Titanium And Titanium Alloys - Forming And Formability Of Titanium And Titanium Alloys
Forming Techniques and Equipment
Titanium and titanium alloys can be formed using techniques and equipment suitable for steel. However they possess lower stretch formability and require more generous bend radii.
Overcoming Galling and Springback
The tendency to galling is significantly greater than when forming stainless steel and can be overcome by hot forming. There is a problem with springback and the great majority of formed titanium components are made by hot forming or cold forming followed by hot sizing to overcome this problem.
The Effect of Oxygen Content on Formability
Formability of commercially pure titanium sheet and strip is dependent on oxygen content and the purer grades containing less oxygen are preferred due to better working properties. Springback is more difficult to predict than with steel and varies depending on the yield strength of the material from heat to heat. A high ratio of yield to tensile strength generally results in springback.
Forming Titanium Sheet and Strip
It is important that sheet or strip used for forming is flat, of uniform thickness, and free from surface contamination. The metal is notch sensitive and areas affected by nicks, scratches or tool marks etc. need to be repaired by sanding with 180 grit emery. Surface oxides or scale present difficulty in forming and must be removed by pickling or fine grit blasting.
The Importance of Cleanliness
Grease, oil, stencils and fingerprints, dust and all chemicals or residues containing hydrogen must be removed from titanium before any heating operation. Salt residues on the surface of the workpiece can lead to stress corrosion problems at temperatures above 320°C. Even the salt from a fingerprint can lead to problems. For this reason, titanium is often handled with cotton gloves after cleaning.
Hot Forming and Contamination
When titanium and titanium alloys are heated for forming to temperatures above 540°C, contamination becomes a problem and the contaminated surface must be removed by chemical means and vacuum treatment for hydrogen removal as for hot forging.
Hot Forging and Mechanical Properties
A good knowledge of the structural changes which occur in titanium and titanium alloys is necessary in order to maintain mechanical properties of titanium components. Some hot forging temperatures may be high enough to age titanium alloy and heat treatable alpha plus beta alloys and beta alloys must be re-heat treated after hot forming.
Because of these microstructural problems plus scaling and embrittlement, hot forming is usually carried out at the lowest temperature which will produce the desired result. When maximum formability is required, forming is carried out at the highest temperature practical that will retain mechanical properties.