The Modification Effects Of Mg-Al-Si Magnesium Alloy

Mg-Al-Si based magnesium alloys are considered as the potentialalloys for the elevated temperature application. If the changes ofmicrostructure for Mg-Al-Si based alloys under the condition ofdifferent composites and treatment states are well known, the basicguidance for the development of new type Mg-Al-Si based heatresistant magnesium alloys would be provided. However, all over theworld the research about microstructure controlling of Mg-Al-Si heatresistant magnesium alloys are not quite systematic and perfect atpresent, so it has very important theoretical significance and practicablevalue to more investigate the microstructure of Mg-Al-Si basedmagnesium alloys on the basis of these existing problems inmicrostructure controlling of magnesium alloys, then according to theresearch results obtained, the guidance basis of microstructurecontrolling of composition design and processing optimization not onlyfor the exploitation of new type Mg-Al-Si based heat resistantmagnesium alloys, but also for other heat resistant magnesium alloys,would be provided.The effects of adding alkaline-earth element Sr、the solid solutionand the aging heat treatment of Mg-Al-Si based magnesium alloy inas-cast microstructure and heat treatment were investigated by means ofmetallurgical phase (OM) analysis, scanning electron microscopy (SEM)analysis, energy dispersive spectrometer (EDS) analysis and X-ray diffraction (XRD) analysis in this paper, and following research resultswere obtained.(1) The microstructure of as-cast Mg-3Al-1Si alloy was mainlyconsisting of grey-write matrix α-Mg, white phase β-Mg17Al12and theblack Chinese characters and the massive Mg2Si phase. In the as-castorganization, Mg2Si was given priority to with massive Chinesecharacters form and blocky phase. When adding0.2wt.%of strontium,the microstructure of Chinese characters reduced gradually, the massivephase also tended to be more granular phase. Divorced eutectic phaseβ-Mg17Al12phase reduced with the addition Sr too. And when thestrontium content was0.4wt.%, it almost did not exist the bulkycharacter in the alloy, but instead of, the particle phase and irregularforms of multilateral, the original particle phase was broken manypieces, and part dendrite arm was evolved as acicular phase, the matrixorganization had gotten significant refinement.(2) Through the solid solution treatment, the microstructure ofMg-Al-Si based magnesium alloy was getting spheroidizing. After4h~12h of T4solid solution heat treatment, Mg2Si phase hadexperienced the process from passivation to dissolved fracture, themicrostructure of Mg-3Al-1Si, Mg-3Al-1Si-0.2Sr andMg-3Al-1Si-0.4Sr alloy got further refinement, and the overallorganizational changed small. Due to the effects of solid solutiontreatment, the mechanical properties of alloy have got improved,especially T4-420×10h solid treatment, with the comprehensivemechanics performance of the three kinds of alloys were better.(3) Through the aging treatment, the microstructure organizationof Mg-Al-Si based magnesium alloy was no longer a significant change.The mechanical property of alloy was slightly elevated than solidsolution treatment. Holding time varies linearly with the alloy aging curve roughly proportional to the tensile properties, the alloy under theaging temperature180℃and200℃with the similar curve ofmechanical properties. The tensile property of the alloy with180℃aging temperature was slightly higher than the performance of the agingtreatment at200℃. Through the solid solution and aging treatment, thehardness of the Mg-Al-Si based had obviously improved than theas-cast alloys, but not two kinds of alloys in heat treatment state withlittle difference. Then the Sr modification had no effects for thehardness of alloys.(4) The alkaline-earth element Sr on the AS magnesium alloy hada significantly refine metamorphism, the metamorphism was mainlyembodied in two aspects: minor amounts of Sr could limit the dendritein the process of solid solution/grain growth, the Sr existed in the formof "refinement agent", it attached to the surface of Mg2Si phase as theadsorption film to form a layer, inhibition of Mg2Si with thespheroidizing effects of Mg2Si phase along the direction of the originaldendrite growth and character formation, and prompted further grainingMg2Si phase, then it achieved the purpose of refinement; On the otherhand, the Sr could partly replace the elements of alloy matrix andcombine with them to form a new binary or multiple phase, theformation of new phase generally for the refinement phase, and wasdifferent from the strengthen phase Mg2Si, but excessive Sr did not playa modification role any more.