1. Unsafe factors in the machining process
During the machining of magnesium alloys, the resulting chips and fine powders are at risk of burning or exploding. The size of the chips generated in the initial processing stage is large. Due to the high thermal conductivity of magnesium, the frictional heat generated can be quickly dissipated, so it is difficult to reach the ignition temperature. In this stage, the accident occurs less. However, in the finishing stage, since the fine chips and fine powders produced have a large specific surface area, it is easy to reach the ignition temperature and cause a combustion or explosion accident.
In the processing of magnesium alloys, the factors that influence the temperature of the chips to reach the flash point or burn are as follows.
a. The relationship between processing speed and cutting rate.
There is a range of processing speeds and feed rates that can cause combustion under any given set of conditions. The feed rate is increased and the chip thickness is increased, 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 machining speed is high enough, it is not possible to heat the chips of any size to the ignition temperature due to the short contact time of the chips with the tool.
b. The relative temperature of the environment. The higher the relative temperature, the greater the likelihood of fire.
c, the composition and state of the alloy. Single-phase alloys are less prone to fire than multiphase alloys. The more uniform the alloy state, the less likely it is to catch fire.
d, other factors. The feed rate or the amount of knife is too small; the pause time during machining is too long; the back angle of the tool and the chip space are too small; the cutting speed is used without the use of cutting fluid; the tool and nesting Sparks may be generated when the dissimilar metal core linings in the casting collide; magnesium chips accumulate around or under the machine tool.
2. Safety operation rules for machining
a. The cutting tool should be sharp and sharpened with a large relief angle and clearance angle; blunt, chipped or broken tools are not allowed.
b. Under normal circumstances, try to use large feeds for machining, avoiding the use of small feeds to produce larger thickness chips.
c. Do not let the tool stop in the middle of the workpiece.
d. When using a small amount of cutting, use mineral oil coolant to reduce the temperature drop.
e. If there is a steel core lining in the magnesium alloy parts, avoid sparks when they collide with the tool.
f. Keep the environment tidy and clean.
g. It is strictly forbidden to smoke, produce fire and weld in the machining work area.
h. A sufficient amount of fire-fighting 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 the air. All possible measures should be taken to ensure the correct collection and disposal of magnesium grinding dust. In the dry grinding of magnesium alloy parts, the magnesium slag must be removed from the work area immediately with a well-designed wet vacuum system. The connection between the vacuum cleaner and the grinder should be short and straight, the vacuum cleaner should be kept clean, and the exhaust vent should be placed outdoors. The magnesium slag in the vacuum cleaner should be cleaned up in time to prevent excessive accumulation. The sludge should be kept in water before it is treated. Keep the working environment clean and tidy at all times, to ensure the grinding of magnesium
The safety of gold parts is of paramount importance. The connecting pipe between the grinding wheel and the vacuum cleaner must be inspected and cleaned at least once a day, and the entire vacuuming system should be thoroughly cleaned at least once a month. Do not allow magnesium powder to collect on seats, windows, piping, and other horizontal surfaces. Too many vacuuming equipment should not be connected to a centralized exhaust system. A central vacuum system with a long drying line and a conventional vacuum system with a filter are not suitable for collecting magnesium powder. If the magnesium alloy parts are to be wet ground on a belt sanding machine or a disc grinder, a sufficient amount of cutting fluid should be used to collect all the 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 "magnesium-specific" label is attached. The vacuum cleaner should be thoroughly cleaned before the grinding wheel is trimmed.
b. Rework grinding of the surface of magnesium alloy parts washed with chromate may cause sparks, so be careful not to allow dust to collect nearby.
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 dust-free and easy to take off.
d. Warning signs should be placed in a conspicuous place.
e. A sufficient amount of fire-fighting equipment should be stored in the work area.
4. Treatment of magnesium chips and fine powder
Dry chips should be placed in a clean and sealed steel container and stored in a location that will not come into contact with water. Wet chips and sludge should be stored in a ventilated steel window placed in a remote location and must be adequately ventilated to allow hydrogen to escape. It is particularly dangerous to pack wet chips and fine powder in a tightly closed container because a high concentration of hydrogen is prone to explosion. At present, the common treatment method for magnesium chips, magnesium powder and sludge is to dissolve with 5% ferric chloride solution (generally 1kg dry magnesium uses 0.6kg ferric chloride), which can transform most magnesium in a few hours. It is a non-burning magnesium hydroxide and magnesium chloride residue. Since hydrogen is generated in this reaction, it should be treated in an open outdoor container, and it is strictly prohibited to make a fire or welding operation around the reactor. When formulating a 5% ferric chloride solution, the water in the sludge should be taken into account.
5, magnesium burning fire extinguishing
a, D-class fire extinguisher.
The material is usually sodium chloride based powder or a passivated graphite based powder, the principle of which is to extinguish the fire by removing oxygen.
a, covering agent or dry sand.
A small area of fire can be covered by it, and the principle is also to extinguish the fire by removing oxygen.
b, cast iron debris.
It can also be used without other good fire-extinguishing materials. The main function is to lower the temperature below the ignition point of magnesium instead of suffocating the fire. In short, no matter what the 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., react with the burning magnesium and strengthen the fire rather than suppress the fire.