Microstructure And Mechanical Properties Of MWCNTs/Mg-14Li-1Al Composite Prepared By Friction Stir Processing And Rolling

Mg-Li alloy has low density,high specific strength,excellent damping performance and electromagnetic shielding performance.Therefore,Mg-Li alloy has great application prospects in aerospace,military,automotive,3C and other fields.The body-centered cubic structure（BCC）high Li content Mg-Li alloy has high plasticity,but low strength,which limits its application.The strength of Mg-Li alloy can be effectively improved by preparing Mg-Li based composites through severe plastic deformation processing and adding reinforcement.Friction stir processing（FSP）is a new type of severe plastic deformation processing technology.It is often used to modify materials to make the material structure uniform and refine the grains.It is also commonly used to prepare metal matrix composites.Multi-walled carbon nanotubes（MWCNTs）,with low density and excellent mechanical properties,are ideal reinforcement materials for preparing metal matrix composites.Therefore,aiming at the problem of low strength of high Li content Mg-Li alloy with BCC structure,MWCNTs are selected as reinforcements,and MWCNTs/Mg-14Li-1Al composite materials are prepared by electrophoretic deposition technology（EPD）and FSP.On this basis,the composites were processed using the rolling,thereby further improving the strength of the Mg-14Li-1Al alloy.In this paper,a uniform MWCNTs film was prepared on the surface of Mg-14Li-Al sheets by EPD.Then,the sheets containing MWCNTs were stacked and rolled to prepare preforms.Finally,they were subjected to FSP respectively,and then the prepared MWCNTs/Mg-14Li-1Al composites were subjected to rolling processing.For comparision,the sheet of Mg-14Li-1Al alloy without MWCNTs was also prepared with the same method.The influences of the voltage,time,Al（NO₃）₃ content,and MWCNTs content of the electrophoretic deposition process on the film were researched,and the best process parameters are determined as follows:voltage is 25 V,MWCNTs content is 0.5 g,Al（NO₃）₃ content is 0.4 g and the time is 4 min.Through the research on the process parameters of FSP,when the process parameters are 1000r/min-40mm/min,the surface macroscopic appearance of the material is the best,smooth and without obvious defects.The microstructure is significantly improved,and the grains are refined obviously.The yield strength and tensile strength of the alloy are respectively 164 MPa and 235 MPa,which are improved by 92.9%,58.8%and 57.7%,46.9%,respectively,compared to the as-cast and rolled Mg-14Li-1Al.But its elongation is low,only 5.8%.The fracture morphology is"rock sugar"block morphology,which is brittle fracture.The surface of the FSP processed MWCNTs/Mg-14Li-1Al composites is smooth and has no obvious defects.The MWCNTs was evenly distributed in the matrix.The yield strength and tensile strength are 190 MPa and 260 MPa,which are higher than those of the as-cast alloy by 123.5%and 75.6%,respectively.But the elongation is only4.1%.After cold rolling,the grains are significantly refined,with an average size of 7μm,and the distribution of MWCNTs is more uniform.A mixed grain structure appears,with a“bimodal distribution”.A large number of ultra-fine grains and dislocations are found in the material.The mechanical properties are further improved,The yield strength and tensile strength are 259 MPa and 313 MPa,which are increased by 204.7%and 111.5%,respectively,compared with the as-cast alloy.The elongation is 9.7%,and the high specific strength of 222 k Nm/kg is obtained.