Investigation Of Microstructure And Properties Of Dual Phase Mg-Li Alloy By Friction Stir Processing And Welding

As the lightest metal structural material available today,magnesium alloys not only decrease in density with the addition of lithium,but also change the crystal structure.The crystal structure changes from close-packed hexagonal（HCP）structure into a body-cube（BCC）structure with more slip systems,which greatly improves the plastic forming ability of magnesium alloys.The dual-phase magnesium-lithium alloy consists ofα-Mg（HCP）andβ-Li（BCC）.The composite structure makes the alloy have better comprehensive mechanical properties.However,the commercial application of magnesium-lithium alloy has been greatly hindered attributing to the low absolute strength and poor corrosion resistance of the dual-phase magnesium-lithium alloy.Therefore,improving the strength and corrosion resistance of the dual-phase magnesium-lithium alloy is an important topic.In this paper,the dual-phase LZ91 magnesium-lithium alloy was used as the research object.The friction stir processing technology was used as the modification process of the alloy.Based on this,the surface phytic acid treatment process of LZ91magnesium-lithium alloy was studied.Finally,joining process of friction stir lap welding the LZ91 magnesium-lithium alloy and AZ31 magnesium were studied.The main work and research results completed in this paper are as follows:（1）The modification of dual-phase LZ91 magnesium-lithium alloy is conducted by friction stir processing method.Process parameters of modification by friction stir process are researched and optimized to find the best parameters.The processed joint structure with a good surface shape and no internal defects can be obtained when the rotation speed ranges from 1600 r/min to 1900 r/min and the stirring processing speed ranges from 30 mm/min to 100 mm/min.The change ofα-Mg phase andβ-Li phase in different positions are different.The grain size ofα-Mg phase andβ-Li phase in the stirred zone（SZ）decreases with the increase of heat input ratio.However,the grain size ofα-Mg phase andβ-Li phase in heat affected zone increases with the increase of heat input ratio.The bending deformation degree of theα-Mg phase and theβ-Li phase grain at the advancing side of thermo-mechanically affected zone（TMAZ）significantly larger than that of the retreating side of advancing side of thermo-mechanically affected zone（TMAZ）.Under the same process parameters,compared with the grain size ofα-Mg phase at the advancing side of thermo-mechanically affected zone（TMAZ）,the grain size of theα-Mg phase at the retreating side of advancing side of thermo-mechanically affected zone（TMAZ）is remarkably refined,while the changing trend of grain size ofβ-Li phase is opposite.The modified LZ91 magnesium-lithium alloy obtained the best comprehensive mechanical properties under the processing parameters of 1800 r/min-60mm/min.（2）The Microstructure,texture Changes,mechanical properties and corrosion behavior ofα-Mg andβ-Li phases of dual-phase LZ91 magnesium-lithium alloy under the optimal friction stir processing parameters are studied.After modification under the processing parameters of 1800 r/min-60 mm/min,the average grain size of theα-Mg andβ-Li phases of the rolled dual-phase LZ91 magnesium-lithium alloy is refined from5.7μm and 11.9μm to 2.1μm and 2.2μm,respectively.Meanwhile,the tensile strength（TD direction）increases from 185.0MPa to 252.0MPa,the elongation increases from19.1%to 31.9%,and the yield strength decreases from 148.0MPa to 132.0MPa.The self-corrosion current drops from 2.734×10^-33 A/cm² to 1.092×10^-3 A/cm².（3）Surface phytic acid treatment of magnesium-lithium alloy is conducted.Through orthogonal test,the influence of phytic acid conversion liquid concentration,pH value,conversion time and conversion temperature on the conversion film is studied and the best film formation condition is explored.The corrosion resistance changes before and after phytic acid treatment were compared and analyzed.The microstructure,composition and formation mechanism of the phytic acid conversion film are analyzed.The corrosion resistance of the conversion film before and after phytic acid treatment was compared and analyzed.The microstructure of the phytic acid conversion film and the composition of the conversion film were analyzed,and the film formation mechanism of the phytic acid conversion film was analyzed.From orthogonal polar difference analysis of phytic acid conversion,it is known that the order of influence on the film formation process is:solution PH value>conversion time>conversion solution concentration>transformation temperature.The best process conditions for conversion coating of phytic acid treatment of LZ91 magnesium-lithium alloy are:PH value of the solution is 6;the conversion time is 5 min;the concentration of the conversion solution is 2 ml/L;the conversion temperature is 35°C.The self-corrosion current of the LZ91magnesium-lithium alloy under the optimal phytic acid treatment drops from 2.734×10^-3 A/cm² to 2.254×10^-5A/cm².The self-corrosion current of optimal FSPed LZ91magnesium-lithium alloy after phytic acid treatment decreased from 1.092×10^-3A/cm²to 6.241×10^-5A/cm².（4）The friction stir lap welding of magnesium-lithium alloy and dissimilar magnesium alloy is conducted.The effects of different process parameters on the surface forming,macro-microstructure,interface characteristics and mechanical properties of the lap joint were studied.The AZ31 plate is placed on the upper layer.When the rotation speed is too low or the welding speed is too high,the welding molding quality is poor.When the rotation speed range of the stir head is1500¹950r/min and the welding speed range is 30¹50mm/min,a lap joint with excellent joint quality can be obtained.When the LZ91 plate is placed on the upper layer,and the friction stir lap joint cannot be connected.The microstructure evolution of the AZ31/LZ91 friction stir welded joint is as follows:the grain size of the SZ of the upper layer AZ31 gradually decreases from top to bottom,and the grain size of the advancing side of thermo-mechanically affected zone（TMAZ）is smaller than that of the retreating side of thermo-mechanically affected zone（TMAZ）.Theα-Mg phase grain of the retreating side of the thermo-mechanically affected zone（TMAZ）of the lower layer LZ91 is smaller than theα-Mg phase grain of the advancing side of thermo-mechanically affected zone（TMAZ）.However,the grain size of theβ-Li phase in the retreating side of the thermo-mechanically affected zone（TMAZ）is larger;the interface of AZ31/LZ91 is well connected,forming a“hook-like interaction structure”.The content of Mg and Al elements decreases from AZ31 side to LZ91 side,however,the content of Zn is substantially unchanged.Under the condition of 1950r/min-120mm/min,the maximum tensile shear strength of the AZ31/LZ91 friction stir welded joint is 155.0 MPa.