Titanium Alloy Chemical Milling Process Technology

Chemical milling is a non-contact special processing technology that removes materials by chemical corrosion. It involves no mechanical stress and cutting tools, and features strong forming capacity. Suitable for thin-walled, large-size and special-shaped titanium alloy  components, it serves as a critical process for precision manufacturing of titanium alloys.

1. Reaction Mechanism

Titanium alloy chemical milling is a controlled chemical corrosion process, mostly adopting mixed hydrofluoric‑nitric acid solution. Hydrofluoric acid breaks passivation films and dissolves titanium substrates via redox reactions. Milling depth and accuracy are regulated by process parameters. Hydrofluoric acid acts as the main corrosive agent, while nitric acid functions as an oxidation corrosion inhibitor to stabilize the corrosion rate and avoid over-corrosion. Additives such as urea, sodium dodecyl sulfate and ethylene glycol n-butyl ether can suppress ripple and groove defects and improve surface quality.

2. Core Technical Advantages

Stress-free processing: Free of cutting forces, it prevents deformation and cracking of thin-walled parts, and enables fabrication of ultra-thin-wall components at the 0.1 mm level.

• Compatibility with complex structures: It can process curved surfaces, cavities and hollow-out structures, and complete overall thickness reduction of large-size wall panels and special-shaped curved surfaces in one operation.
• Good processing consistency: Stable corrosion rate in batch production brings small dimensional errors and higher finished-product rate.

Low cost: No tool wear, simple-configured equipment, suitable for mass production.

1. Surface Pretreatment

Remove oil stains, oxide scales and impurities on workpieces to enhance adhesion of masking coatings and ensure uniform corrosion. Degrease with composite alkaline solution at 70 ℃; remove oxide layers and passivation films by pickling, then rinse with pure water and dry. For forged-rolled parts, the procedure can be simplified; for castings, oxide removal shall be intensified to mitigate surface porosity.

2. Protective Masking Treatment

Since chemical milling corrodes all exposed surfaces of workpieces, non-processing surfaces shall be masked by brushing, pasting or laminating acid-resistant rubber, polyurethane or polytetrafluoroethylene. Seal edges strictly to prevent coating blistering, warping and chemical penetration. Special attention shall be paid to holes, grooves and corners to control dimensional deviation.

3. Chemical Milling

After completion of foregoing procedures, workpieces are vertically hoisted with tooling into a constant-temperature tank. Quantitative corrosion is achieved by adjusting chemical-solution ratio, temperature, immersion time and stirring rate. Tank liquor is circulated and replenished to maintain process stability, and the etching ratio K is adopted to guarantee machining accuracy.

4. Post‑treatment Procedures

Upon reaching target milling depth, take out workpieces and remove residual acid by multi‑stage high-pressure pure-water washing. Strip masking layers and perform preliminary inspection. Subsequently conduct acid neutralization, washing and drying, followed by surface finishing, defect inspection and dimensional measurement. For high‑precision parts, roughness and flatness shall be additionally measured to satisfy design requirements.

1.Chemical-solution formula

Hydrofluoric acid controls corrosion rate; nitric acid provides passivation and prevents pitting corrosion. Add urea, surfactants and corrosion inhibitors to stabilize the corrosion rate within 20-23 μm/min, reduce fluctuation of etching ratio and improve process stability.2.Temperature‑time control

Temperature ranges from 25 ℃ to 45 ℃ with temperature fluctuation ≤±1 ℃. High temperature leads to volatile loss, over‑corrosion and ripple defects; low temperature slows down reactions and impairs uniformity. Corrosion time is set according to corrosion coefficient and plate thickness with time error ≤±5 s to avoid over-milling depth.

2.Material conditions

Metallurgical state, work hardening and original surface roughness affect chemical‑milling performance. Forged/extruded/rolled parts deliver favorable performance; castings tend to form pitting defects due to loose microstructure and thus require enhanced pretreatment. Cold-hardened zones feature variant corrosion rates, so surfaces shall be unified before processing to eliminate dimensional deviation.

1.Surface pitting

Caused by unbalanced tank-solution ratio, insufficient corrosion inhibitors or inadequate pretreatment. Calibrate tank-solution concentration, replenish corrosion inhibitors and surfactants, strengthen degreasing and oxide removal, and strictly control processing temperature.

2.Milling ripples and grooves

Induced by poor fluidity of tank liquor, local concentration non‑uniformity or lack of additives. Turn on circulating stirring, optimize workpiece clamping, and add additives such as sodium dodecyl sulfate to improve corroded surfaces.

3.Dimensional out-of-tolerance and uneven thickness

Result from temperature fluctuation, time‑control deviation and inconsistent chemical‑solution contact. Deploy constant‑temperature systems, precisely control corrosion time, calibrate corrosion coefficients by pre‑specimen tests, and optimize tooling to guarantee uniform chemical‑solution contact.

4.Boundary leakage corrosion

Stem from poor sealing of masking coatings and inadequate edge sealing at corners. Select high‑adhesion acid‑resistant masking materials, reinforce sealing for holes and corners, and inspect coatings for bubbles and warping before processing.

Ruihang is a direct manufacturer of titanium products, supplying optimal quality raw materials for your CNC machining. For more details, please feel free to contact us via email: Sam.Rui@bjrh-titanium.com