What Rolling Contributes to Titanium Alloy Production
Rolling is the main deformation route that turns a titanium ingot or forged billet into plate, sheet, strip, bar and wire rod. Titanium alloys are harder to roll than steel: the hexagonal alpha phase deforms anisotropically, the forming window is narrow, and the deformation resistance is high, so the pass schedule, reheating practice and roll condition all influence the final structure and surface. The process is therefore matched to the product: heavy sections such as plate and large bar go through hot rolling, while high-precision thin sheet and strip are finished by cold rolling.
Strip, plate and sheet are normally ordered against ASTM B265, bar and rod against ASTM B348, and the delivered product is certified for chemical composition and mechanical properties. Because rolling changes texture and grain flow, the rolling direction relative to the component axis has to be specified on the drawing, especially for parts that will be bent or formed afterwards.
Hot Rolling
Hot rolling is a rolling process carried out above the recrystallisation temperature of the metal material. Working above the recrystallisation temperature significantly reduces the deformation resistance of the material and improves rolling efficiency, which is why it is the standard route for the production of large-size titanium alloy plates and bars. Reheating between passes restores workability, and the finishing temperature determines the grain structure that reaches the customer. Scale and oxygen-enriched surface layers formed during reheating are removed by pickling or conditioning before the next operation.
Cold Rolling
Cold rolling is carried out at room temperature and improves the surface quality and dimensional accuracy of the material, but it places higher demands on the plasticity of the alloy. It is normally used for the production of high-precision thin plates and strips, where thickness tolerance and flatness are the critical acceptance criteria. Because the material work hardens during cold reduction, intermediate annealing is scheduled between rolling stages to restore ductility, and roll finish and lubrication have a direct effect on surface defects.
Reversible Rolling and Near-Isothermal Rolling
Reversible rolling means that the rolled piece passes through the rolls several times, and the direction of travel is reversed after each pass. Reversing the direction of each pass reduces the anisotropy introduced by a single-direction pass schedule, which is important for titanium alloys whose properties are strongly orientation dependent.
Near-isothermal rolling is a newer concept in which the amount of deformation, the deformation rate and the rolling cycle are controlled so that the workpiece stays close to a uniform temperature throughout the pass. Keeping the temperature uniform limits the temperature gradient between the surface and the centre of the section, raising both rolling efficiency and the consistency of material properties.
Flexible Rolling, One-Heat Routes and Mill Equipment
Conventional fixed rolling schedules are not efficient for the full range of titanium products, so flexible rolling is used. Flexible rolling allows different rolling hole patterns and rolling regimes to be used from the same mill, so the schedule can be adapted to different material properties and production requirements without changing the whole line.
A high-efficiency one-heat route, in which the material goes from the reheating furnace through the full deformation sequence, has been applied to aerospace-grade titanium alloy production and has also been used to increase the weight of small-size wire rod coils.
| Equipment | Characteristic | Typical duty |
|---|---|---|
| BDM850 fast traverse reversible rolling mill | Shortest pass intervals and high rolling efficiency | Large section titanium alloy rolling |
| Stepping mill | Stepped pass design for controlled reduction | Bar of defined size ranges |
| Spiral mill | Rotational pass geometry | Bar and rod production |
| Y-type mill | Three-roll arrangement with low spread | Precision bar |
| Planetary mill | Large reduction in a single pass | Bar and billet reduction |
The development of titanium alloy rolling technology continues to drive progress in material processing. Through improvements to both the rolling process and the rolling equipment, the comprehensive performance of titanium alloy material is raised to meet the requirements of the high-end market.
Frequently Asked Questions
Q: Why is hot rolling used for large titanium plates and bars?
Hot rolling is carried out above the recrystallisation temperature, which sharply reduces deformation resistance and improves rolling efficiency, so large sections can be reduced in fewer passes.
Q: When is cold rolling chosen instead of hot rolling?
Cold rolling is used when surface quality and dimensional accuracy matter most, typically for high-precision thin plates and strips, although it demands better plasticity from the material.
Q: What is reversible rolling?
It is a process in which the rolled piece passes through the rolls several times and the direction of travel is reversed after each pass, which reduces the anisotropy of the material.
Q: What does near-isothermal rolling achieve?
By controlling the amount of deformation, the deformation rate and the rolling cycle, the workpiece stays close to a uniform temperature, which improves both rolling efficiency and material properties.
Q: What is the advantage of flexible rolling?
Different rolling hole patterns and rolling regimes can be used to suit different material properties and production requirements.
Q: Which mills are used for titanium bar production?
Stepping mills, spiral mills, Y-type mills and planetary mills each have their own characteristics and are selected for different bar specifications, while the BDM850 fast traverse reversible mill is used for large sections.
