How To Control Surface Oxidation Of Titanium Alloy Forgings?

Titanium is highly easy to oxidation during hot forging, forming an embrittlement layer that causes cracking and processing difficulties. That significantly reduces the performance of forgings. Effective oxidation prevention processes are critical for improving the reliability and economy of titanium alloy forgings.

 

(I) Formation Mechanism of Oxide Layer

The surface reacts with oxygen in the furnace to form a three-layer oxide film during the hot forging.

The three-layer oxide film: a loose outer layer of TiO₂, a middle layer of TiO-Ti₃O mixed phase, and an inner oxygen-enriched α-phase embrittlement layer.

The α-phase is the key factor determining to forging failure, and its thickness increases exponentially with temperature rise.

 

(II) Key Influencing Factors
Temperature and Time: The oxidation rate increases by 30% for every 100℃ rise in temperature

Alloy Type: α-titanium alloys undergo significant oxidation at 600℃, while β-titanium alloys only form an obvious oxide layer at temperatures above 980℃.

Atmosphere Environment: The oxygen concentration in the furnace determines the oxidation rate. Nitrogen protection can reduce the rate , and inert gas protection offers better results.

 

Titanium forgings in Ruihang

 

(I) Atmosphere Protection Technology
Inert Gas Protection: Continuously introduce high-purity argon or helium (purity ≥99.99%), reducing the oxidation rate by over 60%. Mitsubishi Electric uses argon protection to stabilize the oxide layer thickness at 0.1-0.3mm, with a qualification rate of 98%.

Vacuum Environment Control: A vacuum degree of ≥10⁻³Pa can completely isolate oxygen, suitable for high-precision key components, but the equipment cost is relatively high.

Loose Material Heating: Granular medium heating enables higher heat transfer efficiency, shortening the high-temperature exposure time and inhibiting oxidation from the source.

 

(II) Process Parameter Optimization
Staged Temperature Control: Control the heating temperature at 20-50℃ below the β-transus temperature to reduce oxide layer thickening.

Precise Time Management: Optimize the deformation path through numerical simulation to shorten the ineffective residence time in the high-temperature zone.

Cooling Strategy: Adopt staged cooling to avoid post-forging high-temperature oxidation and reduce the risk of oxide layer spallation.

 

(III) Surface Modification Protection

 

Process Type	Technical Parameters	Core Advantages	Application Scenarios
Pre-oxidation Treatment	Form a dense oxide film at 300-400℃	Oxidation weight gain rate reduced by 20% at 500℃	Medium-temperature service components
Anodic Oxidation	Voltage 20-60V, H₂SO₄ electrolyte	Corrosion resistance increased by 50%, film thickness 10-30μm	Medical implants, electronic components
Micro-arc Oxidation (MAO)	High-voltage electrolysis at 300-600V, composite TiO₂/Al₂O₃ layer	Temperature resistance ＞500℃, wear and cavitation resistance	Aero-engine blades, marine valves
B+(B-Al) Composite Diffusion	Solid powder embedding, isothermal treatment at 800℃	Oxidation weight gain rate reduced by 83.5%, hardness 1800HV	Gr5 components under high-temperature working conditions

 

Note: The B+(B-Al) composite diffusion technology adopts a synergistic mechanism of TiB₂ outer layer barrier + Al₃Ti inner layer self-protection, resulting in a weight gain of only 7.24 g/m² for Gr5 alloy after oxidation at 800℃ for 100 hours.

 

(IV) Composite Protection System
The combined process of "pre-oxidation + glass enamel coating + argon protection" can simultaneously improve surface quality and plasticity. Experiments show that BT3-1 titanium alloy forgings treated with this combination have a smooth surface without fish-scale defects, and the subsequent cleaning efficiency is increased by 40%.

 

(I) Oxide Layer Detection Methods
Microscopic Analysis: Observe the oxide layer structure using Scanning Electron Microscopy (SEM) with a thickness measurement accuracy of 0.1μm;

Composition Detection: Use spectral analysis to determine the degree of nitrogen and hydrogen contamination, avoiding embrittlement risks;

Performance Evaluation: Test the stability of the oxide film using Electrochemical Impedance Spectroscopy (EIS), suitable for medical implants.

(II) Oxide Layer Cleaning Processes
Mechanical Cleaning: Sandblasting can remove 0.13-0.76mm of oxide scale, requiring a coverage rate of ≥200% to avoid substrate damage;

Chemical Pickling: A mixed solution of nitric acid and hydrofluoric acid removes the α-case layer with a cleaning rate of 0.03mm/min, removing 0.25-0.38mm of the surface layer in a single treatment;

Repair Technologies: Laser cladding repairs pitting defects at a cost of only 25% of new parts; PVD recoating repairs coating spallation, with adhesion meeting ASTM D3359 standards.

 

(I) Aerospace Field
Components: Gr5 titanium alloy engine compressor blades, landing gear;

Process: MAO ceramic layer + shot peening, heating at a vacuum degree of 10⁻³Pa;

Effect: Oxide layer thickness ≤0.2mm, fatigue resistance increased by 30%, coating integrity inspected every 500 flight hours.

 

(II) Medical Field
Components: Artificial joints, bone plates;
Process: Electropolishing+ anodic oxidation;
Effect: Biocompatibility meets standards, oxide film impedance retention rate ＞80% 2 years after surgery.

 

(III) Chemical Industry Field

Components: Reactor agitator paddles, flanges;
Process: Chemical passivation + PTFE coating;
Effect: Corrosion resistance improved, annual corrosion rate ≤0.1mm.

 

Ruihang is specialized in R&D,production and sales of high-quality titanium and titanium alloy products, including forgings, pipes,rings,bars, plates and other titanium products etc. For more details,please contact us via the Email: Sam.Rui@bjrh-titanium.com