The development of secondary magnesium alloys requires a completely different concept compared with standard alloys which obtain their corrosion resistance by reducing the levels of impurities below certain alloy and process depending limits. The present approach suitable for Mg-Al based cast and wrought alloys uses a new concept replacing the ß-phase by τ-phase, which is able to incorporate more impurities while being electrochemically less detrimental to the matrix. The overall experimental effort correlating composition, microstructure and corrosion resistance was reduced by using thermodynamic calculations to optimise the alloy composition. The outcome is a new, more impurity tolerant alloy class with a composition between the standard AZ and ZC systems having sufficient ductility and corrosion properties comparable to the high purity standard alloys.
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 ...
In respect to the current magnesium alloy recycling situation, an increase in post consumer scrap should be expected. Both shredder and dismantled scrap contain copper, nickel and silicon. [2,5] Consequently, after a remelting process, the concentrations rise above the currently specified tolerance limits in AZ 91±type alloys and lead to an accelerated corrosion of the material. Removing the copper, nickel and silicon is complex. A solution lies in the development of new contaminant-tolerant secondary magnesium alloys. The requirements of such alloys include adequate corrosion and strength properties.Various authors [1,3,7,8,10,11] have already studied the corrosion behaviour of magnesium AZ 91±type alloys. They all make reference to a complicated corrosion mechanism, differing from that of pure magnesium and aluminium alloys. Corrosion studies on AZ, AM, AS±types of magnesium alloys carried out in different electrolytes containing chloride, sulphate, carbonate and other anions [15±17] show that the corrosion behaviour of magnesium alloys in different electrolytes varies and strongly depends on the pH of the solution.However, in spite of this varying behaviour, the basic overall reaction is:And, when acid is added for pH-control:These equations are also used in this paper for interpreting all of the corrosion experiments. Furthermore, the most common form of corrosion is localized microgalvanic corrosion, Figure 1.The effect the intermetallics have on corrosion is greater, the more positive the OCP (open circuit potential) is. However, many authors, including, [7] view the tolerance limit for the copper content as being too low.Results: The following possibilities can improve the corrosion resistance: ± preventing the segregation of intermetallic phases, i.e. through rapid solidification.± binding the impurities by chemical combination in base phases, enclosing with the b-phase (Mg 17 Al 12 ).± manipulating the microstructure, i.e. grain size, distribution of the phases, heat treatment, casting conditions. ± developing protective layers using micro-alloying elements.The structure of the AZ 91 is characterized by the a-phase (magnesium matrix), eutectic magnesium and intermetallic compounds, which segregate primary (Al 8 Mn 5 ) or secondary COMMUNICATIONS [**] The authors wish to thank the Deutsche Forschungsgemeinschaft for financial support for the project within the scope of SPP 1168. Fig 1. Microgalvanic corrosion of magnesium alloy. Fig. 2. In-situ atomic force microscopy and cross-section analysis of alloy 1. Corrosion in 3.5 % NaCl solution at pH=11.
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