Research On The Mechanical Behavior For Mg-Li Alloys And The Designment Of LPSO

Mg-Li alloys are the lightest engineering materials with the advantages of low density,high specific strength, high specific stiffness, good heat-conductivity andelectricity-conductivity, good property of electromagnetic shielding, etc. Therefore, Mg-Lialloys have wide prospect of applications in the fields of aerospace, weapon equipment, trafficequipment, and portable equipment, etc. In these applications, the alloys are often processedby welding, and the alloys are often applied with dynamic impacting load. However, theweldability and dynamic mechanical properties for Mg-Li base alloys are seldom researched.The absolute strength and high temperature mechanical properties are somewhat poor, whichrestrict the wide applictions.In this dissertation, the research points are based on the background mentioned above.Mg-Li alloys were welded with TIG welding method, and the welding microstructure andmechanical properties of Mg-Li alloys with different phase composition were studied. Mg-Lialloy was also welded with aluminum alloy, and the welding microstructure and mechanicalproperties of it was also studied. The mechanical properties of Mg-8Li-1Al andMg-8Li-1Al-1Ce under the condition of different strain rate were researched with SHPB.Comparing the mechanical behaviors of the alloys between quasi-static and dynamicconditions, the strain rate effecting of the alloys was obtained. What’s more, to improve thestrength and high temperature mechanical properties, the Mg-Li alloys with LPSO structurewere prepared and their microstructure and mechanical properties were studied. The researchcontents and results show that:1. Mg-Li base alloys possess good weldability. The mechanical properties of the weldingjoint of Mg-Li alloys with different Li content are all good, and the best of them is the alloywith dual phases（Li content is about8wt.%）. During the welding process, some weldingfaults, such as cracks and shrinkage cavities, form in the welding joints. A suitable feedingspeed of the welding wire with the same compositon with that of alloys can be used to avoidthese welding faults. Mg-8Li-3Al-2Zn-1Ce can be welded well with the alloy of Al5083withTIG welding method. The bondings between welding seam and matrix alloys are good, andsome hard brittle Mg-Al phase forms in the welding seam.2. In the experiments of quasi-static compression with the strain rate lower than10^-3/s, when the strain rate is defined, the stress of extruded Mg-8Li-1Al increases with strain, that isstrain-strengthening effect. When the strain is defined, the stress of the alloy increases withstrain rate, showing strong strain rate sensitivity, that is a strain rate effect. When the strainrate is between900/s and1700/s, the strain rate effect of Mg-8Li-1Al is not obvious. However,comparing the results of the alloy under quasi-static and dynamic states, the strain rate effectof the alloy is very sensitive. The strain rate sensitivity coefficients under different strain withthe strain rate between10^-5/s and2000/s show that, the strain rate sensitivity of Mg-8Li-1Alin the direction of extrusion is lower that in the vertical to the extrusion direction. Theextruded Mg-8Li-1Al alloy has obvious mechanical property difference between the cross andlongitudinal directions. The mechanical properties in the direction of parallel to extrusiondirection are better than those in the direction of vertical to extrusion direction.The dynamic stress-strain curves of Mg-8Li-1Al-1Ce in the dynamic compressionexperiments with different loading rates show that, the stress increases with the strain rate,then decreases with it. The yielding strength of the alloy increases with the strain rate, thendecreases with it. During the dynamic impacting, the refined microstructure makes thestrength increase. However, the deformation localization when the strain rate is between2300/s and2700/s makes the yielding strength decrease. According to Hollomon empiredequation and the results of experiments, the strain hardening exponent is about4.6, and thestrength factor increases with strain rate first, then decreases with it.3. The as-cast, as-extruded and as-heat treatment Mg-8Li-6Y-2Zn is prepared. Themicrostructure for them shows that, without LPSO structure is found in the as-cast andas-extruded alloys. After solid solution, the LPSO structure exists in the alloy. The （Mg,Zn）24Y5phase transfers to18R type LPSO phase. Compared with the tensile properties ofas-cast alloy, the strength increases by35MPa, and elongation increases by1times. At150oC,the strength increases by30MPa, and elongation also somewhat increases. The improvementof tensile properties is mainly attributed to the LPSO structure.4. The as-cast Mg-5Li-6Y-2Zn possesses much LPSO structure（existing inα-Mg）.However, the mechanical properties of as-cast Mg-5Li-6Y-2Zn are somewhat poor. This isbecause much （Mg,Zn）₂₄Y₅existing at boundaries of dendrites. The strengthening effect ofLPSO structure is small because of the existence of much brittle （Mg,Zn）₂₄Y₅at boundaries ofdendrites. To improve the mechanical properties, the alloy should be extruded to crush the brittle （Mg,Zn）₂₄Y₅phase.