All oxide scales and alpha shells formed during the forging heat treatment of titanium alloys are very brittle, and cause cracks in subsequent forging or final forging or cause tool wear during subsequent machining. Therefore, it is best to clean the scale and alpha shell between successive forgings, and the scale and alpha shell must be removed before the forging is delivered to the user.
The cleaning technology of titanium alloy forgings has two aspects: one is the removal of oxide scale; the second is the removal of α shell. Rust can be removed by mechanical methods, such as sandblasting; or chemical methods, such as molten salt derusting. The choice of rust removal method depends on the size, complexity and cost of the parts.
Sand blasting is an effective method to remove rust. It can remove rust with a thickness of 0.13 ～ 0.76mm. 100 ～ 150 mesh zircon sand or steel sand can be used. The air pressure can reach 275Pa. Although sandblasting can be used for forgings of various sizes, it is mostly used for medium and large titanium alloy forgings. Sand blasting equipment can use a drum equipped with
abrasives, shot blasting or sand blasting device. After sandblasting, pickling is required to remove the alpha shell.
Dissolved salt rust removal is another effective method to remove scale, and it also pickles to remove the alpha shell. Fig. 14 is a flow chart of rust removal and pickling with dissolved salts. See Table 7 for solution composition and related parameters. The racks used for dissolving salt to remove rust are generally made of wood, titanium or stainless steel to prevent the generation of electrical potential between the workpiece and the rack, which can cause electrical erosion or arcing of the workpiece. Molten salt descaling is often used for small and medium-sized forgings. In the case of large-scale forgings, the operating system can be fully automated.
Pickling is used to remove the alpha shell under the rust, the process is as follows:
- (1) Use sandblasting or alkaline salt for overall cleaning.
- (2) If alkaline cleaning is used, it should be fully washed in clean running water.
- (3) Pickling in acetic acid-hydrofluoric acid aqueous solution for 5 to 15 minutes. The solution contains 15% ~ 40% HNO3, 1% ~ 5% HF, the operating temperature is 25 ~ 60 ℃. Usually the acid content (especially for α + β and β alloys) usually takes the middle value of the above acid content range (for example, 30% to 35% HNO3, 2% to 3% HF, or the ratio of HNO3 to HF is 10: 1 to 15 :1). However, the chemical solution of HNO3 and HF about 2: 1 can achieve the removal effect of 0.025mm / min, and the hydrogen absorption is the least.
When mixed acid is used, the content of titanium in the acid solution keeps increasing, so that the pickling effect is reduced. It is generally considered that the titanium content of 12g / L has reached the maximum limit, 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 extend the life 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 to clean and clean the metal during pickling is mainly determined by several factors such as the thickness of the α shell, the operating conditions of the pickling tank, the requirements of the technical conditions and the trend of the workpiece to absorb hydrogen. Pickling provides conditions for excessive hydrogen absorption of titanium alloys, so it must be carefully controlled. The metal removal rate in pickling is generally 0.03 mm / min or more, and this rate is strongly affected by the following 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 the alpha shell. But sometimes more or less clearance may be required, depending on the type of alloy and the specific conditions of the forged part being treated.
Hydrogen absorption can reach 10 × 10-6 for every 0.03mm of surface metal removal in pickling, which depends on the specific pickling solution and concentration temperature conditions, the trend of alpha alloy ratio (α + β) alloy hydrogen absorption in pickling However, the (α + β) alloy has a smaller tendency to absorb hydrogen than the β alloy in pickling. The trend of hydrogen absorption during pickling increases with decreasing metal removal rate (due to increased titanium content in the solution); with increasing cleaning temperature (above 60 ° C); and with increasing surface area-volume relative ratio of the workpiece increase. In general, 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 maximum allowable hydrogen content in the forging 140-170cm3 / 100g, vacuum dehydrogenation annealing should be added.
Parts that do not require pickling should be painted in advance to protect them. However, it should be noted that the hanger holding the part can only contact the painted part of the forging.