Introduction to the machining of difficult-to-cut materials

This paper introduces what is a hard-to-cut material, factors affecting the machinability of materials, and cutting characteristics of hard-to-cut materials. The basic concept of improving the machinability of hard-to-cut materials.

1 What is a hard-to-cut material?

It is a material with poor machinability. General material performance greater than or less than this indicator (hb>;250, σb>;1000mpa, δ>;30%, ak>;100 mpa, k<41.8w/m-k) of more than one, are difficult to cut materials. It can also be measured by the phenomena in the cutting process (cutting force, cutting heat, tool wear and tool durability, machined surface quality and chip control, etc.).

2 factors affecting the machinability of the material

2.1 Physical properties

Carbon steel k = 48.2 ~ 50.2w/m-k vc = 100 ~ 150m / min High temperature alloy k=8.4～16.7w/m-k vc=7～60m/min Titanium alloy k=6.3～9.6w/m-k vc=15～50m/min Thermal conductivity k: the vc allowed for materials with high thermal conductivity is high. If the carbide tool is used to cut the following materials vc is shown in the table on the right. Coefficient of linear expansion α: affect the degree of thermal expansion and contraction of the material and affect the machining accuracy.

2.2 Chemical composition

The chemical composition and ratio of the material is the fundamental factor that affects the mechanical properties, physical properties, heat treatment properties, metallographic organization and machinability of the material. For example c: the increase of carbon content of the material, its hardness and strength increases. ni: nickel can improve the heat resistance of the material, but make the thermal conductivity of the material significantly decreased: when ni>;8%, the formation of austenitic steel, making the process hardening severe.

v: As the content of vanadium increases, the grinding performance of the material becomes worse.
mo: molybdenum can improve the strength and toughness of the material, but the thermal conductivity of the material decreases.
w: tungsten can improve the thermal strength and high temperature strength and room temperature hardness and strength of the material. However, the thermal conductivity of the material decreases significantly.
mn: manganese can improve the hardness and strength of the material, so that the toughness of the material decreases. When mn>;1.5%, the material’s machinability becomes poor.
si: silicon can make the thermal conductivity of the material decrease.
ti: titanium is easy to form carbide elements, its machinability is also poor.
There are also cr, o, s, p, n, pb, cu, al and other elements have an impact on the machinability of the material.

2.3 Mechanical properties of the material

Hardness and strength: the hardness and strength of the material is moderate, its machinability is relatively good, the higher the hardness and strength, the worse the machinability. Such as normalized 45 steel: hb200, σb = 640mpa: quenched 45 steel: hrc45, σb = 2100 ~ 2600mpa. affect the material machinability and metal material organization of fine impurities, such as al2o3, sio2, tio2, etc., their high microhardness, mechanical wear on the tool: its machinability also becomes poor.

Toughness α k and plasticity δ: toughness and plasticity of the material, the resistance, deformation and heat generated during the cutting process is large, and its machinability is poor.
Modulus of elasticity e: It is an indicator of material stiffness. A large modulus of elasticity means that the material is not easy to produce elastic deformation under the action of external forces. But the material with small elastic modulus in the cutting process, elastic recovery, and tool friction is large, cutting is also difficult. Such as soft rubber e == 2 ~ 4mp: 45 steel e = 200000 mpa: mo material e = 500000 mpa.

2.4 Metallographic organization

Ferrite: it has low hardness and strength (hb50～90, σb=190～250mpa), high plasticity and toughness (δ=40～50%), easy to produce chip tumor when cutting, poor machinability.

Pearl body: spherical pearl body cutting machinability is good. (such as 45 steel)
Carburite: high hardness (hrc66-70), but very brittle (α k=30-35 mpa), easy to chipping due to the increase of fc, making cutting difficult.
Austenite: it is not high hardness (hb200 or so), but plasticity and toughness is very high, processing surface hardening and chips and tools cold welding serious, poor machinability. Such as 1cr18ni9ti, high-temperature alloys, etc.