Precision Machining of Titanium Interference Structures Using Specialized EDM Methods

This study explores the combined electrical discharge machining (EDM) process for creating precision through-holes in titanium alloys with interference structures. The main focus is on solving the machining challenges associated with long cantilever structures, which can lead to deformation and low machining accuracy due to low stiffness.

To optimize chip removal and reduce electrode wear, the study employs EDM technology alongside nesting techniques, utilizing hollow copper tube electrodes and specialized fixtures. Through experimental analysis, the effects of various working media and electrical parameters on machining efficiency and quality are evaluated. This analysis leads to the development of a combined positive and negative polarity machining scheme aimed at improving efficiency while maintaining dimensional accuracy and surface quality of the holes.

In conclusion, the study presents a highly efficient and high-precision machining scheme tailored for titanium alloy parts constrained by interference structures. This approach is significantly important for enhancing machining efficiency and reducing production costs.

 

PART.01 Introduction

Long cantilever structures are commonly used in aerospace applications due to their lightweight characteristics and high load-bearing capacity. However, the special design and low rigidity of these components can lead to difficulties during machining, as the cutting tools are prone to interference. Factors such as cutting force, chip deformation, chatter, and cutting temperature significantly impact the surface quality of the machined parts. The machining process involves complex thermo-mechanical interactions that can result in quality defects, including cracking, springback, wrinkling, and cross-sectional distortion.

Ti-6Al-4V, also known as TC4, is a grade 5 (α+β) titanium alloy. It offers several advantages, including low density, high strength, low thermoplasticity, high hardness, resistance to high temperatures, an excellent strength-to-weight ratio, a poor cost-performance ratio, high tensile strength, strong corrosion resistance, and good biocompatibility. Due to these properties, TC4 is widely used in the aerospace, automotive, and biomedical industries. An example of TC4 titanium alloy is illustrated in Figure 1.

 

The widespread use of TC4 titanium alloy has made electrical discharge machining (EDM) a prominent technique for processing difficult-to-machine materials. Scholars from various countries have conducted extensive research on EDM for titanium alloys. For instance, Tao X.T. and colleagues explored EDM, negative polarity machining, and the combined EDM/electrochemical machining processes on TC4 titanium alloy, examining factors such as machining efficiency, recast layer formation, and surface microcracks. RAHMAN M. and others studied how EDM parameters—such as peak current, pulse on-time, and pulse off-time—affect the surface roughness of the Ti-6Al-4V titanium alloy.

Additionally, PANAGIOTIS O. and his team focused on high-power EDM processes for Ti-6Al-4V using graphite electrodes, identifying optimal parameters through variance analysis and extensive experimentation. Çakiroğlu R. investigated the EDM process for the Ti-4Al-6V titanium alloy, analyzing the effects of discharge current and pulse width on material removal rate, electrode wear rate, and surface roughness. He developed a mathematical model to predict optimal parameters using artificial neural networks and regression analysis.

 

This paper presents an in-depth experimental study on the machining of precision through-holes in titanium alloys with interference structures. It ensures that surface roughness, positional accuracy, and cylindricity meet specified machining requirements. A variety of EDM parameters are employed as key process variables to identify the most suitable settings for machining.

To enhance machining accuracy and efficiency, the research proposes an optimized scheme that combines multiple EDM technologies. This approach not only enriches the fundamental theory of EDM by leveraging the advantages of various techniques but also improves machining efficiency and reduces production costs, thus providing practical benefits for production applications.

 

PART.02 Machining Technology Requirements

2.1 Part Requirements

This paper examines the F-type bracket of a specific UAV, illustrated in Figure 2. The material used is TC4 titanium alloy, with the main chemical composition detailed in Table 1. The bracket features a base plate that is 3 mm thick and two side plates that are each 5 mm thick, incorporating two holes in the middle plate. The component has strict requirements for dimensional and geometric accuracy. Specifically, the cylindricity of the holes must be ≤0.02 mm, the surface roughness should be Ra ≤3.2 μm, and the flatness of the thin plates is required to be ≤0.02 mm. Additionally, the positional accuracy of the holes with respect to reference points A and B must have a center position error of ≤0.02 mm, and the center distance tolerance between the two holes should also be ≤0.02 mm.

 

Due to the special structure of the part, the following points should be noted during machining: ① Machining can only be performed from the long plate end; the other end is solid and cannot be machined. ② Appropriate machining methods must be selected to ensure the stability of machining the long plate end. ③ The two holes are only 2mm from the edge of the long plate; interference between the fixture and the cutting tool must be prevented during machining. ④ Due to the thinness of the part, deformation during machining must be prevented. Machining and electrical discharge machining are shown in Figure 3.

 

2.2 Selection of Machining Method

TC4 titanium alloy is known as a difficult material to machine, primarily due to its low thermal conductivity and high rate of tool wear. To overcome these challenges, electrical discharge machining (EDM) is utilized. This method is advantageous because it is not affected by the hardness of the material, which helps reduce costs and prevents deformation. However, traditional EDM faces issues with chip removal and the risk of short circuits. To address these issues, hollow copper tube electrodes are used, as they can reduce machining allowance, enhance chip removal efficiency, and prevent secondary discharges and electrode wear. Additionally, a specialized fixture is designed to ensure machining accuracy and avoid interference.

A comprehensive machining scheme has been developed that combines various EDM technologies to leverage their specific benefits. Initially, a nesting method is employed for hole preparation, which improves machining efficiency by minimizing machining allowance. Next, EDM is used to enlarge the hole, reducing taper and ensuring the geometry adheres to accuracy standards. Finally, EDM finishing is applied to refine the hole, achieving the desired dimensional accuracy and surface roughness. The machining scheme is illustrated in Figure 4.

 

 

PART.03 Analysis and Solutions to Machining Challenges

3.1 Difficulties in Nesting Machining Process

To reduce electrode wear and enhance machining efficiency, it is essential to select electrodes with thinner walls. We chose a copper tube electrode that has a diameter of 6 mm and a wall thickness of 0.5 mm. Although the etched products can easily compromise machining quality during processing—necessitating stronger flushing pressure—there are challenges. The uneven flow field allows for effective chip removal and low wear on one side of the electrode, while the other side experiences difficulties with chip removal and increased wear. To address this issue, rotating the electrode during machining can significantly help alleviate the problem. The nesting machining principle is illustrated in Figure 5.

(1) Effect of Different Processing Media

To determine the most suitable medium for internal flushing fluid sleeve processing, we compared the efficiency and quality of sleeve processing in two different media: deionized water and EDM oil. Titanium alloy plates with a thickness of 5mm were processed using the same electrical parameters in both deionized water and EDM oil. We then assessed the processing taper angle, electrode wear, and processing efficiency. The parameters for the processing tests are outlined in Table 2.

 

The influence of the working medium on the material removal rate and the relative electrode wear rate is shown in Figure 6. When using deionized water and EDM oil, the machining taper angle of both media is 0.020° to 0.030°, and the difference in aperture between the upper and lower surfaces is 0.10 to 0.15 mm. The stability of the machining waveform is shown in Figure 7. In Figure 7a, when using deionized water as the working medium, the discharge gap is large, chip removal is smooth, the current and voltage waveforms during the machining process are regular, the discharge probability is high, and no machining short circuit occurs. In Figure 7b, when using EDM oil as the working medium, the machining gap is small, chip removal is poor, the current and voltage waveforms during the machining process are unstable, the effective discharge is low, and a large number of short circuit pull-down phenomena occur.

 

 

(2) Influence of Electrical Parameters on Machining

In Electrical Discharge Machining (EDM), the material removal rate primarily depends on the electrical parameters for the same material. The factors that most significantly affect the machining outcome among these electrical parameters are pulse width, pulse interval, and peak current. To investigate the impact of these electrical parameters on the machining process, commonly used values for pulse width, pulse interval, and peak current were selected, and their respective effects on machining performance were analyzed.

During the nesting process, the machining depth in the Z direction is utilized to assess efficiency. The working medium in this case is deionized water, and the machining test parameters are detailed in Table 3. The effects of current, pulse width, and pulse interval on machining efficiency are illustrated in Figure 8.

 

As illustrated in Figure 8a, increasing the current enhances processing efficiency, but it also leads to greater electrode loss. At a current of 25A, processing efficiency increases by 1011.11% compared to 5A, while electrode loss rises by 301.89%. This demonstrates that a higher peak current results in increased discharge energy and a greater volume of material removal.

Figure 8b shows that while increasing pulse width can improve processing efficiency, excessively large pulse widths result in higher electrode loss. A pulse width of 150 μs provides a good balance, offering high processing efficiency along with low electrode loss. However, as the pulse width continues to increase, there is a significant rise in electrode loss, and the processed surface becomes more susceptible to burning.

Meanwhile, Figure 8c indicates that shorter pulse intervals lead to increased efficiency but also contribute to greater electrode loss. A pulse interval of 25 μs achieves the highest efficiency with the lowest electrode loss. Conversely, a pulse interval of 125 μs results in lower electrode loss but significantly decreases processing efficiency. While extending the pulse interval aids in recovering the discharge gap, excessively long intervals can diminish energy utilization.

 

3.2 Analysis of the Challenges in Hole Reaming

The holes created during the nesting machining process often exhibit a certain degree of taper. To minimize this taper and reduce the allowance needed for subsequent finishing processes, electrical discharge milling (EDM) is employed for hole reaming.

Based on experience in machining titanium alloys and considering the machining area and depth, high-precision solid electrodes made from 6mm copper rods were utilized under positive polarity conditions. Hole reaming experiments were conducted using various EDM parameters to identify the most suitable settings for machining.

Figures 9 and 10 illustrate the results of the experiments, showing CNC mechanical parts machined with both positive and negative polarities, as well as the efficiency and losses associated with different polarities. The experiments revealed that while positive polarity machining resulted in lower electrode losses, it also led to the formation of burns and pits that compromised the surface quality of the holes. This effect was particularly pronounced under higher current (15A) and pulse width (150μs) conditions, as demonstrated in Figure 9a, where significant burn pits were observed on the hole surface. However, when the current was decreased to 10A and the pulse width remained at 150μs, the burn issue was reduced, significantly improving the surface quality of the holes.

In contrast, negative polarity machining demonstrated higher efficiency, allowing for quicker enlargement of holes. However, this approach resulted in considerably increased electrode wear, as shown in Figure 9b. Additionally, negative polarity machining often led to the development of rounded corners at both the top and bottom of the holes, which adversely affected their geometric accuracy.

 

To optimize the hole reaming process, we start with negative polarity machining for rapid reaming. As we approach the target size, the electrode must be changed, and the current and pulse width adjusted. At this point, we switch to positive polarity machining for the finishing stage. This change helps reduce burns and improve machining quality. By effectively combining positive and negative polarity machining, we can ensure efficiency while enhancing the surface quality and geometric accuracy of the workpiece.

 

3.3 Analysis of Process Challenges in Hole Finishing

Hole finishing demands high surface roughness and precise dimensional accuracy. During the machining process, it is essential to allocate roughing and finishing allowances appropriately to maintain accuracy and surface quality at each stage. Poor selection of machining parameters can result in excessive hole diameter errors or prolonged machining times, both of which can increase costs. The finishing process is illustrated in Figure 11.

 

(1) Alignment of electrode and workpiece:
To achieve high precision during the machining process, it is essential to continuously adjust the positions of both the electrode and the workpiece, ensuring that the error in any direction remains below 0.03 mm. Multiple sets of fixtures are utilized in combination to aid this alignment. The alignment process, illustrated in Figure 12, allows for quick adjustments of the workpiece in multiple directions, ultimately enhancing machining accuracy.

 

(2) Selection of Electrical Parameters for Finishing:

Electrical parameters in electrical discharge machining (EDM) significantly influence hole diameter and surface quality. While increasing current and pulse width can enhance machining efficiency, they also lead to greater hole diameter errors and increased electrode wear. To address this, electrical parameters were gradually adjusted during the machining process. This adjustment aimed to minimize the machining allowance during the semi-finishing and finishing stages, thereby achieving optimal dimensional accuracy and surface roughness. The machining parameters for hole finishing are detailed in Table 4. Experimental results indicated that the surface roughness value (Ra) of the electrode after finishing was found to be ≤ 2.2 μm.

 

PART.04 Process Flow

After analyzing processing difficulties and experimental data, a comprehensive processing scheme has been proposed. The processing flow includes wire EDM (Electrical Discharge Machining) blanking, nesting and drilling holes, enlarging these holes using EDM, and finishing them to achieve high-precision machining of titanium alloy round holes in long cantilever plates. The CNC machining process for creating round holes in this thin titanium alloy plate consists of three main steps: nesting round holes with copper tube electrodes, enlarging the holes using solid copper rod electrodes, and finishing the holes with solid copper rod electrodes.

 

4.1 Nesting Round Holes with Copper Tube Electrodes

The hole machining process is illustrated in Figure 13. The working medium used is deionized water, with a pulse width of 150 μs and an interval of 50 μs. The current is set at 15 A, and the diameter of the guide fixture hole is 6.5 mm. This setup allows the electrode to rotate within a specified range while maintaining the positional accuracy of the electrode’s center. A 6 mm diameter copper tube electrode is employed to create an initial hole with an approximate diameter of 6.5 mm.

 

Two holes were machined using a nesting tool, with an average machining time of 5 minutes for each hole. After CNC custom machining, an additional feed of 1 to 2 mm was required along the Z-axis. Due to the presence of burrs on the bottom of the workpiece after the core material detached, a deburring process was necessary and took 1 minute. The machining process is illustrated in Figure 14. Measurements indicated that the diameters of the machined holes ranged from 6.4 mm to 6.5 mm.

 

 

4.2 Hole Enlargement and Repair of Copper Rod Electrode

The test platform was mounted onto the spindle of a high-precision machine tool. Hole enlargement and repair procedures were conducted as illustrated in Figure 15. The workpiece and electrode were leveled and aligned properly. Following this leveling and alignment process, multiple adjustments were made. After leveling the workpiece, the electrode positioned over the 6mm copper tube was maintained within an error margin of ±0.02 mm.

 

The allocation of machining allowance, selection of polarity, and choice of electrical parameters are presented in Table 5. Various copper rods, polarities, and machining methods are employed at each stage, depending on the range and increment of the circular interpolation radius. Different plug gauges are utilized to measure the reference hole diameter during each step, and the actual measured hole diameter values are recorded. After each measurement, it is essential to determine whether to advance to the next machining step. The finished machined sample is illustrated in Figure 16.

 

Upon completion of processing, the time, taper angle, hole diameter, and required surface roughness for each step of the process are summarized. The parameters for each step are presented in Table 6. Finally, an inspection is conducted for each step.

 

PART.05 Conclusion

This paper thoroughly examines the challenges associated with machining precision through-holes in titanium alloys with interference structures. Given the complexities of working with these difficult materials and components, we propose an innovative process that addresses the precision and surface quality requirements of such structures. The following conclusions are drawn:

1) A machining scheme has been developed for precision through-holes in titanium alloys with interference structures. This scheme includes nesting, reaming, and finishing processes. It is effective not only for machining TC4 titanium alloys but also for other difficult-to-machine materials, such as high-temperature alloys. This versatility significantly enhances machining efficiency while ensuring accuracy.

2) For specialized parts with long cantilever structures or challenging machining conditions, we have designed suitable machining fixtures and process flows. These designs address the issues of interference and part deformation that are common in traditional machining, ensuring the stability of complex structural parts during the machining process.

3) Nesting machining experiments were conducted using different media. The results indicate that nesting machining in deionized water can greatly improve machining efficiency and reduce secondary discharge and surface defects. Compared to EDM oil, the machining process with deionized water shows greater stability.

4) Electrical parameter test data demonstrate that the selection of electrical parameters significantly impacts processing efficiency and electrode wear. While larger pulse widths and currents can enhance processing efficiency, they also increase electrode wear. By carefully adjusting parameters such as current, pulse width, and pulse interval, it is possible to minimize excessive electrode wear while maintaining processing efficiency.

 

 

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