Machining of Superalloys: Classification, Causes And Specific Measures

Superalloys, also known as heat-resistant alloys or thermal strength alloys, can work under high-temperature oxidation atmosphere of 600 ~ 1000 ℃ and gas corrosion conditions.

They have good thermal strength, thermal stability and thermal fatigue properties.

According to the matrix elements, superalloys are divided into nickel base superalloys, iron base superalloys and cobalt base superalloys, which are widely used in various fields, especially in aviation, aerospace, power generation equipment, shipbuilding and other industries.

1. Classification of Superalloys

The classification and characteristics of superalloys are shown in table 7-14.

Table 7-14 classification and characteristics of Superalloys

Nickel base superalloyIt is an alloy based on nickel. It is the most stable material with high temperature oxidation resistance at present.

Typical brands include GH4033GH4169, K403, etc. The machinability of nickel base superalloy is very poor.
Iron base superalloyIt is an alloy based on iron or iron nickel. The latter is also called iron nickel base superalloy. Their oxidation resistance is not as good as nickel base superalloy, and their high temperature strength is far less than cobalt base superalloy.

However, due to low nickel content and low price, it is widely used. Typical brands include GH2036, GH2135, etc.

The machinability of iron base superalloy is better than that of nickel base or cobalt base superalloy, which is similar to austenitic stainless steel.
Cobalt base superalloyIt is an alloy based on cobalt, which is characterized by high high temperature strength and can withstand temperatures above 1000 ℃.

Typical brands include K640, etc.

The machinability of cobalt base superalloy is slightly better than that of nickel base superalloy.

Note: in addition to classification by chemical composition, superalloys can also be divided into deformation superalloys and casting superalloys according to their production processes.

The former has good high-temperature plasticity and can accept pressure processing and molding such as forging, while the latter contains more strengthening elements such as W, Mo, Ti, Al and high carbon content, high thermal strength and poor plasticity, and can only be used in as cast condition.

Cast superalloys are more difficult to cut than deformed superalloys.

2. Reasons why superalloys are difficult to cut

The reasons why superalloys are difficult to cut are shown in table 7-15.

Table 7-15 reasons for difficult cutting of Superalloys

High temperature strength, high work hardening tendencyDuring cutting, the plastic deformation resistance is large, the cutting load is heavy, and the cutting temperature is high.

Generally, the unit cutting force of nickel base superalloy is 50% higher than that of medium carbon steel;

The work hardening and residual stress of the machined surface layer are large, and the hardening degree can reach 200% – 500%;

The tool tip and boundary wear are extremely serious, and the groove wear of the auxiliary rear knife surface is also very easy to occur.
Poor thermal conductivityThe thermal conductivity is about 1/5~1/2 of 45 steel, so the cutting temperature is high.
Large bonding tendency with the toolIt is very easy to produce chip buildup, which affects the quality of the machined surface.
High content of fortified elementsA large number of hard spots such as metal carbides and intermetallics with strong abrasiveness are formed in the alloy, which has a strong scratch effect on the knife.

3. Specific measures for cutting Superalloys

The specific measures for cutting superalloys are shown in table 7-16.

Table 7-16 specific measures for cutting Superalloys

Selection of tool materialsCemented carbide tools are commonly used, and high-speed steel is only used when machining complex profiles with low cutting speed.

When cutting with cemented carbide tools, it is best to choose new brands with better performance, K20, M30, K30 and other brands can be selected for rough machining, and K30 and other brands can be selected for finish machining.Suitable CVD coatings include TiCN, ticn+al, o+hfn, etc;

When using high-speed steel cutting tools, w10mo4cr4v3a1, w18cr4sia1nb, w12cr4v3mo3co5si and other brands can be selected.

In addition, silicon nitride ceramics are also suitable for semi finishing and finishing of superalloys because of their higher adhesion resistance, heat resistance and hardness than cemented carbide.

PCBN tools are more suitable for continuous cutting of superalloys because of their high hardness and high heat resistance.
Selection of tool geometric parameters1. Selection of geometric parameters of cemented carbide turning tools:

(1) Front corner y.: Rough turning front angle y= 5 0~150, front corner Y during finish turning= 0 ° ~5 °, when turning and casting superalloy, y=3 0~50

(2) Rear corner A.: Rough turning rear corner a= 10 ° ~14 °, rear angle a during finish turning= 14 0~180, when turning cast superalloy, a= 10°~150.

(3) Main deflection angle KX,: generally, x=45 0~750 is used to reduce the radial cutting force.The smaller value shall be adopted as far as possible under the condition that the terminal power and the rigidity of the process system allow.

(4) The inclination angle of the blade is in: in rough turning = 10 °, in fine turning, In order to control the flow of chips to the surface to be machined, it can be taken in =0 ° ~30.

(5) The arc radius of the tool tip R,: r=0.5~0.8mm for rough turning of deformed superalloy, r=0.3~0.5mm for fine turning, r=lmm for cast superalloy.

2. Selection of geometric parameters of high-speed steel knife:

(1) Front corner y.: Take y when milling deformed superalloy= 3 ° ~ 12 0, take y when milling cast superalloy= 0°~50;

(2) Rear corner α.: α0= 130 ~160

(3) Helix angle: cylindrical milling cutter β=450, end milling cutter β=28 ° ~350;3. The grinding purity standard of the cutting tool can be 13~1/2 of that of the ordinary steel cutting tool.
 The selection principle is basically the same as that of cutting stainless steel, and the most important is the cutting speed. When cutting superalloys, the cutting speed is too high or too low, and the tool wear is relatively rapid.

When using cemented carbide tools, the cutting speed is usually vc=20~50mmin;The feed rate should be small, generally f=0.1~0.5mm/r, the large value for rough turning and the small value for fine turning;

The back cutting amount should not be too small. ap=2~4mm for rough turning and ap=0.2~0.5mm for fine turning.

The commonly used cutting parameters for machining superalloys with high-speed steel end mills are vc=5~10m/min;f=0.05~0.12mm/r,ap=1~3mm.

For carbide face milling cutter, vc=20~45m/min;f=0.05~0. 1mm/r,ap=1~4mm.
Conduct proper heat treatmentIron base superalloys can be annealed and nickel base superalloys can be solution treated.
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