The Semi-solid Non-dendritic Microstructure Evolution Of Mg-Cu-Al New Magnesium Alloy

As the lightest structure metal in practice，magnesium alloy is widely used moreand more, but magnesium is an active metal, it is oxidized quickly in molten stateand easily burn, the traditional forming methods are hard to control, when usingsemi-solid forming, because of small proportion of liquid and low-temperaturehomework, the dangers of combustion significantly reduced, The semisolidforing(SSF)has become one of the hotspots in the field of metal processing in recentyears, comparing with traditional die-casting processes, SSF has special technicaladvantages and wide application prospeet. Near net-shape forming(NNF) is one ofthe outstanding advantages of SSF and it can be enhance the utilization efficiency ofmaterials and save resource. In addition, SSF can reduce macro-segregation, avoidintenal defects and ultimately improve the performance of the formed components.And it also can reduce the detrimental heating effect in the used molds and prolongtheir period of validity. Thus, comprehensive the characteristics of the semi-solidprocessing technology and the advantages and disadvantages of magnesium alloys, ithas the very good prospects for development suitable for the development of thesemi-solid forming of magnesium alloy.Aluminum is the most commonly used and effective alloying elements in themagnesium alloy. The toughness and solid solution processing characteristics of thecasting can be improved when copper is added in Mg-Al alloy, can be performedrelatively good mechanics performance in room and high temperature. Mg-Al alloysolid phase line temperature can be improved for the copper’s addition, can be solidsolution at a higher temperature, make more alloying element solid soluble in alloymatrix, enhance the strengthening effect in subsequent limitation, and copper canimprove the ductility of the alloy significantly.Therefore, this paper choosing9kinds of alloy at different copper and aluminum content as an experiment alloy,using isothermal heat treatment method for semi-solid pulp, observation and analysisthe evolution process of the semi-solid crystal branch organization.Experimental results showed that the branch crystallin organization ofMg-Cu-Al alloy is fusing and have a trend of transformation to globular organization,but the different copper and aluminum contents make the organization forms quitedifferent, according to analyzing the semi-solid remelting crystal branchorganization and evolution rule of the Mg-Cu-Al alloys, it can be seen that withthe same Al content the average product circle diameter of the born solid phased particles for the semi-solid remelting tissue of the alloy reduce gradually along withthe increase of Cu element content, but when the content of Cu are same, along withthe increase of Al element content, the same length pellets of the born solid phasedparticles enhance gradually, namely the shape coefficients reduce gradually. Theexperiment obtains the best Cu and Al elements in the5%(wt%),5%(wt%), namelyMg-5%Cu-5%Al alloy relative to other alloy has good semi-solid branchorganization of crystal.Comparison the organization photo of Mg-5%Cu-5%Al alloy in differentisothermal temperature and isothermal time, can obtain that: in different insulationtemperature heat preservation15minutes respectively, along with the increase oftemperature heat preservation, the average product circle diameter of born solidphased particles decrease gradually, shape coefficient decreases first and afterincreases gradually, and steady rheological aspect also decrease gradually; doingheat preservation processing for different time in600℃, along with the extension ofthe heat preservation time, the change process of the alloy organization is: smallpieces organization+nearly spherical organization—globular organization—globularorganization of coarsening, keeping insulation process from15to60minutes, theaverage product circle diameter of born solid phased particles increase gradually,shape coefficients first increases and then decreases. To sum up, the average productcircle diameter of born solid phased particles is47.40μm to heat preservation15minin600℃, the shape coefficient is1.34, and the solid phase volume fraction is48.93%.When the isothermal temperature reach610℃,the dendritic structureorganization of Mg-5%Cu-5%Al alloy has no longer existed with the holding timeextend, but the branch organization did not nodulizing, only a few (nearly) sphericalparticles, born of solid phased particles shapes, the organization has turned intoliquid when the holding time reach to45min. Through comparing the organization,the best semi-solid heating preservation temperature and time of Mg-5%Cu-5%Alalloy is600℃and15min.