Iron Pickup of AZ91 and AS31 Magnesium Melts in Steel Crucibles

Abstract

The application and use of magnesium alloys in the transport sector is unbowed. For this, it is necessary dealing with the topic about behavior of alloys with melting equipment, which is the main focus of the described investigations, and it is closely connected and also interesting for the recycling [1][2][3][4][5] of magnesium alloys.Typically, magnesium alloys are molten in steel crucibles because the solubility of iron in pure liquid magnesium and in magnesium alloys with suitable manganese contents is small. [6][7][8] Manganese is added to control the iron content. But especially at high temperatures and with inappropriate manganese content, magnesium and the alloying elements can react with the steel crucible. Changes in the melt temperature lead to sludge formation. It is necessary to remove sludge and dross, which forms by the reaction of the melt surface with air, getting a clean melt. However, this process causes considerable metal loss. Additionally, improper iron contents and inclusions of intermetallic particles affect negatively the quality of castings. Furthermore an inadequate ingot composition reduces the lifetime of the crucibles. Studies investigating the reactions of magnesium alloys AZ91 and AS31 [9] with steel of the crucible confirm the formation of intermetallic layers between steel of the crucible and magnesium alloy melt. The intermetallic phases are assumed to be in equilibrium with the melt. Thus, using this information, the iron content of the melt is determined. The knowledge of equilibrium phases and their behavior makes statements possible about crucible wear and the formation of sludge. The alloying elements aluminum, manganese and silicon, which are responsible for the formation of intermetallic phases, play a crucial role.In literature, information on the precipitation of aluminum-manganese phases has been reported. The Mg-Al-Mn system was thoroughly investigated by Ohno et al. [10] They identified two equilibrium phases precipitating from the metal, which are Al 8 Mn 5 for 9 % aluminum and b-Mn and Al 8 Mn 5 for 3 % aluminum. Their results also make possible to determine the manganese content, which depends on the aluminum content and temperature. These phases are confirmed by Bakke [11] for 660 and 700°C. Additionally, the influence of 0.005 % Fe is specified. This low iron content shifts the manganese solubility to considerable smaller values and changes the equilibrium phases into Al 8 (Mn,Fe) 5 in alloys with high aluminum content and to a-AlMnFe in alloys with a 3 % aluminum content. The solubility of Mn in Mg-Al-Mn is described by Thorvaldsen et. al. [12] as well, valid for 5-11 % Al, < 1.5 % Zn and < 20 ppm Fe. The mutual solubility of iron and manganese in AZ91 and AM50 melts is stated by Hillis et al. [13] Figures 1 and 2. The knowledge of solubility and precipitating intermetallic phases supports the estimation of sludge amount, located in the melt after decrease of temperature. 0 0,01 0,02 0,03 0,04 0,05 0,06 0 0,1 0,2 0,3 0,4 0,5 %M n %Fe A8 680ºC A8 730ºC A8 770ºC ...

Corrosion Behavior of Pure Magnesium with Low Iron Content in 3.5 wt% NaCl Solution

“…5 The corrosion tolerance limit for Fe in pure Mg is reported to be 170 ppm 4 (throughout the paper, the unit ppm denotes the mass fraction, μg/g.). The corrosion rate of Mg in a neutral chloride-containing aqueous environment is usually very low if the content of Fe is below the tolerance limit, whereas it increases substantially when the Fe content exceeds 170 ppm.…”

Effects of processing variables on melt cleanliness of AZ31B magnesium alloy investigated by quantitative evaluation of Fe and non-metallic inclusions

“…The solubility of Fe in liquid did not change with the Mg content in the Mg-Fe binary alloy. However, the solubility of Fe in liquid dramatically decreased with an increase in the Mg content in the Mg-3.05Al-0.8775Zn-0.35Mn-Fe alloy, which resulted from the addition of Mn, as reported in previous studies [8][9][10][11]. Although the solubility of Fe in liquid decreased remarkably with the addition of Mn, the solubility of Fe in liquid was much higher than the Fe content in the raw material.…”

“…The ZrC x layer acted as an effective diffusion barrier between the Fe and Zr, which prevented the dissolution of Fe into the liquid and the formation of an Fe-Zr intermetallic compound. Scharf and Ditze [10] reported that an intermetallic layer consisting of Al, Fe, and Mn formed on the surface of a steel crucible containing Mg melt, including Al and Mn as major alloying elements, by diffusion of Al and Mn into the steel crucible. The inter- metallic layer was dense and compact and thus could be considered an equilibrium phase of the melt.…”

Directional Solidification of Mg-Al Alloys and Microsegregation Study of Mg Alloys AZ31 and AM50: Part I. Methodology

“…Scharf and Ditze [22] reported recently that sludge and dross formation, as well as a rising iron content and inclusions of intermetallic particles in the melt, are consequences of this effect.…”

“…This transfer of iron-rich particles into the magnesium melt may eventually result in the formation of what is known as intermetallic sludge. [19,22] The alternative V4A steel was also tested for 19 days at 650°C in contact with AZ62 and AZ91 melts. The interface investigation showed a behavior similar to St35 steel.…”