CN1587425A - Magnesium alloy smelting method using difluoromethane as protective atmosphere - Google Patents

Abstract

A magnesium alloy smelting method using difluoromethane as a protective atmosphere, mixing difluoromethane with a volume content of 0.05-5% with dry air or/and carbon dioxide in a gas mixing device as an atmosphere protective agent for magnesium melts, Put pure magnesium or magnesium alloy in a closed crucible and heat up, then pass the mixed gas into the crucible, the flow rate of the mixed gas per minute is 0.05-10% of the volume of the closed gas, so that the gas covers the surface of the magnesium melt to form a layer Dense oxide protective film, continue to heat up and refine and deteriorate under gas protection, then gravity pour or pressure casting after standing still. The protection gas adopted in the invention can effectively prevent the oxidation and combustion of the magnesium alloy during the smelting process, and has good protection effect, low price and low greenhouse effect.

Description

Magnesium Alloy Melting Method Using Difluoromethane as Protective Atmosphere

technical field

The invention relates to a high-temperature oxidation and combustion protection technology in the smelting process of pure magnesium or conventional magnesium alloys, in particular to the protection of pure magnesium or magnesium alloys using a mixed gas containing difluoromethane (its trade name is R32) as the protective atmosphere The smelting method belongs to the technical field of material metallurgy.

Background technique

Due to the oxophilicity of metal magnesium, pure magnesium or magnesium alloys are easily oxidized in the air. Since the density coefficient of its oxidation product, magnesium oxide, is less than 1, this naturally occurring oxide film cannot constitute an effective barrier to prevent the internal Further oxidation of the metal, pure magnesium or magnesium alloys are also prone to combustion at high temperatures. Therefore, in the industry, the mixed gas containing sulfur hexafluoride is generally used as the protective atmosphere for magnesium alloy smelting and production, which is one of the most effective protection methods for magnesium alloy smelting. In recent years, with the rapid development of the magnesium industry, the amount of protective gases such as sulfur hexafluoride required for the smelting and production of pure magnesium or magnesium alloys is increasing. However, the global warming potential (GWP) of sulfur hexafluoride reaches 23900 (the GWP value of carbon dioxide is 1), and the atmospheric life is as long as 3200 years. The pressure is increasing, and it is listed as a substance that needs to be reduced by the international environmental protection organization. The "Kyoto Protocol" signed at the 5th United Nations Framework Convention on Climate Change requires the industry to minimize hexafluoride in the next 10 years. Sulfur gas emissions, to achieve zero emissions of sulfur hexafluoride by 2015 (U.S.EPA, 2003b). Therefore, the International Magnesium Association (IMA) requires that the use of sulfur hexafluoride be reduced by improving the efficiency of use, and that the magnesium industry should achieve zero emission of sulfur hexafluoride before 2015.

比六氟化硫对环境更加友好，但这三种气体的GWP值仍然较高，均超过1000，而且后两种气体的保护效果还在评价当中，目前还未得到确认；同时，HFE7100和Novec^TM612的价格较高，国内尚无生产，明显将限制其工业应用的可行性。因此继续寻找价格更低廉，环境影响更小的替代气体及其熔炼保护工艺一直是镁工业的目标。In order to achieve the above goals without affecting the development of the magnesium industry, international efforts are being made to find a substitute for sulfur hexafluoride and to develop a melting protection process for pure magnesium or magnesium alloys. The results of the survey on the new protective atmosphere with low environmental impact found that only a few alternative atmospheres have been proposed. The most typical representatives are three alternatives that are being evaluated by the International Magnesium Association for their protective effects and environmental loads: AMCover ^TM ( HFC-134a), HFE7100 (C ₄ F ₉ -O-CH ₃ ) and Novec ^™ 612 (C ₃ F ₇ COC ₂ F ₅ ). The former was invented by the Australian CAST Research Center, and the latter two were invented by 3M. Although these three gases and smelting protection processes have been initially confirmed to be effective for pure magnesium or magnesium alloys, and their greenhouse effect is only as strong as that of sulfur hexafluoride.

It is more environmentally friendly than sulfur hexafluoride, but the GWP values of these three gases are still high, all exceeding 1000, and the protective effects of the latter two gases are still under evaluation and have not yet been confirmed; meanwhile, HFE7100 and Novec The price of ^TM 612 is relatively high, and there is no domestic production, which will obviously limit the feasibility of its industrial application. Therefore, it has always been the goal of the magnesium industry to continue to look for alternative gases with lower prices and less environmental impact and their smelting protection processes.

Contents of the invention

The object of the present invention is to address the deficiencies of the prior art, to provide a magnesium alloy smelting method, using a material whose protective effect on the magnesium melt is equivalent to that of sulfur hexafluoride, and whose environmental protection effect is better than that of sulfur hexafluoride. It replaces sulfur hexafluoride as an atmosphere protective agent for magnesium melt, reduces process cost, has good smelting protection effect, and does not cause pollution to the environment.

In order to achieve such purpose, the present invention adopts difluoromethane to replace sulfur hexafluoride as the atmosphere protective agent of magnesium melt, and the smelting process mainly includes the following steps:

1. Mix difluoromethane (its trade name is R32) and auxiliary diluent gas in a gas mixing device, and the volume content of difluoromethane in the mixed gas is 0.05-5%. The auxiliary diluting gas may be dry air, dry carbon dioxide or other inert gases, or a mixture thereof.

2. Put pure magnesium or magnesium alloy in a closed crucible and heat up.

3. When the temperature of pure magnesium or magnesium alloy reaches 400°C, start to pass the mixed gas into the crucible so that the gas covers the surface of pure magnesium or magnesium alloy. The flow rate of the mixed gas per minute is 0.05-10% of the volume of the closed gas .

4. Continue heating until the pure magnesium or magnesium alloy melts, so that the protective gas quickly forms a dense and protective oxide film on the surface of the melt.

5. When the temperature of the pure magnesium or magnesium alloy melt is raised to 700-850°C, it is refined and modified under gas protection, and then it is left to stand for 30-40 minutes before gravity pouring or pressure casting.

During the entire melting and pouring process of pure magnesium or magnesium alloy, the mixed gas containing difluoromethane undergoes a complex chemical reaction with the surface of the molten fresh magnesium liquid, which improves the structure of the natural oxide film and makes the surface film dense, complete and It is elastic, closely combined with the molten metal, and completely covers the surface of the melt, separating the magnesium melt from the air, greatly reducing the oxidation or burning speed of the magnesium melt, so that pure magnesium or magnesium alloys can be melted during smelting be protected. When the old oxide film is removed during smelting or pouring, a dense protective new oxide film will quickly form on the surface of the melt, covering the surface of the magnesium melt again, and continue to effectively protect the magnesium melt from being oxidized or burned.

The characteristics of the present invention are: (1) the protection effect of the atmosphere containing difluoromethane on the magnesium melt is equivalent to that of the atmosphere containing sulfur hexafluoride, and the content of difluoromethane is 0.05-5% (volume) in the whole melting temperature range It has a good protective effect on magnesium melt immediately. (2) Under optimized conditions, the density, smoothness, and thickness of the formed surface film are very similar to those formed by the atmosphere containing sulfur hexafluoride, and the protective effect on the magnesium melt is almost the same. (3) Difluoromethane is a stable, non-toxic, and non-corrosive substance at room temperature. It will decompose at high temperature, and its decomposition products can be consumed by chemical reaction with magnesium. The exhaust gas will not cause harm to the environment and operators. Hazard; (4) The global warming potential (GWP) of difluoromethane is about 650 (about that of sulfur hexafluoride ), the atmospheric life is only 5 years, even if difluoromethane is emitted into the atmosphere during the magnesium alloy smelting process, its greenhouse effect is far lower than that of sulfur hexafluoride. (5) Difluoromethane has been industrialized in China, and it is easy to buy and cheap. Its market price is about that of sulfur hexafluoride. Compared with sulfur hexafluoride, the amount and efficiency of the former in magnesium smelting are almost the same, and the smelting protection process is simple. Therefore, the economic effect of replacing sulfur hexafluoride with difluoromethane as the protective atmosphere of pure magnesium or magnesium alloy is very considerable.

Detailed ways

With reference to the description of the following specific embodiments, the technical solutions and advantages of the present invention can be clearly understood.

Embodiment 1: Protective smelting of AM60 magnesium alloy by difluoromethane

The implementation steps are: (1) difluoromethane is mixed with dry air or carbon dioxide in the gas mixing device according to the various mixing ratios given in Table 1; (2) 3 kg of clean AM60 magnesium alloy is mixed in a 5 kg resistance crucible Heating in the furnace; (3) When the temperature of the alloy exceeds 400°C, start to feed the mixed gas into the crucible, and feed the mixed gas into the crucible according to the flow parameters given in Table 1, so that the gas covers the surface of the alloy; (4) Magnesium After the alloy continues to heat up to melting, the protective gas quickly forms a layer of dense and well-protected oxide film on the surface of the melt; (5) The magnesium alloy melt is heated to 70-850 ° C, and under the protection of the gas Carry out refining and metamorphism, then after standing still for 30 to 40 minutes, carry out gravity pouring or pressure casting.

During the whole melting process, the protection effect of various melting process parameters was continuously observed, and the observation results were also recorded in Table 1. It can be seen from Table 1 that when AM60 magnesium alloy is smelted under the protection of a mixed atmosphere containing difluoromethane, under the various smelting temperatures, various blowing flow rates and holding times set in Table 1, even if surface agitation occurs, all Can quickly form a dense, non-combustible surface protection layer.

Table 1 serial number Protect process parameters Smelting protection effect Melt temperature °C Mixed gas ratio (volume) Mixed gas flow(%/min) diluent gas Hold time (minutes) surface agitation film color, smoothness Film formation rate film thickness Flammability 1 670 0.05% 0.05 Air 1 have bright, flat quick Thin non-combustible 2 700 0.12% 0.5 Air 19 have bright, wrinkled quick Thin non-combustible 3 720 0.05% 1 carbon dioxide 14 have bright, flat quick Thin non-combustible 4 750 0.05% 1 carbon dioxide 11 have bright, flat quick thicker non-combustible 5 780 0.1% 2 carbon dioxide 7 have bright, wrinkled quick thick non-combustible 6 800 2% 2 air + carbon dioxide 4 have bright, wrinkled quick thick non-combustible 7 820 2% 3 air + carbon dioxide 5 have bright, wrinkled quick thick non-combustible 8 820 5% 10 air + carbon dioxide 10 have gray, wrinkled quick thick non-combustible

Embodiment 2: Protective melting of difluoromethane to AZ91 magnesium alloy

The implementation steps are: (1) difluoromethane is mixed with dry air or carbon dioxide in the gas mixing device according to the various mixing ratios given in Table 2; (2) 3 kg of clean AZ91 magnesium alloy is mixed in a 5 kg resistance crucible Heating in the furnace; (3) When the temperature of the alloy exceeds 400°C, start to feed the mixed gas into the crucible, and feed the mixed gas into the crucible according to the flow parameters given in Table 2, so that the gas covers the surface of the alloy; (4) Magnesium After the alloy continues to heat up to melting, the protective gas quickly forms a dense and well-protected oxide film on the surface of the melt; (5) The magnesium alloy melt is heated to 700-850 ° C, and under the protection of the gas Carry out refining and metamorphism, then after standing still for 30 to 40 minutes, carry out gravity pouring or pressure casting.

The protection effect of various melting process parameters was continuously observed during the whole melting process, and the observation results were also recorded in Table 2. It can be seen from Table 2 that when the AZ91 magnesium alloy is smelted under the protection of a mixed atmosphere containing difluoromethane, under the various smelting temperatures, various blowing flow rates and holding times set in Table 2, even if surface agitation occurs, all Can quickly form a dense, non-combustible surface protection layer.

Table 2 serial number Protect process parameters Smelting protection effect Melting temperature ℃ Mixed gas ratio (volume) Mixed gas flow(%/min) diluent gas Hold time (minutes) surface agitation Film color, smoothness Film formation rate film thickness Flammability 1 650 0.05% 0.05 Air 40 have bright, wrinkled quick Thin non-combustible 2 700 0.15% 0.5 Air 17 have bright, wrinkled quick Thin non-combustible 3 720 0.4% 1 carbon dioxide 25 have bright, flat quick Thin non-combustible 4 750 1% 1 carbon dioxide 10 have bright, flat quick Thin non-combustible 5 780 1% 2 carbon dioxide 25 have bright, flat quick Thin non-combustible 6 800 5% 10 air + carbon dioxide 10 have bright, wrinkled quick Thin non-combustible

Embodiment 3: the protective smelting of difluoromethane to pure magnesium

Implementation steps: (1) difluoromethane is mixed with dry air or carbon dioxide in the gas mixing device according to the various mixing ratios given in Table 3; (2) 3 kg of clean pure magnesium is mixed in a 5 kg resistance crucible furnace Heating to raise the temperature; (3) when the temperature of pure magnesium exceeds 400°C, begin to feed the mixed gas into the crucible, and feed the mixed gas into the crucible according to the flow parameters given in Table 3, so that the gas is covered on the surface of the magnesium; (4) After the pure magnesium continues to heat up to melting, the protective gas quickly forms a dense and well-protected oxide film on the surface of the melt; Carry out refining and metamorphism, then after standing still for 30 to 40 minutes, carry out gravity pouring or pressure casting.

The protection effect of various melting process parameters was continuously observed during the whole melting process, and the observation results were also recorded in Table 3. It can be seen from Table 3 that when pure magnesium is smelted under the protection of a mixed atmosphere containing difluoromethane, under the various smelting temperatures, various blowing flow rates and holding times set in Table 3, even if surface agitation occurs, the Forms a dense, non-combustible surface protection layer relatively quickly.

table 3 serial number Protect process parameters Smelting protection effect Melting temperature ℃ Mixed gas ratio (volume) Mixed gas flow(%/min) diluent gas Hold time (minutes) surface agitation Film color, smoothness Film formation rate film thickness Flammability 1 680 0.05% 0.05 Air 1 have bright, flat quick Thin non-combustible 2 680 0.2% 0.5 Air 40 have bright, wrinkled quick thick non-combustible 3 700 0.1% 0.5 carbon dioxide 17 have bright, flat quick Thin non-combustible 4 720 0.4% 1 carbon dioxide 25 have bright, flat quick Thin non-combustible 5 750 0.4% 1 carbon dioxide 10 have bright, flat quick Thin non-combustible 6 780 5% 2 air + carbon dioxide 15 have bright, flat quick Thin non-combustible 7 800 5% 10 air + carbon dioxide 20 bright, wrinkled quick thicker non-combustible

The above examples illustrate that using a mixed gas containing difluoromethane as a protective atmosphere to smelt pure magnesium or magnesium alloy melts has a good protective effect in the entire temperature range of its smelting, which is comparable to the protection of sulfur hexafluoride. The effect is equivalent, and the smelting process is simple, and it has more environmental protection and economic advantages than sulfur hexafluoride.

Claims (2)

1, a kind of methylene fluoride that adopts is the magnesium alloy smelting method of protective atmosphere, it is characterized in that may further comprise the steps:

1) methylene fluoride is mixed in gas mixing device with auxiliary diluent gas, the volume content of methylene fluoride in hybrid protection gas is 0.05-5%;

2) pure magnesium or magnesium alloy are placed sealing crucible heat temperature raising;

3) when pure magnesium or magnesium alloy temperature surpass 400 ℃, begin mixed gas is fed crucible, make gas cover magnesium or Mg alloy surface, the mixed gas per minute flow of feeding is confining gas volumetrical 0.05-10%;

4) continue heat temperature raising to pure magnesium or magnesium alloy fusing, make shielding gas form the protective oxide layer of one deck densification at bath surface;

5) when pure magnesium or magnesium alloy fused mass are warming up to 700～850 ℃, under gas shield, carry out refinement and denaturation, leave standstill 30～40 minutes then after, carry out gravity casting or pressure die casting.

2, the employing methylene fluoride as claimed in claim 1 magnesium alloy smelting method that is protective atmosphere is characterized in that described auxiliary diluent gas is dry air, dry carbonic acid gas or its mixture.