Study On The Effect Of Different Surface Modification Processes On The Microstructure,corrosion And Mechanical Properties Of Mg-Dy-Ni Alloy

Magnesium alloys have a wide application prospect in automotive lightweight aerospace and medical equipment fields owing to its low density,high specific strength and good biocompatibility.However,Mg matrix itself has a lower equilibrium potential and higher chemical activity,and is very easy to form galvanic corrosion with the second phase,significantly reducing the corrosion resistance of the alloy,which also greatly limits the engineering application.Surface modification technology can improve the corrosion and mechanical properties of the alloy through reducing the grain size of the alloy and change properties of the second phase(morphology,size,quantity and distribution.Therefore,the as-cast Mg-12Dy-1.1Ni alloy was prepared in this paper.The microstructure,corrosion and mechanical properties of the alloy was studied by three surface modification methods:micro-plasma remelting modification(MPRM),laser surface remelting modification(LSM)and sliding friction machining(SFT).The relationship between alloy structure and corrosion and mechanical properties were discussed,the corrosion mechanism and mechanical strengthening mechanism of the alloy were explained,and theoretical guidance and technical support were provided for the opening of high-strength corrosion-resistant Mg alloys.The as-cast Mg-12Dy-1.1Ni alloy was composed of?-Mg,a small amount of Mg₂₄Dy₅ phase and lamellar long period stack ordered(18R-LPSO)phase distributed along the dendrite boundaries.After MPRM,the grain size of the as-cast alloy decreased form 37?m to 7?m,the 18R-LPSO phase is refined,the volume fraction increases and presents a continuous distribution.Meanwhile,the distribution of alloying elements in the matrix is more uniform.The electrochemical and immersion tests indicated that the corrosion current density of the MPRM alloy was reduced by38%compared with that of the as-cast alloy,and the corrosion resistance of the MPRM alloy was significantly improved.The enhancement of MPRM alloy corrosion resistance the is primarily ascribe to the uniform microstructure structure,the denser corrosion product film and the precipitation of the higher amounts fine 18R-LPSO phases acted as a corrosion barrier.In order to further optimize the microstructure of the alloy,laser surface remelting modification(LSM)with higher cooling rate was used to modify the as-cast alloy surface.Compared with MPRM,LSM has a higher supercooling due to rapid heating and solidification.Therefore,the grain refinement degree of the LSM alloy(4?m)is more significant than that of the MPRM alloy.The higher volume fraction(34.9%)18R-LPSO phase with a continuous network structure is formed at the dendritic grain boundary.Electrochemical and immersion tests show that the corrosion rate of the LSM alloy(2.8±0.16 mg/cm²/h)was lower than that of the as-cast alloy(4.2±0.12 mg/cm²/h).This was because the grain refinement contributes to the formation of a dense corrosion product film(Mg(OH)₂,Mg O and Dy₂O₃)during the corrosion process and prevents the corrosive solution from penetrating into?-Mg matrix surface.On the other hand,the continuously distributed18R-LPSO phase with the high amount could act as a corrosion barrier.In consideration of the influence of severe plastic deformation on the corrosion property while maintaining excellent mechanical properties,the as-cast alloy is used for single extrusion and sliding friction machining.The extruded Mg-12Dy-1.1Ni alloy is processed by SFT,the lamellar 18R-LPSO phase is severely broken and distributes evenly in the?-Mg matrix,and the content of the 14H-LPSO phase increased.This contributed to improve the mechanical strength and corrosion resistance of the SFT alloy.The ultimate tensile strength()of the SFT alloy was 356 MPa,which was higher than that of the as-extruded alloy(336 MPa),and the elongation of SFT alloy increased by 82%.The improvement of the SFT alloy tensile strength mainly originates from the grain refinement of alloy surface layer and the precipitation strengthening of LPSO phase.Additionally,the weight loss rate of immersion in 0.1 M Na Cl for 1 h decreased from 3.02 mg/cm²/h of the extruded alloy to 1.86 mg/cm²/h of the SFT alloy.The better corrosion resistance of the alloy was related to the grain refinement induced by SFT,the good surface quality and the morphology and distribution of LPSO phase.