Research on Precision Control of Hot Forming and Surface Oxide Scale Treatment Processes for Titanium Alloy Rods

During the hot forming process of titanium alloy rods, geometric accuracy deviations mainly originate from the elastic recovery (springback) behavior of the material. Although titanium alloys exhibit improved plasticity at elevated temperatures, they still possess relatively high deformation resistance. In addition, the high-temperature yield strength of titanium alloys increases rapidly as the temperature decreases. As a result, significant elastic recovery occurs after unloading, causing deviations between the actual contour of the formed component and the mold surface, making it difficult to fully satisfy the required geometric accuracy.

Although the elevated temperature during hot forming can significantly reduce deformation resistance, relieve residual stress, and suppress springback to a certain extent, it cannot completely eliminate elastic recovery. For example, in processes such as resistance heating drop forming and die press forming, even with strict control of heating temperature, holding time, and pressure parameters, a certain degree of springback remains after the component is cooled and removed from the die. Traditionally, additional manual correction and adjustment are required to achieve dimensional requirements, which not only reduces production efficiency but also affects consistency and stability during batch manufacturing.

To improve dimensional accuracy during hot forming, process optimization methods can be adopted to reduce the influence of springback. For instance, springback compensation technology can be introduced during die design. By predicting shape changes after forming through finite element simulation, the die surface can be pre-compensated accordingly. In addition, process methods such as curve compensation and step-by-step forming can be combined to make the forming process closer to the target geometry, thereby reducing subsequent manual adjustment and improving dimensional accuracy and production efficiency.

Titanium alloys have a strong affinity for oxygen and nitrogen at elevated temperatures, resulting in the formation of oxide layers on their surfaces during heating and forming. The oxidation behavior is closely related to the forming temperature, and the color, thickness, and bonding characteristics of the oxide scale change significantly with increasing temperature. When the hot forming temperature is approximately 600°C, a relatively thin blue oxide film is generally formed on the surface. As the temperature increases to around 850°C, the oxide layer gradually changes to a grayish-red color. When the temperature reaches 900°C or above, a darker gray oxide layer is formed. With further temperature increases, the oxide scale becomes thicker and more strongly bonded to the substrate, making subsequent removal increasingly difficult.

Practical experience has shown that the dense gray oxide layer formed at temperatures above 900°C usually requires more intensive chemical treatment for effective removal. However, excessive chemical treatment may damage the substrate surface and reduce surface quality. Therefore, when establishing hot forming parameters, it is necessary to comprehensively consider the effects of forming temperature on material properties, forming accuracy, and oxidation behavior, while minimizing oxidation without compromising forming performance.

At present, the removal of titanium alloy oxide scale mainly adopts a two-step process combining molten alkali cleaning and acid pickling. The first step involves molten alkali treatment at approximately 450°C, where the oxide layer is initially loosened and partially removed through the dissolution and destructive effects of the alkaline medium. The second step is acid pickling, typically using a mixed solution of nitric acid and hydrofluoric acid, to remove residual oxide layers and restore the metallic appearance of the titanium alloy surface.

During oxide scale removal, special attention must be paid to the influence of process parameters on the microstructure and mechanical properties of the material. The molten alkali cleaning temperature of approximately 450°C is close to the aging temperature range of certain α+β titanium alloys and β titanium alloys. If the treatment temperature is excessive or the holding time is prolonged, undesirable precipitation of strengthening phases may occur, increasing the difficulty of subsequent cold working and potentially causing cracking. In addition, during acid pickling, improper control of hydrofluoric acid concentration or insufficient cleaning after alkaline treatment may allow hydrogen atoms to penetrate into the titanium alloy substrate, resulting in hydrogen embrittlement and surface corrosion. These issues can significantly reduce the fatigue life and overall mechanical performance of titanium alloy components.

Therefore, precision control of hot forming processes for titanium alloy rods requires full consideration of material springback characteristics during process design, along with optimized heating temperatures, forming parameters, and die compensation strategies. Meanwhile, surface oxide scale treatment should achieve effective oxide removal while strictly controlling alkali cleaning temperature, acid pickling time, and subsequent cleaning procedures to prevent adverse effects on material microstructure and properties. Differentiated hot forming and surface treatment specifications should be developed for titanium alloys with different grades, dimensions, and thicknesses. Furthermore, the introduction of online dimensional inspection and surface quality monitoring technologies can enable high-precision, high-efficiency, and highly consistent batch production of titanium alloy rods.