1. Unsafe factors in the machining process
During the machining of magnesium alloys, the resulting chips and fine powders are in danger of burning or exploding. The chip size generated in the initial processing stage is large. Due to the high thermal conductivity of magnesium, the generated frictional heat can be quickly dissipated, so it is difficult to reach the ignition temperature, and fewer accidents occur at this stage. But in the finishing stage, because the fine chips and fine powder produced have a large specific surface area, it is easy to reach the ignition temperature and cause combustion or explosion accidents.
In the processing of magnesium alloys, the factors that cause the chips to heat up to the flash point or burn are as follows.
a. The relationship between processing speed and cutting rate.
Under any given set of conditions, there is a range of processing speed and feed rate that may cause combustion. As the feed rate increases, the chip thickness increases, making it less likely to reach the ignition temperature. As long as the processing speed is low enough, chips of any size cannot be ignited. If the processing speed is high enough, because the contact time between the chips and the tool is very short, it is impossible to heat chips of any size to the ignition temperature.
b. The relative temperature of the environment. The higher the relative temperature, the greater the possibility of fire.
c. The composition and state of the alloy. Compared with multi-phase alloys, single-phase alloys are less prone to misfire. The more uniform the alloy state, the less likely it is to misfire.
d. Other factors. The feed rate or the amount of knife eaten is too small; the dwell time during the process is too long; the back angle of the tool and the chip space are too small; a high cutting speed is adopted without using cutting fluid; the tool and the nest Sparks may be generated when the dissimilar metal core linings in the casting collide; magnesium chips accumulate around or below the machine tool.
2. Safe operation rules for mechanical processing
a. The cutting tool should be kept sharp, and a large clearance angle and clearance angle should be ground; it is not allowed to use blunt, chipped or broken tools.
b. Under normal circumstances, try to use a large feed for processing, avoid using a small feed to produce chips with a larger thickness.
c. Do not let the tool stop on the workpiece halfway.
d. When using a small amount of cutting, use mineral oil coolant to reduce the temperature drop.
e. If there is steel core lining in the magnesium alloy parts, avoid sparks caused by collision with the tool.
f. Keep the environment tidy and clean.
g. Smoking, firing and electric welding are strictly prohibited in the machining work area.
h. A sufficient amount of fire extinguishing equipment should be stored in the work area.
3. Safety issues in grinding
Magnesium powder is easy to burn and can cause an explosion when suspended in air. All possible measures should be taken to ensure the correct collection and disposal of magnesium grinding dust. When performing dry grinding of magnesium alloy parts, the magnesium slag must be immediately removed from the work area using a properly designed wet dust extraction system. The connecting pipe between the vacuum cleaner and the grinder should be short and straight, the vacuum cleaner should be kept clean, and its exhaust vent should be set outdoors. The magnesium slag in the vacuum cleaner needs to be cleaned out in time to prevent excessive accumulation. The sludge should be kept in water before being treated. Keep the working environment clean and tidy at all times
The safety of gold parts is critical. The connecting pipe between the grinding wheel and the vacuum cleaner must be inspected and cleaned at least once a day, and the entire vacuum system should be cleaned at least once a month. Do not allow magnesium powder to accumulate on seats, windows, pipes and other horizontal surfaces. There should not be too many suction equipment connected to a centralized discharge system. The central vacuum system with a long drying line and the ordinary vacuum system with a filter are not suitable for collecting magnesium powder. If you want to wet-grind magnesium alloy parts on a belt grinding device or disc grinder, you should use a sufficient amount of cutting fluid to collect all dust and transport it to the collection point.
Therefore, the following precautions must be taken when grinding magnesium alloy parts.
a. There must be a grinding machine specially used for the processing of magnesium alloy parts, and the label «special for magnesium» should be affixed. Before dressing the grinding wheel, the vacuum cleaner should be thoroughly cleaned.
b. When reworking and grinding the surface of magnesium alloy parts washed with chromate, sparks may be caused, so special care must be taken to never allow dust to gather in the vicinity.
c. Grinding equipment operators should use smooth hats, smooth gloves and smooth flame-retardant clothing without pockets and cuffs. The apron or protective clothing used should be clean and free of dust and easy to take off.
d. Warning signs should be placed in a conspicuous place.
e. A sufficient amount of fire extinguishing equipment should be stored in the work area.
4. Treatment of magnesium dust and fine powder
Dry chips should be placed in clean and sealed steel containers and stored in a place where they will not come into contact with water. Wet cuttings and sludge should be stored in a ventilated steel window placed in a remote place, and there must be sufficient ventilation to allow hydrogen to escape. It is particularly dangerous to pack wet chips and fine powders in tightly closed containers because the accumulation of high concentrations of hydrogen is prone to explosion. At present, the common treatment method for magnesium shavings, magnesium powder and sludge is to dissolve with 5% ferric chloride solution (generally 1kg dry magnesium uses 0.6kg ferric chloride), which can convert most of the magnesium within a few hours Into non-combustible magnesium hydroxide and magnesium chloride residues. Since hydrogen gas is generated in this reaction, it should be handled in an open container outdoors, and it is strictly forbidden to fire, smoke or weld around the reactor. When preparing 5% ferric chloride solution, the water in the sludge should be taken into account.
5. Fire extinguishing with magnesium scrap burning
a. Class D fire extinguisher.
The material usually uses sodium chloride-based powder or a passivated graphite-based powder. The principle is to extinguish the fire by excluding oxygen.
a. Covering agent or dry sand.
Small areas can be covered with fire. The principle is to extinguish the fire by removing oxygen.
b. Cast iron debris.
It can also be used without other good extinguishing materials. The main function is to lower the temperature below the ignition point of magnesium, rather than smother the fire. In short, under any circumstances, you can not use water or any other standard fire extinguisher to extinguish the fire caused by magnesium. Water, other liquids, carbon dioxide, foam, etc. will react with the burning magnesium and strengthen the fire rather than suppress it.