Grain Refiners And Their Refining Behaviors In Mg-Al Based Mg Alloys

Magnesium alloys have been considered as green engineering materials in21st century, the refinement of the Mg-Al based alloys by means of the refiner not only reduces the processing costs to meet the increasing demand for various kinds of magnesium alloys, but also promots their wider application in the automotive, aerospace, communications, electronics, mechanical fields and so on. Therefore, a lot of studies have been done by the worldwide researchers, but the breakthrough was not obtained until now. In this work, the refinement behavior has been investigated through optical microscope, X-Ray diffraction(XRD), scanning electron microscopy (SEM) and energy dispersive spectropy of X-ray (EDS) when the ZnCO3, Al-Mn alloy and Al-Co alloy were added to the AM60and AZ31magnesium alloys. The main results are as follows:There is the refining effect in the AZ31and AM60magnesium alloys added by the ZnCO3. When ZnCO3was added to the AZ31magnesium alloy from0.3wt.%to1.2wt.%, the grain size remarkably decreases. When the more addition of the ZnCO3was added, the grain size again grows. The grain size of AZ31cast Mg alloy is remarkably refined with the addition of ZnCO3(1.2wt.%) when held at the moulding temperature of750℃for30minutes, i.e., decreased from90μm to52μm. When ZnCO3was added to the AM60magnesium alloy from0.3wt.%to1.2wt.%, the grain size remarkably decreases. The grain size of AM60cast Mg alloy is remarkably refined with the addition of ZnCO3(1.2wt.%) when held at the moulding temperature of750℃for45minutes, i.e., decreased from90μm to45μm. It is shown from the microstructure observation and composition analysis of the as-cast AM60Mg alloy that the refinement is contributed to the heterogeneous nucleation by the ZnO formed by the decomposition of the ZnCO3.When the Al-66wt.%Mn master alloy, which is composed of Al8Mn5and ε-AlMn phases, is added to the AM60magnesium alloy, the grain refinement is remarkable. Moreover, the refining efficiency is hardly affected by the adding amount of Al-66wt.%Mn alloy after its weight content is larger than1wt.%. The most refining efficency is obtained with the addition of Al-66wt.%Mn alloy(1wt.%) and the holding time of30minutes at the moulding temperature of750℃, i.e., decreased by50%from90μm to45μm. On the other hand, when the Al-70wt.%Co master alloy, which is composed of single AlCo phase, is added to the AM60magnesium alloy, the grain refinement is also remarkable. Moreover, the refining efficiency is hardly affected by the adding amount of Al-70wt.%Co alloy after its weight content is larger than1wt.%. The most refining efficency is obtained with the addition of Al-70wt.%Co alloy(1wt.%) and the holding time of30minutes at the moulding temperature of750℃, i.e., decreased from90μm to48μm. This shows that the Al-Mn master alloy containing ε-AlMn phasewith the hcp structure and the Al-Co master alloy composed of single AlCo phase with the bcc structure has the same refinement effect when added into the AM60Mg alloy, respectively. This means that the lattice match between the intermetallic compound and a-Mg matrix affects the grain refinement, as well as the growth constraining factor of the element.In summary, the grain size of the AZ31or AM60Mg alloys can be effectively refined by the addition of either ZnCO3, Al-Mn master alloy, or Al-Co master alloy, respectively. The grain sizes of AZ31or AM60Mg alloys decreased by44%,50%50%and46%, respectively. The hardness of the AZ31and AM60magnesium alloy is also improved by the either addition of the three kinds of the refiners mentioned above.