Why Titanium Distorts and Springs Back
Titanium alloys have an elastic modulus of roughly 100 to 114 GPa, about half that of steel. For the same applied moment, a titanium bar therefore bends about twice as far elastically, and when the load is released it recovers roughly twice as much of the deflection. That elastic recovery is the springback that makes straightening counter-intuitive: the ram must travel well past the point at which the part looks straight.
The distortion itself comes from upstream processing. Asymmetric cooling after forging or extrusion, uneven reduction across the width in rolling, and localised heating during welding all lock residual stresses into the part. These stresses do not necessarily show up on the dimensional report, because they only bow the part once subsequent machining removes material from one side and unbalances the stress field.
Three-Point Reverse Bending Mechanics
In the classic setup the workpiece rests on two supports and a punch applies load at mid-span. The outer fibres yield first, the plastic zone grows inward as the punch descends, and the material that remains elastic stores the energy that drives springback. Three quantities govern the result:
Support span (L). A long span bends gently and spreads the plastic zone; a short span localises strain and risks surface cracking.
Deflection at yield. Proportional to yield strength times span squared, divided by modulus times section depth. Because titanium has a low modulus, this deflection is large.
Overbend factor. The ratio of the applied deflection to the deflection that would just produce yielding. Typical practice for titanium runs 2 to 4, compared with 1.2 to 2 for carbon steel.
For round bar the deflection at which the outer fibre reaches yield is approximated by d = (sigma_y * L squared) / (6 * E * D), with sigma_y the yield strength, E the modulus, L the span and D the diameter. For a 50 mm Grade 5 bar with a 275 mm span and a 900 MPa yield strength, this yields a purely elastic deflection of only a few millimetres, which is why practical overbending is expressed as a multiple of a small number and demands fine ram control.
Equipment and Process Parameters
| Element | Practical setting | Notes |
|---|---|---|
| Press type | Hydraulic press with position control, or a multi-roll straightener | Position control is essential; force control cannot resolve springback |
| Punch radius | 8 - 15 times the local section thickness | Tight radii generate tensile strains that initiate cracks |
| Support span | 6 - 12 times the section depth | Longer span gives gentler, more uniform plasticising |
| Ram speed | Slow, with a dwell at full stroke | Dwell lets plastic flow continue and reduces elastic snap-back |
| Overbend | 2 - 4 times the yield deflection | Established empirically for the grade and section |
| Temperature | Ambient for thin bar, 400 - 600 C for heavy or high-strength sections | Warm straightening lowers required overbend and reduces cracking risk |
Multi-roll straightening is preferred for long bar and tube because it applies many small alternating bending cycles instead of one large one. Each pass imposes a decreasing curvature, and the accumulated plastic strain leaves the part straighter than any single three-point stroke could achieve, with a much lower risk of surface damage.
Cold Versus Warm Straightening
Alpha-beta alloys such as Ti-6Al-4V have limited room-temperature ductility and are notch sensitive, so cold straightening of heavy sections is risky. Localised tensile strain at the contact points can nucleate cracks that are invisible under a straightedge but fatal in fatigue service. Warm straightening at 400 to 600 C lowers the flow stress, reduces springback, and lets strain distribute over a longer arc. Two cautions apply: heating must be uniform and controlled, because titanium absorbs oxygen above roughly 600 C and forms a hard, brittle alpha case; and the warm operation should be followed by a stress relief rather than a rapid cool.
Residual Stress Relief and Verification
Straightening redistributes rather than eliminates residual stress, so a stress relief anneal is normally specified after the geometry is set. For commercially pure grades, 540 to 650 C for one to three hours in vacuum or inert atmosphere is typical; for Ti-6Al-4V the treatment is usually in the same window, staying well below the beta transus so the microstructure is unaffected. Straightness is then verified with a dial gauge on a surface plate or with a laser alignment system, and the acceptance limit is stated as deviation per unit length, for example 1 mm per metre for bar and a tighter figure for tube.
Common Defects and Misconceptions
"Straightening removes residual stress." It redistributes it. Machining after straightening can release the remaining stress, so the sequence matters.
"More force gives a straighter part." Excessive single-stroke deflection creates local plastic hinges and can reverse the bow when the part is later machined.
"Cold straightening is safe for any grade." Not for heavy, high-strength, or notched sections; warm straightening or multiple-roll passes are safer.
"Ovality in tube is normal." Flattening beyond the tolerance of the product standard indicates that punch radius or span is too small, not that the material is at fault.
"A crack would be visible." Titanium cracks at straightening contacts often stay shallow and tight, so fluorescent penetrant inspection is used where the part is safety critical.
FAQ
Q: Why is titanium harder to straighten than steel?
Because its elastic modulus is roughly half that of steel, so about twice as much of the bending deflection is recovered elastically and the overbend factor is correspondingly larger.
Q: What is the overbend factor?
It is the ratio of applied deflection to the deflection that just yields the outer fibre. Titanium practice typically uses 2 to 4, established by trial on representative sections of the same grade and temper.
Q: Can titanium tube be straightened cold?
Thin-wall tube is commonly straightened on multi-roll machines at ambient temperature, but heavy walls and high-strength grades are better handled warm at 400 to 600 C to avoid cracking and ovality.
Q: What temperature is used for stress relief after straightening?
Roughly 540 to 650 C for one to three hours in vacuum or inert atmosphere, always below the beta transus so that the microstructure and strength level are preserved.
Q: Does straightening change mechanical properties?
It introduces plastic strain, which raises yield strength slightly and reduces ductility in the worked zones. Where properties are certified, straightening is performed before the final anneal.
Q: How is straightness measured?
Normally as total indicated runout on a surface plate with a dial gauge, or with laser alignment for long product, expressed as deviation per unit length against a limit agreed in the purchase specification.
