The oxide scale and α-shell layer formed during the forging heat treatment of all
titanium alloys are very brittle, and induce cracks in the subsequent forging or in the final forging or cause tool wear in the subsequent machining process. Therefore, it is best to clean the scale and α-shell between continuous forgings, and the scale and α-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 skin; the other is the removal of α shell. The scale can be removed mechanically, such as sandblasting; or chemically, such as molten salt derusting. The choice of rust removal method depends on the size, complexity and cost of the part.
Sandblasting is an effective method to remove scale. It can remove scale with a thickness of 0.13-0.76mm. It can use 100-150 mesh zircon sand or steel grit, and the air pressure can reach 275Pa. Although sand blasting is available for forgings of various sizes, it is mostly used for medium and large titanium alloy forgings. Sandblasting equipment can use abrasive drums, shot peening or sandblasting units. After sandblasting, it is pickled to remove the alpha shell.
Dissolved salt rust removal is another effective method for removing scale, and pickling is also used to remove the α-shell. 1. The flow chart of rust removal and pickling by molten salt, solution composition and related parameters. The frame used for molten salt derusting is generally made of wood, titanium or stainless steel, in order to prevent electric potential from being generated between the workpiece and the frame, resulting in electrical erosion or arcing of the workpiece. Molten salt derusting is often used for medium and small forgings, and in the case of large quantities of titanium forgings, the operating system can be fully automated.
Pickling is used to remove the α shell layer under the rust scale, and the process is as follows:
(1) Use sandblasting or alkali salt for overall cleaning.
(2) If alkaline cleaning is used, it should be fully cleaned in clean running water.
(3) Pickling in 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 to 60°C. Usually the acid content (especially for α + β and β alloys) often 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 at a ratio of about 2:1 can achieve a removal effect of 0.025mm/min, while absorbing less hydrogen.
When mixed acid is used, the titanium content in the acid solution increases continuously, which reduces the pickling effect. It is generally considered that the titanium content of 12g/L is the upper 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 prolong the life of the pickling solution.
(4) Wash the forgings thoroughly in clean water.
(5) Wash with hot water to speed up drying, and let it dry after washing.
The time required to remove and clean the metal in 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 process and technical conditions, and the tendency 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 0.03mm/min or more, and this rate is strongly affected by the following factors, such as alloy type, acid concentration, solution temperature and titanium content. A metal removal thickness of 0.25 to 0.38 mm per surface is usually sufficient to remove the alpha shell. Sometimes, however, more or less removal may be required, depending on the type of alloy and the specific conditions present in the forging being treated.
For every 0.03mm surface metal removed in pickling, the hydrogen absorption can reach 10×10-6, which depends on the specific pickling solution and concentration temperature conditions. In pickling, the hydrogen absorption trend of α alloy ratio (α+β) alloy Small, and the (α+β) alloy has a smaller hydrogen absorption tendency than the β alloy in pickling. The hydrogen absorption tendency in pickling increases with the decrease of metal removal rate (due to the increase of titanium content in the solution); increases with the increase of cleaning temperature (above 60 °C); and increases with the increase of the relative surface area-volume ratio of the workpiece. Increase. 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 maximum hydrogen content of 140-170cm3/100g in the forging, it is necessary to increase the vacuum dehydrogenation annealing.
