Machining And Safety Of Magnesium Alloy Parts

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Magnesium alloy with a density of 36% is lighter than aluminum alloy, 73% lighter than zinc alloy, and 77% lighter than steel. It is recognized as the structural metal material with the smallest mass. The machining of small batches of magnesium alloy parts can be carried out on small manually operated machine tools; when processing magnesium alloy parts in large batches and with high efficiency, it will be more economical to use dedicated large-scale automated machining centers or computer numerical control machine tools. Compared with those metal materials with poor machining properties, magnesium alloys with good cutting properties have very outstanding advantages. For magnesium alloys, powerful cutting can be performed at high cutting speeds and large feeds, so that the number of machining hours can be reduced. Therefore, when completing the same task, if magnesium alloy is used as raw material, the number of processing equipment can be reduced, infrastructure investment can be saved, the floor space can be reduced, and labor costs and management expenses can be reduced.

1Machining of magnesium alloy

Magnesium alloy with a density of 36% is lighter than aluminum alloy, 73% lighter than zinc alloy, and 77% lighter than steel. It is recognized as the structural metal material with the smallest mass. The machining of small batches of magnesium alloy parts can be carried out on small manually operated machine tools; when processing magnesium alloy parts in large batches and with high efficiency, it will be more economical to use dedicated large-scale automated machining centers or computer numerical control machine tools. Compared with those metal materials with poor machining properties, magnesium alloys with good cutting properties have very outstanding advantages. For magnesium alloys, powerful cutting can be performed at high cutting speeds and large feeds, so that the number of machining hours can be reduced. Therefore, when completing the same task, if magnesium alloy is used as raw material, the number of processing equipment can be reduced, infrastructure investment can be saved, the floor space can be reduced, and labor costs and management expenses can be reduced.

1.1 Cutting power consumption of magnesium alloy. Because magnesium alloy has good thermal conductivity and low cutting force, it dissipates heat very quickly during the processing. Therefore, the tool life is long and the amount of tool sticking is less, which can reduce tool costs and shorten the time required to replace tools. of downtime. Because magnesium alloy is easy to cut, its chip breaking performance is very good. Generally, only one finishing process is needed to achieve the required final surface roughness.

1.2 Effect of magnesium alloy materials on processing performance

1.2.1 Effect on chip formation

The type of chips formed during machining is related to factors such as material composition, part shape, alloy state, and feed speed. When single-edged tools are used for turning, boring, planing, and milling of magnesium alloys, the chips generated can be divided into three categories: a. Large and well-broken chips are formed at large feed rates: b. Under the feed rate, chips with short length and good chip breaking are formed; c. Long and curled chips are formed under the small feed rate.

1.2.2 Effect on distortion and deformation

Since magnesium has high specific heat and good thermal conductivity, the heat generated by friction will quickly spread to all parts of the part, so high temperatures will not be generated when cutting magnesium alloys. However, at high cutting speeds and large feeds, the heat generated by the parts is also quite high, and distortion and deformation may occur due to excessive temperature.

1.2.3 Effect on thermal expansion

If the dimensional tolerance requirements for finished parts are strict, the thermal expansion coefficient of magnesium must be taken into consideration in the design. If a considerable amount of heat is generated under the above processing conditions, it is likely to affect the processing accuracy of the part. The thermal expansion coefficient of magnesium is slightly higher than that of aluminum and significantly higher than that of steel, which is 26.6-27.4μm/m℃ in the range of 20200℃.

1.3 The influence of cutting tools on the machining of magnesium alloy parts

1.3.1 Influence of tool material

The choice of tool material for machining magnesium alloys depends on the amount of machining work required. When processing small batches, ordinary carbon steel tools with particularly long service life are generally used; when processing large batches, tools inlaid with carbide are usually preferred; when processing batches are large and tolerance requirements are very strict, higher-cost diamond-inlaid tools can be used. This eliminates the need for tedious reset and compensation adjustments.

1.3.2 Tool design

Tools used for machining steel and aluminum are usually also suitable for machining magnesium alloys. However, due to the small cutting force and low heat capacity of magnesium, its machining tool should have a larger external relief angle, a larger chip clearance, a smaller number of cutting edges and a smaller rake angle. In addition, it is also very important to ensure that all surfaces of the tool are smooth.

1.3.3 Tool sharpening

An important principle for machining magnesium alloys is that the tool should be kept as sharp and smooth as possible and must be free of scratches, burrs and curling edges. If the tool has cut other metals, it should be re-sharpened and sharpened even if the cutting angle has not changed.

For the initial grinding of the tool, a medium-grained grinding wheel can be used, and then a fine-grained grinding wheel can be used for sharpening, and if necessary, a fine or ultra-fine whetstone can be used for manual honing. For high-speed steel tools, a 100-mesh aluminum oxide grinding wheel can be used for fine grinding to obtain satisfactory results; for sharpening tools inlaid with carbide, a 320-mesh silicon carbide grinding wheel or a 200-300-mesh diamond grinding wheel is generally used.

1.4 Effect of cutting fluid on machining

Cutting fluid has two major functions: cooling and lubrication. Because magnesium dissipates heat very quickly, it can keep the surface being processed at a lower temperature. In addition, magnesium’s easy machinability makes it difficult to bond with steel, and lubrication is generally not required during cutting.

When machining magnesium alloy parts, a smooth machined surface can be obtained regardless of high or low cutting speed, with or without cutting fluid. The main purpose of using cutting fluid is to cool the workpiece and minimize the distortion of the part and the risk of chip ignition. possibility. Therefore, in the machining of magnesium alloy parts, the cutting fluid is generally called coolant. Coolant is one of the factors that extends tool life when production runs are large.

The coolant is generally mineral oil. Mineral seal oils and kerosene have been used successfully as coolants for magnesium alloy machining. In order to achieve better cooling effect, the cutting oil should have a lower viscosity. In order to prevent corrosion of magnesium alloy parts, the free acid content in the cutting fluid should be less than 0.2%.

  1. Safety operating procedures for machining

2.1 Unsafe factors during mechanical processing

During the machining of magnesium alloys, the chips and fine powder produced have the risk of burning or exploding. The size of the chips generated in the initial processing stage is large. Since magnesium has a high thermal conductivity, the friction heat generated can be quickly dissipated, so it is difficult to reach the ignition point temperature. There are fewer accidents at this stage. However, during the finishing stage, due to the large specific surface area of the generated fine chips and fine powder, it is easy to reach the ignition temperature and cause combustion or explosion accidents.

During the processing of magnesium alloys, the influencing 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 speeds and feed rates that may cause combustion. As the feed rate increases, the chip thickness increases, making it less likely to reach the ignition point 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 impossible to heat chips of any size to ignition temperature due to the short contact time between the chip and the tool.

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. Single-phase alloys are less prone to misfire than multi-phase alloys. The more uniform the alloy state, the less likely it is to catch fire.

d. Other factors. The feed rate or tool engagement is too small; the dwell time during machining is too long; the clearance angle and chip space of the tool are too small; very high cutting speeds are used without using cutting fluid; the tool is not nested Sparks may occur when dissimilar metal core liners in castings collide; magnesium chips accumulate around or under machine tools, etc.

2.2 Safety operating procedures for machining

a. Cutting tools should be kept sharp and ground with large clearance angles and clearance angles; blunt, chip-adhered or broken tools are not allowed.

b Generally speaking, try to use a large feed amount for processing and avoid using a small feed amount to produce larger-thickness chips.

c. Do not let the tool stop on the workpiece midway.

d. When using small cutting amounts, mineral oil coolant should be used to reduce cooling.

e. If there is a steel core lining in the magnesium alloy part, avoid sparks from collision with the tool.

f. Keep the environment neat and clean.

g. Smoking, lighting fires, and electric welding are strictly prohibited in the machining work area.

h. Sufficient fire-fighting equipment should be stored in the work area.

2.3 Safety issues in grinding processing

Magnesium powder is flammable and can explode when suspended in the air. All possible measures should be taken to ensure the correct collection and disposal of magnesium grinding dust.

When dry grinding magnesium alloy parts, magnesium slag must be immediately removed from the work area using a properly designed wet vacuum 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 outlet should be located 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 until it is disposed of.

Keeping the working environment clean at all times is crucial to ensuring the safety of grinding magnesium alloy parts. 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 thoroughly cleaned at least once a month. Do not allow magnesium powder to collect on seats, windows, pipes and other horizontal surfaces.

Too many vacuum cleaners should not be connected to a centralized exhaust system. Central vacuum systems with long drying lines and ordinary vacuum systems with filters are not suitable for collecting magnesium powder.

If magnesium alloy parts are to be wet ground on a belt grinder or disc grinder, sufficient cutting fluid should be used to collect all dust and transport it to a collection point.

Therefore, the following precautions must be taken when grinding magnesium alloy parts.

a. There must be a grinding machine specially used for processing magnesium alloy parts and affixed with the label “Special for Magnesium”. Before dressing the grinding wheel, the vacuum cleaner should be thoroughly cleaned.

b. When re-grinding the surface of magnesium alloy parts that have been washed with chromate etching, sparks may occur, so special care must be taken to ensure that no dust is allowed to accumulate nearby.

c. Grinding equipment operators should use smooth hats, smooth gloves and smooth flame-retardant clothing without pockets and cuffs. The aprons or protective clothing used should be clean, dust-free and easy to take off.

d. Warning signs should be placed in conspicuous places.

e. Sufficient fire-fighting equipment should be stored in the work area.

2.4 Treatment of magnesium chips and fine powder

Dry cuttings should be placed in clean and sealed steel containers and stored out of contact with water.

Wet chips and sludge should be stored in a ventilated steel window in a remote location, and there must be sufficient ventilation to allow hydrogen gas to escape. Containing wet chips and fine powders in tightly closed containers is particularly dangerous because high concentrations of hydrogen gas can cause explosions.

At present, the common treatment method for magnesium chips, magnesium powder and sludge is to dissolve it with 5% ferric chloride solution (generally 0.6kg ferric chloride is used for 1kg dry magnesium), which can convert most magnesium within a few hours. into non-combustible magnesium hydroxide and magnesium chloride residues. Since hydrogen gas will be produced in this reaction, it should be handled in an open container outdoors, and it is strictly prohibited to light a fire, smoke or weld around the reactor. When preparing a 5% ferric chloride solution, the water in the sludge should be taken into account.

2.5 Magnesium chips burning fire extinguishing

a. Class D fire extinguisher. The material usually uses sodium chloride-based powder or a passivated graphite-based powder, and its principle is to extinguish the fire by excluding oxygen.

b. Covering agent or dry sand. It can be used to cover small area fires, and its principle is to extinguish the fire by extinguishing oxygen.

c. Cast iron chips. It can also be used when there are no other good fire extinguishing materials. Its main function is to reduce the temperature below the ignition point of magnesium, rather than suffocating the fire.

In summary, under no circumstances should water or any other standard fire extinguisher be used to extinguish magnesium fires. Water, other liquids, carbon dioxide, foam, etc. will react with burning magnesium and intensify the fire rather than suppress it.