Hot Working And Cold Working Of Titanium Alloys

Hot working and cold working, as the two core forming methods for titanium alloys, exhibit distinctly different process characteristics, performance advantages and application scenarios based on different temperature mechanisms and deformation principles.

 

 

The key difference between hot and cold working is the processing temperature relative to the recrystallization temperature. It directly determines the material's microstructure and properties.

 

Hot working: Conducted above the recrystallization temperature, it eliminates work hardening through dynamic recrystallization and enables easy forming.

 

Cold working: Conducted at room temperature or below the recrystallization temperature, it deforms via dislocation slip with obvious work hardening and no significant dynamic recrystallization.

 

 

(I) Hot Working

To realize the forming of large-sized billets, eliminate as-cast defects, optimize the microstructure, reduce deformation resistance and improve production efficiency.

 

Typical processes and characteristics

Main processes: hot forging, hot rolling, and hot extrusion.

 

Core control points

Environmental : Titanium alloys are prone to oxidation and hydrogen embrittlement at high temperatures. It requires processing in an inert or vacuum environment.

Temperature change: Strictly control the heating temperature, holding time and cooling rate to avoid coarse grains, insufficient plasticity or excessive internal stress.

 

Technical difficulties

Coordinately control "temperature, environment and deformation", and solve the problems of oxidation, hydrogen embrittlement, heat accumulation and part defects.

 

(II) Cold Working

To accurately control dimensional accuracy, improve strength and optimize surface quality through work hardening, and adapt to small-sized precision parts.

 

Typical processes and characteristics

Cold rolling: Refines the thickness of plates with a tolerance of up to ±0.05mm and improves dimensional accuracy.

Cold drawing: Produces high-precision titanium alloy wires and bars by drawing through molds.

Cold rotary swaging: Features a smooth surface and high precision, suitable for small-batch special-shaped bars, and can refine grains to improve strength.

 

Core control points

Deformation amount: The single-pass deformation amount is less than 15% to prevent material fracture.

Intermediate annealing: Annealing at 650-700℃ for 1-2 hours is required between multi-pass processing to eliminate stress and restore plasticity.

Surface protection: Strictly control the surface finish of tools; high-precision products require subsequent polishing to avoid scratches.

 

Technical difficulties

Overcome the plastic decline caused by work hardening to prevent cracking; accurately control the dimensional and residual stress, especially for high-strength titanium alloys, to balance strength and plasticity.

 

 

(I) Differences in Microstructure

Hot working: Dynamic recrystallization occurs at high temperatures, forming uniform equiaxed grains, eliminating work hardening and improving isotropy.

Cold working: No dynamic recrystallization occurs, with the accumulation of dislocation density; grains are elongated and refined, and some can form nanoscale grains and preferred orientation texture, thus improving strength.

 

(II) Differences in Macroscopic Properties

Mechanical properties: Hot working yields excellent comprehensive properties with a balanced combination of strength, plasticity and toughness, suitable for complex stress scenarios; cold working improves strength, hardness and wear resistance while reducing plasticity, suitable for high-strength, high-precision and single-stress scenarios.

 

Surface quality: The surface after hot working has an oxide layer and high roughness, requiring subsequent treatment; the surface after cold working is free of oxide layer and low in roughness with a smooth finish, without the need for complex subsequent treatment.

 

Dimensional accuracy: Hot working is affected by thermal expansion and contraction, resulting in low accuracy and large tolerance; cold working features a stable temperature, high accuracy and small tolerance, suitable for the production of precision parts.

 

 

1. Applications of Hot Working

Aerospace: Wing structures, aero-engine turbine disks, blades, etc. Complex forming is achieved through hot forging/isothermal forging to ensure uniform microstructure and high strength and toughness.

 

Marine engineering: Propellers, titanium alloy components for offshore platforms, which improve toughness and corrosion resistance to adapt to the marine environment.

 

Preparation of base billets: Titanium alloy ingots are made into plates, bars and tubes through hot forging and hot rolling to provide billets for subsequent processing.

 

2. Applications of Cold Working

Medical industry: Artificial joints, orthopedic implants, etc. Cold rolling/cold rotary swaging ensures accuracy, surface finish and strength to meet the requirements of biocompatibility.

 

Precision instruments: Precision gears, sensor housings, etc., achieving high-precision forming and matching stability.

 

High-end equipment: Precision components for new energy and intelligent equipment, improving strength and wear resistance and extending service life.

 

3. Hot-Cold Synergistic Processing

In production, titanium alloys are usually processed by hot working for forming and cold working for property improvement.Taking Gr5 titanium alloy plates as an example:

 

• Hot rolling at high temperature to refine as-cast defects

 

• Cold rolling for thickness reduction, with intermediate annealing to eliminate work hardening

 

• Final vacuum annealing for stress relief

 

This produces high-precision, high-performance plates that meet the strict requirements of aerospace and medical applications.

 

Ruihang Group mainly produces Titanium and Titanium Alloy products. For more details, please reach us to the email: Sam.Rui@bjrh-titanium.com