Titanium rods, bars, wires and tubes often exit rolling, drawing or extrusion with wavy shape defects and residual curvature. Straightening restores straightness so that downstream machining, cutting and final inspection can proceed reliably. Two complementary processes are used: tension (stretch) straightening and multi-roller straightening, frequently preceded by pressure straightening for severely bent pieces.
Tension (Stretch) Straightening
Tension straightening applies a longitudinal tensile stress above the yield strength of the material, producing plastic extension that corrects shape defects. The mechanism is straightforward:
Along the length of a wavy bar, some zones carry residual tensile stress (short, "extended" regions) and others carry residual compressive stress (long, "loose" regions).
When a uniform tensile load is superimposed, the actual deformation stress is reduced in the tensile-stressed zones (less plastic extension) and increased in the compressive-stressed zones (more plastic extension).
The result is that every part of the bar receives approximately the same total extension, and the wavy defect is eliminated.
Extension is normally controlled within a small range (typically 0.5%–3%, depending on alloy and section) to avoid over-stretching. Because titanium has a low elastic modulus and high springback, tension straightening is particularly effective for small-diameter wire, thin-wall tube and bar with gentle waviness.
Pressure Straightening
For workpieces with large curvature, pressure straightening is performed first: the piece is bent in the opposite direction between supports so that, after springback, it becomes approximately straight. The straightening pressure depends on the alloy strength and the degree of bending - higher-strength alloys and larger bends require greater pressure. Pressure straightening is usually followed by roller straightening to refine the result.
Multi-Roller Straightening
Products with simple cross-sections are commonly straightened on multi-roller machines with more than four rolls (typically 5–29 rolls). The rolls repeatedly apply three-point bending along the length, gradually reducing the variation of residual curvature until the product is straight. Basic operating principles:
Smaller roll diameter and more rolls give higher straightening accuracy. A small roll pitch helps the workpiece enter the rolls and stabilizes the process.
The first few rolls mainly reduce the difference in residual curvature along the length; the final rolls smooth the curvature toward uniformity.
Quality depends mainly on the anti-bending (overbend) rate. The second and third rolls are set with a large anti-bending rate; subsequent rolls are set so that each straightens the maximum residual curvature left by the preceding rolls.
Harder materials (higher strain-hardening coefficient n) are more difficult to straighten and need a larger anti-bending rate, more rolls and smaller roll diameters.
Titanium-Specific Considerations
High springback: titanium's elastic modulus (about 105–120 GPa) is roughly half that of steel, so elastic recovery after bending is large. Roller settings must compensate with significantly higher overbend than steel.
Surface protection: titanium surfaces are easily scratched and galled. Rolls should be smooth, well lubricated, and the machine set to avoid surface marking, which can initiate fatigue cracks.
Section and grade effects: straightening force and roll settings must be adjusted for alloy strength and cross-section; alpha-beta alloys such as Ti-6Al-4V require higher forces than commercially pure grades.
Residual stress: straightening introduces a controlled residual-stress pattern. For critical applications, follow straightening with a stress-relief anneal to stabilize dimensions during machining.
Typical Process Sequence
Incoming inspection: measure straightness, ovality and surface condition.
Pressure straightening for large-curvature pieces (if required).
Tension straightening for wire, thin-wall tube and small bars; multi-roller straightening for bars and tubes with simple sections.
Straightness verification against the tolerance in the applicable standard (e.g., ASTM B348, B861, B338).
Optional stress-relief annealing for dimensionally critical products.
