Estudo comparativo do desempenho da fresagem de componentes de liga de titânio impressos em 3D e de peças forjadas em liga de titânio

3D-printed liga de titânio, as an emerging material in the aerospace field, has always received great attention in terms of its cutting performance. In this paper, orthogonal milling experiments were conducted on 3D printed titanium alloy components and peças forjadas em liga de titânio. The similarities and differences between the two materials regarding milling force, milling temperature, and tool wear were analyzed.

1. Comparison of milling processing performance between 3D printed titanium alloy components and titanium alloy forgings

3D printing is a rapid prototyping technology based on digital model files and uses powder metal or plastic and other adhesive materials to construct objects by layer printing. In recent years, titanium alloy 3D printing technology has developed rapidly in the aerospace field due to its fast speed, low cost, unlimited materials used for prototype production, the ability to process parts of various shapes, and high flexibility and integration.
Due to the anisotropy of 3D printing titanium alloy, this paper analyzes the milling performance of 3D printing titanium alloy in different directions, measures the cutting force, tool wear, and other data during the test, and optimizes the processing parameters according to the measured values, and compares the difference of the cutting performance of the two titanium alloy materials, guiding for future processing and production.

2. Test conditions and plan

(1) Test materials

As shown in Figure 1, the test specimens were made of 3D printed titanium alloy blocks and forged TC4 titanium alloy blocks with dimensions of 40mm x 30mm x 20mm. Table 1 shows the chemical composition of TC4 titanium alloy material.

Figure.1 Test piece

Table.1 Chemical composition of TC4 titanium alloy material (wt.%)

Material	Ti	Fe		C		N		H
TC4	allowance	≤0.30		≤0.10		≤0.05		≤0.015
Material	O		Al		V		Outros
TC4	≤0.20		5.5-6.8		3.5-4.5		≤0.4

(2) Test equipment

• 1) Processing machine tool: VMC850E machining center produced by Shenyang Machine Tool Factory;
• 2) Processing tool: PVD diamond-coated hard alloy end milling cutter, with tool parameters of D8 * 25 * 70 * 4T, circumferential edge front angle of 12 °, back angle of 10 °, and helix angle of 40 °;
• 3) Force measuring system: KISTLER9272 four-way piezoelectric force measuring instrument, KISTLER5017B charge amplifier, and corresponding data acquisition and processing system;
• 4) Other: SJ201 surface roughness meter and A615 infrared thermal imager from Sanfeng, Japan.

(3) Orthogonal test plan
Basic milling experiments were conducted on 3D printed titanium alloy materials and TC4 titanium alloy materials, with processing parameters shown in Table 2, totaling 18 groups. There is no coolant during the testing process, making it easy to measure temperature during the processing, adopting the reverse milling processing method.

Table.2 Milling Processing Parameters

Número	linear velocity v(m /min)	Feed per tooth fz(mm/z)	Axial cutting depth ap(mm)	Feed Speed f(mm/min)
1	40,60,80	0.06	1	382,73,64
2	40,0,0	0.06	2	382,73,764
3	40,0,0	0.08	1	509,764,020
4	40,0,0	0.08	2	509,764,020
5	40,0,0	0.1	1	637,55,274
6	40,0,0	0.1	2	637,55,274

3. Test results and analysis

3.1 Analysis of milling force results

Analyze the milling force F_x of the two materials with a line chart diagram, as shown in Figure 2.

Figure.2 Milling Force Line

Figure.3 Flow stress-strain curve at 5000/s
Under the same parameters, the milling force F_x of forged TC4 titanium alloy is greater than that of 3D printed titanium alloy. The decrease in the milling force of the former during high-speed milling is not as significant as that of the latter. This is because the forged TC4 titanium alloy has a slightly poor strength (hardness). Still, its plasticity is very good, so there will be large plastic deformation, tool sticking, and other phenomena in the processing process, aggravating the tool wear and increasing the milling resistance, resulting in greater milling force. Although 3D printed titanium alloy has high strength, its plasticity is low. Under high strain rates, it will generate an adiabatic temperature rise (see Figure 3), resulting in a significant softening of the material’s temperature rise and a significant decrease in milling force.

Comparing and analyzing the chips of two materials (see Figure 4), it was found that the 3D printed titanium alloy chips have a smaller curling amplitude and are in the shape of C-shaped sheets, making them easy to break and less prone to knife entanglement. Additionally, the plastic deformation during chip breaking is relatively small, resulting in less heat generation; TC4 titanium alloy chips have a large curling amplitude and are cylindrical, making them easy to entangle and stick to the knife. When chips are broken, plastic deformation is large, generating more heat and high deformation resistance. In summary, forged TC4 titanium alloy has good plasticity and high milling force during processing.

Figure.4 Chip morphology of two titanium alloys under the same processing parameters

Figure.5 Effect of processing parameters on milling forces of two materials
The range analysis of milling force F_x is shown in Figure 5. It can be found that the linear speed has the greatest impact on the milling force of 3D printed titanium alloy, followed by the axial cutting depth and, finally, the feed rate per tooth. When the linear speed increases from 40m/min to 80m/min, the milling force F_x first increases and then decreases. The increase in feed rate per tooth and axial cutting depth will lead to an increase in milling force F_x, where the increment of milling force caused by axial cutting depth is significantly greater than the increment of milling force caused by feed rate per tooth; For forged TC4 titanium alloy, the linear speed has the greatest impact on its milling force F_x, followed by the feed rate per tooth, and finally the axial cutting depth.
3.2 Analysis of milling temperature

Figure 6 shows that for 3D printed titanium alloys, due to their lower plasticity than forged TC4 titanium alloys, the milling temperature of the former is lower than that of the latter under the same processing parameters. At a lower linear speed, the milling temperature decreases to a certain extent with the increase of axial cutting depth, which is consistent with the trend of forging TC4 titanium alloy. As the linear speed gradually increases, especially when the linear speed increases to 60m/min, the temperature suddenly increases from 470 ℃ to 580 ℃ because the linear speed is large. The axial cutting depth is small, the extrusion effect between the tool and the workpiece surface is more obvious, and the chips are small, leading to a certain temperature rise. As the linear speed increases to 80m/min, the change in milling temperature is no longer significant but shows a small amplitude oscillation. This is because 3D printed titanium alloys have poor plasticity, and higher spindle speeds can significantly reduce the plastic deformation of the material. Even if the spindle speed is increased again, the plastic deformation is no longer significant.

Figure.6 Milling Temperature of Two Materials
3.3 Tool wear detection and analysis

From Fig. 7a, it can be found that the tool wear for forging TC4 titanium alloy is severely worn, the edge collapse phenomenon is very obvious, the matrix material falls off seriously, and a large amount of titanium alloy matrix material is bonded to the edge of the tool. This is because, during the milling process of titanium alloy, the high temperature in the cutting area and the high strain rate of the chips can cause a certain degree of material softening, thereby adhering to the tool. As the accumulation of chips increases, the roughness of the tool edge increases and is no longer sharp, further increasing the cutting resistance. In addition, there are some cracks on the cutting surface after the cutting edge. As the cracks increase, the cutting-edge material slowly falls off and exposes the tool substrate material. The phenomenon of coating peeling off on the rear cutting surface is also relatively obvious, causing direct contact between the tool substrate material and the workpiece material, accelerating abrasive wear and diffusion wear on the rear cutting surface around the tool, and reducing the service life of the tool. Figure 7b shows that the wear on the front face of the milling cutter is relatively light, with only chipping and chip sticking near the edge. The phenomenon of abrasive wear is not obvious, which is more in line with the characteristics of reverse milling.

Fig.7 Tool wear for Forging TC4 Titanium Alloy

The 3D printing titanium alloy machining tool also has microchipping and matrix material detachment at the cutting edge, and there are obvious abrasive wear scratches on the rear cutting surface. However, the bonding phenomenon of the titanium alloy matrix material is not obvious on the cutting edge after the cutting edge (see Figure 8). This is due to the high hardness of 3D printed titanium alloy material during the milling process of titanium alloy, resulting in severe abrasive wear during the machining process and very obvious abrasive wear scratches on the rear cutting surface. Moreover, due to the high hardness and brittleness of the material, the matrix material will fall off after small plastic deformation occurs during the processing, resulting in less heat generated during the material removal process and less tendency to adhere to chips. The wear on the cutting surface after cutting is consistent with that of forged TC4 titanium alloy.

Fig.8 Tool wear in 3D Printing Titanium Alloy Processing

Compared with the forged TC4 titanium alloy processing tools, the 3D printing titanium alloy processing tool wear is less, the microchipping at the peripheral edge is less, and the chip sticking on the tool surface after the peripheral edge is not obvious. Therefore, 3D printing titanium alloy material processing tools have a longer lifespan under the same processing parameters.

4. Conclusão

• (1) Under the same milling parameters, the milling force F_x of forged TC4 titanium alloy is greater than that of 3D printed titanium alloy, and the decrease in milling force of the former during high-speed milling is not as significant as that of the latter;
• (2) Under the same processing parameters, the milling temperature of 3D printed titanium alloy materials is lower than that of titanium alloy forgings;
• (3) Under the same processing parameters, 3D printing titanium alloy material processing tools have a longer lifespan;
• (4) When processing these two titanium alloy materials, the cutting speed should be avoided at around 60m/min. Low spindle speed and high feed speed can be used if conditions permit.

Author: Wang Lei

Fonte: China Fabricante de peças forjadas em liga de titânio: www.titaniuminfogroup.com