Weakening And Suppressing Effects Of Shot Peening On A Rolled 2060-T8 Aluminum-Lithium Alloy

As a new kind of high-performance and lightweight metallic material,aluminumlithium(Al-Li)alloys are playing important roles in producing lightweight aircraft structures and improving components’ properties.Since Al-Li components usually undergo harsh service conditions,damage would occur on their surfaces and cause early failure.Then,the safety and durability of structures can be influenced.Improving the surface mechanical properties of Al-Li alloys and enhancing the safety and reliability of critical components are of great significance to the protection of national economy and the improvement of national defense security.At present,shot peening(SP)is widely employed in engineering fields as a kind of surface treatment technique to process the Al-Li alloys used in civil aircrafts.However,current researches on strengthening mechanisms of SP are mostly concentrated on stress strengthening and microstructure strengthening.While research on weakening factors induced by SP is not enough.Additionally,there is a lack of broad understanding of the suppression methods of the weakening factors induced by conventional shot peening(CSP).As a result,a systematic investigation considering the characteristics of Al-Li alloys is necessary and important to understand the modification mechanisms of SP.This work investigated the effects of CSP on mechanical properties and microstructures of a rolled 2060-T8 Al-Li alloy.The weakening effects of CSP on mechanical properties of 2060-T8 Al-Li alloys were quantitatively characterized by tensile tests and microhardness on longitudinal-sections.The thermal effects on mechanical properties in the surface layer of 2060-T8 Al-Li alloy were analyzed with the help of a coupled thermal-mechanical finite element model.And the weakening mechanisms were analyzed from the prospective of surface morphology,microstructural evolution and compressive residual stress relaxation.In this case,a wet shot peening(WSP)method was proposed to suppress the weakening effects induced by CSP in Al-Li alloys.And the suppression mechanisms of WSP were mainly discussed from the perspective of grain sizes,grain orientations,dislocations and the suppression of residual stress relaxation.Moreover,based on previous analysis,the formation mechanisms and deformation characteristics of gradient microstructures produced by CSP and WSP as well as their influences on mechanical properties were analyzed via the combination of theoretical and numerical methods.As a result,a modified model of gradient materials considering various microstructures and the characteristics of gradient materials was established to predict mechanical properties.Verified by yield strength and the microhardness at surface,the modified model can reasonably predict the mechanical properties of gradient materials produced by SP.In line with theoretical analysis,numerical simulation and experimental observations,main conclusions are as follows:(1)The weakening effects induced by CSP were discovered and the weakening mechanisms were analyzed.The average yield strength,average tensile strength and the microhardness at the surface of a 2060-T8 Al-Li alloy respectively decreased by16.98%,13.57% and 5.74%,but the average elongation rate increased by 28.17% after the treatment of CSP with an intensity of 0.2 mm A and a coverage of 100%.These phenomena implied that the 2060 Al-Li alloy was weakened by the high-intensity CSP.It’s well-known that surface morphology and microstructures played important roles in determining macroscopic properties.The weakening effects included the increment of surface roughness,dynamic recovery and dynamic recrystallization owing to the comprehensive influences of plastic deformation and thermal effects as well as the increment of weighted average Schmid factors derived from textural change from rolling textures to recrystallization textures.Moreover,it was indicated that the thermal effects produced by CSP can cause residual stress relaxation in the surface layer of material by the calculation of a coupled thermal-mechanical finite element model.Residual stress relaxation can also weaken mechanical properties in the surface layer of the 2060-T8 Al-Li alloy.(2)The suppression method of WSP was proposed and the suppression mechanisms were discussed.The average yield strength,average tensile strength and microhardness at the surface of a 2060-T8 Al-Li alloy respectively increased by 11.83%,16.15% and 9.43%,but the average elongation rate decreased by 30.18% after the treatment of WSP with an intensity of 0.2 mm A and a coverage of 100%.These phenomena indicated that the 2060-T8 Al-Li alloy was work hardened by the highintensity WSP.During the process,the surface roughness can be reduced and the thermal effects can be suppressed by the cooling and lubricant effects of water in WSP.The refinement of microstructures can then be promoted and the residual stress relaxation can be suppressed in the surface layer of 2060-T8 Al-Li alloy.As a result,based on Hall-Petch equation,Schmid equation and Taylor dislocation theory,the refined grains,decrement of weighted average Schmid factors,proliferation of dislocations and the suppression of residual stress relaxation were the fundamental physics behind the suppression effects of WSP.(3)The formation mechanisms and deformation characteristics of gradient materials in the Al-Li alloy processed by CSP and WSP were discussed with the help of numerical simulation,and a modified model was proposed to predict their mechanical properties.It was indicated that heterogeneous plastic strain and strain rates induced by SP were the principal extrinsic factors affecting the formation of gradient microstructures.At surface,relatively large plastic strain can induce more crystal defects while high strain rates can increase the density of dislocations,thus promoting the refinement of microstructures.With the increment of depth from surface,the impact energy of SP was absorbed by material and the extent of plastic deformation was declined so that gradient microstructures were formed.During the process of deformation,the incompatibility between gradient microstructures put the material in a complex stress state.As a result,the stress gradient can be formed to enhance mechanical properties while the strain gradient can promote the accumulation of geometrically necessary dislocations(GNDs)and cause the corresponding back stress hardening.Then,the suppression effects induced by CSP can be suppressed.In this case,a modified model of gradient materials considering various microstructures and the characteristics of gradient microstructures was established.Verified by yield strength and the microhardness at surface,the modified model of gradient materials can reasonably predict the mechanical properties of gradient materials processed by SP.