WC-Co cemented carbide tools are usually used for titanium alloy cutting, and the material properties of the tools directly affect tool life and cutting efficiency. Extending the service life of tools for titanium alloy processing and improving processing efficiency have always been research hotspots in the industry.

In this paper, two matrix materials with different high-temperature properties were prepared by adding trace alloy carbides TaC (NbC) to the cemented carbide matrix. The high-temperature hardness and high-temperature fracture toughness of the two cemented carbides were tested using a high-temperature Vickers hardness tester.

Solid carbide end mills with the same geometric parameters were prepared based on the two matrix materials, and titanium alloy TC4 cutting tests were carried out. By analyzing tool life and wear forms, the influence of TaC (NbC) on the wear performance of cemented carbide tools in high-speed milling was studied.

Contenido ocultar

1 Experimental Conditions and Testing Methods

2 Experimental Results and Discussion

3 After end mills A and B cut 200 m, a scanning electron microscope was used to analyze the tool tip wear location. The tip wear morphologies of the two end mills are shown in Figure 6. Figure 6 Failure morphology of cutting edge by SEM (a) Wear morphology of end mill A (b) Wear morphology of end mill B

4 Conclusions

Experimental Conditions and Testing Methods

Two types of WC-Co-based cemented carbide materials coded A and B were prepared. Trace alloy carbides TaC (NbC) were added to material A, with a mass fraction of less than 0.1%. The compositions of the two cemented carbides are shown in Table 1.

Influence of TaC (NbC) on Wear Performance of Cemented Carbides Tools in High-Speed Milling 2

Table 1 Carbide composition

The high-temperature hardness of the two cemented carbide materials was tested using an HTV-PHS30 high-temperature Vickers hardness tester. The test load was 98.07 N, the holding time was 10 s, and three measurements were taken at each temperature, with the average value calculated.

A scanning electron microscope (SEM) was used to measure the crack propagation length l at the vertex of the indentation at the same magnification, as shown in Figure 1, where C is the distance from the crack tip to the center of the indentation. The formula for calculating the fracture toughness of the two materials at different temperatures is:

(1)

In the formula: K_IC is the fracture toughness of the material, MPa·m⁰·⁵; H is the material hardness, MPa; E is the material elastic modulus, GPa; 2re is the indentation diagonal length, mm; φ=3.

Influence of TaC (NbC) on Wear Performance of Cemented Carbides Tools in High-Speed Milling 4

Figure 1 Schematic of toughness measurement

The high-temperature hardness and high-temperature fracture toughness of materials A and B at different temperatures are shown in Figure 2 and Figure 3.

Influence of TaC (NbC) on Wear Performance of Cemented Carbides Tools in High-Speed Milling 5

Figure 2 Vickers hardness of the carbide with varied temperature

Figure 3 Fracture toughness of the carbides with varied temperature

During the cutting of titanium alloy with solid carbide end mills, the tool edge bears severe thermal shock and mechanical load. The high-temperature hardness and high-temperature fracture toughness of the cemented carbide material determine the tool performance at relatively high cutting temperatures.

It can be seen from Figure 2 that at 400 °C, the high-temperature hardness of cemented carbide A is 5% higher than that of cemented carbide B, and the fracture toughness is 4% higher; at 800 °C, the high-temperature hardness of cemented carbide A is approximately 14.5% higher than that of cemented carbide B, and the fracture toughness is approximately 10.2% higher.

Studies have shown that the mechanism by which adding trace alloy carbides TaC (NbC) to WC-Co-based cemented carbide affects the high-temperature hardness of the material is that the addition of trace alloy carbides TaC (NbC) can increase the solid solubility of WC grains in the bonded Co phase, inhibit the grain growth of WC grains during high-temperature sintering, thereby improving the high-temperature hardness of WC-Co-based cemented carbide.

Based on the two cemented carbide materials A and B, four-flute flat-end solid carbide end mills A and B with the same geometric parameters were prepared accordingly. The tool specifications and parameters are shown in Table 2.

Table 2 Cutting tool parameters

The workpiece material for the cutting test was annealed titanium alloy TC4, with a material hardness of 28.2 HRC. The machine tool used for the cutting test was a Mazak Nexus 430A-Ⅱ vertical machining center. Climb milling was adopted, with water-based emulsion cooling. A heat-shrinkable tool holder was used to ensure stable clamping. The processing parameters are shown in Table 3. After cutting a certain distance, a Keyence VHX-950F optical microscope was used to measure the maximum flank wear value of the end mill. After end mills A and B cut 200 m, an S-3700N scanning electron microscope (SEM) was used to observe the wear morphology of the tool edge.

Table 3 Processing parameters of TC4 alloy

Experimental Results and Discussion

Under the same cutting conditions, the flank wear curves of end mills A and B for machining titanium alloy TC4 are shown in Figure 4. Figure 5 shows the flank wear morphology photos of the two end mills.

Influence of TaC (NbC) on Wear Performance of Cemented Carbides Tools in High-Speed Milling 8

Figure 4 Wear curve of flank face of cutting tools

Figure 5 Wear morphology figures of two tools
Influence of TaC (NbC) on Wear Performance of Cemented Carbides Tools in High-Speed Milling 9
(a) Wear photo of end mill A after cutting 60 m

(b) Wear photo of end mill A after cutting 180 m

(c) Wear photo of end mill B after cutting 60 m

(d) Wear photo of end mill B after cutting 180 m

It can be seen from Figure 4 and Figure 5 that when the cutting distance is less than 60 m, both cemented carbide end mills are in the normal wear stage, and the flank wear of the tools increases slowly. In this stage, mainly at the initial cutting stage, the tool coating has good wear resistance and heat insulation, so the tool matrix is effectively protected during this stage.

However, due to the very high cutting temperature of titanium alloy, part of the cutting heat will inevitably be transferred to the tool matrix. Since the high-temperature hardness of the matrix of end mill A is better than that of end mill B, the wear amount of end mill A is slightly less than that of end mill B.

With the progress of cutting, the coating on the tool edge is worn away, exposing the tool matrix, and the tool edge directly bears severe thermal shock and mechanical load impact. Since trace alloy carbides TaC (NbC) are added to the matrix of end mill A, its high-temperature hardness and high-temperature fracture toughness are higher than those of end mill B, so end mill A has an advantage over end mill B in terms of flank wear.

When end mill A cuts to 180 m, the flank wear is approximately 0.062 mm, while when tool B cuts to 180 m, the flank wear reaches 0.089 mm. When end mill A cuts to 200 m, the flank wear is approximately 0.076 mm, while when tool B cuts to 200 m, the flank wear has reached 0.13 mm.

Brief Analysis of Tool Failure

After end mills A and B cut 200 m, a scanning electron microscope was used to analyze the tool tip wear location. The tip wear morphologies of the two end mills are shown in Figure 6.
Figure 6 Failure morphology of cutting edge by SEM

(a) Wear morphology of end mill A

(b) Wear morphology of end mill B

It can be seen from Figure 6 that the tip of end mill A remains in good condition, while the tip area of end mill B is severely damaged, exposing the matrix material.

Through comparative analysis of the local enlarged images in Figure 6, no obvious cracks appear in the exposed matrix part of end mill A, while there are multiple cracks on the matrix surface at the edge of end mill B. This is because trace alloy carbides TaC (NbC) are added to end mill A, which improves the high-temperature fracture toughness of the WC-Co-based cemented carbide, inhibits the generation and propagation of cracks in the tool matrix, and makes the tool edge retention better, thereby extending the tool life.

However, the high-temperature fracture toughness of molino de extremo B is worse than that of molino de extremo A. When the cutting edge bears severe thermal shock and mechanical load, cracks are prone to generate and expand continuously, leading to chipping of the tool edge matrix material and ultimately tool failure.

Conclusions

(1) Two types of cemented carbide materials A and B with different contents of trace alloy carbides TaC (NbC), and corresponding cemented carbide end mills A and B were prepared, and high-speed milling tests on titanium alloy TC4 were carried out.

(2) Under the condition that the main elements are the same, compared with cemented carbide material B without trace alloy carbides, cemented carbide material A with trace alloy carbides TaC (NbC) has higher high-temperature hardness and high-temperature fracture toughness. At 800 °C, the high-temperature hardness is increased by approximately 14.5%, and the high-temperature fracture toughness is increased by approximately 10.2%.

(3) When cutting titanium alloy TC4 under the same conditions, the tool with trace alloy carbides TaC (NbC) added has better wear resistance. When the cutting distance is 200 m, the flank wear of end mill A is 0.076 mm, and the flank wear of end mill B is 0.13 mm.

(4) Cemented carbide material A with TaC (NbC) added has better high-temperature fracture toughness. Under the same cutting conditions, end mill A has better edge retention, and the number of cracks in the wear failure area is significantly less than that of end mill B.