Effect of Steels on the Purity of Molten Mg Alloys

Abstract

Magnesium (Mg) alloys have received extensive attention in industrial applications due to their low density, low modulus of elasticity, high-specific strength, and high-specific stiffness. [1-4] However, the low-temperature formability [5] and low corrosion resistance [6-8] of Mg alloys limit their large-scale applications. Many researches have indicated that the impurity elements, Fe, Ni, Cu, Co, in Mg alloys can strongly increase their corrosion rate, [9-11] and also severely deteriorate the mechanical [12-14] and damping properties. [15] When the impurities content in alloys exceeds the "tolerance limit" (10 ppm magnitude), the corrosion rate of Mg alloys will be exponentially accelerated. [16] Therefore, it is of great significance to strictly control the purity of the Mg alloys, especially the purity of the molten alloys before casting. Unfortunately, it is almost impossible to completely avoid these impurities during the production of Mg alloys. First, during Mg-metal smelting, no matter whether the Pidgeon method or the electrolytic magnesium method is used, there are always some impurities picked up from the raw materials. Second, these elements existing in the steel tools and steel crucibles can be dissolved in the molten Mg alloys during the melting process. [17-20] The purification of the molten Mg and Mg alloys has been studied extensively. [21-24] However, the effect resulting from defined tools on the Mg alloys production is rarely studied. [19,25] Scharf and Ditze [19] studied the interaction layers between AZ91 and AS31 with steel crucibles. The dissolution of iron, lowcarbon steel, and SUS316 (11wt% Ni, 17wt% Cr, remainder Fe) crucibles in commercial pure molten Mg was investigated by Taninouchi et al. [25] However, the influence of the types of used steels on the purity of molten Mg alloys has not been studied. Only by understanding the specific impact of elements coming from the tooling materials on the purity of molten Mg alloys can assure the purity of Mg alloys. This work aims to quantitatively determine the effect of tooling materials on the purity of Mg alloys melt. The most commonly cast Mg alloys, AZ91D, and AM50A, were studied. Three types of common tooling materials used in the production of Mg alloys: 20# steel, H13 steel, High-CrNi (022Cr25Ni7Mo4N) steel, were studied. Low carbon steel (here 20#) is the most common material used as the crucible materials for the Mg alloys synthesis. H13 steel is commonly used for Mg alloys casting molds. High-CrNi heat-resistant steel is commonly seen for the reduction tank in the Pidgeon process. The interactions between these tooling materials and Mg alloys were pre-evaluated using the Thermo-Calc 2019 software [26] based on the TCMG5 thermodynamic database. The influence of tooling materials on the purity of molten Mg alloys was experimentally studied using the liquid-solid diffusion method. What is more, the thermodynamic information is very important for the effective alloy design as well as the prediction of the distribution of impurities in al...

Effects of Alloying Elements on the Dissolution and Precipitation Behaviour of Fe in Mg-Al Alloy Melts

“…However, with the increased requirements regarding the quality of raw materials, the effects of impurities in the alloys have attracted increasing attention. As shown in our previous study, elements Fe, Ni, and Cr can all be dissolved in Mg melts and then remain in the alloys [32]. Our previous study successfully resulted in the Ni in the Mg material being below 5 ppm using a directional solidification purification method [25].…”

“…As Fe impurities are inevitably incorporated into the Mg alloys during the commercial melting and casting process, the Fe solution in the Mg alloys and the effects from Fe on the properties of Mg alloys have been studied for a long time [18][19][20][21][22]. Based on the current available synthesizing process, many micro-alloying methods have been proposed and have been proven to be successful strategies for Fe removal in Mg alloys [29][30][31][32][33].…”

Dissolution Behavior of Ferritic Stainless Steel in Liquid Magnesium

“…Representative studies on the quantitative dissolution of steel materials in Mg or Mg-alloy melts. 9,[16][17][18][19][20] Authors, Year Steel/Melt Temperature Ref. 21) The concentration of Cr in the SUS430 was approximately 16 mass%.…”

“…Quantitative data about the dissolution of various steel materials in Mg and its alloy melts and about changes in the steel surfaces due to contact with the melts can provide insights for improving impurity control in industrial processes such as Mg alloy production and Ti smelting. However, the number of studies on this subject are limited, [9][10][11][12][13][14][15][16][17][18][19][20] and the details are still unclear.…”

“…Table 1 lists previous studies on the quantitative dissolution of steels in Mg-based melts. 9,[16][17][18][19][20] For ferritic stainless steels, Kaitoh et al melted high-purity Mg in a SUS430 crucible and reported the impurity concentration in the Mg. 16) The surface of the SUS430 crucible was pre-treated to form a MgO layer; after melting Mg at 973 K for 1.5 h, the concentrations of Fe and Cr in the Mg were determined to be 0.001 mass% and <0.001 mass%, respectively. Sonoda et al melted a non-combustible Mg alloy (AZX912) in a SUS430 crucible and reported the impurity uptake and its effect on the corrosion resistance of the Mg alloy.…”