Heat treatment of titanium
by Eric Guay
(Cobbs Creek VA)
Q : Is it better to heat treat and then machine TI or machine and then heat treat?
A : In order to understand the heat treatment of titanium, you’d have to understand why manufacturers consider heat treatment in the first place and what properties of titanium are affected by heat treatment.
Also, it is worthwhile noting that the response of titanium and titanium alloys to heat treatment depends on the composition of the metal and the effects of alloying elements on the α-β crystal transformation of titanium. In addition, not all heat treating cycles are applicable to all titanium alloys, because the various alloys are designed for different purposes.
• Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr and Ti-6Al-2Sn-4Zr-6Mo are designed for strength in heavy sections.
• Alloys Ti- 6Al-2Sn-4Zr-2Mo and Ti-6Al-5Zr-0.5Mo-0.2Si for creep resistance.
• Alloys Ti-6Al-2Nb-1 Ta-1Mo and Ti-6Al-4V, for resistance to stress corrosion in aqueous salt solutions and for high fracture toughness.
• Alloys Ti-5Al-2.5Sn and Ti-2.5Cu for weld ability;
• Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.
In order to better understand the types of titanium alloys, check the ASTM titanium grades list . The most widely used form of titanium is grade 2, with moderate strength and excellent corrosion resistance and formability. Grade 2 titanium is slightly weaker than Grade 3, but stronger than Grade 1 and equally resistant to corrosion. These properties account for Grade 2 titanium’s versatility in regard to its applications.
Overall, there are a lot of factors that influence heat treatment in titanium mental,
but, basically, heat treatment alters titanium ductility and/or strength and toughness.
Generally, titanium is subjected to heat treatment in order to achieve the following:
1. Relief of stresses set-up during cold forming or machining.
I think this is kind of self-explanatory in the short version. The long version is that titanium ductility decreases while being subjected to machining operation, and, depending on the requirements of the particular application titanium is used in, a machining subsequent heat treatment can restore or enhance ductility properties to the desired levels.
This type of treatment is usually employed in primary fabrication of titanium, when titanium ingots are transformed into titanium plates or sheets. The temperatures applied typically range from 650*F. to 1000*F, while the time span varies from a few minutes for a very thin sheet to an hours or more for heavier sections. Typically, we’re talking about 30 minutes sessions at 900*F, followed by a cooling phase.
2. To enhance maximum titanium ductility before cold working procedures.
Cold working efficiency is directly related to the ductility of titanium metal, prior to the start of the process. That’s why in some situations, heat treatment is required before the actual machining begins, in order to achieve the desired results.
3. Thermal hardening to improve strength and toughness.
Obviously, if this is the case, then the heat treatment is done after all other machining and forming of titanium. This procedure is commonly used in titanium alloy aircraft components, and the additional strength usually comes at some sacrifice in ductility.