The quality of tool material performance is a fundamental factor affecting the surface quality, cutting efficiency, and tool life of machining. When cutting, the cutting part of the tool is directly responsible for the cutting work. The cutting performance of cutting tools mostly depends on the materials that make up the cutting part of the tool, the geometric parameters of the cutting part, and the reasonable selection and design of the tool structure. The level of cutting productivity and tool durability, the amount of tool consumption and processing cost, the quality of processing accuracy and surface quality, etc., largely depend on the reasonable selection of tool materials. The correct selection of cutting tool materials is one of the important aspects of designing and selecting cutting tools.

Each type of tool material has its own specific processing range, which can only be applied to a certain range of workpiece materials and cutting speeds. There are often significant differences in tool life between different tool materials and the same type of tool when machining different workpiece materials. For example, when machining aluminum pistons, the life of diamond tools is several to dozens of times that of YG type hard alloy tools; There is also a significant difference in the lifespan of YG type hard alloy cutting tools when machining aluminum alloys with high, medium, and low silicon content. Therefore, the reasonable selection of cutting tools is the key to successful cutting processing. Each cutting tool material has its optimal processing object, which means there is a reasonable matching problem between the cutting tool and the processing object.

1. The properties that tool materials should possess

1.1 High hardness and wear resistance

Hardness is a fundamental characteristic that tool materials should possess. The tool needs to cut chips from the workpiece, and its hardness must be greater than the hardness of the workpiece material. The cutting edge hardness of the cutting tool used for cutting metal is generally above 60HRC.

Wear resistance is the ability of a material to resist wear and tear. Generally speaking, the higher the hardness of tool materials, the better their wear resistance. The higher the hardness and quantity of hard points (carbides, nitrides, etc.) in the organization, the smaller and more evenly distributed the particles, and the better the wear resistance. The wear resistance is also related to the chemical composition, strength, microstructure, and temperature of the friction zone of the material. The wear resistance WR of the material can be expressed by the formula:

WR=KIC0.5E-0.8H1.43

In the formula: H - Material hardness (GPa). The higher the hardness, the better the wear resistance.

KIC - fracture toughness of material (MPa · m ½)。 The larger the KIC, the smaller the fracture caused by stress on the material, and the better its wear resistance.

E - Elastic modulus of the material (GPa). When E is very small, the microstrain caused by abrasive particles helps to generate lower stress and improve wear resistance.

1.2 Adequate strength and toughness

To make the cutting tool work under high pressure, as well as the impact and vibration conditions that often occur during the cutting process, without causing edge collapse and fracture, the tool material must have sufficient strength and toughness.

1.3 High heat resistance (thermal stability)

Heat resistance is the main indicator for measuring the cutting performance of tool materials. It refers to the performance of tool materials to maintain a certain degree of hardness, wear resistance, strength, and toughness under high temperature conditions.

The tool material should also have the ability to resist oxidation at high temperature and good ability to resist adhesion and diffusion, that is, the tool material should have good chemical stability.

1.4 Good thermophysical properties and thermal shock resistance

The better the thermal conductivity of the tool material, the easier it is for cutting heat to dissipate from the cutting zone, which is beneficial for reducing the cutting temperature.

When cutting intermittently or using cutting fluid, the tool is often subject to great thermal shock (temperature changes violently), so cracks will occur inside the tool and cause fracture. The ability of tool material to resist thermal shock can be expressed by the thermal shock coefficient R, which is defined as:

R= λσ b(1-µ)/E α

In the equation: λ—— thermal conductivity;

σ B - tensile strength;

µ - Poisson's ratio;

E - elastic modulus;

α—— Coefficient of thermal expansion.

The high thermal conductivity makes it easy for heat to dissipate and reduces the temperature gradient on the tool surface; Small coefficient of thermal expansion can reduce thermal deformation; The small elastic modulus can reduce the amplitude of alternating stress caused by thermal deformation; It is beneficial to improve the thermal shock resistance of the material.

Cutting fluid can be used for cutting tool materials with good thermal shock resistance.

1.5 Good process performance

In order to facilitate the manufacturing of cutting tools, it is required that the tool material has good process performance, such as forging performance, heat treatment performance, high-temperature plastic deformation performance, grinding processing performance, etc.

1.6 Economy

Economy is one of the important indicators of tool materials. Although the cost of a single piece of high-quality tool material is high, the cost allocated to each part may not be very high due to its long service life. Therefore, when selecting cutting tool materials, it is necessary to comprehensively consider their economic effects.

2 Tool material

2.1 High speed steel

High speed steel is a type of high alloy tool steel that contains a large amount of alloying elements such as tungsten, molybdenum, chromium, and vanadium. High speed steel has high strength and toughness, as well as certain hardness and wear resistance. Suitable for the requirements of various types of cutting tools. The manufacturing process of high-speed steel cutting tools is simple and easy to grind into sharp cutting edges. Therefore, despite the continuous emergence of various new tool materials, high-speed steel cutting tools still account for a large proportion in metal cutting. Can process non-ferrous metals and high-temperature alloys. Due to the above properties of high-speed steel, cutting tools such as milling risers, milling transverse grooves, milling expansion grooves, and drilling bits for oil holes in piston processing are all made of high-speed steel materials.

2.2 Hard alloy

Hard alloy is made by powder metallurgy of refractory metal carbides (such as WC, TiC, TaC, NbC, etc.) and metal binders (such as Co, Ni, etc.) powder.

Due to the presence of a large amount of metal carbides in hard alloys, which have the characteristics of high melting point, high hardness, good chemical stability, and good thermal stability, the hardness, wear resistance, and heat resistance of hard alloy materials are all very high. The hardness of commonly used hard alloys is 89-93HRA, which is higher than that of high-speed steel (83-86.6HRA) and can be cut at 800-1000 ℃. At 540 ℃, the hardness of hard alloy is 82-87HRA, and at 760 ℃, the hardness can still maintain 77-85HRA. Therefore, the cutting performance of hard alloys is much higher than that of high-speed steel, and the tool durability can be improved by several to dozens of times. When the durability is the same, the cutting speed can be increased by 4-10 times.

At present, the hard alloy cutting tools used by our company are mainly YG6 and YGX in the YG class (WC TiC Co). Hard alloys such as YT15 in the YT class (WC TiC Co) are used for piston rough machining, semi precision machining, and partial precision machining processes.

2.3 Diamond

Diamond is currently the material with the highest hardness and best thermal conductivity among known mineral materials. The wear amount when paired with various metal and non-metallic materials is only 1/50-1/800 of that of hard alloys, making it the most ideal material for making cutting tools. However, natural single crystal diamonds are only used for ultra precision machining of jewelry and certain non-ferrous metals. Although artificial large particle single crystal diamond for cutting tools has been industrialized by De Beers, Sumitomo Electric, and others, it has not yet entered a large-scale application stage.

The cutting edge of diamond cutting tools is very sharp (which is important for cutting chips with very small cross-sections), the roughness of the edge is very small, the friction coefficient is low, and it is not easy to generate chip accumulation during cutting, resulting in high surface quality machining. When processing non-ferrous metals, the surface roughness can reach Ra0.012 µ m, and the processing accuracy can reach IT5 level or above.

There are three types of diamond cutting tools: natural single crystal diamond cutting tools, overall artificial polycrystalline diamond cutting tools, and diamond composite cutting tools. Due to its high cost, natural diamond cutting tools are rarely used in practical production. Artificial diamond is formed by converting graphite under high temperature and pressure through the action of an alloy catalyst. Diamond composite blade is a layer of diamond about 0.5-1 µ m thick sintered on a hard alloy substrate through advanced processes such as high temperature and high pressure. This material is made from hard alloy as the substrate, and its mechanical properties, thermal conductivity, and expansion coefficient are similar to hard alloy. The diamond crystals in the artificial polycrystalline diamond abrasive on the substrate are irregularly arranged, and its hardness and wear resistance are uniform in all directions.

Polycrystalline diamond (PCD) is formed by sintering selected artificial diamond microcrystals under high temperature and pressure. During the sintering process, due to the addition of additives, a bonding bridge composed mainly of TiC, SiC, Fe, Co, and Ni is formed between diamond crystals. Diamond crystals are firmly embedded in a strong framework composed of structural bridges by combining covalent bond, which greatly improves the strength and toughness of PCD. Its hardness is about 9000HV, bending strength is 0.21~0.48GPa, thermal conductivity is 20.9J/cm · s µ ℃, and coefficient of thermal expansion is 3.1 × 10-6/℃。 Most of the polycrystalline diamond tools currently used are composites formed by sintering PCD with a hard alloy matrix, that is, sintering a layer of PCD on the hard alloy matrix. The thickness of PCD is generally 0.5mm and 0.8mm, and due to the bottom layer being made of hard alloy, welding is convenient; Due to the conductivity of PCD combined with the bridge, it is easy to cut and process PCD into various shapes and make various cutting tools, with a cost far lower than that of natural diamonds.

Polycrystalline diamond (PCD) can process all kinds of non-ferrous metals and highly wear-resistant non-metallic materials with high performance, such as aluminum, copper, magnesium and their alloys, hard alloys, fiber reinforced plastics, metal matrix composite, wood composites, etc. The average size of diamond grains in PCD tool materials varies, and the impact on performance is also different. The larger the grain size, the higher its wear resistance. Under similar cutting edge processing quantities, the smaller the grain size, the better the quality of the cutting edge. Selecting PCD tools with grain sizes of 10-25 µ m can achieve high-speed cutting of silicon-aluminum alloys with Si content of 12-18% at a speed of 500-1500m/min, while PCD tools with grain sizes of 8-9 µ m can process aluminum alloys with Si content less than 12%. For ultra precision machining, PCD tools with small grain sizes should be selected. The wear resistance of PCD will weaken when it exceeds 700 ℃, as its structure contains metal Co, which promotes the "reverse reaction", i.e. the transition from diamond to graphite. PCD has good fracture toughness and can carry out intermittent cutting. It can end mill aluminum alloy with 10% Si content at a high speed of 2500m/min.

The high hardness, high wear resistance, high thermal conductivity, and low friction coefficient of diamond materials can be utilized to achieve high-precision, high-efficiency, high stability, and high surface finish machining of non-ferrous and wear-resistant non-metallic materials. When cutting non-ferrous metals, the lifespan of PCD tools is dozens or even hundreds of times that of hard alloy tools, making it an ideal tool for precision machining of aluminum pistons. For example: finishing turning piston ring groove, finishing boring piston pin hole, finishing turning piston excircle, finishing turning piston top surface, etc.

2.4 cubic boron nitride

Polycrystalline cubic boron nitride (PCBN) is formed by sintering CBN micropowder with a small amount of bonding phase (Co, Ni or TiC, TiN, Al203) under high temperature and pressure with a catalyst. It has very high hardness (second only to diamond) and heat resistance (1300~1500 ℃), excellent chemical stability, much higher thermal stability (up to 1400 ℃) and thermal conductivity than diamond tools, low friction coefficient, but low strength. Compared with diamond, the outstanding advantage of PCBN is its much higher thermal stability, reaching up to 1200 ℃ (700~800 ℃ for diamond), and being able to withstand higher cutting speeds; Another prominent advantage is its high chemical inertness, which does not react with iron group metals at temperatures ranging from 1200 to 1300 ℃. It can be used for processing steel. Therefore, PCBN tools are mainly used for efficient processing of black difficult to machine materials.

In addition to the above features, PCBN tools also have the following advantages: ① high hardness, especially suitable for processing hardened steel above HRC50, heat-resistant alloy above HRC35, and gray iron below HRC30 that can only be grinded before, but other tools are difficult to machine. ② Compared with hard alloy cutting tools, the cutting speed is high and can achieve high-speed and efficient cutting Good wear resistance, high tool durability (10-100 times that of hard alloy tools), can achieve good surface quality of the workpiece, and achieve the use of turning instead of grinding. The disadvantage lies in the poor impact resistance of PCBN tools compared to hard alloys. Therefore, attention should be paid to improving the rigidity of the process system and avoiding impact cutting as much as possible when using it.

PCBN can be made into integral blades or combined with hard alloys to make composite blades. PCBN composite blade is a layer of PCBN sintered on a hard alloy substrate with a thickness of 0.5-1.0mm, which has good toughness, high hardness, and wear resistance.

The performance of PCBN is mainly related to the particle size of CBN, the content of CBN, and the type of binder. According to its structure, it can be roughly divided into two categories: one is formed by directly combining CBN grains, with a high CBN content (over 70%) and high hardness, suitable for cutting heat-resistant alloys, cast iron, and iron based sintered metals; The other type is mainly composed of CBN grains and sintered through ceramic binders (mainly TiN, TiC, TiCN, AlN, Al203, etc.). This type of PCBN has a low CBN content (below 70%) and low hardness, making it suitable for cutting hardened steel.

In our company, cubic boron nitride cutting tools are used in the turning process of cast iron ring grooves for cast iron ring pistons, as well as in the processing of piston three-dimensional profiles.

2.5 Ceramics

The main advantages of ceramic tool materials are:

Has high hardness and wear resistance, with a room temperature hardness of 91-95HRC;

Has high heat resistance, with a hardness of 80HRC at a high temperature of 1200 ℃; Moreover, the reduction in bending strength and toughness under high temperature conditions is minimal;

It has high chemical stability, low affinity between ceramics and metals, good high-temperature oxidation resistance, and does not interact with steel even at melting temperature. Therefore, the adhesion, diffusion, and oxidation wear of the cutting tools are less;

Has a low coefficient of friction, making chips less likely to stick to the knife and less prone to chip buildup.

The disadvantages of ceramic knife are:

Due to its high brittleness, low strength and toughness, and only 1/2 to 1/5 of the bending strength of hard alloy, it is necessary to choose appropriate geometric parameters and cutting amount when using it; Avoid bearing impact loads to prevent blade breakage and damage; In addition, the thermal conductivity of ceramic knife is low, only 1/2~1/5 of that of cemented carbide, but its coefficient of thermal expansion is 10~30% higher than that of cemented carbide, and its thermal shock resistance is poor.

At present, ceramic cutting tools have not been used in the processing of aluminum pistons.

3 Summary

The development of tool materials plays a decisive role in the progress of cutting technology. This article introduces the properties and applicability of cutting tool materials such as diamond, polycrystalline cubic boron nitride, ceramics, hard alloys, and high-speed steel used in cutting. The mechanism of tool damage is the theoretical basis for the rational selection of tool materials. The reasonable matching of tool materials and workpiece materials is the key basis for the selection of cutting tool materials. Tool materials should be selected based on the mechanical, physical, and chemical properties of the tool materials and workpiece materials in order to achieve good cutting effects. This article elaborates on the selection of tool materials for piston cutting.

High speed steel: High speed steel materials are used for milling and pouring risers, milling transverse grooves, and milling expansion grooves in piston machining, as well as drill bits for drilling oil holes.

Hard alloy: The YG and YD series of hard alloy cutting tools are widely used in various processes of aluminum piston machining, especially in piston rough machining and semi precision machining processes.

Cubic boron nitride: Cubic boron nitride tools are used in the turning process of cast iron ring grooves for cast iron ring pistons. It is also applied in the processing of piston three-dimensional profiles.

Diamond: Diamond cutting tools can utilize the high hardness, high wear resistance, high thermal conductivity, and low friction coefficient of diamond materials to achieve high-precision, high-efficiency, and high stability of non-ferrous and wear-resistant non-metallic materials