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The process of titanium alloy forging cleaning

The oxide rust formed during all titanium alloy forging and heating treatments α The shell is very brittle and can induce cracks during subsequent forging or final forging, or cause tool wear during subsequent machining processes. Therefore, it is best to clean the rust and α The shell layer must be removed before the forgings are delivered to the user α Shell.

There are two aspects to the cleaning technology of titanium alloy forgings: one is the removal of oxide skin; The second is that α Removal of the shell layer. Rust can be removed by mechanical methods, such as sandblasting; Alternatively, chemical methods such as molten salt rust removal can be used. The selection of rust removal methods depends on the size, complexity, and cost of the parts.

Sandblasting is an effective method for removing rust, which can remove 0.13-0.76mm thick rust. It can be used with 100-150 mesh zirconium sand or steel sand, and the air pressure can reach 275Pa. Although sandblasting is suitable for forgings of various sizes, it is mostly used for medium and large titanium alloy forgings. Sandblasting equipment can use drums, shot blasting, or sandblasting devices equipped with abrasives. After sandblasting, it needs to be pickled to remove α Shell.

Salt dissolution rust removal is another effective method for removing oxide scale, and it is also accompanied by acid washing for removal α Shell. A typical process diagram for rust removal and acid washing using dissolved salt, solution composition, and related parameters. The racks used for salt rust removal are generally made of wood, titanium, or stainless steel to prevent the generation of electric potential between the workpiece and the rack, which may cause electrical erosion or arc formation on the workpiece. Salt dissolution rust removal is often used for small and medium-sized forgings, and in the case of large-scale forgings, the operating system can be fully automated.

Pickling is used to remove rust from the subcutaneous area α Shell, its process is as follows:

(1) Use sandblasting or alkaline salt for overall cleaning.

(2) If alkaline cleaning is used, it should be thoroughly cleaned in clean flowing water.

(3) Wash in a sand acid hydrofluoric acid aqueous solution for 5-15 minutes. The solution contains 15% to 40% HNO3, 1% to 5% HF, and the operating temperature is 25-60 ℃. The usual acid content (especially for α+β and β The middle value of the above acid content range is often taken for alloys (such as 30% to 35% HNO3, 2% to 3% HF, or the ratio of HNO3 to HF is 10:1 to 15:1). However, a chemical solution with HNO3 and HF ratios of approximately 2:1 can achieve a clearance effect of 0.025mm/min, while hydrogen absorption is minimal.

When using mixed acids, the titanium content in the acid solution continuously increases, resulting in a decrease in the pickling effect. It is generally believed that the titanium content of 12g/L is at its maximum limit, and if it exceeds this value, the solution should be discarded. Solution treatment can be carried out by filtering or adding other organic chemical additives to extend the lifespan of the pickling solution.

(4) Thoroughly clean the forgings in clean water.

(5) Wash with hot water to accelerate drying, and let it dry after washing.

The time required for metal removal and cleaning in acid pickling is mainly determined by α The thickness of the shell, the operating conditions of the pickling tank, the requirements of the process technology, and the trend of hydrogen absorption of the workpiece are several factors that determine it. Acid washing provides conditions for excessive hydrogen absorption in titanium alloys, therefore careful control is necessary. The metal removal rate during pickling is generally 0.03mm/min or more, which is strongly influenced by factors such as alloy type, acid concentration, solution temperature, and titanium content. Each surface metal removal thickness of 0.25-0.38mm is usually sufficient to remove α Shell. But sometimes more or less clearance may be required, depending on the type of alloy and the specific conditions present in the processed forging.

During pickling, hydrogen absorption can reach 10 × 10-6 for every 0.03mm of surface metal removed, depending on the specific pickling solution and concentration temperature conditions α Alloy ratio（ α+β) The hydrogen absorption trend of alloys is small, while（ α+β) Alloy is better than β The hydrogen absorption trend of alloys during acid washing is small. The hydrogen absorption trend during acid washing increases with the decrease of metal removal rate (due to the increase of titanium content in the solution); As the cleaning temperature increases (above 60 ℃), it increases; And it increases with the increase of the relative ratio of surface area to volume of the workpiece. Generally speaking, at a certain solution concentration and temperature, the metal removal rate must exceed the hydrogen diffusion rate. After cleaning, if the hydrogen content exceeds the maximum allowable hydrogen content of 140-170cm3/100g in the forging, vacuum dehydrogenation annealing needs to be added.

The parts that do not require acid washing should be painted in advance to protect them. However, it should be noted that the hanging bracket for clamping parts can only come into contact with the painted parts of the forging.

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