Effect Of Laser Shock Processing On Microstructure And Properties Of IMI834 Alloy

Near-? titanium alloys are often used in key components of aero engines due to their high specific strength and excellent high temperature performance.IMI834 is a classic high temperature titanium alloy used for 600 ?,which is often used in the manufacture of engine compressor disc parts.However,the surface of metal components in a complex service environment often produces localized fatigue cracks that cause component failure.In order to prolong the service life of component materials and improve the reliability of materials,surface modification technology has been extensively studied.Laser shock processing(LSP)is also known as laser shot peening.As a new surface modification technology,it has significant technical advantages compared with traditional processing methods,which plays an irreplaceable role in strengthening key components.In order to improve the service life of IMI834 alloy under high temperature service conditions,IMI834 titanium alloy was strengthened by laser shock processing.Therefore,studying the effect of laser shock processing on the microstructure and properties of IMI834 alloy develops the application of laser shock processing technology in the aviation industry.The main research contents and related results of this thesis are as follows:The surface morphology and mechanical properties of IMI834 alloy before and after LSP were studied by surface roughness meter,confocal laser scanning microscope,micro Vickers hardness tester and X-ray diffractometer.The results show that the roughness of the surface increases after the specimen is subjected to a single LSP impact and two LSP impacts.Compared to the untreated sample,the microhardness and residual stress of the material increases after the specimen is subjected to LSP impact.As the number of LSP impact increases,the microhardness maximum,the depth of the strengthening layer and the residual compressive stress in the plastic deformation layer of the IMI834 alloy increase further.The microstructure of IMI834 alloy in the depth direction induced by LSP was characterized by transmission electron microscopy(TEM).The dislocation configuration characteristics at different strain rates in the plastic deformation layer were summarized.The plastic deformation influential layer with a certain thickness is formed on the surface layer of IMI834 alloy subjected to LSP,and a large number of dislocation structures are generated in the strengthening layer.As the number of impacts increases,the plastic deformation becomes more severe and the dislocation density further increases.During the laser shock processing,the strain rate decreases with increasing depth from the surface layer,and the dislocation density and microstructure characteristics also produce gradient changes,including polygonal substructures,dislocation cells(DCs),and highdensity dislocation walls(DDWs),grid-like mechanical twins(MTs),dislocation arrays(DAs),dislocation tangles(DTs)and dislocation lines(DLs).The orderly distribution of dislocation structures in primary ?-phase and ?-transformation is mainly based on laterally dividing the matrix to achieve the purpose of reducing the total energy state and coordinating plastic deformation.Laser shock processing induces changes in microstructure,which plays an important role in the improvement of mechanical properties of materials.The effects of laser shock processing on the thermal stability of IMI834 alloy were investigated by microstructure observation and mechanical properties test.The results show that the residual stress and microhardness of the surface layer of IMI834 alloy subjected to LSP will be relaxed at 600 ?,but it still maintains a high strengthening effect in a certain period of time.The microstructure evolution of the alloy was observed by transmission electron microscopy(TEM).The results show that the dislocations in the grains are redistributed and the density is reduced with the increase of the holding time,and the lattice distortion and internal stress are also reduced.However,there are still many crystal defects in the matrix for a certain period of time,which has good thermal stability.In addition,the effect of temperature causes the reduction of dislocation density in the matrix to be a major cause of the decrease in mechanical properties.The oxidation behavior of IMI834 alloy subjected to LSP at medium and high temperatures was studied.The results show that the increase of dislocation density of IMI834 alloy induced by LSP accelerated the diffusion of film-forming elements,and the oxidation cycle of the alloy became shorter,which accelerated the formation of steady oxide film.