Analysis And Influencing Factors of Material Machinability

With the continuous development of manufacturing technology, a variety of new materials have been developed and widely used.

Many of these new materials are difficult to machine materials, such as titanium alloys, superalloys and composites.

On the one hand, it greatly improves the product performance, on the other hand, it also brings great difficulties to processing and manufacturing.

According to the characteristics of various difficult to process materials, it is of great significance to take corresponding measures to complete the processing tasks with high efficiency, low cost, quality and quantity, meet the processing requirements, and meet the needs of modern science, technology and industrial development.

Analysis of material machinability and influencing factors

1. Machinability classification

The machinability of metal materials refers to the degree of difficulty in cutting metal materials under certain cutting conditions.

Machinability depends not only on the material itself, but also on the specific machining requirements and cutting conditions.

The machining requirements and production conditions are different, and the indicators for evaluating the machinability of materials are also different.

There are many methods for evaluating the indicators.

The commonly used method is to measure the machinability by cutting speed, that is, on the premise of the same tool service life T, the allowable cutting speed VT for cutting a certain material is high, and the machinability is good;

On the contrary, VT is small and the machinability is poor.

If the tool service life T=60min is taken, VT can be written as V60.

The relative machinability kV is usually measured in production.

KV is based on the V60 of 45 steel (normalized) with strength Rm=0.598GPa, written as (V60) j.

The V60 of other materials to be cut is compared with it, that is, kv=v60/ (V60) j.

The greater the Kv value, the better the machinability.

The relative processability of common materials is divided into 8 levels, as shown in table 7-1.

Table 7-1 classification of machinability

Machinability grade

Name and type

Relative machinability coefficient KV

Representative workpiece materials

1

Materials that are very easy to cut

General nonferrous metals

>3.0

Cu-Pb copper alloy, Al-Cu aluminum alloy, Al-Mg zinc alloy

2

Materials that are easy to cut

Free cutting steel

2.5~3.0

Annealed 15Cr steel(Rm=370-440MPa),

 Y12 steel(Rm=390~490MPa),

 

3

Relatively free cutting steel

1.6~2.5

Normalized 30 steel(Rm=448~558MPa),

 

4

Common materials

General steel and cast iron

1.0~1.6

45 steel, grey cast iron

5

Slightly difficult to cut materials

0.65~1.0

Quenched and tempered 2Cr13 Steel(Rm-835MPa),

85 hot rolled steel (Rm=888MPa)

6

Materials that are difficult to cut

Relatively difficult to cut material

0.5~0.65

Quenched and tempered 45Cr steel(Rm=1030MPa),

 

Quenched and tempered 65Mn Steel(Rm=930~980MPa)

7

Difficult to cut material

0.15~0.5

Quenched and tempered 50CrV, 1Cr18Ni9Ti stainless steel (final quenching), industrial pure titanium, α-phase titanium alloy

8

Very difficult to cut material

<0.15

β-phase titanium alloy casting, nickel base superalloy, Mnl3 high manganese steel

2. Factors affecting the machinability of materials

See table 7-2 for the factors affecting the machinability of materials.

Table 7-2 factors affecting the machinability of materials

FactorsInstruction
Strength and hardnessThe higher the strength or hardness of the workpiece material, the greater the cutting force, the higher the cutting temperature, and the tool wear will intensify.In addition, when cutting hard materials, the tool chip contact length is short, and the cutting force and cutting heat are concentrated near the cutting edge, which is easy to cause the cutting edge to peel off or even collapse. This is particularly obvious for brittle tool materials such as cemented carbide and ceramics. Therefore, the cutting machinability of materials is poor.
Plasticity and toughnessThe greater the plasticity and toughness of the workpiece material, the greater the chip deformation, the more cutting heat generated, and the chips are also easy to bond with the tool. Therefore, the tool wear will be aggravated. However, if the plasticity and toughness of the workpiece material are too small, the tool chip contact length becomes very short, which will also cause serious tool wear.Therefore, the machinability of workpiece materials with too large or too small plasticity and toughness is poor.
resistance to effect of heatThe better the heat resistance of the workpiece material, the higher the strength and hardness can be maintained at high temperature, and it will be very difficult to cut.
Ability to scratch the toolThe stronger the scratch ability of the workpiece material, the greater the wear of the tool and the worse the machinability.
conductivity factor The smaller the thermal conductivity of the workpiece material, the less the cutting heat is transmitted, the higher the cutting temperature, the more serious the tool wear, and the worse the machinability.

Cutting characteristics of difficult to machine materials

See table 7-3 for machining characteristics of difficult to machine materials.

Table 7-3 cutting characteristics of difficult to machine materials

Machining characteristicsCause
Large cutting force1. The material has high hardness, high strength, good plasticity and toughness, and large cutting deformation;2. The contact length between the tool and the chip is short, which increases the contact stress of the cutting edge.
High cutting temperature1. Much deformation work is consumed during cutting;2. The thermal conductivity is small, and the cutting heat is not easy to be transmitted, which is concentrated at the tool tip of the cutting edge.
High work hardening tendencyThe plasticity and toughness of the material are large, the cutting deformation caused by the cutting force is large, and the lattice is distorted, resulting in work hardening.
Serious tool wear1. Due to the large cutting force and high cutting temperature, the tool is prone to oxidation wear and diffusion wear;2. Hard spots, cementite and other structures cause strong friction damage to the tool;3. The elasticity of the material is large, which increases the affinity between the tool and the processing material, so it is easy to produce adhesive wear and diffusion wear;4. The work hardening layer is easy to cause boundary wear of the tool.
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