How Do Wire Cutting Parameters Affect the Crack of Cemented Carbide?

In view of the phenomenon of cracks in wire cutting cemented carbide products, combined with the actual situation of the company’s products, by adjusting wire cutting processing parameters, the microstructure of the alloy surface is analyzed, and on the basis of experimental comparison, the main factors affecting the cracks and solutions are found out.

1 .  Preface

At present, in the processing of cemented carbide as mold material and wear-resistant parts, WEDM process is often used to process the workpiece surface with small size, complex shape and unable to be machined by grinding wheel, and compared with traditional methods, it can greatly improve the processing efficiency.

In the process of practical application, especially when machining alloy parts with complex structure on-line, only dimensional accuracy, surface roughness and production efficiency are generally considered, and the influence on the microstructure area of the alloy surface is often ignored.

The actual situation is that WEDM will greatly change the microstructure of the machined surface of the alloy, which will have a great impact on the performance of the machined cemented carbide workpiece itself.

In recent years, there have been many research results in this area, pointing out that micro cracks and other defects will occur on the surface of die products processed by WEDM, which will seriously affect the performance of the workpiece itself. On this basis, researchers have put forward corresponding improvement measures.

Wang Zhenxing described the characteristics of WEDM of cemented carbide materials.

By using composite cutting fluid in the first two cuts and kerosene as the working medium in the final finishing, the cemented carbide YG8 was cut many times, and the workpiece surface roughness Ra was less than 1mm.

According to the principle of EDM, Liu Yike analyzed the test results of WEDM machining cemented carbide molds in detail.

It was concluded that when the discharge voltage and discharge current remained unchanged, the discharge energy increased with the increase of pulse width, which could improve the machining efficiency, but when the pulse width increased to a certain value, cracks occurred.

Luo Binhui and others analyzed the alloy crack problem of cemented carbide template cutter, and conducted a comparative test on the EDM process that may affect the crack.

On the basis of the test and comparison, they found the main factors and solutions to the crack problem, which provided an important process and technical support for the production of high-quality template cutter.

On the basis of the above research, combined with the previous research results and the actual situation of wire cutting of cemented carbide products in our company, this post further analyzes the mechanism of cracks in cemented carbide products caused by wire cutting, and analyzes the microstructure of the machined alloy surface by changing the electrical parameters of wire EDM, and obtains the influence of wire cutting electrical parameters on the microstructure cracks of the alloy surface, and put forward the methods to reduce and avoid cracks.

2 . Crack generation mechanism

Different from general cutting, wire cutting does not directly contact the workpiece, but relies on the continuous pulse spark discharge between the cutting wire and the workpiece, and uses the partial and instantaneous high temperature generated during the discharge to gradually etch away the metal materials.

Spark discharge is carried out in an insulated liquid medium (such as emulsion).

During wire cutting, the current density in the discharge area is as high as 10000a/mm2, and the temperature is as high as 10000 ~ 12000 ℃.

The filled dielectric liquid cools sharply.

Under the action of EDM energy, the material surface produces a non-uniform time-varying temperature field, resulting in great thermal stress, which has strong thermal shock properties.

In WEDM, the machining surface is heated and cooled suddenly, and the material expansion and contraction are uneven, which can cause great thermal stress.

Especially when machining some hard and brittle materials (cemented carbide, cermet, etc.) and selecting incorrect electrical parameters, once the thermal stress exceeds the strength limit, the workpiece surface will produce cracks.

For the phenomenon of micro cracks, it is shown that tensile stress is generated on the surface of EDM due to the action of instantaneous high temperature and rapid cooling.

Micro cracks often appear on the surface.

Generally, cracks only appear in the melting layer, and they can expand to the heat affected layer only when the pulse energy is large (during rough machining).

The influence of the discharge energy received by the workpiece on the microcrack is very obvious.

The larger the energy is, the wider and deeper the microcrack is;

The smaller the pulse energy is, the narrower and shallower the microcrack is, and the less and smaller the hole distribution is.

Different workpiece materials show different sensitivity to cracks with the change of thermal conductivity of the material itself.

Brittle materials such as cemented carbide are prone to surface microcracks.

Wire cutting discharge is shown in Fig. 1.

wire cutting discharge

Fig. 1 wire cutting discharge

According to the thermal stress model, the amount of heat entering the material is directly proportional to the peak stress, which is directly related to the input electric pulse energy under the same other conditions.

The larger the input power is, the more heat the material absorbs.

Therefore, the greater the power is, the greater the stress is, and the easier it is to produce cracks. Ignoring the loss of energy, the energy acting on the workpiece in the machining process can be simplified as wire cutting discharge pulse energy, which is

How Do Wire Cutting Parameters Affect the Crack of Cemented Carbide? 1

Where,

  • W is pulse energy (J);
  • U is intermittent instantaneous discharge voltage (V);
  • I is intermittent instantaneous discharge current (A);
  • t is the time (s);
  • tk is the discharge duration (pulse width, s).

It can be seen that the discharge pulse energy w of wire cutting is proportional to the discharge voltage U, the discharge current I and the discharge duration, that is, the pulse width tK.

It shows that when the discharge duration (pulse width) is constant, increasing the discharge voltage and current will aggravate the generation and propagation of microcracks;

When the discharge voltage and current are fixed, increasing the discharge duration (pulse width) will produce the same result.

3 . Test plan

Cemented carbide shaft sleeves have good hardness, excellent wear resistance and corrosion resistance, and are widely used in electric submersible pump motors, centrifugal pumps, protectors, separator shafts and other components of oil production equipment, such as sliding bearing sleeves, motor shaft sleeves, stabbing bearing sleeves, thrust bearing sleeves and sealing shaft sleeves, which play the role of rotating support, stabbing, thrust and sealing.

conventional cemented carbide shaft sleeve

Fig. 2 conventional cemented carbide shaft sleeve

In the test, CTP350, a medium wire cutting equipment, is used.

The cutting fluid is an emulsion with a concentration of 8%, and the cutting wire is φ 0.18mm molybdenum wire.

One piece is clamped for processing each time.

See Table 1 for the processing parameters of wire cutting.

Test group No

NO.

Voltage/V

Current/A

Pulse duration/μs

Pulse interval/μs

Wire feeding speed/(m/s)

One

1

100

1.5

40

320

15

2

100

1.5

36

282

15

3

100

1.5

24

192

15

4

100

1.5

12

96

15

Two

5

100

3.5

20

160

15

6

100

2.8

20

160

15

7

100

2.4

20

160

15

8

100

2.0

20

160

15

Three

9

120

2.0

20

160

15

10

110

2.0

20

160

15

11

90

2.0

20

160

15

12

70

2.0

20

160

15

Four

13

100

2.8

16

128

15

14

80

1.0

6

48

6

15

50

0.3

4

32

3

4 . Test result

4.1 Effect of pulse width on microcracks on alloy surface

1 # ~ 4 # product metallographic photos are shown in Fig. 3 ~ Fig. 6.

It can be seen that with the decrease of pulse width, the microcracks on the alloy surface gradually become smaller.

When the pulse width is 40ms, the microcrack depth reaches 15mm; When the pulse width is 12ms, there is basically no microcrack.

1# metallographic photos of products

Fig. 3 #1 metallographic photos of products

metallographic photos of products

Fig. 4 #2 metallographic photos of products

metallographic photos of products

Fig. 5 #3 metallographic photos of products

metallographic photos of products

Fig. 6 #4 metallographic photos of products

4.2 Effect of current on microcracks on alloy surface

5# ~ 8# product metallographic photos are shown in Fig. 7 ~ Fig. 10.

It can be seen that when the processing current is 3.5A, the crack depth is more than 30mm; When the processing current is 2.8A, the crack depth is 30mm; When the processing current is 2.4A, the crack depth is 20mm;

When the processing current is 2.0A, the crack depth is 10mm.

The greater the machining current, the greater the crack depth.

product metallographic photos

Fig. 7 #5 product metallographic photos

metallographic photos of products

Fig. 8 #6 metallographic photos of products

metallographic photos of products

Fig. 9 #7 metallographic photos of products

8# metallographic photos of products

Fig. 10 #8 metallographic photos of products

4.3 Effect of voltage on microcracks on alloy surface

9# ~ 12# product metallographic photos are shown in Fig. 11 ~ Fig. 14.

It can be seen that when the current is 2A, the pulse width is 20ms, and the pulse width is 8 times the pulse width, the processing voltage is 70 ~ 120V, and no alloy microcrack is found in the cutting section, that is, when the current and pulse width are constant, the influence of voltage on alloy microcrack is not obvious.

metallographic photos of products

Fig. 11 #9 metallographic photos of products

10 metallographic photos of products

Fig. 12 #10 metallographic photos of products

metallographic photos of products

Fig. 13 #11 metallographic photos of products

12 metallographic photos of products

Fig. 14 #12 metallographic photos of products

4.4 Effect of cutting times on microcracks on alloy surface

13# ~ 15# metallographic photos of products are shown in Fig. 15 ~ Fig. 17.

It can be seen that through multiple cutting processes, the surface quality of products has been significantly improved, and the depth of microcracks has been significantly reduced.

Through 2 times of cutting and processing with medium wire walking, the depth of micro cracks in the product is within 15mm;

Through 3 times of cutting and processing of medium wire walking, the depth of micro cracks in the product is within 10mm.

Through two times of cutting, it can meet the current requirements that the micro crack depth of cemented carbide shaft sleeve products is less than 20mm.

#13 metallographic photos of products

Fig. 15 #13 metallographic photos of products

#14 metallographic photos of products

Fig. 16 #14 metallographic photos of products

#15 metallographic photos of products

Fig. 17 #15 metallographic photos of products

5. Conclusion

During EDM, the workpiece surface is subjected to the violent temperature change caused by the sudden change of cold and heat.

This change has the characteristics of fast speed and large amplitude, which makes the machining show the characteristics of strong transient thermal shock process.

When machining brittle materials such as cemented carbide, to reduce or avoid the occurrence of cracks, fundamentally speaking, all methods to reduce the stress amplitude and its time-varying, that is, thermal shock, are effective.

Pulse width and processing current have obvious effects on the surface microcracks of the alloy. The larger the pulse width and current are, the deeper the cracks are, while the effect of voltage on the surface microcracks of the alloy is not obvious.

No microcracks were found when the pulse width was 12ms.

Since cracks occur to a certain extent when the current is above 2A, the processing current above 2A should be avoided when selecting electrical parameters.

High peak narrow pulse electrical parameters should be adopted, and the superposition effect of temperature field should be fully used to reduce the size of thermal stress and its impact property, so that the workpiece material can be thrown out in the gas phase, and the gasification heat is much higher than the melting heat, so as to take away most of the heat and avoid overheating of the workpiece surface.

Multiple cutting is a very effective method to reduce and remove surface microcracks.

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