Titanium alloys are strong, but difficult to machine, especially thin-plate parts. Cutting them easily leads to stress deformation and dimensional inaccuracies, causing headaches for many! Don't panic, a combination of techniques can solve this: adjust wire EDM and CNC milling paths, optimize machining plans, and use positioning fixtures + closed-loop cutting to improve part rigidity, reducing deformation at its source and ensuring consistent product quality!
1. Introduction
Titanium alloys are widely used in aerospace due to their high strength, corrosion resistance, heat resistance, and hardness. Their disadvantages include poor thermal conductivity and high machining difficulty.
The titanium alloy rib part is 43mm long, 25mm wide, and 3.5mm thick. The thickness and two inner cavities are CNC milled, while the eight ribs are machined by wire EDM, ensuring a rib width of (0.3±0.05)mm and a symmetry of 0.05mm with the inner cavities. This is classified as a fine-rib part. Ten parts were initially processed according to the process documents. Inspection personnel found that four parts had issues with rib width and symmetry, failing to meet design requirements.
2. Root Cause Analysis
The original process documents required a raw material thickness of 5mm. However, due to inventory limitations, only 18mm thick raw materials were available. Therefore, the blank size was required to be 250mm × 80mm with a thickness of 18mm, as shown in Figure 1. A wire EDM process was added to divide the material thickness in two (see Figure 2), resulting in each piece being 9mm thick. This was then machined to a thickness of 3.5mm using CNC milling. During CNC milling, the operator used a vacuum chuck clamping method (see Figure 3). One surface was first precision milled, removing a 3mm allowance. The part was then flipped over for suction, and the second surface was milled to a thickness of 3.5mm. Finally, the internal cavity in the middle of the part was machined.
Figure 1. Blank
Figure 2. Blank split in two
Figure 3. Vacuum suction cup clamping
Ten small parts are arranged on each piece of material (see Figure 4). A 3mm wire-threading hole is drilled at one end of each row of parts, and then the parts are processed by wire EDM.
Figure 4. Part layout
Before processing, the wire EDM operator checks the flatness of the material and finds stress deformation (see Figure 5), with a maximum deformation of 3.05mm. Using a clamping plate for cutting, since there is only one wire-threading hole, each small part is interconnected after cutting. The material is cut, and therefore, under stress, material deformation occurs during processing (see Figure 6), causing the part's rib width to exceed tolerances, thus affecting the symmetry with the internal cavity.
Figure 5. Material deformation before cutting
Figure 6. Material deformation after cutting
3. Taking effective measures
Analysis revealed that the main problem was stress deformation of the material. Titanium alloy materials generate cutting heat during machining. The material dissipates heat slowly, and the more allowance is removed, the greater the deformation. This can only be solved by changing the cutting method [1]. The original machining scheme was optimized by taking the following effective measures.
1) Replacing high stress with low stress. In CNC milling, the larger the cutting allowance, the greater the stress and the greater the material deformation. The wire cutting process of the raw material was changed from splitting it into two parts to splitting it into three parts (see Figure 7), so that the thickness of each piece of material was about 6mm, which greatly reduced the machining allowance of CNC milling and thus reduced the material deformation.
Figure 7 Blank split into three parts
2) Changing the CNC milling clamping method. When machining the thickness in CNC milling, the vacuum chuck clamping method was changed to a side-top clamping method [2] (see Figure 8). By repeatedly flipping the part and milling both sides, the cutting amount was ≤0.2mm each time, ensuring that the thickness met the drawing requirements and reducing the material machining deformation. According to the calculation, after CNC milling, as long as the deformation of the whole piece of material is controlled within 0.5mm, the flatness requirements of a single small part can be met. The operator processed the parts according to the optimized method, inspecting them as they processed to ensure flatness ≤ 0.2mm.
Figure 8 Side-top clamping
3) Make special tooling to increase the number of wire-threading holes. In the wire cutting process, to prevent material deformation during processing, the number of wire-threading holes was increased to 10, ensuring that each rib part has an independent wire-threading hole, which is then machined in one step by CNC milling to ensure consistency. A wire cutting tooling was made, and the workpiece was positioned on the tooling plate by positioning pins (see Figure 9). Each rib was processed independently, without cutting through each other, increasing material rigidity and reducing part deformation [3].
Figure 9 Positioning the workpiece on the tooling plate by positioning pins
4 Effect verification
20 parts were processed according to the improved scheme. After testing by professional testing equipment, the rib width and symmetry all met the drawing requirements. Finally, a total of 120 parts were processed, all of which met the requirements, with a pass rate of 100%, indicating that the improved scheme was effective. 5 Conclusion
This paper introduces a machining route and deformation control method for titanium alloy thin plate parts. By optimizing the machining scheme and clamping method, changing the wire EDM path and CNC milling strategy, and adopting positioning fixtures and closed cutting to reduce cutting stress deformation, the rib width and symmetry requirements of the parts are effectively guaranteed, accumulating experience for the machining of such parts.
