1. 1 magnesium chips are active and easy to burn at high temperature. In the process of magnesium alloy cutting, magnesium chips are mostly unoxidized magnesium and magnesium alloys. Because magnesium metal is a first-class flammable material when wet, its ignition point and minimum ignition energy are low, and the chip is thin and small, with large specific surface area, which is easy to burn at high temperature in air.
1.2 high-speed cutting will produce high temperature and ignite magnesium chips. In machining, in order to give full play to the cutting performance of tools and improve production efficiency and workpiece quality, higher cutting speed is generally required. However, high-speed cutting often makes the temperature of metal chips as high as 700 C ~1000 C 000 C. Without effective supply of cooling liquid, the high temperature will be enough to ignite the fire of magnesium chips.
1.3 The burning temperature of magnesium chips is high, the fire spreads quickly, and it is difficult to put out the fire. Once a fire happens to magnesium, its combustion temperature can reach 3000℃ and its combustion calorific value can reach 25 12 1 kJ/kg. When magnesium chips are in powder form, they will explode when mixed with air in case of fire. In addition, because magnesium can react with water to release hydrogen at high temperature, water, foam, carbon tetrachloride and other extinguishing agents are all limited in magnesium metal fire, and dry powder and halon extinguishing agents have no obvious extinguishing effect, so it is difficult to put out.
Factors affecting cutting temperature in metal cutting process
In the process of metal cutting, 99% of the work consumed by cutting deformation and friction is converted into heat energy, which can be expressed by (1):
Q total =Q 1+Q2+Q3( 1)
Among them: qtotal-the total heat generated in the cutting process, also known as cutting heat;
Q 1- heat of work conversion consumed by deformation;
Q2—— refers to the heat consumed by the friction between the rake face and the chip surface;
Q3—— the heat consumed by friction between the side surface and the chip surface.
Cutting heat is mainly conducted through chips, tools, workpieces, cutting fluids (such as cooling fluid and lubricating fluid) and the surrounding air. When cutting fluid is not added, most of the heat is transferred through chips.
Fire prevention in magnesium alloy cutting process
3. 1 Control the cutting speed. The generation of cutting heat increases in direct proportion to cutting speed, so cutting speed has a great influence on cutting temperature. In practical work, we find that the cutting heat generated by different cutting speeds will change the color of the oxide film on the chip surface. Therefore, the safe spindle speed of lathe can be estimated by cutting the oxide film color of magnesium alloy at different cutting speeds.
3.2 Correct selection of cutting fluid. Unless the mechanical structure itself is limited, cutting fluid should always be fully supplied in magnesium alloy cutting to reduce the chip temperature. Considering the chemical characteristics of magnesium, the choice of cutting fluid should avoid using flammable, strong oxidizing and high water content liquid to prevent the coolant from burning with high-temperature magnesium chips or catching fire due to exothermic reaction.
3.3 Strengthen the supervision of flammable and combustible materials. Lubricating oil for cold working and hydraulic oil, guide rail oil and spindle oil for precision machine tools are mostly flammable liquids, and the oil storage capacity is generally large. For example, the hydraulic oil of ordinary machine tools is about 20 kg ~70 kg. Therefore, it is necessary to strengthen the inspection and maintenance of equipment in daily work, keep the machine tool in good condition and prevent oil leakage. Oilcloth, oily cotton yarn, etc. The magnesium chips left after cleaning the machine tool should be cleaned and isolated in time.
Fire extinguishing of magnesium chips
4. 1 Water, foam, carbon tetrachloride and carbon dioxide fire extinguishing agents are strictly prohibited.
4.2 Class D fire extinguishers should be selected for the burnt metal magnesium chips. Such as 7 150, d-type dry powder, dry sand, etc. Considering that 7 150 and D dry powder fire extinguishers are not common in the domestic market at present, and dry sand is very harmful to machine tools (especially precision machine tools), according to the actual situation, 75 %~80% of covering flux powder and 20 %~25% of sulfur powder can be selected from local materials and filled in dust removal flux.
4.3 When putting out the magnesium chip fire, a certain distance should be kept between the nozzle of the fire extinguisher and the burning magnesium chip, so as to reduce the impact of the fire extinguisher on the magnesium chip and prevent the magnesium chip from spreading to form an explosive mixture.
Application of magnesium alloy sheet
① Magnesium alloy sheet is an indispensable material in aerospace industry. The economic benefits and performance improvement brought by weight reduction of aviation materials are very significant. The fuel cost savings brought by the same weight reduction of commercial aircraft and automobiles are nearly 100 times that of the latter, while the fuel cost savings of fighter aircraft are nearly 10 times that of commercial aircraft. More importantly, the improvement of its mobility can greatly improve its combat effectiveness and viability. Because of this, the aviation industry will take various measures to increase the application of magnesium alloys. At present, the aluminum used in aircraft accounts for about 85% of the total weight of aircraft. The performance of magnesium alloy sheet with high strength and corrosion resistance is better than that of aluminum sheet, and it has more advantages in aircraft application.
② Magnesium alloy is an ideal structural material to reduce the quality of weapons and equipment, realize the lightweight of weapons and equipment and improve the tactical performance of weapons and equipment. Military applications, such as helicopters and fighters, should be widely used; Tanks, armored vehicles, military jeeps, firearms and weapons, etc. The single soldier integrated combat system can be reduced to 6.37Kg, and magnesium plates are used to make bullet shells and artillery shells, which doubles the load of single soldier bullets.
(3) the application of transportation, such as cars, trains, ships, etc. We can lose weight, save energy and reduce pollution. The country is developing vigorously.
④ Widely used in 3C products.
⑤ In terms of power supply application, magnesium power products are all high-energy pollution-free power supplies, such as manufacturing magnesium-manganese dry batteries, magnesium air batteries, magnesium seawater batteries, torpedo power supplies and power batteries.
⑥ High potential magnesium alloy sacrificial anode plate is used for metal protection.
⑦ It is also widely used in civil use. Such as environmental protection building decorative boards, sporting goods, medical instruments, tools, advanced spectacle frames, watchcases,
Advanced travel goods, etc. Recently, Ube Corporation of Japan carried out research and development of semi-solid extrusion casting of aluminum alloy and magnesium alloy on extrusion casting machine. The semi-solid new technology developed by the company is as follows: firstly, molten metal is melted in a high-precision temperature-controlled melting and holding furnace, and poured into an intermediate container for heat preservation, and the temperature is slightly higher than the melting point of the alloy; Then, the temperature of the semi-liquid metal in the intermediate container is adjusted to the required temperature by controlling the air flow to prevent supercooling, and then the semi-solid slurry in the intermediate container is inverted and poured into an inclined cup to keep the oxide layer on the surface of the extrusion head in the cup and prevent it from entering the casting; Then, the extrusion head presses the semi-solid slurry into the cavity for molding, so as to realize solidification into castings under pressure. Compared with conventional squeeze casting, the castings produced by this process have denser structure and better properties, and can form thinner castings.
Squeeze casting, also known as liquid die forging, is a precision forming casting technology that makes liquid or semi-solid metal fill and solidify under high pressure. This process effectively improves the feeding and forming ability of castings, and has the advantages of avoiding or reducing casting defects such as air holes, improving the mechanical properties of castings, wide application range and saving energy. At present, this technology has been widely used in the production of important safe and high-performance parts such as automobiles and motorcycles, and it has important application prospects in the case of increasingly tight energy and higher mechanical performance requirements in the future.
The improvement of squeeze casting equipment is an important condition for the popularization of squeeze casting technology. At present, there are about 1000 hydraulic presses used in squeeze casting production all over the world. The advanced special squeeze casting equipment is mainly distributed in developed countries such as Japan and the United States, among which Japan is the largest. Extrusion casting machines can be roughly divided into three types: vertical clamping and vertical extrusion; Horizontal clamping and horizontal extrusion; Horizontal clamping and vertical extrusion. The research and development of squeeze casting equipment mainly focuses on the following aspects: (1) The squeeze casting method has changed from traditional direct extrusion and indirect extrusion to compound, which has the advantages of both methods, forming a new squeeze casting process with a wider application range. Another example is the combination of die casting and semi-solid casting to form extrusion die casting process and semi-solid extrusion casting. (2) The pouring method and pouring system device should pay attention to improving the automation level and pouring conditions, develop high-precision hydraulic control valve and closed-loop control injection system, and establish a special and efficient pouring system. (3) Diversification of clamping force specifications, improvement of pressure control mode of injection system, improvement of its control accuracy and stability, and accurate control of extrusion process; Broaden the extrusion speed and pressure range of the equipment.
With regard to the development of new squeeze casting products, all countries follow the following directions: (1) to replace the conventional die casting process to make it have a denser structure, and heat treatment can be carried out through solution treatment to improve its mechanical properties or improve its wear resistance and leakage resistance; (2) instead of sand mold and metal mold casting, the internal structure of the casting is denser, the surface contour is clearer and the dimensional accuracy is higher; (3) Replacing forging, hot extrusion and other processes, reducing the cost and simplifying the process. In recent years, the research work of squeeze casting aluminum matrix composites is very active all over the world. Squeeze casting technology is the cheapest and most practical process to prepare metal matrix composites, so it has attracted much attention. Advantages: ① Low density, lighter than aluminum 1/3, and higher specific strength (ratio of tensile strength to density) than aluminum alloy; ② High fatigue limit; ③ It can bear greater impact load than aluminum alloy; ④ Good thermal conductivity; ⑤ Good casting performance; ⑥ Good dimensional stability; ⑦ Easy to recycle; 8 Good machinability; ⑧ It has good vibration damping performance; Attending is superior to engineering plastics in many aspects and can replace engineering plastics; It has high corrosion resistance in kerosene, gasoline, mineral oil and alkali.