Titanium alloy forging cleaning process!
Oxide scale formed during forging and heating treatment of all titanium alloys α The shell is very brittle and may cause cracks in subsequent forging or final forging or tool wear in subsequent machining. Therefore, it is good for z to remove rust scale and α The shell, and the rust scale and α Shell.
The cleaning technology of titanium alloy forgings has two aspects: one is the removal of oxide scale; The second is α Shell removal. Rust skin can be removed by mechanical methods, such as sand blasting; Or use chemical methods, such as dissolving salt derusting. The selection of rust removal method depends on the size, complexity and cost of parts.
Sand blasting is an effective method to remove rust scale. It can remove 0.13~0.76mm thick rust scale. It can use 100~150 mesh zirconium sand or steel sand. The air pressure can reach 275 Pa. Although sand blasting is applicable to forgings of various sizes, it is mostly used for medium and large titanium alloy forgings. Sand blasting equipment can adopt roller, shot blasting or sand blasting device with abrasive. Pickling is required after sand blasting to remove α Shell.
Dissolved salt derusting is another effective method to remove oxide scale, and also followed by pickling α Shell. 1. Flow chart, solution composition and relevant parameters of rust removal and pickling with dissolved salt. The racks used for salt dissolving derusting are generally made of wood, titanium or stainless steel to prevent the electric potential between the workpiece and the rack from generating electric erosion or electric arc to the workpiece. Salt solution derusting is often used for medium and small forgings. In the case of large quantities of forgings, the operating system can be fully automated.
Pickling is used to remove rust under the skin α Shell, its process is as follows:
(1) Conduct overall cleaning with sand blasting or alkali salt.
(2) If alkali cleaning is used, it shall be fully cleaned in clean flowing water.
(3) Pickle for 5~15min in sand acid hydrofluoric acid aqueous solution. The solution contains 15%~40% HNO3, 1%~5% HF, and the operating temperature is 25~60 ℃. Usually acid content (especially for α+β and β Alloy) usually take the middle value of the above acid content range (for example, 30% ～ 35% HNO3, 2% ～ 3% HF, or the ratio of HNO3 to HF is 10:1 to 15:1). However, the chemical solution with HNO3 and HF of about 2:1 can achieve the removal effect of 0.025 mm/min, while absorbing less hydrogen.
When mixed acid is used, the titanium content in the acid solution increases continuously, thus reducing the pickling effect. It is generally believed that the titanium content of 12g/L is high, and the solution should be discarded if it exceeds this value. Solution treatment can be carried out by filtering or adding other organic chemical additives to prolong the service life of pickling solution.
(4) Thoroughly clean the forgings in clean water.
(5) Wash with hot water to speed up drying, and let it dry after washing.
The time required for metal cleaning in pickling is mainly determined by α The thickness of the shell, the operating conditions of the pickling tank, the need for technical conditions and the trend of hydrogen absorption of the workpiece are determined by several factors. Pickling provides conditions for excessive hydrogen absorption of titanium alloys, so it must be carefully controlled. The metal removal rate in pickling is generally 0.03mm/min or more, which is strongly affected by the following factors, such as alloy type, acid concentration, solution temperature and titanium content. The metal removal thickness of each surface is 0.25~0.38mm, which is usually enough for removal α Shell. However, sometimes more or less removal may be required, depending on the type of alloy and the specific conditions that exist for the treated forging.
When 0.03mm surface metal is removed in pickling, hydrogen absorption can reach 10 × 10-6, which depends on the specific pickling solution and concentration temperature conditions α Alloy ratio（ α+β) The hydrogen absorption trend of alloy is small, while（ α+β) Alloy β The tendency of hydrogen absorption of alloy in pickling is small. The trend of hydrogen absorption increases with the decrease of metal removal rate (due to the increase of titanium content in solution); It increases with the increase of cleaning temperature (higher than 60 ℃); And increase with the increase of the relative ratio of surface area to volume of the workpiece. Generally speaking, the metal removal rate must exceed the hydrogen diffusion rate at a certain solution concentration and temperature. After cleaning, if the hydrogen content exceeds the allowable hydrogen content of 140~170cm3/100g in the forging, vacuum dehydrogenation annealing shall be added.
The parts that do not need pickling shall be painted in advance for protection. However, it should be noted that the hanger holding the parts can only contact the painted part of the forging.
