I. Titanium alloys are widely used in aerospace, medical, and other fields due to their high specific strength and corrosion resistance. However, their high chemical reactivity makes them prone to reacting with oxygen and nitrogen during high-temperature heating to form a brittle oxide layer, leading to decreased material plasticity and increased machining allowance. Achieving minimal or no oxidation during the heating process of titanium alloy forging billets has become a key technical challenge for improving material utilization and reducing production costs. We explored methods for controlling surface oxidation of titanium alloy forgings through systematic experimental research.
II. Experimental Materials and Methods BT3-1 titanium alloy extruded billets were selected as the main research object, with simultaneous comparisons of the performance changes of BT20, OT4-1 alloy plates, and PT7M alloy tubing. All samples were mechanically polished and then heated in an electric furnace to 950℃～980℃ (close to the allotropic transformation temperature of titanium alloys), with a holding time controlled within 1 hour. Experimental variables included: pre-oxidation treatment, glass enamel protective coating, heating medium type (ordinary electric furnace/loose material pseudo-liquefaction layer), and post-forging surface treatment method (sandblasting).
III. Key Technologies for Surface Oxidation Control
1. Pre-oxidation Treatment Process:
Experiments show that the surface of untreated billets exhibits a fish-scale oxide layer, while the surface smoothness of pre-oxidized billets is significantly improved. Pre-oxidation treatment, by forming a uniform and dense oxide film on the billet surface, effectively inhibits deep oxidation during subsequent heating. Furthermore, the adhesion of the glass enamel coating on the pre-oxidized billet surface is reduced, making subsequent removal more than 30% easier and significantly improving production efficiency.
2. Glass Enamel Protective Coating Technology:
Applying a glass enamel coating on top of the pre-oxidation treatment can further reduce the oxidation rate during heating. This coating reduces the contact between the billet and oxidizing gases through physical isolation. Experimental data shows that coating protection can reduce the oxide layer thickness on the billet surface by 50%–70%. Notably, the synergistic effect of the coating and the pre-oxidation layer can improve the surface plasticity of the billet, increasing the elongation of the forged samples by 15%–20%.
3. Heating medium optimization technology:
(1) Ordinary electric furnace heating control: When heating in a conventional electric furnace, the temperature is strictly controlled above the allotropic transformation temperature and the holding time is ≤1 hour to avoid obvious gas absorption on the surface. The formed oxide layer can be effectively removed by sandblasting, and the material loss rate is controlled within 5%. (2) Loose material pseudo-liquefaction layer heating technology: This technology heats the billet by burying it in a pseudo-liquefaction layer composed of granular media (such as alumina powder), and uses the intense relative motion between the media particles to enhance heat exchange. Experiments show that its heat transfer efficiency is 1.5 orders of magnitude higher than that of a forced convection furnace, approaching the level of a molten salt furnace. This technology can achieve rapid and uniform heating of the billet, shortening the heating time by 40% to 60%, and at the same time significantly reducing the oxidation tendency through the isolation effect of the medium, reducing the thickness of the surface oxide layer by more than 80%.
Application Case: We used Y₂O₃ dispersion strengthening + thermal diffusion coating on titanium-niobium alloy turbine disks, which increased the creep strength at 650°C by 35% and reduced the creep rate to 1×10⁻⁸/s.
IV. Surface Treatment Process Optimization:
Sandblasting after die forging is a key step in improving the performance of forgings. Conventional sandblasting can remove the surface oxide layer and gas-absorbing layer, reducing the surface roughness Ra value to below 3.2μm, while simultaneously improving plasticity through surface strengthening. For blanks with glass enamel coatings, the sandblasting pressure must be controlled within the range of 0.3–0.5MPa to avoid excessive damage to the base material.
V. Conclusions:
1. The synergistic application of pre-oxidation treatment and glass enamel coating can construct a dual-layer protection system of "active oxidation control + passive isolation protection," significantly improving the surface quality of titanium alloy forgings.
2. The loose material pseudo-liquefaction layer heating technology achieves the dual goals of efficient heating and oxidation control by optimizing the heat transfer mechanism, making it particularly suitable for the mass production of complex-shaped forgings.
3. Precise control of process parameters (temperature, time, sandblasting pressure, etc.) is crucial to ensuring the comprehensive performance of titanium alloy forgings; standardized process specifications need to be established according to specific alloy grades.
Controlling surface oxidation of titanium alloy forgings is essentially a comprehensive system engineering project integrating "process, environment, and post-treatment."
With the support of local industries in Baoji, vacuum forging + inert gas protection + pickling and passivation has become the mainstream solution, while high-temperature coating and digital control are driving it towards the goal of "zero oxidation."
For high-end fields such as aerospace and nuclear power, vacuum forging + PVD coating is the ultimate path to achieving "service-grade zero oxidation."
