Research On Numerical Simulation,microstructure And Properties Of Mg-Li Alloy By Friction Stir Processing

Magnesium lithium（Mg-Li）alloy is one of the lightest metal structural materials at present.Due to its low density,high specific strength and stiffness,good damping performance,and good electromagnetic shielding performance,it has huge potential in the fields of aerospace,military,medical equipment and 3C industry.Friction stir processing（FSP）is a very potential metal surface modification technology based on friction stir welding（FSW）.The key work of this paper included:a three-dimensional thermal-mechanical coupling FSP model of Mg-Li alloy was established by Deform-3D software,the temperature field and strain field of the alloy were obtained,the metal flow in different parts of the stirring tool was studied by tracer particle method;Combined with literature data and finite element simulation results,reasonable process parameters were designed,and the effects of different rotation speed and welding speed on the microstructure and properties of the alloy were studied.Metallographic microscope（OM）,X-ray diffraction technique（XRD）,scanning electron microscope（SEM）and transmission electron microscope（TEM）were used for comprehensive characterization analysis to explore the internal relationship between numerical simulation-process-microstructure-properties.The main work and results of the paper are as follows:（1）The high-temperature compression test of the dual-phase Mg-7.8Li-0.7Zn-0.4Sr-0.4Ca Mg-Li alloy was done by the Gleeble-3500 thermal simulator,and the true stress-strain curves at different temperatures（T=423 K,473 K,523 K,573 K and 623K）and different strain rates(0.001s^-1,0.01 s^-1,0.1 s^-1 and 1 s^-1)were obtained,combined with the curves and Deform-3D software,the Arrhenius constitutive equ=ation（ε（5）A[si nh（ασ）]ⁿ exp（-Q/R T））of the alloy was established by fitting.（2）A three-dimensional thermal-mechanical coupling FSP finite element model was established using Deform-3D software.The point tracking method was adopted to visually present the metal flow phenomenon in different parts of the stirring tool,and initially revealed the law of plastic flow in the stirring zone（SZ）.In the horizontal direction,the flow of particles on the advancing side was stronger than that on the retreating side and in front of the stirring tool;in the thickness direction,the points near the weld center at h=4 mm on the advancing side migrated downwards around the stirring pin.The middle points migrated upward around the stirring pin,and the points at the far end did not migrate because they were not stirred by the stirring pin.（3）The fine-grained regions in the middle and lower part of SZ under different process parameters were analyzed by SEM,and the corresponding grain sizes ofα-Mg phase andβ-Li phase were counted,and the results showed thatα-Mg phase andβ-Li phase have been significantly refined and close to equiaxed after FSP,The800r/min-100mm/min sample had the smallest average grain size ofα-Mg phase andβ-Li phase,the average grain size ofα-Mg phase was 3.01μm,and the 3.71μm ofβ-Li phase;the distribution of the second phase has become relatively dispersed and the size was reduced in varying degrees.（4）The grain region of 800r/min-100mm/min sample’s SZ was analyzed by FEM,the selected area electron diffraction results of the TEM bright-field image showed that there was the phenomenon of BCC-β-Li changing to FCC-β-Li,which was rarely reported before;in addition,HAADF-STEM results showed that there were a large number of nano-scale second phase particles at the grain boundaries,and the corresponding EDS surface scan results showed that the granular nano-scale second phase mainly contained Mg and Zn elements.（5）Theα-Mg phase andβ-Li phase macro-textures of the base material（BM）and5 groups with different process parameters were analyzed.The results showed that the textures ofα-Mg phase andβ-Li phase were more sensitive to the process parameters of FSP,and the texture intensity of theα-Mg phase andβ-Li phase was significantly weakened after FSP,and some new texture components of theβ-Li phase were formed.（6）Finally,the room temperature mechanical properties of the BM and the weld after FSP were studied.The results showed that the best comprehensive mechanical properties were the sample with the parameter of 800r/min-100mm/min,and the highest tensile strength was 210 MPa,which was 32.3%higher than that of the BM159MPa.FSP process had a great influence on the plasticity of the alloy,and the elongation decreased after FSP;the microhardness of the SZ was significantly improved after FSP.