Magnesium and magnesium alloy are the lightest structural metals with low density of only 1.74g/cm3. They are easy to be machined because of the good machinability, especially when the amount of the materials to be machined is large. When machining magnesium alloy, high speed, large cutting depth and high feed speed should be adopted. At the same cutting amount, the energy required for cutting magnesium alloy is much lower than that of other metals.
Machining Characteristics of Magnesium Alloy Castings
1. Chip Formation
The chips formed by turning, boring, planing and milling with a single tool can be divided into three categories: thick and short chips formed by large feed rate, medium chips formed at medium feed rate and long, curly chips formed at small feed rate. Magnesium alloy castings are easy to form broken or partially broken chips. Chip morphology is closely related to heat treatment state. Forgings and extrusions will produce partially broken or curled chips, which are related to feed speed.
Generally, when machining magnesium alloy castings, the temperature that the workpiece can reach is not high, so the workpiece is hardly deformed or the deformation is small, because magnesium alloy has good heat dissipation performance. If the cutting speed and the feed rate are high, great cutting heat will be generated, so the workpiece may be distorted.
3. Thermal Expansion
If the heat generated by magnesium alloy castings during processing is large and the precision of dimensional deviation of workpieces is very strict, the characteristic of large thermal expansion coefficient of magnesium alloy should be considered. At 20℃~200℃, the thermal expansion coefficient of magnesium is 26.6um/(m℃)~27.4um/(m℃). The thermal expansion coefficient of magnesium is slightly larger than that of aluminum, but much larger than that of steel.
4. Stress Relief Annealing
Magnesium alloy castings, especially workpieces with complex shapes, often have some internal stress after machining. Although this insignificant internal stress is unfavorable to the dimensional deviation of precision workpieces, the residual stress can be eliminated by the low temperature annealing.
It should be noted that when clamping parts during machining, the clamping position should be the thick section of the parts. For die-casting parts, it is advisable to place the clamping positioning pad in the part area formed by the same half mold to minimize the influence of parting line. The clamping force should not be too large to avoid deformation of the workpiece. If necessary, a gasket should be placed between the workpiece and the bracket. Pay special attention to machining thin cross-section workpieces, because improper clamping or excessive cutting amount may cause deformation.