In cutting, a variety of materials will be used, and difficult cutting materials are also one of them. It mainly refers to materials with poor machinability. Compared with the widely used high-quality carbon structural steel 45 steel, the difficult to cut material has higher strength and hardness, high work hardening degree, large cutting resistance during cutting, and it is difficult to form and remove chips, thus reducing the durability of the tool and poor surface quality. Common difficult to cut metal materials include: high manganese steel, high strength steel, stainless steel, superalloy, titanium alloy, etc.
Difficult to cut materials have the following main milling characteristics:
1. High milling force
Difficult to cut materials generally have high strength, especially their high-temperature strength is much greater than that of ordinary steel (45 steel), coupled with large plastic deformation and serious hardening during machining, so the milling force is generally much greater when milling difficult to cut materials than when milling ordinary carbon steel. For example, under the same conditions, the milling force required for milling stainless steel is about 50% greater than that for milling 45 steel.
2. High milling temperature
The thermal conductivity of difficult to cut materials is relatively low, and the cutting heat generated during milling is not easy to dissipate, causing a large amount of heat to accumulate in the cutting area (mainly concentrated at the tool tip).
3. Severe work hardening
The deformation coefficient of difficult cutting materials is generally large, such as stainless steel, titanium alloy and superalloy. The milling speed starts from 0.5m per minute, and the deformation coefficient increases with the increase of milling speed. When the milling speed is about 6m per minute, the deformation coefficient of chip reaches the maximum.
4. Easy to stick knife
Due to the severe work hardening of difficult to cut materials, the chips are strong and tough (that is, the strength and hardness of the chips are high, and the toughness is good). At high milling temperature, when strong and tough chips flow through the front of the milling cutter, it is easy to produce cold welding, fusion welding and other sticking phenomena. Sticking the knife is not conducive to the removal of chips, which is easy to block the chip holding groove, and the knife is easy to collapse or hit the knife, as well as cause bonding wear of the knife. In addition, if the strong chips are serrated, the cutting edge of the tool is easy to be damaged.
5. The wear speed of milling cutter is fast, and the durability is reduced
Due to the high strength, high thermal strength, high plasticity, high milling temperature and severe work hardening of difficult to cut materials, some materials have strong chemical affinity and tool sticking phenomenon, so the wear speed of milling cutter is very fast, which reduces the durability of milling cutter.
When milling difficult cutting materials, because their properties have their own characteristics, corresponding measures should be taken according to the object when formulating the machining plan. Selecting appropriate tool materials, selecting reasonable geometric parameters of milling cutter, using appropriate cutting fluid, selecting reasonable milling parameters, selecting reasonable milling methods and so on, can be good for milling difficult cutting materials.