Aircraft engines need to withstand huge thrust and extremely high operating temperatures. The application of titanium alloys, high-temperature alloys and composite materials is of great help to increase the flight speed and load capacity of aircraft. In the petroleum and chemical industries, low-alloy steel, stainless steel, corrosion-resistant alloy steel, titanium alloy and other materials play an important role in pipes, valves, flanges, shafts and other components with their excellent corrosion resistance.
Difficult-to-process materials usually refer to materials with poor cutting properties. Due to its excellent heat resistance, wear resistance, high hardness, ductility and other characteristics. It often plays an important role in special components in the aerospace, shipbuilding, petroleum, chemical and other industries.
What Materials Make Metalworking Difficult?
Chilled Cast Iron and Hardened Steel
The extremely high hardness of chilled cast iron is the main reason why it is difficult to process. Its plasticity is very low, the contact length between the cutter and the chip is very small, and the cutting force and cutting heat are concentrated near the cutting edge, so the cutting edge is easily damaged. The structural dimensions and machining allowances of chilled cast iron parts are generally large, and the blank precision is low, which further increases the difficulty of processing.
After heat treatment, hardened steel will have a very high hardness and will also have extremely high requirements for the tool. The milling cutter must not break easily during processing and must have a long service life. This requires that the tool material itself has extremely high requirements, and the coating must be resistant to high temperatures and wear.
SAMHO has launched SHG and SHH series end mills, which are specially designed for hard materials. For hard materials within HRC63, both cutting efficiency and tool life are very good.
High Strength Steel
Compared with ordinary carbon structural steel, high-strength steel/ultra-high-strength steel has high strength and low thermal conductivity, so the cutting force is large, the cutting temperature is high, the tool wear is fast, the tool life is short, and chip breaking is also slightly difficult.
When processing such materials, high-performance carbide, ceramic or PCBN tools are selected. At the same time, high-pressure cooling or MQL is used to reduce cutting heat and tool wear.
Pure Metal
Commonly used pure metals such as copper, pure aluminum, and pure iron have low hardness and strength, and high thermal conductivity, which are favorable for cutting. However, they have high plasticity, large chip deformation, large tool-chip contact length, and are prone to cold welding, resulting in built-up edge. Therefore, the cutting force is large, it is not easy to obtain a good machined surface quality, and chip breaking is difficult. In addition, their linear expansion coefficient is large, and it is difficult to control the machining accuracy of the workpiece during fine machining.
Stainless Steel
According to the metallographic structure, stainless steel can be divided into three types: ferrite, martensite, and austenite. The main components of ferrite and martensite stainless steel are chromium, and cutting is generally not difficult. The main components of austenitic stainless steel are chromium, nickel and other elements. After quenching, it is austenite, and the cutting processability is relatively poor, which is mainly manifested in:
- High plasticity, severe work hardening, easy to generate built-up edge and deteriorate the quality of the processed surface. The degree of hardening of the processed surface and the depth of the hardened layer are large, which often brings difficulties to the next process. And it is not easy to break chips.
- The thermal conductivity is small, and the heat generated is not easy to transfer, so the cutting temperature is high.
- Due to the high cutting temperature, severe work hardening, and the presence of carbides (TiC, etc.) in the steel, hard inclusions are formed, and it is easy to cold weld with the tool, so the tool wears quickly and the service life is reduced.
High Temperature Alloys
According to their chemical composition, high-temperature alloys are divided into three types: iron-based, nickel-based, and cobalt-based. Their machinability is worse than that of stainless steel. High-temperature alloys contain many high-melting-point alloying elements, such as iron, titanium, chromium, cobalt, nickel, vanadium, tungsten, molybdenum, etc., which together with other alloying elements form austenitic alloys with high purity and dense structure. Some elements are combined with non-metallic elements such as carbon, nitrogen, and oxygen to form high-hardness compounds with low specific gravity and high melting point.
Some high-hardness intermetallic compounds with certain toughness can also be formed. At the same time, some alloying elements enter the solid solution to strengthen the matrix. After long-term aging, high-temperature alloys can precipitate hard phases from the solid solution, further distorting the lattice, which not only increases the resistance to plastic deformation, but also aggravates the wear of the tool due to the presence of hard particles.
How End Mill Tools Meet the Challenge of Difficult-to-Machine Materials
Select High Quality Materials
Polycrystalline diamond (PCD) tools and cubic boron nitride (CBN) tools are also effective ways to deal with difficult-to-machine materials. In recent years, their market share has gradually increased. PCD tools are widely used in milling non-ferrous metals, composite materials, plastics, and extremely difficult-to-machine super alloys. CBN tools are used for continuous or intermittent cutting of hardened ferrous metals, as well as cutting of welded metals and composite metals.
High Quality Coating on Cutting Tools
Tool coating is the most economical and targeted technology for difficult-to-process materials. A wide variety of new materials bring more complex processing requirements. At the same time, it is also promoting the continuous development of coating technologies such as CVD and PVD. Tool coating itself is a technology developed to address the adverse effects of excessive force and heat generated during the chip travel on the tool. Coated tools can increase the service life of uncoated tools by 2-10 times in different processing materials.
Using Advanced Cutting Technology
Conventional processing temperatures are normal room temperature. However, in the case of difficult-to-process materials, changing the processing temperature sometimes brings unexpected results. The heating cutting method is to apply low voltage and high current in the circuit between the workpiece and the tool to generate heat in the cutting area. There is also plasma heating cutting. That is, the workpiece material near the tip of the tool is heated with a plasma arc to reduce its hardness and strength, thereby improving the cutting conditions.
The low-temperature cutting method uses liquid nitrogen (-180 ℃) or liquid CO2 (-76 ℃) as a cutting fluid. The temperature of the cutting zone can be reduced. Using this method, the main cutting force can be reduced by 20%, and the cutting temperature can be reduced by more than 300 ℃. At the same time, the built-up edge disappears, the quality of the processed surface is improved, and the tool durability can be increased by 2~3 times. It is effective when processing high-strength steel, wear-resistant cast iron, stainless steel, and titanium alloys.
Ultrasonic processing using ultrasonic tool holders and ultrasonic tools is a special cutting technology that makes the tool vibrate at a high speed along the cutting direction at a frequency of 20-40KHz. Ultrasonic vibration cutting is a kind of pulse cutting from a microscopic point of view. In one vibration cycle, the effective cutting time of the tool is very short. In most of the time in one vibration cycle, the tool and the workpiece chips are completely separated, and the tool and the workpiece chips are in intermittent contact, which greatly reduces the cutting heat. It is a technology that is more suitable for dealing with brittle and hard materials. The workpiece roughness and processing accuracy are greatly improved.
