Since magnesium is active, it is necessary to cover the protective gas when smelting magnesium. These protective gases can have a variety of effects on the environment or the foundry. In order to reduce the burden on the environment and workers caused by magnesium die casting, the new protective gas Novec 612 was tested in the casting room of Volkswagen, which significantly reduced the greenhouse gas value compared with the R134a currently used. The experiment successfully showed that the Novec 612 was also able to stably cover the melt of the magnesium furnace with a lower concentration of carrier gas. The protection mechanism has proven to be very effective. However, based on cost considerations, the new protective gas Novec 612 is currently not an economical solution.

Taking into account the environmental protection of cars and their emissions and the increasing environmental awareness of consumers, environmental protection regulations are becoming more stringent, so car manufacturers must take measures in terms of weight and fuel economy. At the same time, the public consumer's voice that the safety and comfort of the car can not be ignored, the weight of the car body needs to be reconsidered again. This requires automakers to give reasonable solutions that allow the car to be lighter in weight, while at the same time ensuring that the cost remains competitive. To achieve this goal, designers can take advantage of different lightweight construction options such as system lightweight construction, formwork lightweight construction, and lightweight construction. When it comes to lightweight construction of materials, that is, the use of low-density metals, such as magnesium, magnesium castings have outstanding advantages in terms of weight. Due to the active nature of magnesium, magnesium processing has extremely high requirements for casting between other lightweight structural materials.

Processing of molten magnesium

The processing of molten magnesium becomes difficult due to the oxidation characteristics of magnesium. The magnesium element is located in the second main group of the periodic table. Since magnesium has a potential of Uh=-2.372V in electrochemical electrokinetic properties, it is a non-noble metal and is highly reactive with oxygen. It is important to form a passivated oxide layer on the metal surface. Whether a passivated oxide layer is formed depends on the ratio of the molar volume (Vm) of the oxide formed to the molar metal volume. This ratio is also known as "PBV," (English: PBR). Described is the stability of the oxide layer. When this ratio is less than 1, the oxide layer will generate tensile stress and will continue to be torn. Next, look at the magnesium melt (PBV = 0,78), the magnesium melt exposed to the air (Figure 1), and the exothermic oxidation is not affected. In addition to the fact that the melt produces oxides due to contamination, it may also result from a chain reaction on the surface of the combustion pool.

When the PBV-ratio is greater than 1, a sealed passivation layer is generated due to the compressive stress of the oxide layer depending on various conditions. For example, when PBV of aluminum is 1.28. Since magnesium does not have such a property, it is protected by oxidation by a protective technique when reacting on the surface of the molten pool. There are special treatments for this, such as covering the surface of the bath with liquid salts or fusing in an inert-and protective gas atmosphere, thus resisting the generation of oxide layers that are not beneficial to magnesium and magnesium alloys.

Magnesium is melted in a protective atmosphere, and a fluorine- or sulfur-containing protective gas can be used. First, since this reaction MgO+F2=MgF2+1/2O2 (ΔG°1000K=-456766J/mol) has a very high formation 焓ΔG°1000K, it is necessary to calculate how many MgF2 in the protective layer to form a dense passivation with MgO. Layer, so that PBV>1 is reached.

In contrast, the corresponding reaction MgO+1/2S2=MgS+1/2O2 is positive.

(ΔG°1000K=178731 J/mol). The formation enthalpy of Mg+1/2S2=MgS (ΔG°1000K=-314899J/mol) is also very negative. However, the protected MgF2 reacts with the oxygen present to form MgO. Therefore, the content of SO2 in the shielding gas must be significantly higher than the content of the fluorine-containing compound. Figure 2 graphically illustrates the protection mechanism by which a fluorine-containing gas is supplied upon melting.

Larger MgF2 molecules are embedded in cracks in the ruptured oxide layer, thus causing compressive stress, which is extremely advantageous for oxidative behavior. That is to say, the development of oxidation behavior is hindered.

It is true that the use of protective gas imposes a burden on the environment and people, so the Volkswagen Group continues to find solutions that are environmentally and climate-friendly. An important parameter in these relationships is called the Global Warming Potential (GWP) of the shielding gas. The definition of the global warming potential value, compared with the reference gas CO2, the amount of protective gas gas investigated makes the greenhouse effect increase or decrease. Based on prospects, the Volve Casting Room in Kassel conducted a study on the use of Novec 612. There has been another alternative to the magnesium casting protection scheme, and the current situation does not require further development. An overview of alternative protection technologies is already available

Volkswagen's die casting workshop

The Volkswagen Group has 70 die casting machines in the casting room in Kassel, making it the largest light metal foundry in Europe. In addition to a wide range of aluminum die-cast parts, there are eight magnesium die-casting machines for the production of magnesium die-cast parts. A "magnesium-cold box-die casting" process is used to develop the final profile of the liquid magnesium and then machined into outer casings and structural parts. The order of the processing steps for the application of the casting depends on the part and the subsequent delivery to the assembly.

In 2008, with the option of replacing SF6 shielding gas, Kassel's Volkswagen casting room had to be traced back to how many magnesium processing plants used SF6 shielding gas when it was banned. Through strict health protection regulations, the Volkswagen Group believes that even though SO2 has a global warming potential of zero, it is not considered to be harmful because it is harmful to workers. For this reason, the Kassel Volkswagen Casting Room has put into use a more environmentally friendly protective gas R134a after its ban on the use of protective gas SF6 (GWP=23900), its GWP=1300. In the face of strict environmental regulations and special guidelines 2006/40/EG (EG: European Community), the use of R134a in air conditioners for certain models has been banned since January 1, 2011, and industrial use is also required to find alternatives to R134a. Program. An overview of alternative protection mechanisms is already available.

For the above reasons, the Volkswagen Group has carried out research on alternative protection gas R134a and SO2 solutions in many casting rooms. An alternative, under the trade name Novec612, is available at the 3M plant in Hilden, Germany. A brief introduction to this famous protective gas will be given below.

Novec 612 (C3F7C(O)C2F5) is a sulfur-containing protective gas and is liquid at room temperature. The protective effect of Novec 612 is based on the formation of the fluorine phase of MgF2 when R134a is used. Due to the extremely high activity of Novec 612, the manufacturer stated that it is usually only necessary to use 10% of the R134a concentration. Novec 612 is non-toxic and does not impose an additional burden on the foundry. The two characteristics of high magnesium activity and a GWP of 1 make Novec612 highly promising as a protective gas replacement for magnesium smelting.

Protected gas sulphur dioxide (SO2) is still considered an effective "successor" to SF6 in many magnesium processing plants. Its GWP value and residence time in the Earth's atmosphere is zero. It is stated in the technical documents that SO2 with a concentration of 0.5% to 1.5% is used together with dry air as a carrier gas, and it is sufficient to form a stable protective layer on the surface of the molten pool. The protective layer produced by magnesium oxide and magnesium sulfide prevents the contact of liquid magnesium with oxygen. The melting process can be steadily protected until the highest melting temperature of 740 degrees SO2. The disadvantage is that it is toxic and will put a health burden on the foundry workers. SO2 can cause coughing, inflammation of the respiratory tract and nausea. In addition to this, it can cause corrosion to machines and equipment. In order for the foundry to be protected from health, the maximum working chamber concentration (MAK) is not allowed to exceed 2 ppm SO2.

There is also a popular protective gas called tetrachloroethylene, also known as R134a. It is used by many foundries, and the Volkswagen Group is no exception. R134a is a fluorine-containing gas whose protective effect is based on the formation of magnesium fluoride (MgF2). In order to protect the surface of the molten pool, it is sufficient to use a concentration of 0.2% to 0.5%. At the same time, nitrogen acts as a carrier gas. Compared to SO2, R134a is not toxic and imposes minimal burden on foundry workers. Its MAK value is 1000 ppm. From an environmental point of view, R134a has a GWP of 1300, which is an excellent alternative to protective gas SF6 (GWP=23900). The most important characteristics of shielding gas are summarized in Figure 1. At present, in addition to toxic SO2, all available shielding gas can be used to form MgF2 as a protection mechanism.

Nitrogen and synthetic air are used as carrier gases. During the test, the flow rate of the carrier gas, the concentration of Novec, and the composition changed. The Novec concentration will vary from 1000 ppm to 80 ppm. In addition, different gas treatment equipment should be tested to optimize the gas distribution in the furnace space. In this case, the most detailed and uniform gas distribution is achieved by using special nozzles, which directly affects the Novec612 protection effect and achieves the best results. Tests have shown the suitability of Novec612 for magnesium casting protection. In addition, the initial parameters of the Novec 612 batch use should be ascertained and clarified. Convinced conclusions are: Novec 612 is suitable for the protection of the surface of the molten pool, and under the precise adjustment, it can protect the surface of the molten pool together with suitable gas treatment equipment.

Batch adoption

For the batch application of Novec612, it is necessary to modify the scheme slightly, and the gas should be finely and evenly distributed through special nozzles, which has been proved effective in laboratory tests. The processing of gas processing aids uses structural steel as a material. At the same time, the traditional gas distribution magnesium furnace must be modified. The series of equipment used for the test consisted of a pig iron preheating device and a die casting machine in a furnace (having a volume of approximately 2,500 kg). The melt is extracted by a pump. The shielding gas is made from a mixture of carrier gases from ASKI, a gas technology company in Schwerte (see Figure 4). Novec612 is used for 8 weeks. During the start-up phase of the endurance test, the Novec-concentration was added for smoking. The cleaning of the molten pool surface has brought about satisfactory results, such as changes in Novec-concentration, reduction in total volume flow, and changes in carrier gas composition. Good protection requires only 15g/h Novec612, which corresponds to a shielding gas flow with a gas concentration of 200ppm. It should be noted that the surface area of the protected molten pool is about 1.65 m2 and the furnace is filled by a simple chute. Almost all parts of the surface of the bath are completely cleaned, and the top cover is briefly opened under protective conditions before the melt loss (Figure 5). The entire continuous process of the machine is run without interference. The results produced in this time zone are not outstanding but of good quality. During the test period, the Volkswagen Group measured and tested the amount of hydrofluoric acid produced in the computer room environment as evidence for the health protection of employees. It was subsequently found that even the values measured directly on the top cover were less than 0.01 mg/m3. Hydrogen fluoride cannot be confirmed. In any case, the Volkswagen Group undoubtedly followed the limit of 0.83mg/m3. Novec 612 also has a positive impact on mud development, so the amount of combustion is significantly reduced.

The gas treatment system should be designed and constructed for use as a special steel pipe system for other purposes. In contrast to the use of R134a, the Novec612 input must have a higher total volume flow (carrier gas) to protect the magnesium melt. Therefore, the gas treatment cost of Novec612 exceeds the gas treatment cost of R134a. The reason for the higher cost of Novec612 gas treatment is not that it is expensive (44.75 euros / kg), but the use of a large amount of carrier gas, which is necessary for effective protection. Therefore, the cost of Novec612 is more than twice that of R134a-gas treatment.

The gas treatment cost of Novec 612 based on the parameters determined in the test can be seen in Figure 7 and compared to R134a.

Discussion of effectiveness and prospects

Practical research using the environmentally friendly protective gas Novec612 indicates that its protective mechanism is similar to that of R134a. The protection mechanisms for both gases are based on the generation of the MgF2-stage.

Study of metal crumb layers, Novec 612 with 0.35% fluorine and R134a with 0.15% fluorine. Note that the gas treatment with Novec612 has a higher fluorine content in the metal chip layer. Novec 612 is an effective shielding gas due to its high fluorine content and heat resistance. Due to these two properties, Novec612 was separated at 575 degrees by the analysis of by-products, such as CF4, C2F4, C3F6. Of course, the heat resistance of the gas also has advantages in mixing synthetic air and carrier gas. The oxygen in the synthetic air acts as an oxidant to destroy the strong C-F bond. The fluorine molecules are then released, they react with magnesium and magnesium oxide, and then form a protective layer with the magnesium oxide. The reactions that occur are as follows:

2CF4+O2→2 COF2+2F2

It is important to note that the mixture must be in clear contact with the surface. In the tests carried out, this precaution was achieved through a fine and uniform gas distribution.

Another test followed the use of Novec 612, the gas was distributed through a conventional gas treatment unit (seven input lines) located in the furnace. This method requires a higher concentration of Novec than a fine distribution. Negative effects are strong corrosion fatigue. Conclusion: It is not recommended to dispense gases under confinement conditions using conventional gas treatment equipment. The high activity of Novec molecules in the melt makes people very concerned about the safety of the working environment when the machine fails. By using Novec 612, the fluorine atoms contained in the exhaust gas stream combined with the shielding gas are greatly reduced by 54%. This perception has led to a significant reduction in the dangerous potential of globalization in the magnesium casting process.

The highly active chemical advantage is reflected in the formation of mud. The schematic of Figure 1 can be taken as an illustration. As shown in the figure, the oxide particles formed on the surface sink to the bottom of the crucible and are subsequently deposited. This principle is supported by the ratio of the density of magnesium to magnesium oxide. When the magnesium oxide particles formed on the surface exceed the ultimate mass, the buoyancy is insufficient to keep the particles on the surface. The surface of the bath using the Novec 612 for gas treatment is significantly better, with the result that a smaller amount of oxide settles from the surface of the bath to the bottom of the crucible.