The Microstructure And Mechanical Properties Of Low-cost Ultrahigh Mg-Al-Ca-Mn Alloys

The current ultrahigh magnesium alloys mainly own high contents of rare-earth element. It is essential to develop the low-cost, free rare-earth element and ultrahigh magnesium alloys for widening its scope of application. In this thesis, alloys which own different contents of Al and Ca element was fabricated and extruded. T he influence mechanism of alloying elements and processing technology to the microstructure and mechanical properties has been investigated by observe the microstructure and test the mechanical properties. The low-cost ultrahigh wrought Mg-Al-Ca-Mn has been manufactured.The casting process significantly affects the microstructure and mechanical properties of Mg-Al-Ca-Mn alloys. The casting process influences the microstructure and mechanical properties of as-extruded alloys by exerting a direct influence on the as-cast microstructure. Because the cooling rate of the direct chill casting is significantly higher than the permanent mould casting, the grain size of alloy which is fabricated by water-cooling casting are smaller the alloy smelted by permanent mould casting. As the cooling rate increase, the structure of second phase transforms from(Mg,Al)2Ca to Mg2 Ca and the average inter-lameller spacing get smaller. After extrusion, the alloys fabricated by direct chill casting own more smaller recrystallization grain and the ratio of unrecrystallization zones and strength increase. The yield strength of Mg-3Al-2.7Ca-0.4Mn alloy increases from 332 MPa to 365 MPa through different casting process.Alloying element Al and Ca strongly affect the microstructure and mechanical properties of Mg-Al-Ca-Mn alloys. The majority of the alloys’ microstructure is second phase when the total content of Al and Ca is 18 wt.%. Howev er, when the total content is 6wt.%, the matrix is mainly composed of α-Mg and the alloy own the dendritic structure with second phase along the grain boundary. As the ratio of Ca and Al content increasing, the grain size decreases and content of second ph ase increases. After hot extrusion, the size of the recrystallization grain become smaller and the intensity of alloys’ texture increase, which lead to the increase of alloys’ strength.The grain size decreases and strength increase with the increasing of Mn’s content. The strength of as-extruded Mg-3Al-2.7Ca-0.4Mn alloy, which owns 0.4wt.% Mn elemen, is 40 MPa higher than Mg-3Al-2.7Ca alloy. Regrettably, the addition of micro alloying of Zn element hardly affect the microstructure and mechanical properties of Mg-Al-Ca-Mn alloys.The extrusion temperature strongly influence the mechanical properties of alloys. The size of the recrystallization grain decrease and the strength of alloys increase as the extrusion temperature decrease. For Mg-2.7Al-3.5Ca-0.4Mn alloy, the ultimate tensile strength increase from 421 MPa to 454 MPa with extrusion temperature decrease from 350 oC to 300 oC.