February 14, 2002
Lots of strength, lots of uses: Ti-6AL-4V, combined with the flowforming process can make sense for a wealth of industrial and consumer applications.
Flowforming is a cold metalworking process for manufacturing seamless tubular and rotationally symmetrical products.
In the process, a cylindrical workpiece, or preform, is attached to a rotating mandrel. Three CNC-controlled rollers compress the preform while moving along the length of it, which causes the metal to be deformed plastically in the axial direction along the rotating mandrel.
The result is a seamless, cylindrical product with increased mechanical properties and good surface finishes with repeatable accuracy on a part-to-part and a lot-to-lot basis.
Tubular shapes in the Ti-6Al-4V alpha-beta alloy now are in regular production in commercial quantities in conventional and extra-low-interstitial (ELI) composition grades.
Ring sections from production flowflormed Ti-6Al-4V (conventional chemistry) tubular parts were tensile-tested in a longitudinal direction (parallel to the axis of the tube). Pieces from two different mill lots were tested in the as-flowformed condition after they were vacuum-annealed at 1,300 degrees F for one hour and then argon-cooled.
Specimens were cut from a certain lot in the as-flowformed condition for micro-examination in both the longitudinal and transverse direction. These were identified as 61L and 61T. Photomicrographs were taken at magnifications of 400x and 800x. The microstructures show very fine primary alpha grains that are elongated along the axis of the tube in the direction of the metal flow from the flowforming process. A transformed beta surrounding the primary alpha grains also can be seen with a micrograph.
In one test, the material was vacuum-annealed at 1,300 degrees F for one hour and then argon-cooled to simulate annealing of a flowformed finished part. As expected, the annealing partially recrystallized the flowformed structure without completely eliminating the axial directionality produced in the flowforming process. The grain size is extremely fine-ASTM 10 or finer.
No internal cracking has been observed in any of the specimens examined to date by the tube's manufacturer. This, coupled with the absence of any surface cracking on the flowformed parts, demonstrates that the plastic deformation of flowforming processes was accomplished well within the limits of the ductility of the metal.
High-strength steels, titanium, zirconium, niobium, tantalum, nickel alloys, stainless steels, and other exotic metals can be flow-formed. Some of the current applications for which the flowforming process is used include:
1. Rocket motor cases for missiles-for flowforming's ability to form high-strength steels seamlessly and precisely.
2. Structural aerospace components-for the elimination of welds and the ability to form high-strength titanium and aluminum components.
3. Nuclear waste containers-for seamless construction and no welds.
4. Rollers for copier machines-for very balanced, concentric tubes; thin walls with work-hardened mechanical properties.
5. Food equipment cylinders-for closed-bottomed cylinders with no need for honing or welding.
6. Softball bat sleeves-for ability to form thin-walled, precise titanium tubes.
7. Bicycle frames-for high strength, lightness, and ability to form seamless, thin-walled Ti 6/4 tubes.
8. Cartridge cases for Navy battle ships-for ability to replace conventional deep-drawing, which had been used since World War II.
Rockwell C hardness test data for the two conditions (average of three readings) show HRC 35.7 for the as-flowformed state and HRC 33.6 for the flowformed-plus-vacuum-anneal-argon-cooled state.
Conventional chemistry Ti-6Al-4V now joins the wide range of available alloys in regular commercial production that are being used to flowform a variety of open- and closed-end tubular parts.
The plastic deformation of the flowforming process increases the strength while still maintaining good tensile ductility. Further, enhanced yield and tensile ductility levels in the mid-140-ksi and mid-150-ksi levels, with elongation in the upper teens, is produced by vacuum annealing at 1,300 degrees F for one hour and then cooling the material with argon.
Hence the flowforming process offers designer of tubular annealed parts made with this alloy an opportunity to incorporate the higher strengths -usually associated with a full solution and aging heat treatment-at a high level of tensile elongation.
Consultant George L. Durfee is a life member of ASM Intl. and can be reached at P.O. Box 168, Grafton, MA 01519, phone-fax 508-839-4689, e-mail email@example.com. Durfee holds a B.S. degree from Michigan Technological University and was a Tau Beta Pi Fellow at MIT. He pioneered the alpha-beta forging of Ti-6Al-4V at Wyman-Gordon Co. in 1955 and also pioneered and patented the use of electroless nickel coating of titanium forging blanks for minimizing surface cracking during forging. This process was especially useful for the structural forgings of the SR-71 Blackbird spy plane in the 1960s.