscope（OM）,transmission electron microscope（TEM）and scanning electron microscope（SEM）,respectively,and the mechanism of microstructure evolution was analyzed based on theoretical basis.The strengthening mechanism of mechanical properties was revealed through microstructure.The results show that the heat treatment generates a large number of fine and uniformγ’andγ’’phases and relatively large carbide particles,and the carbide particles become the nucleation points of the fracture cavities and weaken the strengthening effect ofγ’’phase on the alloy.Laser shock peening induces graded grain size,a large number of dislocation structures and ultrafine grains,and these microstructures can effectively improve the mechanical properties of the alloy.Based on this heat treatment,theγ’’phase is missing an orientation under residual compressive stress,while theγ’’phase tends to heterogeneously nucleate in regions with high dislocation density,leading to the existence of strong and weak regions within the grains,and the inhomogeneous microstructure allows the strength of IN718 alloy to be improved while the ductility is maintained.（4）A study of the high-temperature oxidation properties of IN718 combined concentration after untreated,heat treatment,laser shock peening,and laser shock and heat treatment combined strengthening was carried out.The structure and growth of oxide films were analyzed by XRD,SEM and energy spectrometer（EDS）,respectively,and the oxidation kinetic curves of each specimen were derived by oxidation weighing,and the growth pattern of oxide films under different microstructures was analyzed by combining microstructure and oxidation kinetic curves.The results show that the large number of precipitated phases generated by heat treatment reduces the contact area between the substrate and oxygen,the oxidation rate is low in the early stage,the oxide film is not dense enough,and the phenomenon of internal oxidation exists.Laser shock peening induced grain refinement and a large number of dislocation structures can increase the atomic diffusion rate and form oxide films rapidly in the early stage of oxidation.The large number of diffusion channels in the combined laser shock and heat treatment process increases the growth rate of oxide films at the early stage of oxidation,and the resulting precipitation phase inhibits the formation of Ni O and Fe₂O₃ at the early stage of oxidation,which makes it more difficult for these oxides to reach the reaction concentration for the formation of（Ni,Fe）Cr₂O₄spinel and inhibits the formation of needle-like spinel oxides,thus forming uniform and dense oxide films and improving the high temperature oxidation resistance of IN718 alloy.