Physical and Mechanical Properties and Cutting Performance of Typical Difficult-to-Machine Materials

With the advancement of materials science and technology and the continuous improvement of material preparation techniques, a large number of new materials have emerged in the production field. Most of these materials have relatively good overall physical and mechanical properties, but extremely poor metal cutting performance, and are therefore referred to as difficult-to-machine materials. The unique mechanical properties of difficult-to-machine materials present numerous difficulties in cutting, especially in ultra-slender, deep-hole machining, which has become a technical bottleneck in production. This chapter focuses on the physical and mechanical properties and cutting performance of several typical difficult-to-machine materials.

Basic Concepts of Metallic Machinability

Metrics for Measuring the Machinability of Metallic Materials

The machinability of a metal material refers to the ease with which it can be cut under certain cutting conditions. The degree of machining difficulty varies depending on the cutting conditions and requirements. The machinability of a metal material is a relative concept and is generally measured using four indicators: surface quality, tool life, specific cutting force, and chip control.

1. Surface Quality

The machinability of mechanical parts is generally measured by surface roughness. The lower the surface roughness after machining, the higher the machinability. For precision parts with special requirements, the depth of the metamorphic layer on the machined surface, the residual stress and the degree of hardening are used to measure their machinability. This is mainly because the depth, residual stress and degree of hardening of the metamorphic layer directly affect the stability of the shape and size of the parts, as well as the magnetic conductivity, electrical conductivity and creep resistance. 2. Tool durability The machinability of metal materials is measured by tool durability. Tool durability refers to the total cutting time from the start of cutting until the wear reaches the tool blunting standard. Tool durability (1) Under the same tool durability conditions, the allowable cutting speed for cutting the workpiece material is examined. This indicator is a commonly used indicator for measuring machinability. The allowable cutting speed is expressed as vy, which means: when the tool durability is T (min), the allowable cutting speed value for cutting the metal material. The larger the vy value, the better the machinability of the workpiece. In general, T=60min; for difficult-to-cut materials, T=30min or T=15min. If T=60min, the cutting speed v can be expressed as v60.

(2) Under the same cutting conditions, examine the value of the tool durability when cutting the workpiece material. The larger the tool durability value, the better the workpiece machinability.

(3) Under the same cutting conditions, examine the volume of metal removed when cutting the workpiece material to the tool blunting standard. The larger the metal cutting volume, the better the workpiece machinability.

3. Unit cutting force

When the machine tool power is insufficient or the machine tool-fixture-tool-workpiece system is insufficiently rigid, the unit cutting force is often used to measure the machinability of the workpiece.

4. Chip control difficulty

The ability to effectively control the flow direction of the chips and reliably break the chips is good in machinability, such as deep hole drilling, deep hole boring, deep hole trepanning drilling, etc.; otherwise, the machinability is poor. In actual production, relative machinability is usually used to measure the machinability of the metal material being processed. Taking the V value of 45 steel (hardness 170-229 HB, strength θ = 0.637 GPa) as the benchmark, denoted as (V60), the ratio k of the V value of other metal materials to (V60) is called relative machinability, i.e., k = V60/(V60)j.

The relative machinability of commonly used metal materials can be divided into eight levels, as shown in Table 3.1.

Factors Affecting the Machinability of Metal Materials

The machinability of metal materials is related to the material’s physical and mechanical properties, chemical composition, heat treatment status, metallographic structure, machining requirements, and processing conditions.

1. Hardness

1) Influence of Workpiece Material Hardness at Room Temperature

Generally speaking, materials of the same type with higher hardness have lower machinability. When the material hardness is high, the contact length between the chip and the rake face decreases, increasing the normal stress on the rake face and concentrating frictional heat on the smaller tool-chip contact surface, leading to higher cutting temperatures and increased wear. Excessively high workpiece hardness may cause tool tip burn and chipping. Figure 3.1 shows the relationship between hardness and machinability of carbon steel.

2) The Impact of Workpiece Material High-Temperature Hardness on Machinability

The higher the high-temperature hardness of the workpiece material, the lower its machinability. As the cutting temperature increases, the hardness of the tool material decreases, and the ratio of tool material hardness to workpiece material hardness also decreases, increasing tool wear. For example, high-temperature nickel-based alloys and heat-resistant steels have relatively high high-temperature hardness and extremely low machinability.

3) The Impact of Hard Spots in the Workpiece Material on Machinability

The sharper and more widely distributed the hard spots in the workpiece material, the lower its machinability. Hard spots affect tool wear in two ways: first, the high hardness of hard spots can cause abrasions on the tool; second, the fine hard spots at the workpiece grain boundaries increase the material’s strength and hardness, increasing its resistance to shear deformation during cutting, thus reducing the material’s machinability.

4) The Impact of Material Work Hardening Properties on Machinability

The higher the work hardening properties of the workpiece material, the lower its machinability. For example, austenitic stainless steel exhibits significant surface hardening after machining, with its surface microhardness 1.4 to 2.2 times higher than the original substrate hardness. As the material’s work-hardening properties increase, cutting forces and temperatures rise; the tool is scratched by the hardened chips, causing edge wear on the secondary flank, and tool wear increases.

2. Strength

Workpiece material strength includes both room-temperature strength and high-temperature strength. Higher room-temperature strength increases cutting forces, higher cutting temperatures, greater tool wear, and poorer machinability. Generally, machinability decreases as metal strength increases. For example, at room temperature, the σ of 20CrMo alloy steel is slightly lower than that of 45 steel (650 MPa). However, at 600°C, 20CrMo alloy steel’s σ is higher than that of 45 steel (180 MPa), reaching 400 MPa. Therefore, 20CrMo alloy steel has poorer machinability than 45 steel at high temperatures.

3. Plasticity and Toughness

For materials with high plasticity, plastic deformation increases due to the increased plastic deformation area, resulting in increased plastic deformation work. For materials with high toughness, the plastic area may not increase during plastic deformation, but the plastic deformation work absorbed does increase. Although the causes differ, increased plasticity and toughness both lead to increased plastic deformation work. For workpiece materials with the same strength, greater plasticity results in greater plastic deformation and greater plastic deformation work consumption. Consequently, when cutting such workpieces, cutting forces and temperatures are higher, and the tool is more susceptible to adhesion, increasing tool wear and surface roughness. Therefore, the greater the plasticity of the workpiece material, the worse the machinability. Excessively low plasticity shortens the contact length between the tool and the chip, concentrating cutting forces and heat near the tool edge, which in turn increases tool wear. Both excessive and low plasticity (or brittleness) of the workpiece material can reduce machinability. The greater the toughness of a workpiece material, the more work energy and cutting forces are consumed during cutting, and the greater the impact of toughness on chip breaking. Therefore, the greater the toughness of a workpiece material, the worse its machinability.

4. Thermal Conductivity of Workpiece Material

The greater the thermal conductivity of a workpiece material, the more heat is carried away by the chips and conducted by the workpiece, which helps reduce the temperature in the cutting zone and improves machinability. For example, stainless steel and high-temperature nickel-based alloys have very low thermal conductivity, only one-third to one-quarter that of 45 steel, making these workpiece materials poorly machinable. However, workpiece materials with high thermal conductivity experience a high temperature rise during machining, making it difficult to control machining dimensions.

5. Chemical Composition

The strength and hardness of steel generally increase with increasing carbon content, while its plasticity and toughness decrease. High-carbon steel has higher strength and hardness, resulting in greater cutting forces and increased tool wear. Low-carbon steel has higher plasticity and toughness, resulting in greater cutting deformation, difficulty breaking chips, and greater surface roughness. Medium-carbon steel falls between the two, offering better machinability. To improve the performance and machinability of steel, alloying elements such as chromium (Cr), nickel (Ni), vanadium (V), molybdenum (Mo), tungsten (W), manganese (Mn), silicon (Si), and aluminum (Al) may be added. Cr, Ni, V, Mo, W, and Mn increase the strength and hardness of steel, while Si and Al tend to form hard particles such as silicon oxide and aluminum oxide, which increase tool wear. Low content of these elements (generally limited to 0.3%) has little effect on the machinability of steel. However, exceeding 0.3% detrimentally affects machinability. Adding trace amounts of sulfur (S), selenium (Se), lead (Pb), bismuth (Bi), and calcium (Ca) to steel can form inclusions, embrittle the steel, or act as a lubricant, reducing tool wear and improving the machinability of the workpiece. While the addition of phosphorus (P) increases the strength and hardness of steel, it significantly reduces its toughness and ductility, making it more susceptible to chip breakage. Figure 3.2 shows the effects of different elements on the machinability of structural steel.

6. Metallographic Structure

Different metallographic structures have a direct impact on machinability. Generally, the ratio of ferrite to pearlite in steel influences machinability. Ferrite has high plasticity, pearlite has high hardness, and martensite is harder than pearlite. Therefore, a lower pearlite content allows for higher machining speeds, greater tool durability, and better machinability. Conversely, a higher martensite content results in poorer machinability.