Professor Pinmei (122): Machining Of Magnesium Alloy Workpieces
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The density of magnesium and magnesium alloys is low, only 1.74g/cm3. It is the lightest structural metal. It has good machining properties and is easy to machine. Especially when the processing volume is large, the advantages are obvious. High speeds, large depths of cut and high feed rates can be used when machining magnesium alloys. Under the same cutting amount, the energy required for cutting magnesium alloy is much lower than that of other metals.
Magnesium alloy has good machinability and can perform high-speed and powerful cutting at large feed rates. It can be processed by large-scale automated machining centers or computer numerical control machine tools. The production efficiency is high, the number of machine tools is reduced, the floor space is small, and infrastructure investment is saved. , labor costs and administrative expenses will decrease. Due to the high thermal conductivity of magnesium alloy, small cutting force, and fast heat dissipation during processing, the tool life is long and the viscosity is low, which can reduce tool costs and shorten the downtime of tool replacement. Typically only one finishing operation is required to achieve the desired final surface finish, and magnesium materials also have very good cutting properties.
Magnesium alloy machining processes include turning, boring, boring, drawing, drilling, reaming, countersinking, polishing, tapping, milling, sawing, grinding, etc., as well as combined processing. Alloys that often require processing include: die-cast AZ91B, AC91D alloy parts, AZ91C (AC91E), AZ92A, and deformed magnesium alloys AZ31B, AZ61A, AZ80, ZK21A and ZK60A.
Processing characteristics
●Chip formation
The chips formed when turning, boring, boring and milling with a single tool can be divided into three categories: thick chips and short chips formed at large feed rates; The chips are not too long and can be broken up at moderate feed rates; long, curled chips are produced at low feed rates. Castings are prone to forming broken or partially broken chips, and the shape of the chips is closely related to the heat treatment state. Forgings and extrusions may produce partially broken or curled chips, depending on feed speed.
●Deformation
Usually, when cutting magnesium alloy workpieces, the temperature that the workpiece can reach is not high, so the workpiece has almost no deformation or a small amount of deformation, because magnesium alloy has good heat dissipation properties. If the cutting speed is high and the feed rate is large, a large amount of cutting heat will be generated, and the workpiece may be distorted and deformed.
●Thermal expansion
If the heat generated during magnesium alloy processing is large and the dimensional deviation accuracy requirements of the workpiece are very strict, the large thermal expansion coefficient of the magnesium alloy must be considered. At 20℃~200℃, the thermal expansion coefficient of magnesium is 26.6um/(m℃)~27.4um/(m℃). The coefficient of thermal expansion of magnesium is slightly larger than that of aluminum, but much larger than that of steel.
●Stress relief annealing
Magnesium alloy workpieces, especially workpieces with complex shapes, tend to retain more or less internal stress after machining. Although this internal stress is not large, it is detrimental to the dimensional deviation of precision workpieces. Residual stress can be calculated as follows: Low temperature annealing eliminates:
When machining, attention should also be paid to the clamping of parts. The clamping section should be a thick section of the part. For die cast parts, clamping positioning pads should be placed within the area of the part formed by the same mold half to minimize the impact of the parting line. The clamping force should not be too large to avoid deformation of the workpiece. If necessary, place shims between the workpiece and the carriage. Special attention should be paid when processing thin-section workpieces. Improper clamping or excessive cutting may cause deformation.
