| Residual tensile stress formed by rapid melting and rapid solidification during laser selective melting(SLM)are important shortcomings that limit the application of the technology.Surface strengthening techniques such as shot peening and laser shock strengthening introduce residual compressive stress in the near surface layer of the alloy,and to some extent improve the microstructure of the surface and near surface layers,thereby improving the fatigue performance of the material.They are expected to become important post-processing techniques for SLM products.This article focuses on the surface strengthening of GH4169 alloy formed by SLM.Firstly,appropriate SLM process parameters of GH4169 alloy were optimized through density and mechanical properties testing.Secondly,the influencing mechanism of SLM building direction on the anisotropy of tensile strength was studied through microstructure characterization,tensile performance testing,and theoretical yield strength calculation.A comparative study was conducted on the effects of shot peening(SP),laser shock strengthening(LSP),and LSP+SP composite strengthening(FH)on surface roughness,microstructure,residual stress,microhardness,and fatigue performance of longitudinally formed specimens with high plasticity.The strengthening mechanism of LSP+SP composite strengthening on the alloy was revealed.The research results of SLM forming process indicate that when maintaining the same powder layer thickness and scanning spacing,to change the laser energy density through adjusting the laser power and scanning speed can have a significant impact on the microstructure,especially the density of SLM formed samples,thereby affecting the tensile properties of the samples.The optimal SLM forming process parameters for GH4169 alloy are as follows:the laser power of 283 W,the scanning speed of 960mm/s,the powder layer thickness of 40μm,and the scanning spacing of 0.11 mm.The tensile strength of the formed transverse specimen is 1472 MPa,and the elongation after fracture reaches 18%.The building direction of SLM significantly affects the tensile properties of GH4169 alloy.Among the three types of formed specimens in the transverse,longitudinal,and oblique(60°)directions,the oblique specimen has the highest tensile strength and yield strength which is 0.57%and 10.61%higher than that of the transverse and longitudinal specimens,respectively.The tensile fracture surface also contains a large number of quasi cleavage planes and deep dimples.The longitudinal specimen has the highest elongation at break,which is 24.6%and 9.47%higher than that of the transverse and oblique specimens,respectively.There are numerous deep dimples in the tensile fracture surface.The theoretical calculation values of the yield strength differences of the samples formed in three different building directions are consistent with the experimental test results.The generation of yield strength anisotropy is mainly attributed to the dislocation strengthening mechanism(with a contribution rate of about50%),followed by precipitation strengthening and grain boundary strengthening,which are related to the Taylor factor,dislocation density,and effective grain size differences of samples formed in different building directions.Among the three surface strengthening methods,LSP strengthening has no significant effect on the surface morphology and roughness of the sample,while SP and FH strengthening significantly increases the surface roughness of the sample,presenting typical morphology of pits and bullet marks.The EBSD test results indicate that among the three strengthening methods,FH strengthening has the most significant effect on the refinement of the average grain size near the surface layer of the sample and the improvement of the geometrically necessary dislocation density.The TEM test results also indicate that there are a large number of dislocation lines entangled and dislocated walls in the SP sample at a depth of 0.05 mm,while the dislocation lines are significantly reduced at a depth of 0.5 mm,while there are fewer dislocation lines in the LSP sample.The FH sample has a large number of dislocation lines at depths of0.05 mm and 0.5 mm.The three strengthening methods significantly improved the surface microhardness of the sample,but as the distance from the surface depth increased,the microhardness gradually decreased and stabilized.Among them,the microhardness at the center depth of the SP sample was similar to that of the AR sample,while the microhardness at the center depth of the LSP and FH samples was higher than that of the AR sample.The peak residual compressive stress(CRS)of the SP specimen reaching 745.7 MPa is located at a depth of approximately 150μm.The peak CRSes of both LSP and FH samples reach 609.7 MPa and 1022.9 MPa,which is located at a distance of 200μm from the surface.The CRS layer thicknesses of the LSP and FH samples are about twice that of the SP sample,respectively.The CRS distribution of FH strengthened specimens combines the CRS distribution characteristics of LSP and SP strengthened specimens,with the highest peak residual compressive stress and deeper CRS depth.At a stress level of 680 MPa,the vibration median fatigue lives of AR and LSP specimens are 2.58×10~5 and 3.87×10~6 cycles,respectively,while there was no failure for the FH sample undergoing 1×10~7 cycle loading.The fatigue life of LSP and FH specimens was about 15 times and 38 times higher than that of AR specimens,respectively.At a stress level of 920 MPa,the median fatigue life of FH specimens under vibration is 3.86×10~5 cycles,which is still higher than the fatigue life of AR specimens under the lower stress levels of 680 MPa.There are two crack initiation sources on the fracture surface of AR fatigue specimens,both located on the surface of the specimen,while LSP and FH fatigue specimens have only one crack initiation source on the fracture surface,located approximately 200μm and 100μm depth from the surface,respectively.Compared to LSP strengthening,FH strengthening introduces high surface roughness,making its fatigue crack initiation source closer to the surface.Grain refinement,high dislocation density,high work hardening degree,high peak CRS and deep CRS depth in the surface and near surface of the specimen are the essential reasons why FH strengthening can significantly improve the fatigue life. |
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