Forming commercially pure titanium sheet
At room temperature titanium 's hexagonal crystal lattice has only one slip direction, and mechanical twinning also results in only slight slipping. For this reason, titanium materials can only be cold formed within set limits. The commercially pure titanium materials no. 3.7025, 3.7035 and 3.7055 are cold formable. For 2 to 5% reductions it is recommendable to heat materials 3.7035 and 3.7055 to between 200°C and max. 500°C. Higher reductions should always be performed in this temperature range, and at temperatures over 650°C for material 3.7065.
Compared with other common forming materials, the ratio of yield point to tensile strength Rp0,2 to Rm is relatively high. A high degree of elasticity is therefore to be expected, resulting in pronounced springback, especially after cold working.
For material 3.7025 the springback angle after cold working is around 5 to 8°.
This gives rise to residual stress in the forming zone composed of residual tensile and compressive stresses. Loading or heating the component may result in creep-induced deformation.
The residual stresses caused by cold working can be alleviated by stress relieving. Soft annealing at approx. 700°C (above recrystallization temperature) also reverses work hardening. After reductions in excess of 5%, it is only possible to dispense with soft annealing if the mechanical properties specified in DIN 17860 are demonstrably met. Material 3.7065 must always be soft annealed after forming. Heat treatments are governed by the DIN 65084 standard.
CP titanium grade 3.7025 is the only titanium material suitable for deep drawing. The harder CP titanium grades require greater radii and higher forces for bending, spinning, flanging etc. Sheet thickness must be taken into account when selecting the forming method. Guideline values for minimum bend radii are provided in DIN/NL 9003 part 3. This standard specifies the minimum bend radii for a bend angle of 90° as a function of material grade and sheet gage. Failure to observe the recommended minimum bend radii may result in surface cracks on the formed component and even to fractures when heavy reductions are involved.
Forming results are greatly enhanced by good and uniform lubrication. When deep-drawing and stretch-forming sheets, coating with a deep-drawing film permits higher drawing ratios without material failure.
Forming speeds should be kept as low as possible.
Uniformity and duration of preheating (approx. 0.5 - 1 hour) are important for hot working.
As hydrogen, oxygen and nitrogen absorption occurs from approx. 200°C, rendering the material brittle, suitable protective measures must be taken.
When forming by hand, the workpiece can be preheated, e.g. using a slightly oxidizing flame.
Minor discoloring can be removed by pickling in an acid mixture (HNO3/HF).
When forming on hydraulic presses the die must be pre-warmed, as generally the die has a considerably greater mass than the workpiece. In addition, dies should have adequately rounded edges and a polished surface.
Proven lubricants for sheet forming at room temperature include special deep-drawing foils, and at higher temperatures colloidal graphite as well as the commonly used hot pressing greases with graphite or molybdenum sulfide additives.
Further information on forming titanium and its alloys can be found in the publications listed below:
- H. Wilhelm "Das Umformverhalten von Blechen aus Titan und Titanlegierungen im einachsigen Zugversuch und beim Tiefziehen" Bänder, Bleche, Rohre 16 (1975), p. 421/26 and pp. 473/76
- H. Wilhelm "Das Verhalten von Blechen aus Titan und Titanlegierungen beim Biegen im V-Gesenk" Bänder, Bleche, Rohre 7 (1976), p. 284/88
- H.W. Wagner, H.J. Engel "Beitrag zum Kaltfließpressen von Titan" VDI-Fortschr.-Ber. VDI-Reihe 2, No. 101, (1985)
- K. Kutzsche, W. Hennig "Umformfestigkeit von Titan" Neue Hütte 35 (1990), p. 149/51
Deutsche Titan, Nov. 2000