Research On Precision Machining Technology For 2-mm Thin-Walled Cavity Cover Plate Of Gr5 Titanium Alloy

The Gr5 titanium alloy thin-walled cavity cover plate  is used for pressure-bearing sealing, which imposes high requirements on profile accuracy, flatness and wall-thickness uniformity. The workpiece with 2-mm wall thickness features low structural rigidity.

Current domestic and international research on titanium alloy thin-walled components mostly focuses on cutting simulation, chatter and residual-stress prediction. The complete set of integrated precision-machining processes targeting this 2-mm-thick cavity cover plate carries high engineering research value.

1. Part Structural Characteristics

The research object is an annealed Gr5 titanium alloy cavity cover plate, a plate-type component with a complex enclosed cavity. The side-wall thickness of the cavity is 2 mm. The blank is a Gr5 titanium alloy forging. Strict requirements are set for profile tolerance, flatness, wall-thickness uniformity and surface roughness. The mating surface of the cover plate serves for sealing; chatter marks, scratches and local collapse are strictly prohibited.

2. Major Machining Difficulties

• Spring-back and tool deflection of thin wall: The 2-mm side wall has poor rigidity. Under cutting force during milling, it elastically yields, resulting in insufficient cutting depth. Post-machining spring-back causes out-of-tolerance wall thickness and cavity profile dimensions.
• Cutting chatter: Low structural rigidity and low natural frequency of the component readily induce regenerative chatter, producing chatter marks on side walls, degrading surface quality and accelerating tool wear.
• Deformation induced by residual stress: A large volume of material is removed in rough machining, generating cutting-induced residual stress. Stress release after fixture removal leads to overall warpage of the cover plate and unacceptable flatness.
• Tool built-up edge and wear: Titanium alloy exhibits high chemical activity at elevated temperatures. Heat concentrates at the tool tip, easily causing built-up edge and cutting-edge wear, hence poor dimensional stability.
• Fixture-induced deformation: Conventional rigid pressure-plate clamping generates local compressive stress. The thin-walled part undergoes plastic deformation under clamping, and spring-back upon unclamping leads to dimensional deviation.

1. Process Route

The Gr5 forging blank first undergoes stress-relief annealing, followed by pre-milling of reference surfaces. The cavity is rough-machined by constant-height layered milling with a unilateral allowance of 0.25-0.3 mm. Unclamping and aging are performed to release machining stress. Re-clamping is carried out for semi-finishing, leaving a uniform unilateral allowance of 0.08-0.1 mm. Contour-offset peripheral milling is adopted for finish machining; symmetric layered machining is applied to cavity side walls to guarantee the 2-mm thin wall. Deburring, ultrasonic cleaning are implemented subsequently, followed by inspection using a coordinate measuring machine and roughness tester.

2. Tooling and Fixtures

A vacuum chuck is employed for primary positioning, with surrounding stop blocks to bear lateral cutting force. Removable resin/low-melting-point supports are fitted inside the cavity and removed before finish machining. Tooling datums are precision-ground; dial-gauge alignment is conducted to control positioning error, improve workpiece rigidity and suppress chatter and tool deflection.

3. Cutting Tools

TiAlN-coated solid carbide end mills with variable pitch and unequal helix are utilized, featuring a rake angle of 10°-15° and large helix angle for vibration reduction and anti-built-up-edge performance. A φ6 tool is used for rough machining, and a φ4 finishing end mill for semi-finishing and finish machining.

4. Cutting Parameters and Cooling

• Rough milling: Cutting speed 45-60 m/min, feed per tooth 0.06-0.10 mm/z, depth of cut 0.4-0.6 mm, radial width of cut accounting for 20%-30% of tool diameter.
• Semi-finish milling: Cutting speed 40-50 m/min, feed per tooth 0.04-0.07 mm/z, depth of cut 0.2-0.3 mm.
• Finish milling: Cutting speed 30-40 m/min, feed per tooth 0.03-0.05 mm/z, depth of cut 0.05-0.1 mm, radial width of cut accounting for 5%-10% of tool diameter, so as to reduce elastic deformation of thin walls.
• Cooling: High-pressure internal cooling combined with soluble cutting fluid is applied for heat dissipation and lubrication to mitigate thermal deformation.

• Layered symmetric milling: Z-direction alternate layer-by-layer machining for cavity side walls. Material support for thin walls is retained to balance cutting forces and reduce deflection-induced deformation.
• Phased stress release: Unclamp for stress release after rough machining; separate roughing and finishing operations. Allow standing after semi-finishing prior to finish machining to restrain workpiece warpage.
• Uniform allowance control: Ensure consistent finishing allowance through semi-finishing to avoid wall-thickness deviation.
• Chatter suppression: Adopt variable-pitch cutters; reduce cutting speed and radial width of cut in finish milling to avoid resonance; deploy auxiliary supports to enhance rigidity.
• Error compensation: Test thin-wall spring-back magnitude in advance, and apply minor profile compensation in CAM programs to improve wall-thickness accuracy.

The above-mentioned process is applied to machine the 2-mm thin-walled cavity cover plate of Gr5 titanium alloy. Results show that wall thickness is stably maintained at 2 ± 0.02 mm, profile tolerance meets specifications, flatness ≤ 0.03 mm, inner-wall surface roughness Ra ≤ 0.8 μm, with no chatter marks, scratches or built-up edge. After unclamping and 72-hour standing, dimensional variation measured in re-inspection is less than 0.02 mm, indicating favorable residual-stress control performance.

• Test conclusions: Slight tool wear in finish machining will cause wall-thickness offset. Regular cutting-edge inspection and tool-change specifications shall be formulated for mass production. Edge burrs tend to occur after support removal; a flexible deburring procedure should be added to protect sealing surfaces.

References

[1] Wang Tonggang, Cheng Lei, Yang Song. CNC Machining of Titanium Alloy Thin-Walled Cavity Cover Plate Parts[J]. CNC Machine Tool Market, 2020.

[2] Chen Guang, Wang Jian. Research Progress on Machining-Deformation Control of Titanium Alloy Thin-Walled Parts[J]. Manufacturing Technology & Machine Tool.

[3] Li Yuan. Research on Milling Chatter and Machining Deformation of TC4 Titanium Alloy Thin-Walled Parts[D]. Shenyang Ligong University.

[4] Zhang Hao. Research on Residual-Stress Regulation for Low-Temperature Milling of Titanium Alloy Thin-Walled Parts[J]. Manufacturing Technology & Machine Tool.