With the thorough implementation of the “Magnesium Alloy Application Development and Industrialization” project, companies have experienced many problems in actual production. Process and safety issues, etc., are due to the fact that most domestic magnesium alloy CNC machining and die casting companies come into contact with magnesium alloys for the first time. Therefore, this article outlines the machining techniques and safe operating procedures for magnesium alloy die-cast parts for reference.
Magnesium alloy machining
Magnesium alloy densities are 36% lighter than aluminum alloy densities, 73% lighter than zinc alloy densities, and 77% lighter than steel densities. Magnesium alloys are recognized as the smallest structural metal material. Machining of small lots of magnesium alloy parts can be performed on small machine tools that are manually operated. When machining magnesium alloy parts in large quantities and with high efficiency, it is more economical to use a dedicated large automatic machining center or computer numerical control machine. Compared to metal materials with poor machinability, magnesium alloys with good machinability have excellent advantages. In the case of magnesium alloy, strong cutting can be performed at a high cutting speed and a large feed rate, so that the number of machining hours can be reduced.
1.1 Cut the power consumption of magnesium alloy
When machining magnesium alloy parts, the power consumption per unit cutting volume is lower than other common metals. Table 1 shows the power consumption of various metals for magnesium at some typical cutting speeds.
Magnesium alloy has good thermal conductivity and low cutting force, so the heat dissipation rate during machining is fast, and the tool life is long, thereby reducing the tool cost and the pause required to change the tool. Save time. Magnesium alloy is easy to cut, so its breaking performance is very good. Under normal conditions, the required final surface roughness can only be achieved in a single finishing process.
1.2 Effect of magnesium alloy material on workability
1.2.1 Impact on chip formation
The tip type formed during the machining process is related to the material composition, part shape, alloy condition and feed rate. When single-edged tools are used for magnesium alloy lathing, boring, planing, and milling, the chips generated can be divided into three categories.
- a) Under high feed rates, coarse chips with good chip fracture are formed.
- b> Chips with short length and good chip breakage are formed at moderate feed rates.
- c> Long bent chips are formed at low feed rates.
1.2.2 Impact on distortion
Magnesium has a high specific heat and good thermal conductivity, so the heat generated by friction diffuses rapidly to various parts of the part and the magnesium alloy is not exposed to high temperatures during cutting. However, in the case of high cutting speeds and high feed rates, the heat generated by the part is also very high, which can be distorted due to excessive temperature.
1.2.3 Impact on thermal expansion
If the dimensional tolerance requirements of the finished part are stringent, the factors that influence the coefficient of thermal expansion of magnesium should be considered in the design. If a considerable amount of heat is generated under the above machining conditions, it may affect the machining accuracy of the part. The coefficient of thermal expansion of magnesium is slightly higher than that of aluminum, which is considerably higher than that of steel. It is 26.6 to 27.4 μm / m ° C in the range of 200 ° C.
2.4 Impact on cold deformation
During machining, magnesium alloy parts are rarely strained or warped by cold deformation. However, if the tool becomes too dull, it will slow down the feed rate, and if the tool pauses during machining, it can cause distortion and warpage.
1.3 Effect of tools on machining magnesium alloy parts
1.3.1 Effect of tool material
The choice of tool material for machining magnesium alloys depends on the amount of machining required to be done.
Small lot machining, general long life of general steel cutters.
For batch machining and tools, embedded hard alloys are usually preferred.
For high throughput and very tight tolerances, the expensive diamond inlay cutter head can be used to eliminate the hassle of reset correction adjustments.
1.3.2 Tool design
Tools for machining steel and aluminum are also generally suitable for machining magnesium alloys. However, since magnesium has a small cutting force and a relatively low heat capacity, the machining tool should have a large lateral clearance, a large chip clearance, a small number of blades, and a small rake angle. In addition, it is important to ensure that the various surfaces of the tool are smooth.
1.3.3 Tool sharpening
An important principle in machining magnesium alloys is that cutting tools should be kept as sharp and smooth as possible, and no scratches, burrs and winding edges are required. If the tool cuts other metal, it should be sharpened even if the cutting angle has not changed.
The tool can be ground first with a medium grain grindstone, then with a fine grain grindstone, and if necessary, by hand with a fine or ultrafine stone. For high speed steel tools, a 100 mesh alumina grindstone can be used for fine grinding to obtain satisfactory results. A 320 mesh silicon carbide grindstone or a 200-300 mesh diamond grindstone is commonly used for sharp carbide inserts.
1.4 Impact of cutting fluid on machining
Due to the rapid heat dissipation of magnesium, the machined surface can be kept at lower temperature levels. In addition, the machinability of magnesium makes it difficult to bond with steel, and lubrication is generally not required in cutting.
When machining magnesium alloy parts, you can obtain a smooth surface with or without cutting fluid at high or low cutting speeds. The main purpose of using cutting fluid is to cool the workpiece and minimize the possibility of distortion and chip ignition. Therefore, in machining magnesium alloy parts, the cutting fluid is generally called coolant. For high production volumes, coolant is one of the factors that extend tool life.
Mineral oil is commonly used as the coolant. Mineral seal oils and kerosene have been successfully used as coolants for magnesium alloy machining. To achieve a better cooling effect, the cutting oil should have a lower viscosity. The free acid content in the cutting fluid should be less than 0.2% to prevent corrosion of magnesium alloy parts.
2.1 Hazardous factors in the machining process
During the machining of magnesium alloys, the chips and fine powders obtained are at risk of burning or exploding. The chip size produced in the initial machining stage is large. The thermal conductivity of magnesium is so high that the frictional heat generated can be quickly dissipated, so it is difficult to reach the ignition temperature, and there are fewer accidents at this stage. However, in the finishing stage, since the produced fine powder and the specific surface area of the fine powder are large, it is easy to reach the ignition temperature, which may cause combustion or an explosion accident.
In the machining of magnesium alloys, the factors that affect the temperature of the insert to reach the flash point or combustion are:
- a> Relationship between machining speed and cutting speed There is a range of machining speeds and supply speeds that can cause combustion under a series of related conditions. The feed rate is increased, the tip is thicker, and the chances of reaching the ignition temperature are reduced. It is not possible to ignite a chip of any size as long as the machining speed is slow enough. If the machining speed is fast enough, it is not possible to heat a tip of any size to the ignition temperature due to the short contact time between the tip and the tool.
- b> Relative temperature of the environment. The higher the relative temperature, the higher the chance of fire.
- c> Alloy composition and condition. Single-phase alloys are harder to burn than multi-phase alloys. The more uniform the alloy is, the less likely it is to catch fire.
- Feed amount or knife feed is too low.
- The pause time during machining is too long.
- The back angle and tip space of the tool are too small.
- High cutting speed is used without using cutting fluid.
- Sparks can occur when the tool collides with a dissimilar metal core liner embedded in the casting.
- Magnesium debris deposits around or under the machine.