Effects Of Mn On The Structure And Melt Solidification Characteristics Of Carbon-inoculated Mg-3Al Alloy

Mn is an important alloying element in commercial Mg-Al base alloys.The main purpose of Mn addition is to avoid the negative effect of impurity Fe element on the corrosion resistance of Mg alloys.There existed an ambiguous research result about the effect of Mn on the microstructure of Mg-Al alloys.In the present study,the effect of Mn on the microstructure of Mg-3Al alloy was studied by adding different Mn additives.The effect of operating sequence of Mn addition and carbon inoculation on grain refining efficiency of Mg-3Al alloy was investigated.Finally,computer-aided cooling curve analysis(CA-CCA)was utilized to disclose the solidification characteristics of Mg-3Al melt treated by different routes.The relationship between grain refining efficiency and solidification characteristics was explained.The influence of Mn content on the grain size of Mg-3Al varied with different Mn additives.Mg-3Al alloy could be refined by Mg-10 Mn and Al-20 Mn when the Mn addition content was 0.3%.However,obvious coarsening occurred when MnCl2 was added into Mg-3Al melt.The Al-Mn phase formed in the alloy modified by Mg-10 Mn and Al-20 Mn master alloys mainly contained Al8Mn5 when Mn addition was 0.3%.And Al-Mn phase turned to be Al0.89Mn1.11 when Mn addition was 0.5%.When modified by MnCl2,the Al-Mn phase was mainly Al10Mn3.The crystallography misfitting results showed that Al8Mn5 can act as crystallize nuclei with unsignificant efficiency.Al0.89Mn1.11 and Al10Mn3 cannot act as crystallize nuclei.The grain size of Mg-3Al was coarsened after being modified by MnCl2 due to Al consumption in the formation of Al10Mn3.The effect of Mn on the grain refinement of carbon-inoculated Mg-3Al alloy was associated with the operating sequence of Mn addition and carbon inoculation.When Mn was added before carbon inoculation,there existed two kinds of partilces in the sample.They were Al-C-O particles and particles with duplex phase structure of Al-C-O coated on Al-(C)-Mn.Consequently,the grain was refined.When carbon inoculation was treated before Mn addition,the particle of Al-C-Mn-rich metallic compounds was formed in addition to the particles with duplex phase structure of Al-C-O coated on Al-(C)-Mn.The Al-C-Mn-rich metallic compounds cannot act as potent nuclei,thus the grain was coarsened.Occurrence of this phenomenon was independent with the Mn additives.Carbon inoculation could remarkably refine the Mg-3Al and Mg-3Al-0.3Mn alloy.With prolonging the holding time,the nuclei in Mg-3Al were stable thus less fading effect occurred.But the nuclei in Mg-3Al-0.3Mn were poorly stable and hence significant fading effect occurred.Carbon inoculation has evident influence on solidification characteristics of the alloys.Without carbon inoculation,recalescence phenomenon occurred during solidification and the melt had small nucleation undercooling(the difference between strating and minimum nucleation temperature).After carbon inoculation and being held for a short time,the grain was remarkably refined.In addition,recalescence phenomenon disappeared and large nucleation undercooling was obtained.The grain was coarsened when the holding time was prolonged or the operating sequence was turned reversely.Meanwhile,recalescence phenomenon occurred again and the nucleation undercooling decreased.When treated by carbon inoculation combining with Sr addition,the grain was significantly refined and recalescence phenomenon disappeared regardless of the operating sequence of Mn(Fe)addition and carbon inoculation.Moreover,the nucleation undercoolings of the melt treated by carbon inoculation combining with Sr addition were larger than those of the melt treated only by carbon inoculation.The recalescence and nucleation undercooling can reflect the grain refining efficiency of Mg-Al alloy under the same solidification condition.Recalescence phenomenon disappeared if the melt has been effectively refined.There exists a positive correlation between the nucleation undercooling and grain refining efficiency,i.e.,the larger the nucleation undercooling was,the better the grain was refined.The results obtained in the present study have important reference value on the selection of Mn additives and widening the applications of carbon inoculation refining technology applying in the commercial Mg alloys.And also,the results can provide guidance to apply thermal analysis technology to rapid pre-furnace detection of grain refining efficiency of the inoculated melt and quality control in Mg alloys industry.