Some parts of construction machinery are made of difficult to machine materials. Fully understanding the types of difficult to machine materials, the characteristics of cutting processing, and the tool materials that should be used, developing advanced tools and materials, and adopting new processing technologies are effective ways to solve the problem of difficult to machine material processing.

1、 Types of difficult to process materials

The difficult to machine materials used for construction machinery parts mainly include high-strength steel, ultra high strength alloy structural steel, high manganese steel, quenched steel, chilled cast iron, stainless steel, high-temperature alloys, engineering plastics, composite materials, etc. The reasons for its difficulty in processing are generally high hardness, high strength, high plasticity and toughness, low plasticity and high brittleness, low thermal conductivity, microscopic hard spots or hard inclusions, and active chemical properties. Therefore, new processing technologies should be adopted based on the characteristics of difficult to process materials to ensure processing efficiency and quality.

2、 Characteristics of cutting difficult to machine materials

1. High strength and ultra high strength steel

Compared with ordinary carbon structural steel, high-strength and ultra high strength steel has a hardness and strength more than twice that of 45 steel, a higher impact value, and a lower thermal conductivity, resulting in greater cutting force and higher cutting temperature.

When processing high-strength and ultra high strength steel, different grades of YT hard alloy cutting tools should be used according to the requirements of rough machining, semi precision machining, and precision machining. During high-speed precision machining, YT type alloys with high titanium carbide (TiC) content should be used, and YN type alloys (metal ceramics), coating alloys, and Al2O3 ceramics can also be used. When the rigidity of the process system allows, the rake and rake angles of the tool should be small, and the radius of the tool tip arc should be large. When processing, it is necessary to use cutting parameters and cutting speeds lower than those used for processing carbon normalized steel.

2. High manganese steel

Typical grades of common high manganese steel include Mn13, 40Mn18Cr3, and 50Mn18Cr4. Although its original hardness is not very high, its plasticity and toughness are particularly high. After work hardening, it can reach HBW500, and its thermal conductivity is only 1/4 of 45 steel. The cutting force is increased by 60% compared to processing medium carbon steel, and the cutting temperature is very high.

When cutting high manganese steel, tool materials with high hardness, certain toughness, high thermal conductivity, and good high-temperature performance should be selected. YG and YW hard alloys are generally used for rough machining, while YW or YT14 alloys can be used for precision machining. If Al2O3 ceramic cutting tools are used for high-speed precision machining, the effect will be very good. When processing, it is advisable to use smaller front angles, main deviation angles, and larger rear angles; The cutting speed should be low and the feed rate should be large.

3. Quenched steel and chilled cast iron

The hardness of quenched steel can reach HRC60 or above, with extremely low plasticity and thermal conductivity; The hardness of chilled cast iron can reach HRC52-HRC60, and its other properties are similar to those of quenched steel. These two materials should be made of YG type alloys (YG type alloys have a higher elastic modulus than YT type alloys). If Al2O3 or Si3N4 based ceramic cutting tools are used for precision machining or semi precision machining of quenched steel and chilled cast iron, the effect is better than that of hard alloys. During processing, the tool should have a smaller rake angle, main rake angle, and lower cutting speed. CBN cubic boron nitride cutting tools can be used for semi precision machining and precision machining.

4. Stainless steel and high-temperature alloys

Austenitic stainless steel (such as 1Cr18Ni9Ti) is difficult to process, as its original hardness and strength are not very high, but its plasticity and toughness are high. During processing, there is severe hardening and a certain amount of hard inclusions, with a thermal conductivity of only 1/3 of 45 steel, high cutting force, and high cutting temperature.

The processing difficulty of high-temperature alloys is greater, with higher initial hardness and strength, low thermal conductivity (1/3 to 1/4 of 45 steel), more hard inclusions, severe work hardening, high cutting force, and high cutting temperature. YG and YW hard alloys should be used for processing stainless steel and high-temperature alloys, rather than YT alloys containing Ti. During processing, an appropriate tool rake angle should be used, the cutting speed should be low, and the feed rate should be large.

5. Engineering plastics

There are many types of engineering plastics, which can be divided into two categories based on their properties: thermoplastic plastics and thermosetting plastics. Although its hardness and strength are not high, its thermal conductivity is extremely small, only 1/175 to 1/450 of carbon steel, which can easily cause burns and thermal deformation during processing; The elastic modulus is small, making it difficult to ensure the processing size. YG hard alloy or high-speed steel are generally used as cutting tools for processing engineering plastics.

6. Composite materials

Composite materials can be artificially synthesized from any two of metals, polymers, and ceramics. Composite materials include fiber-reinforced materials and particle reinforced materials, among which fiber-reinforced materials include carbon fiber (CFRP), glass fiber (GFRP), and Kevlar fiber (KFRP). Its elastic modulus and thermal conductivity are both very small, and the processed surface is prone to rebound, tearing, and burrs; Fibers have a certain scratching effect on the cutting force of cutting tools, and YG type hard alloys or high-speed steel should be selected as tool materials; Although the matrix of particle reinforced materials (such as aluminum alloy) is relatively soft, the particles (such as SiC) are very hard and have impact and scratching effects on the cutting force of the tool, so the tool is greatly damaged and CBN or diamond tools should be used.

3、 New technology for processing difficult to machine materials

1. Adopt high-performance new cutting tools and materials

The application of new cutting tools (see Figure 1) has significantly improved the efficiency of processing difficult to machine materials. The new high-speed cutting steel includes various types of superhard high-speed steel, powder high-speed steel, and coated high-speed steel, and its cutting performance is greatly improved compared to ordinary high-speed steel. New hard alloys include various WC based alloys with added elements such as tantalum and niobium, WC based alloys with fine and ultrafine grains, TiC based and Ti (C, N) - based alloys, coatings and rare earth hard alloys, as well as hot pressed composite ceramics and superhard tool materials such as CBN and diamond. They can be used to cut various difficult to machine materials, but attention should be paid to the reasonable matching of workpiece and tool materials.

2. Adopting unconventional cutting methods

The performance of the above-mentioned new tool materials under conventional cutting conditions cannot meet the cutting needs of some difficult to machine materials. For example, for the processing of certain high hardness materials, the hardness and wear resistance of new hard alloys are still insufficient, so the cutting speed must be reduced, resulting in low processing efficiency. Although CBN and diamond cutting tools have high hardness, their strength is insufficient, and diamond cannot process black metals, so they can only be used for processing difficult to machine materials under certain cutting conditions. For the above situations, unconventional new cutting methods can be used.

(1) Heating cutting method

One type of heating cutting method is conductive heating cutting, which applies low voltage (about 5 V) and high current (about 500 A) in the circuit between the workpiece and the tool (the workpiece must be a conductive body) to generate heat in the cutting area, thereby causing changes in the mechanical properties, contact and friction conditions of the local workpiece material.

Another type is plasma heating cutting, which uses plasma arc to heat the workpiece material near the cutting edge, reducing its hardness and strength, thereby improving cutting conditions.

These two methods have similar effects, both of which can significantly reduce cutting force, eliminate chip deposits, improve surface roughness technical standards and tool durability. Therefore, using this method for processing hard materials with large cutting depth and feed rate is effective. Shenyang University of Technology and Beijing Institute of Technology have used plasma heating cutting method to process high manganese steel and high-strength steel, while South China University of Technology and Anhui Institute of Technology have used electric heating cutting method to process high-strength steel, obtained systematic experimental data, and started to apply it in production. In addition, there is a laser assisted cutting method, as shown in Figure 2.

In recent years, the "electric melting explosion" cutting method has been invented in China. The charged cutter head generates violent discharge with the machined surface, rapidly melting and exploding the machined surface, thereby cutting off the excess. This method ensures that the internal materials of the workpiece are not affected by heat, has high efficiency, and is suitable for hard, soft, and viscous materials. It can be used for both rough and precision machining.

(2) Low temperature cutting method

The low-temperature cutting method uses liquid nitrogen (-180 ℃) or liquid CO2 (-76 ℃) as the cutting fluid, which can reduce the temperature in the cutting zone, as shown in Figure 3. According to experiments, using this method can reduce the main cutting force by 20%, lower the cutting temperature by more than 300 ℃, and eliminate chip deposits, improving the quality of the processed surface. The tool durability can be increased by 2-3 times, and it is effective in processing high-strength steel, wear-resistant cast iron, stainless steel, and titanium alloys.

(3) Vibration cutting method

Vibration cutting method uses different forms of vibration generators to force the tool to vibrate. F>10 kHz is considered high frequency, and f<200 Hz is considered low frequency. Vibration cutting can significantly reduce the friction coefficient and cutting force between chips, decrease the deformation coefficient and cutting temperature, eliminate chip deposits, and reduce work hardening, thus improving the quality of the machined surface. But it is somewhat disadvantageous for the durability of the cutting tool, and it is necessary to use tool materials with strong toughness (such as high-speed steel, ultra-fine hard alloy, etc.). More than 10 universities and research institutes in China have conducted research on vibration cutting, with processed materials including titanium alloys, quenched steel, stainless steel, thermal spray coatings, purple steel, ceramics, and GFRP. Processing methods include turning, tapping, drilling, and reaming, all of which have achieved good results. If cutting fluid is used simultaneously, the effect is particularly good.

(4) Vacuum cutting method

Toyo University in Japan conducted research on vacuum cutting, and found that the vacuum degree has no effect on the deformation coefficient, cutting force, and surface roughness of processed copper and aluminum; When processing carbon steel and titanium alloys, the higher the vacuum degree, the greater the deformation coefficient and cutting force, and the lower the surface roughness. This is because in vacuum, the interface between the cutting chips cannot produce oxides that are conducive to reducing friction.

(5) Inert gas protection cutting method

This is a measure taken for cutting materials such as titanium alloys. Nanjing University of Aeronautics and Astronautics once sprayed argon gas in the cutting zone of titanium alloy to isolate the cutting material from the air, thus avoiding the production of compounds that are unfavorable for processing and improving the machinability of titanium alloy. This method has a certain effect on the processing of chemically active metals. Similarly, if certain special components of cutting fluid are used, it will also have an effect.

(6) Insulation cutting method

During the cutting process, if the workpiece and tool are connected in a circuit and there is a thermal current in the circuit, the wear of the tool will intensify; If the workpiece, tool and machine tool are insulated and the current is cut off, the durability of the tool will be improved. Northwestern Polytechnical University used this method to drill high-temperature alloy K14, and Xi'an Huanghe Machinery Factory used this method to cut 1Cr13 and 2Cr13 steels, both of which achieved certain results. Although the mechanism of this method has not been elucidated, it is simple and practical, and has practical value.

(7) Ultra-high speed cutting method

Under conventional cutting, increasing the cutting speed will reduce the durability of the tool. Someone has proposed that when the cutting speed reaches a critical value, the cutting temperature reaches its highest value, and then the temperature will decrease as the speed continues to increase, and the cutting force will also decrease, resulting in higher surface quality. This is the theoretical basis of high-speed cutting. Schools and factories in the United States, Germany, and Japan have many practices in this area, using hard alloys and ceramic cutting tools to cut materials such as steel, cast iron, titanium, aluminum alloys, etc. This cutting method is often limited by equipment conditions and cannot be promoted. Whether it can play a role in cutting difficult to machine materials remains to be explored.

3. Adopt special processing methods

In addition to the various methods mentioned above, some machining methods that are completely different from cutting principles have been developed for the processing of engineering machinery parts, such as electrical discharge machining, electrochemical machining, ultrasonic machining, laser machining, electron beam machining, ion beam machining, etc., which are called special machining methods.

In special processing methods, the tool and workpiece are basically not in contact, and there is no obvious mechanical force during processing. It can process brittle materials, precision fine parts, thin-walled and weakly rigid parts, etc. This method utilizes electrical energy, chemical energy, acoustic energy, and thermal energy to remove the processed material, with a high instantaneous energy density and the ability to process any high hardness material.

At present, electrical discharge machining is mostly used for processing molds and irregular holes; Electrochemical machining is commonly used for machining special surfaces and irregular holes, which can achieve higher surface quality; Ultrasonic processing can process many non-metallic hard and brittle materials, especially for machining abnormal holes, cutting, etc; Laser processing is mainly used for drilling and cutting various metal and non-metal materials; Electron beam processing is mainly used for making micro holes and cutting seams; Ion beam machining can perform ultra precision and ultra micro machining on the surface of parts.

These special processing methods require specialized equipment, are expensive, consume a lot of energy, and therefore have a limited range of applications. However, they have played a significant role in the processing of difficult to machine materials and have broad development prospects.

Responsible Editor: Ben