Research On Shot-peened Surface Integrity And Service Performance Of Al-Li Alloy 2A97

As a new type of lightweight metal materials,aluminum-lithium alloys have been widely used in aerospace and marine industries due to their high specific strength,low density,excellent processability and convenient recycling.However,the poor corrosion resistance and friction and wear properties have seriously restricted the popularization and application of these materials.The development of surface strengthening technology provides an opportunity to extend its service life.Shot peening surface strengthening technology is widely used because of its simple operation and high productivity.The surface integrity of shot peening is affected by the thermo-mechanical coupling effect in the shot peening process,that is,by the shot peening process parameters.When inappropriate shot peening process parameters are used,the surface integrity of shot peening may be deteriorated,thus further weakening the surface corrosion resistance and friction and wear properties.Therefore,in order to improve the surface quality and service performance of Al-Li alloy components,it is necessary to study the shot peening process of Al-Li alloy on the premise of considering the material and its service environment.In this paper,the influence of shot peening process on the surface integrity of 2A97 Al-Li alloy was studied,the qualitative relationship between process parameters and surface integrity was determined,the correlation between surface integrity and service performance was grasped,and the mechanism of shot peening surface strengthening was revealed.The reasons for improving the bonding strength between the coating and the substrate,the surface corrosion resistance and wear resistance were analyzed,which provided a theoretical basis for the processing of Al-Li alloy 2A97 with high corrosion resistance.Firstly,the milling integrity of Al-Li alloy 2A97 was studied by single factor experiment.The results showed that a low cutting speed and a low feed rate could reduce surface roughness,and the residual stress on the machined surface was of a small compressive stress.The work hardening of the surface ranged from 120% to 124%,but milling was not enough to refine the large grains in the alloy’s original microstructure,thus the cross-sectional microstructure failed to show grain refinement.Through the electrochemical corrosion test,it is found that when the cutting depth is 2 mm,the feed speed is 1000 mm/min and the feed per tooth is 0.1 mm/z,the optimal surface milling parameters are obtained.Secondly,the finite element simulation models of 2A97 Al-Li alloy under different shot peening process parameters were established by ABAQUS to explore the influence of shot peening parameters on the surface integrity.The results show that the influence of shot peening process parameters on the surface roughness Ra is in the order of impact angle > shot velocity >coverage > shot size,and the greatest influence factor on the surface residual stress is the coverage.with the increase of shot peening coverage,the surface residual stress amplitude increases.Coverage has the greatest influence on surface residual stress,followed by impact angle and shot velocity,and shot size has the least influence on surface residual stress.In the PEEQ value fitting model,the gain effect of coverage is the most significant.Then,the shot peening test of Al-Li alloy 2A97 was carried out,and the results based on the test characterization were compared with the model results,and it was found that the model error was relatively small,which verified the reliability of the shot peening model.Based on the single factor shot peening test,it is found that with the increase of shot peening coverage,the surface defects increase and the surface roughness increases.The surface residual compressive stress and the depth of residual stress field increase,and the maximum residual stress appears in the subsurface;The surface work hardening degree of the workpiece fluctuates between 141% and 273%,and the surface work hardness tends to be saturated when the shot peening coverage is greater than 600%.Through the surface temperature measurement test,it is found that the surface temperature increases significantly,and the mass fraction of oxygen element on the peened surface is 17.40%,while the mass fraction of oxygen element on the milled surface is only 8.31%,which is easy to form alumina film on the surface;the thickness of plastic deformation layer increases with the increase of coverage,and the maximum thickness of deformation layer is up to 198 μm.The results of XRD and TEM show that the surface of the alloy has serious plastic deformation and produces nano-grains.Finally,by comparing the results of scratch test,electrochemical corrosion test and friction and wear test,it is found that the surface shot peening can effectively improve the bonding strength between the surface and the coating;The corrosion potential of the alloy sample surface is obviously increased,the size and the number of corrosion pits are obviously reduced,the wettability of the alloy surface is improved,the contact angle is increased,and the corrosion resistance of the surface is enhanced;Compared with the milling specimen,the wear mass loss of the shot peening specimen is reduced to 0.0031 g,with a reduction of 74.59%,and the surface wear resistance is optimal when the shot peening coverage is 700%.