Bending characteristics of different grades of titanium alloys
The bending characteristics of different grades of titanium alloys vary significantly, depending on their alloy type (α, near-α, α-β, β), microstructure and mechanical properties (especially yield strength, elastic modulus, elongation and work hardening rate). Understanding these properties is critical to successful bending and avoiding cracking, excessive springback, or inaccurate shapes.
So, how to achieve titanium alloy bending processing? It is necessary to understand the core bending characteristics of different grades of titanium alloys (rebound, minimum bending radius, temperature sensitivity), and combine the specific application requirements (strength, shape complexity, cost) for material selection and process design, which is the key to the successful bending of titanium alloys.
Bending characteristics of titanium alloy grades
01.Industrial Pure Titanium (CP Ti: Gr1, Gr2, Gr3, Gr4)
Type: α alloy
Characteristics: Best bendability: CP titanium has the best bending performance of all titanium alloys. It has the lowest strength (Gr1 is the softest, Gr4 is the hardest) and the ductility is the highest.
Low rebound: The relatively low yield strength and elastic modulus mean that the rebound is smaller than that of high-strength titanium alloys, making it easier to control the final shape.
Low cracking tendency: High ductility makes it not easy to crack under a large bending radius.
Minimum bending radius: It is usually possible to achieve a relatively small bending radius (for example, for thin plates, the minimum inner radius R of 90° bending can be 1-2 times the thickness of the plate t).
Applications: Chemical, marine, medical implants (Gr2, Gr4), consumer goods and other fields that require corrosion resistance and good formability.
02.Ti-3Al-2.5V(Gr9)
Type: Near α Alloy
Properties: Good bendability, with about 50% higher strength than Gr2, while still maintaining good ductility. Bendability is between CP titanium and Ti-6Al-4V.
Moderate Springback: Springback is greater than CP titanium but less than Ti-6Al-4V.
Minimum bending radius: Slightly larger than CP titanium but still better than most α-β alloys. For example, R≥2t may be required.
Application: Aviation hydraulic tubes, bicycle frames, sports equipment (balancing strength, formability, and weight).
03. Ti-6Al-4V(Gr5)
Type: α-β alloy (most commonly used)
Characteristics: Medium to poor bending: This is the most widely used titanium alloy, but it is also one of the most challenging common grades of bending.
High rebound: High yield strength and high elastic modulus lead to great rebound. The mold design must be significantly compensated (cornering), otherwise it will be difficult to achieve the target angle. The resilience may be several times that of mild steel.
High cracking tendency: relatively low ductility (especially compared with CP titanium).
Sensitive to bending radius: A larger minimum bending radius is required. For 90° bending, the minimum inner radius R≥3t (plate thickness) is usually required, and even R≥4t or greater, especially when the bending direction is perpendicular to the rolling direction. Trying a smaller radius can easily cause cracking on the outside.
Sensitive to surface defects: scratches, gaps, etc. can become sources of cracks.
Work hardening: The work hardening rate is high, and multiple bends or small-step bends will increase the risk of cracking.
Temperature effect: Heating (150-300°C) can significantly improve its bending performance：
Reduce the rheological stress and reduce the required bending force.
Improve ductility and allow a smaller bending radius (which may be reduced to R≥2t).
Reduce the amount of rebound.
Thermal bending is a common method for machining Ti-6Al-4V complex shapes or small radii.
Application: Aerospace structural parts, engine parts, high-performance automobiles, medical implants, military industry and other fields that require high strength, good fatigue performance and medium heat resistance.
04.Ti-6Al-4V ELI(Gr23)
Type: α-β alloy (ultra-low interstitial version of Ti-6Al-4V)
Characteristics: Basic characteristics similar to Gr5 (high rebound, large bending radius required, easy to crack).
Slightly better bending properties: lower oxygen, nitrogen, iron content makes its ductility slightly better than standard Gr5. This means that under the same conditions, the cracking tendency may be slightly lower and the allowable bend radius may be slightly reduced (but still much larger than CP titanium).
The rebound is still strong.
Applications: Mainly used in areas requiring extremely high fracture toughness, such as surgical implants, cryogenic containers. Bending properties have improved only marginally and caution is still required.
05. β alloy (such as Ti-15V-3Cr-3Al-3Sn, Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C), Ti-5Al-5V-5Mo-3Cr)
Type: β alloy (usually formed in the solution-treated state (ST))
Characteristics: Excellent cold bending properties (in the solution state): This is one of the biggest advantages of β alloy.
Very low yield strength/high ductility: The strength is low in the ST state, and the ductility is very high (up to 20%+).
Very small minimum bending radius: In the ST state, a very small bending radius can be achieved, even close to R=0.5t (depending on the specific alloy and thickness), which is far better than α-β alloys.
Low rebound: Low yield strength means that the rebound is relatively small.
Low cracking tendency: High ductility makes it not easy to crack under severe bending.
Key point: Aging hardening is required after forming: Bending is usually carried out in a soft state (ST) to obtain the best formability. After forming, aging treatment must be carried out to achieve the required high strength. Aging treatment will lead to dimensional changes (shrinkage), which must be considered in mold design and process planning.
High cost: The cost of raw materials is usually higher than that of Ti-6Al-4V.
Application: Mainly used for aerospace structural parts, springs, etc. that require extremely complex shapes, small bending radii, or high cold formability. Use its excellent cold formability to process complex parts, and then obtain high strength through aging.
