After the removal of the deposition matrix or vacuum annealing treatment, the subsequent cooling process is not well controlled, it leads to the generation of transverse cracks on the deposition layer surface which along the vertical axial direction of tungsten tube.

Cause of tungsten tube surface transverse cracks as follows:

During cooling deposition, the deposition substrate shrink which would lead to the deposition of tungsten tube generate uneven deformation and subsequent release of residual stress.

When cooling deposition is completed, the deposition matrix contraction will cause the adjacent tungsten atoms deformed. As the further deformation of tungsten tube decreases, elastic deformation occurs.

After removing the deposit matrix, the gradual release of elastic deformation in tungsten tube inside will change the state of the surface stress, resulting in residual compressive stress decreases and even turn into the residual tensile stress. When the residual tensile stress exceeds the strength to withstand, depositing tungsten tube surface will produce transverse cracks.

 

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On the market today , for tungsten carbide end mill, the well accepted common coatings comprise Titanium Nitride (TiN), Titanium Carbon Nitride( TiCN) and Aluminum Titanium Nitride (AlTiN) , as well as Titanium Aluminum Nitride (TiAlN).

Since the coatings like AlTiN and TiSAlN have extra aluminum content,which are more effective at evacuating heat, they are more popular in end mill applications. Specially ,both TiAlN and AlTiN is a thin film coating that was delveloped from Titanium Nitride. It is recommended that when machining aluminum, brass, plastics and other nonferrous materials it is better to use AlTiN or TiAlN coated end mill with polished flutes to prevent edge buildup.


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Since the TiAlN is a thin film coating that was developed from Titanium Nitride with more aluminum content, TiAlN coated tungsten carbide end mills are more effective in heat removing and higher heat resistance.



During the milling processes, the temperature of the cutting edge could soar up to 1800°F , which would make tungsten carbide end mill suffer from damaging effects of heat. However, with the TiAlN coating, the aluminum in the coating provides a gaseous aluminum oxide layer for the cutting edge. In addition, the dry machining is recommended, as the heat is removed so well into the chips with TiAlN coating, and the high-speed or hard milling is also ideal match with TiAlN coated tungsten carbide end mill.

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It is known that the AlTiN is a thin film coating that was developed from Titanium Nitride with more aluminum content, which enables AlTiN coated tungsten carbide milling tools offer higher temperature resistance than the ones with TiN, as well as slightly higher hardness and especially higher hot hardness. In general, the advantages of AlTiN coated tungsten carbide milling tools comprise as follow.



It proves that AlTiN coated tungsten carbide milling tools could withstand elevated temperatures up to 800°C in air. It operates well in high temperature applications, as it could offer an outstanding bound-a low in friction and oxidation resistant thin Al2O3 surface layer, which could constantly rebuilt and require high temperatures to form. AlTiN coated tungsten carbide milling tools typically last 3 to 10 times longer than uncoated tools.


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In order to get a better milling performance and longer tool life, coating is a vital process for the tungsten carbide milling Cutter to resist wear and heat. It is indicated that coatings enable the tungsten carbide milling cutters to remove the cutting chips from the flutes, as well as the end mill’s ground surface.

In nowadays, the hard coatings for cutting tools int terms of commercial acceptance is driven by demands on machining productivity, environmental protection and up to date hard materials. Improved cutting performance is derived from synergies of machine tool system and cutting tool development. The latter strives for an optimized combination of tool material, hard coating and cutting edge geometry. As a result the cutting-edge technology offer more heat- and wear-resistant coatings has promoted longer tool life and increased productivity.


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Cutting ring method is one of the methods for measuring the residual stress of tungsten tube, which is implementing by measuring dependent variable before and after the cut, the average distribution of residual stress thereby overall reflected.

Cutting ring method steps:
Taking deposition samples →obtaining the cutting ring by cutting the samples in the axial direction → removing the deposition matrix →cutting tungsten tube in the axial direction to get cutting ring → vacuum annealing (temperature of 1200 ℃, holding time of 1h) to get cutting ring

The incision of tungsten tube got after the removal of deposition matrix and the cut along the axial direction is significantly reduced, indicating that in the subsequent cooling process, shrinkage of deposition matrix produce great residual stress in tungsten tube, and the residual stress is present in the circumferential direction of tungsten tube. The residual stress decreases with the deposition temperature increases and the deposition of tungsten tube diameter increases.

After vacuum annealing, change of annular incision diameter is not obvious.

 

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CVD tungsten deposition rate factors tube following main points:

Reduction reaction rate;

Reduction reaction: WF₆ + H₂ → W + 6HF (125Kj/mol)

Deposition temperature: 500 ℃, 600 ℃, 700 ℃, 800 ℃

As the deposition temperature rises, the reduction reaction speed is also increasing, that is to say, W atoms generation amount is increased in unit time.

When the deposition temperature up to 700 ℃, WF6 has almost been completely consumed.

When the deposition temperature increases around 700 ℃ -800 ℃, CVD tungsten tube deposition rate trend is not very clear.

The diffusion movement speed of WF₆, H₂ and HF; as the deposition temperature increasing, WF₆, H₂ and HF diffusion speed is evident.

W atomic diffusion movement speed along the deposition surface; as the deposition temperature increases, the diffusion rate of W atoms significantly accelerated.

Speed of Crystallization.

 

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CVD tungsten tube process parameters:

Reaction gas: keep the inlet amount and ratio of WF6 and H2 unchanged

Deposition temperature: 500 ℃, 600 ℃, 700 ℃, 800 ℃ (hold heating way the same)

Deposition matrix: copper with the same diameter

Reduction reaction: WF₆ + H₂ → W + 6HF (125Kj/mol)

Effect of CVD tungsten tube process parameters on the deposition rate:

There are some aspects can be seen from the analysis of CVD tungsten tube test results:

CVD tungsten tubes’ deposition rate rises quickly with the deposition temperature increases.

When the deposition temperature is lower than 500 ℃, the speed of reduction reaction is very slow, and the growth rate of depositing tungsten tube film is also very low.

When the deposition temperature is controlled at 500 ℃ -700 ℃ around, the increase of the CVD tungsten tube deposition rate is obvious.

When the deposition temperature is about 700 ℃ -800 ℃, the acceleration of deposition rate of CVD tungsten tube is more slower.

If the deposition temperature is over 800 ℃, the organizational structure of the deposited layer of tungsten tube is so loose that there is no practical value, therefore, the deposition temperature should be controlled at 500 ℃ -700 ℃ around.

 

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Tungsten carbide shell mill refers to the variety milling cutters, such as face mill or end mill, with a modular construction form, connected to the shank by standardized joining methods. Specially, the shank and the cutter body are designed separately while both of them including cutting edges are all adopting modular construction.

It is known that modular construction is proper choice for large milling cutters. There are two reasons could be addressed, firstly instead of machining all the features, it is more practical for the operator to make the individual pieces, as making the whole units would require a larger machine tool work envelope and most likely it will be way expensive than individual piece; secondly, the user can change some pieces while keeping other pieces the same rather than changing the whole unit.


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As the strengthened tungsten carbide cutting tips shows more stable properties, in order to get a desired performance of tungsten carbide cutting tips and to a extent to expand the tool life, a proper way to strengthen the carbide tips is required.



A most popular new way to strengthen the cutting tips in nowadays comprises steps as follow, firstly, put the carbide cutting tips in the kerosene or alcohol to remove oil, then put it into an open container and sprinkle some urea. And then, heated it to 180℃. After that, urea will melt and flow into the tungsten carbide through the blowhole. Next, keep the status for about 15minutes and then cool the tips to a temperature lower than the melting temperature of urea. At last, suddenly heated the tips to about700℃，the urea would depolymerize, and carbon and nitrogen would come into being. In this way, the hardness and strength of carbide tips will be improved.

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