Analysis of Titanium Alloy Forging Cleaning Process

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Analysis of Titanium Alloy Forging Cleaning Process

Molten salt descaling is another effective method for removing oxide scale, and it is followed by pickling to remove the alpha case layer. A flowchart is typically used for molten salt descaling and pickling, including solution composition and relevant parameters. The racks used for molten salt descaling are generally made of wood, titanium, or stainless steel to prevent electrical potential between the workpiece and the rack, which could cause electrical erosion or arcing on the workpiece. Molten salt descaling is often used for small and medium-sized forgings. In the case of large batches of forgings, the operating system can be fully automated.

Pickling is used to remove the alpha case layer beneath the scale. The process is as follows:

(1) Perform overall cleaning using sandblasting or alkaline salt.
(2) If alkaline cleaning is used, thoroughly rinse in clean flowing water.
(3) Pickle in a nitric acid-hydrofluoric acid aqueous solution for 5–15 minutes. The solution contains 15%–40% HNO₃ and 1%–5% HF, with an operating temperature of 25–60°C. Typically, the acid concentration (especially for α+β and β alloys) is taken from the middle range of the above concentrations (e.g., 30%–35% HNO₃, 2%–3% HF, or an HNO₃-to-HF ratio of 10:1 to 15:1). However, a chemical solution with an HNO₃-to-HF ratio of approximately 2:1 can achieve a removal rate of 0.025 mm/min with minimal hydrogen absorption.

When using mixed acids, the titanium content in the acid solution continuously increases, reducing the pickling effectiveness. Generally, a titanium content of 12 g/L is considered the upper limit. Beyond this value, the solution should be discarded. The solution can be treated by filtration or by adding other organic chemical additives to extend the life of the pickling solution.

(4) Thoroughly rinse the forgings in clean water.
(5) Rinse with hot water to accelerate drying and allow them to dry after washing.

The time required for metal removal during pickling is mainly determined by the thickness of the alpha case, the operating conditions of the pickling tank, the requirements of the technical process, and the tendency of the workpiece to absorb hydrogen. Pickling provides conditions for excessive hydrogen absorption in titanium alloys, so it must be carefully controlled.

The metal removal rate during pickling is generally 0.03 mm/min or more. This rate is strongly influenced by factors such as the alloy type, acid concentration, solution temperature, and titanium content. A metal removal thickness of 0.25–0.38 mm per surface is usually sufficient to remove the alpha case layer. However, more or less removal may be needed depending on the alloy type and the specific conditions of the forgings being processed.

During pickling, hydrogen absorption can reach 10×10⁻⁶ for every 0.03 mm of surface metal removed, depending on the specific pickling solution and concentration-temperature conditions. Alpha alloys tend to absorb less hydrogen during pickling than (α+β) alloys, and (α+β) alloys tend to absorb less hydrogen than β alloys.

The tendency for hydrogen absorption during pickling increases as the metal removal rate decreases (due to increased titanium content in the solution); as the cleaning temperature rises (above 60°C); and as the surface area-to-volume ratio of the workpiece increases. Generally, at a given solution concentration and temperature, the metal removal rate must exceed the hydrogen diffusion rate. After cleaning, if the hydrogen content exceeds the allowable hydrogen content in the forgings (140–170 cm³/100g), vacuum dehydrogenation annealing must be added.

Parts that do not require pickling should be coated with paint in advance for protection. However, note that the racks holding the parts should only contact the painted areas of the forgings.

Titanium product forms available from GNEE