Processing of difficult-to-cut materials (1)

I. Introduction :

1. What is difficult to cut material?

It is a material with poor machinability. The performance of general materials is greater than or less than one of the above indicators (HB>250, σb>1000 MPa, δ>30%, α k>100 MPa, K<41.8 W/m?k), all of which 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 materials :

1 physical properties:

1) Thermal conductivity K: Thermal conductivity The high allowable Vc is high. For example, use a cemented carbide tool to cut the Vc of the following materials:

Carbon steel K=48.2～50.2 W/m?k Vc=100～150 m/min

Superalloy K=8.4～16.7 W/m?k Vc= 7～60 m/min

Titanium alloy K=6.3～9.6 W/m?k Vc= 15～50 m/min

2) Linear expansion coefficient α: affects the degree of thermal expansion and contraction of the material and affects the processing accuracy.

2 chemical composition: the chemical composition and ratio of the material is the fundamental factor affecting the mechanical properties, physical properties, heat treatment properties, metallographic structure and machinability of the material. Such as:

C: The increase in the carbon content of the material increases the hardness and strength.

Ni: Ni can improve the heat resistance of the material, but the thermal conductivity of the material is significantly reduced; when Ni>8%, austenitic steel is formed, which makes the work hardening severe.

V: As its content increases, the grinding performance of the material deteriorates.

Mo: Improves the strength and toughness of the material, but the thermal conductivity of the material decreases.

W: It can improve the heat strength and high temperature strength of the material and the hardness and strength at room temperature. However, the thermal conductivity of the material is significantly reduced.

Mn: It can increase the hardness and strength of the material and reduce the toughness of the material. When Mn is >1.5%, the machinability of the material is deteriorated.

Si: The thermal conductivity of the material can be lowered.

Ti: Titanium is an element which is easy to form carbides, and its workability is also poor.

Also, elements such as Cr, O, S, P, N, Pb, Cu, and Al have an influence on the machinability of the material.

3 Mechanical properties of the material: 1) Hardness and strength: The hardness and strength of the material are moderate, the machinability is better, the higher the hardness and strength, the worse the machinability. Such as normalizing 45 steel: HB200, σb 640 MPa; quenching 45 steel: HRC45, σb 2100 ~ 2600 MPa. Also affecting the machinability of the material are fine impurities in the metal material structure, such as A1203, SiO2, TiO2, etc., which have high microhardness and mechanical wear on the tool; and the machinability is also deteriorated. 2) Toughness α k and plasticity δ: A material having high toughness and plasticity has large resistance, deformation, and heat generation during cutting, and also has poor machinability. 3) Elastic modulus E: It is an index indicating the stiffness of the material, and the modulus of elasticity is large, indicating that the material is less prone to elastic deformation under the action of external force. However, in the material with small modulus of elasticity, the elastic recovery is large during the cutting process, and the tool friction is large, and the cutting is also difficult. Such as soft rubber E == 2 ~ 4MP; 45 steel E = 200000 MPa: Mo material E = 500000 MPa.

4 metallographic structure: 1) ferrite: its hardness and strength are very low (HB50 ~ 90, σb = 190 ~ 250 MPa), plasticity and toughness (δ = 40 ~ 50%), easy to produce chips when cutting tumor. Poor machinability. 2) Pearlite: Spherical pearlite has good machinability. (such as 45 steel) 3) cementite: high hardness (HRC66-70), but very brittle (α k = 30-35 MPa), due to Fc increase and easy to collapse, making cutting difficult. 4) Austenite: Its hardness is not high (about HB200), but the plasticity and toughness are very high, the surface hardening of the surface and the cold welding of the chips and the tool are severe, and the machinability is poor. Such as 1Cr18Ni9Ti, high temperature alloys, etc. 5) Martensite: Hardened steel belongs to this type of metallographic structure. It has high hardness and brittleness, and its relative machinability is 1/3 to 1/10 of 45 steel.

3. Cutting characteristics of difficult-to-cut materials:

1 Large cutting force: 1.5 to 2.5 times of 45 steel

2 Cutting temperature is high: When Vc=75m/min, the cutting material is higher than the cutting temperature of cutting 45 steel:

TC-4 435°C GH132 320°C GH36 270°C 1Cr18Ni9Ti 195°C

3 The tendency of work hardening is large: due to the toughness and high plasticity of some difficult-to-cut materials, the strengthening coefficient is large; in addition, under the action of cutting heat, the atoms of H, O, N, etc. in the surrounding medium are absorbed, so that the cutting surface and the processed surface are formed. Hard and brittle layer, the degree of hardening is 50 ~ 200% higher than the substrate. The depth is greater than 0.1mm, which is several times that of 45 steel.

4. Ways to improve the processability of difficult-to-cut materials:

1 Use excellent tool materials. 2 Select reasonable tool geometry parameters. 3 Choose a reasonable amount of cutting. 4 Select a good cutting fluid. 5 Improve the fixture structure. 6 Appropriate heat treatment of the material to be processed. 7 using other processes

Second, the turning of difficult-to-cut materials:

1. Turning of hardened steel:

1 Cutting characteristics of hardened steel: 1) High hardness: HRC45-70. 2) Large brittleness. 3) High strength: σb = 2100 ~ 2600 MPa, about 4 times that of 45 steel. 4) Large cutting force: The unit cutting force Kc is 4500 MPa, which is 2.5 times that of 45 steel. 5) High cutting temperature: its thermal conductivity is 1/7 of 45 steel.

2 Tool and cutting amount: 1) Tool material: PCBN, ceramic, hard alloy (600, 610, YS8, YT05, 758, 813). 2) Tool geometry parameters: γ0=0～-10°, workpiece hardness and continuous cutting γ0=-10～-30°, α 0=8～10°, κr=30～60°, λs=0～-3 °. 3) Cutting amount: cemented carbide Vc=30～50m/min; ceramic tool Vc=60～120m/min; PCBN cutter Vc=100～200 m/min, Vc is 1/2, α in the intermittent cutting p and f are 1/2 of the cut steel.

Note: The heat resistance of a hardened steel is 200-400 ° C, and its hardness decreases with increasing temperature. The heat resistance of tool materials (hard alloy, ceramic, PCBN) is 800-1000 °C, 1200 °C, 1400-1500 °C, respectively. With this characteristic, Vc cannot be selected too low during continuous cutting; Vc should be reasonable, and the knife should be retired; f is small when interrupted cutting, κr is appropriately reduced, γε is increased, and λs is negative.

3. Turning of high temperature alloy

Since the superalloy contains many high melting point alloying elements such as Fe, Ti, Cr, Ni, V, W, Mo, etc., this element forms a high-purity, densely organized austenitic alloy with other alloying elements. The elements and non-metallic elements C, B, N, etc. constitute a metal and non-metal compound with high hardness, small specific gravity and high melting point. The other machinability is made poor. Its relative machinability is only 5-20% of 45 steel.

1 Cutting characteristics: 1) Large cutting force: 2 to 3 times larger than general steel. 2) High cutting temperature: 50% higher than cutting 45 steel. 3) Severe work hardening: the hardness of the cut surface and the machined surface is 50-100% higher than that of the substrate. 4) The tool is easy to wear: it is easy to produce bonding, diffusion, oxidation, and groove wear.

2 Tool materials: 1) High-speed steel: high-vanadium, high-carbon, aluminum-containing high-speed steel should be used. 2) Cemented carbide: YG with fine and ultrafine particles should be used

9 tool geometry parameters: deformation superalloy: γ0 = 10 °, casting superalloy: γ0 = 0 °, generally do not take negative chamfer. α 0 = 10 ~ 15 °, rough car λs = -10 °, fine car λs = 0 ~ 3 °, κr = 45 ~ 75 °.

4 cutting amount: 1) high speed steel cutter: Vc = 3 ~ 8m / min. 2) Carbide tool: Vc = 10 ~ 60 m / min. Deformed superalloy: Vc = 40 ~ 60m / min. Cast superalloy: Vc = 7 ~ 10m / min. α p and f > 0.1 mm.

5 cutting fluid: same with stainless steel.

6 high-temperature alloy drilling: Carbide drill bits or S-type and shallow-hole drills should be used as much as possible; if using high-speed steel drills, increase 2 towels, repair α 0 and b, Vc is 3 m/min, f is 0.2 ~0.3mm/r; the drill bit should be sharp, and the blunt standard is 1/2 to 1/3 of the general steel. It is best to use automatic feed and do not stay on the cutting table.

4. Turning of titanium alloy:

Titanium alloy is a new metal developed in recent decades. Because of its specific strength (σb/ρ=1680/4.5=373, which is 4.5 times that of 45 steel), it has high heat strength (500°C long-term work). It has good corrosion resistance and excellent low temperature performance, and is used in aerospace, chemical, medical and so on.

1 Cutting characteristics: 1) High cutting temperature: under the same cutting conditions, it is 1 times higher than that of 45 steel. 2) The affinity is large, the bonding is serious, and the bonding phenomenon occurs at high temperatures. 3) Large chemical activity: Under high temperature cutting conditions, it reacts with O, N, H, and C in the air to form a hard skin layer of TiO2, TiN, TiH, etc., which causes difficulty in cutting.

2 Tool materials: 1) High-speed steel: In addition to ordinary high-speed steel, it is best to use high-vanadium, high-cobalt, aluminum-containing high-speed steel. 2) Cemented carbide: YG8, YG6X, YG6A, 813, 643, YS27, YD15.3) PCD, PCBN natural diamond.

3 tool geometry parameters: γ0 = 5 ~ 15 °, α 0 ≥ 15 °.

4 cutting amount: 1) high speed steel cutter: Vc = 8 ~ 12 m / min. 2) Carbide tool: Vc = 15 ~ 60 m / min. 3) PCD, PCBN natural diamond cutter: wet cut Vc=200 m/min: dry cut Vc=100 m/min, α p>0.05mm, f>0.05mm/r.

5 cutting fluid: one emulsion, extreme pressure emulsion. Extreme pressure cutting oil is used for finishing.

6 Note: When turning the slender shaft (rod), use the live tip and nylon as the claw holder of the tool holder and the center frame; when reaming, apply 40% of castor oil + 40% of kerosene as cutting fluid; when tapping, Appropriately increase the diameter of the bottom hole.

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