Minimizing Scrap Rates by Controlling Thermal-Mechanical Stresses in Titanium Machining

Although titanium alloys are strong, they can be challenging to process. This is especially true for thin plate components, as cutting can easily cause stress deformation, leading to inaccurate dimensions. This poses a significant challenge for many manufacturers. However, there is a solution: employing a set of “combined techniques.” By adjusting wire cutting and CNC milling paths, optimizing the machining plan, and utilizing fixture positioning along with closed cutting methods, we can enhance the rigidity of the parts. This approach reduces deformation at the source and ensures stable product quality.

 

1. Introduction

Titanium alloys are known for their high strength, good corrosion resistance, heat resistance, and relatively high hardness. These properties make them widely used in the aerospace industry. However, they have some drawbacks, including poor thermal conductivity and challenges with machining.

The titanium alloy ribbed component has dimensions of 43 mm in length, 25 mm in width, and 3.5 mm in thickness. The thickness and the two inner cavities in the middle were formed through CNC milling, while the eight outer ribs were created using wire cutting. This process ensures a rib width of (0.3 ± 0.05) mm and maintains a symmetry of 0.05 mm relative to the inner cavity, categorizing it as a fine-rib component.

Previously, ten parts were machined according to the process documentation. Upon inspection, it was noted that four of these parts had rib widths and symmetry that exceeded the specified tolerances, failing to meet the design requirements.

 

2. Cause Analysis

According to the original process documents, the required material thickness for the parts was 5mm. However, due to limitations in the available stock model, only 18mm thick material was accessible. Consequently, the blank cutting size was set to 250mm × 80mm with a thickness of 18mm, as illustrated in Figure 1. A wire cutting process was introduced to reduce the material thickness by half (see Figure 2), resulting in each piece having a thickness of 9mm. This was then CNC milled down to a final thickness of 3.5mm.

During CNC milling, the operator utilized a vacuum suction cup for clamping (see Figure 3). The first surface was precision milled to remove 3mm of excess material. After flipping and repositioning the part, the second surface was milled to achieve the final thickness of 3.5mm, followed by machining the inner cavities.

Minimizing Scrap Rates by Controlling Thermal-Mechanical Stresses in Titanium Machining1

Layout 10 small parts on each piece of material (see Figure 4). Drill a 3mm threaded hole at one end of each row of parts, then proceed to wire cutting to shape the parts.

Minimizing Scrap Rates by Controlling Thermal-Mechanical Stresses in Titanium Machining2

 

Before processing, the wire-cutting operator checks the flatness of the material and discovers that it has undergone stress deformation, with a maximum deformation of 3.05 mm (see Figure 5). A pressure plate is used for clamping during the cutting process. Since there is only one wire piercing hole, the small parts remain interconnected after cutting, resulting in a severed material. Consequently, material deformation occurs during processing due to stress (see Figure 6), leading to rib-width dimensions that are out of tolerance. This deviation, in turn, affects the symmetry of the parts with respect to the inner cavity.

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3. Implement effective measures

Analysis showed that the primary issue stems from the stress deformation of the material. During machining, titanium alloy generates cutting heat, and since the material dissipates heat slowly, removing more stock leads to greater deformation. This problem can only be addressed by modifying the cutting method. The original processing plan was optimized with the following effective measures.

1) To minimize stress during CNC milling, it’s important to replace high stress with low stress. When larger cutting allowances are used, they increase stress levels, resulting in greater material deformation. To address this, the initial wire cutting process for the raw material was revised from splitting it into two parts to splitting it into three parts (see Figure 7). Each resulting piece of material is approximately 6 mm thick, which significantly reduces the CNC milling allowance and, in turn, minimizes material deformation.

Minimizing Scrap Rates by Controlling Thermal-Mechanical Stresses in Titanium Machining4

 

2) Change the clamping method for CNC milling.

When milling thick parts with a CNC machine, switch from vacuum chuck clamping to side-push clamping (refer to Figure 8). To achieve precision, mill both sides of the workpiece by flipping it repeatedly, taking a cutting depth of no more than 0.2 mm with each pass. This approach helps meet the thickness requirements outlined in the drawing while minimizing material deformation during processing. Calculations indicate that if the overall material deformation is kept within 0.5 mm after CNC milling, the flatness requirements for individual small parts can still be satisfied. Operators should adhere to this optimized machining method and conduct inspections throughout the process to ensure the flatness remains within 0.2 mm.

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3) To enhance the manufacturing process of specialized tooling, we will increase the number of wire holes to 10 during wire cutting. This adjustment is crucial to prevent material deformation while processing. Each rib component will have its own independent wire hole, and all holes will be formed in a single CNC milling operation to ensure consistency.

Wire cutting fixtures will be constructed, and the workpiece will be positioned on the fixture plate using positioning pins (see Figure 9). Each rib will be machined independently, avoiding any cuts through adjacent ribs. This method increases material rigidity and minimizes part deformation.

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4. Effect Verification

Under the improved scheme, a trial processing of 20 parts was conducted. Tests performed with professional inspection equipment indicated that the rib width and symmetry of all parts complied with the drawing specifications. Ultimately, a total of 120 parts were processed for this batch, and all met the required standards, achieving a 100% pass rate. This result confirms the effectiveness of the improvement scheme.

 

5. Conclusion

This paper presents a new processing route and deformation control method for titanium alloy thin plate parts. By optimizing the processing plan and clamping technique, altering the wire cutting path, and refining the CNC milling strategy, we utilized positioning fixtures and closed cutting to minimize cutting stress and deformation. As a result, we effectively ensured the rib width and symmetry requirements of the parts. This research has provided valuable insights for the processing of similar components.

 

 

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