The production of titanium bolts is a precise and multi-step process that combines metallurgy, machining and surface treatment techniques to take full advantage of the excellent properties of titanium alloys. The main process links include the following steps.
1. Raw material selection and preparation:
According to the required strength, corrosion resistance, working temperature and other requirements of the bolt, select the appropriate titanium alloy grade. Commonly used ones include industrially pure titanium (e.g. Gr.1, Gr.2) and high-strength titanium alloys (e.g. Ti-6Al-4V, Gr.5).
Raw materials are usually supplied in the form of coils (wires) or rods. These materials are subjected to rigorous chemical composition and mechanical property testing to ensure compliance with standards.
2. Hot working (optional):
For some high-alloy or large-gauge titanium bars, hot working operations such as forging or hot extrusion may be required. This is usually done above the β phase change temperature in order to break up the as-cast structure, refine the grains, improve the density and mechanical properties of the material, and prepare for subsequent cold processing. After processing, a corresponding heat treatment (e.g. annealing) is required to relieve internal stresses.
3. Cold forming (cold heading/cold forging):
This is the core process for making the prototype of the bolt head and rod part. Titanium wires/bars are precisely cut to length (called "blanks").
The blank is fed into the multi-station cold heading machine, at room temperature or slightly higher temperature, through a series of continuous stamping of the die, the metal is plastically deformed by the huge pressure, and the head of the bolt (such as hexagonal head, round head, countersunk head, etc.) and rod part are gradually formed. This process requires extremely high mold precision and pressure control.
Key points: Titanium alloys have a strong tendency to harden in cold work, and their plasticity is poor compared to steel, so cold heading process parameters (such as deformation, speed, mold design) need to be carefully optimized, and intermediate annealing may be required to restore plasticity and prevent cracking.
4. Threading Processing:
Rolling (thread rolling): This is the most common, efficient and effective method of increasing thread strength. After cold heading, the smooth rod bolts are fed into the thread rolling machine and extruded under strong pressure by means of a pair of thread rolling plates (or rotating thread rolling rollers) with threaded tooth profiles. The metal is extruded and flows to form a thread profile. Rolling hardens the surface metal of the thread, which significantly improves the fatigue and tensile strength of the thread.
Turning: Precision turning threads are used for oversized sizes, special threads, or when rolling is not possible. Turning is not as good as rolling to improve the properties of the material.
5. Heat Treatment:
Annealing: For industrial pure titanium or bolts requiring high plasticity, recrystallization annealing is mainly carried out to eliminate cold working stress and improve plasticity, toughness and dimensional stability.
Solution treatment: The bolt is heated to a specific high temperature below the β phase zone, α+β the two-phase zone and insulated, so that as many alloying elements as possible are dissolved in the β phase (solid solution), and then rapidly cooled (usually water quenched) to obtain a supersaturated metastable β phase (martensite α' phase or supersaturated α phase).
Aging (precipitation strengthening): The solution-treated bolts are kept warm for a period of time at a lower temperature (usually in the range of 480-595°C). At this time, fine and diffuse α-phase particles are precipitated in the supersaturated phase, which hinder the dislocation motion, thereby significantly improving the strength and hardness of the bolt while maintaining a certain toughness.
The heat treatment process must be carried out in a tightly controlled atmosphere, usually a vacuum or an inert gas such as argon, preventing titanium from absorbing hydrogen, oxidizing and nitriding at high temperatures.
6. Surface Treatment:
Descaling/pickling: The oxide layer formed on the surface after heat treatment must be removed, usually by pickling (e.g. nitric-hydrofluoric acid mixture) or sandblasting.
Anodizing: In a specific electrolyte, the bolt acts as an anode to generate a dense oxide film (TiO₂) on the surface through an electrochemical reaction. This film significantly improves the abrasion resistance, corrosion resistance, insulation (galvanic corrosion resistance) of the surface, and is available in a variety of colors (e.g. blue, purple, gold, gray) for identification or aesthetics.
Coatings: Solid lubricating coatings (e.g. MoS₂, PTFE) or corrosion-resistant coatings may be applied in extreme abrasion or specific chemical environments.
Passivation: Treatment with nitric acid or nitric acid-dichromate solution to remove contaminants such as free iron ions on the surface, enhance the integrity and uniformity of the natural oxide film, and improve corrosion resistance.
7. Quality inspection and testing:
Strict dimensional tolerances (e.g. head size, thread accuracy, rod diameter, length) are carried out throughout the production process.
Tests are carried out on key mechanical properties of finished products: tensile tests (tensile strength, yield strength, elongation), hardness tests (Rockwell or Vickers hardness).
If necessary, metallographic examination, chemical composition review, non-destructive testing (e.g. ultrasonic, magnetic particle - special methods are required because titanium is non-magnetic).
For critical applications such as aerospace and medical, additional tests such as hydrogen content analysis, salt spray testing, stress corrosion cracking testing, fatigue testing, etc. are required.For more information, please contact catherine@hiriger.com.
