The oxide scale and alpha shell formed in all titanium alloy forging heat treatments are very brittle and induce cracking in subsequent forging or in final forging or causing tool wear during subsequent machining. Therefore, it is preferable to clean the scale and the alpha shell between the continuous forging, and the scale and the alpha shell must be removed before the forging is delivered to the user.
There are two aspects to the cleaning of titanium alloy forgings: one is the removal of scales; the other is the removal of the alpha shell. The scale can be removed by mechanical means, such as sand blasting; or by chemical methods such as leaching of salt. The choice of rust removal method depends on the size, complexity and cost of the part.
Sand blasting is an effective method for removing scales. It can remove 0.13~0.76mm thick scales, and can use 100~150 mesh zircon sand or steel sand, and the pressure can reach 275Pa. Although sand blasting is available for all sizes of forgings, it is mostly used for medium and large titanium alloy forgings. The blasting equipment can be equipped with abrasive drums, shot blasting or sand blasting devices. After sand blasting, it is pickled to remove the alpha shell.
Desalination of dissolved salts is another effective method for removing scales, and is followed by pickling to remove the alpha shell. A flow chart for the derusting and pickling of dissolved salts, solution composition and related parameters. The shelf for leaching of dissolved salts is generally wood, titanium or stainless steel to prevent potential build-up between the workpiece and the shelf, resulting in electrical erosion or arcing of the workpiece. Dissolved salt rust 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 the alpha shell under the scale. The process is as follows:
(1) Whole cleaning with sandblasting or alkali salt.
(2) If alkaline cleaning is used, it should be thoroughly cleaned in clean flowing water.
(3) Pickling in a hydrochloric acid-hydrofluoric acid aqueous solution for 5 to 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 intermediate value of the above range of acid content (eg 30% to 35% HNO3, 2% to 3% HF, or the ratio of HNO3 to HF is 10:1 to 15) :1). However, HNO3, a chemical solution with an HF of about 2:1 can achieve a removal effect of 0.025 mm/min, while hydrogen absorption is minimal.
When a mixed acid is used, the titanium content in the acid solution is continuously increased, so that the pickling effect is reduced. It is generally considered that the titanium content of 12 g/L has reached the maximum limit, and the solution should be discarded if it exceeds this value. Solution treatment can be carried out by filtration or by addition of other organic chemical additives to extend the life of the acid wash.
(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 remove and clean the metal during pickling is mainly determined by the thickness of the alpha shell, the operating conditions of the pickling tank, the technical requirements of the process, and the tendency of the hydrogen absorption of the workpiece. Pickling provides conditions for excessive hydrogen absorption by titanium alloys and must be carefully controlled. The rate of metal removal in pickling is typically 0.03 mm/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 to 0.38 mm is usually sufficient to remove the alpha shell. However, sometimes more or less removal may be required depending on the type of alloy and the specific conditions in which the forged part is treated.
For every 0.03mm of surface metal removed in pickling, the hydrogen absorption can reach 10×10-6, depending on the specific pickling solution and concentration temperature conditions, and the hydrogen absorption tendency of α-α alloy in the pickling. Small, and the (α + β) alloy has a smaller tendency to absorb hydrogen in the pickling than the beta alloy. The tendency of hydrogen absorption in pickling increases with the decrease of metal removal rate (due to the increase of titanium content in solution); increases with the increase of cleaning temperature (above 60 °C); and with the increase of surface area-volume relative ratio of workpiece increase. In general, the metal removal rate must exceed the hydrogen diffusion rate at a given solution concentration and temperature. After cleaning, if the hydrogen content exceeds the maximum allowable hydrogen content of 140-170 cm3/100g in the forging, vacuum dehydrogenation annealing is required.
Parts that do not require pickling should be painted in advance to protect them. However, it should be noted that the pylon holding the part can only be in contact with the lacquered part of the forging.
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