Chemical Milling Process Of Titanium Alloys

Titanium alloy is difficult to machine. Traditional mechanical machining tends to cause stress deformation and surface damage, making it hard to adapt to the manufacturing of complex and precision parts. Chemical milling process relies on chemical corrosion to achieve precise material removal. It has significant advantages in machining complex-shaped parts and lightweight manufacturing. It is one of the key methods for precision machining of titanium alloys.

 

 

 

Chemical milling of titanium alloys removes materials in preset areas for forming through the selective reaction between corrosive solution and matrix. The core is the "etching-dissolving-passivation" cycle: hydrofluoric acid dissolves the surface oxide film and exposes the fresh matrix. Nitric acid forms a dense passivation film to inhibit over-corrosion and hydrogen embrittlement. The two work together to precisely control the corrosion rate.

The structure of titanium alloy affects the corrosion behavior microscopically. For example, α+β type titanium alloy (Gr5) forms a micro-battery with β phase as cathode and α phase as anode, and the α phase dissolves preferentially. Therefore, it is required to adjust the process according to the metallurgical state and phase ratio of the alloy to ensure machining uniformity.

 

 

(I) Pretreatment

The core is to obtain a clean and uniform surface to ensure the adhesion of the protective coating:

Degreasing removes surface oil stains to prevent uneven local corrosion.

Descaling improves the bonding force between the coating and the matrix.

Annealing eliminates the internal stress caused by work hardening to avoid excessive differences in corrosion rate.

 

(II) Coating Protective Coating and Pattern Scribing

A strippable protective coating is applied on the surface of the workpiece and cured to form a film. The coating in the area to be processed is peeled off through pattern scribing or photosensitive technology to achieve precise definition of the corrosion area. Automated coating and laser scribing can improve coating uniformity and scribing accuracy.

 

(III) Chemical Milling

The workpiece is hung and immersed in HF-HNO₃ type mixed acid milling solution. Quantitative material removal is realized by controlling concentration, temperature, stirring speed and corrosion time.

Additives such as sodium dodecyl sulfate (to prevent corrosion grooves and ripples), urea and ethylene glycol n-butyl ether are added. Process parameters are strictly controlled to prevent acid volatilization and coating failure.

 

(IV) Post-treatment and Quality Inspection

After corrosion, we need to wash the workpiece with water to remove residual acid and peel off the coating.Then, dimensional inspection, surface roughness testing and hydrogen content analysis are carried out in sequence. Secondary milling or mechanical vibration treatment is performed on the edge stress concentration problem to balance precision and structural reliability.

 

Image source: The IQS Directory

 

 

(I) Decisive Role of Material State

Different types of titanium alloys have different phase compositions and corrosion activities. Generally, the higher the β phase content, the faster the corrosion rate. The surface of forged or rolled workpieces is dense, resulting in lower roughness after milling. Cast workpieces require adaptive adjustment of corrosion parameters. The development of new titanium alloys needs to simultaneously optimize the milling solution formula and process parameters to adapt to their special chemical activity.

 

(II) Core Influence of Milling Solution Performance

HF dominates the corrosion rate and HNO₃ controls the surface roughness. A volume ratio of 2:1~3:1 between the two can balance the corrosion rate and surface quality. We can also adjust the ratio as needed. During processing, the concentration of titanium ions in the solution will increase, so it is necessary to replenish or replace the solution to maintain performance and ensure milling consistency.

 

(III) Guarantee Role of Equipment Performance

Specialized milling equipment must have precise temperature control, efficient stirring and corrosion-resistant structure. Equipment for processing large parts is equipped with flexible hanging devices. Intelligent equipment monitors process parameters in real time through sensors and automatically adjusts to reduce manual intervention and human error, improving machining precision and efficiency.

 

 

The core application field of titanium alloy chemical milling is aerospace. It can realize lightweight machining of large thin-walled parts such as aircraft wings and fuselage panels. It also can reduce weight and improve efficiency while ensuring structural strength.

It is also suitable for machining complex parts such as aero-engine casings and blades, meeting the precision requirements of high-temperature and high-speed working conditions.

 

This process can also be applied to the manufacturing of titanium alloy porous parts and micromechanical products, as well as the surface defect repair of forgings and castings.

 

Ruihang Group – a specialist in titanium and titanium alloy products, supplies custom high-precision workpieces as per your demands. For cooperation inquiries, reach us via email: Sam.Rui@bjrh-titanium.com

 

Reference

IQS Directory. (n.d.). Chemical milling: Types, uses and products. IQS Directory. https://www.iqsdirectory.com/articles/metal-etching/chemical-milling.html