Heat Treating

Heat treatments for titanium alloys include stress relieving, annealing, and solution treating and aging (STA). All titanium alloys can be stress relieved and annealed, but only the alpha-beta and beta alloys can be STA to increase their strength. Since alpha and near-alpha alloys do not undergo a phase change during processing, they cannot be strengthened by STA. Since the response to STA is determined by the amount of beta phase, the beta alloys, with higher percentages of beta phase, are heat treatable to higher strength levels than the alpha-beta alloys. In addition, the beta alloys can be through-hardened to thicker sections than the alpha-beta alloys.

One great disadvantage of heat treating titanium alloys is the lack of a nondestructive test method to determine the actual response to heat treatment. While

Inboard/Outboard Landing Gear Doors

4-Sheet SPF/DB

Fig. 4.23. SPF and SPF/DB Applications on F-15E Source: The Boeing Company

L/R Forward Engine Door

2-Sheet SPF/DB

Inboard/Outboard Landing Gear Doors

4-Sheet SPF/DB

Fig. 4.23. SPF and SPF/DB Applications on F-15E Source: The Boeing Company hardness may be used with steel alloys and a combination of hardness and conductivity for aluminum alloys, there are no equivalent methods for titanium. Therefore, if verification is required, actual mechanical property tests have to be conducted to determine heat treat response.

4.8.1 Stress Relief

Stress relief is used to remove residual stresses that result from mechanical working, welding, cooling of castings, machining, and heat treatment. Stress relief cycles may be omitted if the part is going to be annealed or solution treated. All titanium alloys can be stress relieved without affecting their strength or ductility. Like most thermal processes, combinations of time and temperature may be used with higher temperatures requiring shorter times and lower temperatures requiring longer times. Thicker sections require longer times to insure uniform temperatures. Either furnace or air cooling is usually acceptable. While the cooling rate is not critical, uniformity of cooling is important, especially through the 600-900° F range. The stress relief temperature for the near-alpha and alphabeta alloys is in the range of 850-1500° F. Ti-6-4 is normally stress relieved at 1000-1200° F with the time being dependent on the temperature used and the percent stress relief desired. At 8 h, 1000, 1100, and 1200° F will result in 55, 75 and

100% stress relief respectively. During stress relieving of STA parts, care has to be taken to prevent overaging to lower than desired strength levels.

4.8.2 Annealing

Annealing is similar to stress relief but is done at higher temperatures to remove almost all residual stresses and the affects of cold work. Common types of annealing operations for titanium alloys include mill annealing, duplex annealing, recrystallization annealing, and beta annealing.

As the name implies, mill annealing is conducted at the mill. It is not a full anneal and may leave traces of cold or warm working in the microstructure of heavily worked products, particularly sheet. For near-alpha and alphabeta alloys, this is the heat treatment normally supplied by the manufacturer. Mill annealing of Ti-6-4 can be achieved by heating to 1300-1440° F and holding for a minimum of 1 h. Beta alloys are not supplied in the mill annealed condition because this condition is not stable at elevated temperatures and can lead to the precipitation of embrittling phases.

Duplex annealing can be used to provide better creep resistance for high temperature alloys such as Ti-6242S. It is a two-stage annealing process that starts with an anneal high in the alpha + beta field followed by air cooling. The second anneal is conducted at lower temperature to provide thermal stability, again followed by air cooling.

Recrystallization anneals are used to improve the fracture toughness. The part is heated into the upper range of the alpha + beta field, held for a period of time and then slowly cooled.

Beta annealing is conducted by annealing at temperatures above the beta transus followed by air cooling or water quenching to avoid the formation of grain boundary alpha. This treatment maximizes fracture toughness at the expense of a substantial decrease in fatigue strength.

A summary of these different annealing procedures and their effects on properties are given in Table 4.3. With the appropriate use of constraint fixtures, operations such as straightening, sizing, and flattening can be combined with annealing. The elevated temperature stability of alpha-beta alloys is improved by annealing because the beta phase is stabilized.

4.8.3 Solution Treating and Aging

The purpose of the solution treatment is to transform a portion of the alpha phase into beta and then to cool rapidly enough to retain the beta phase at room temperature. During aging, alpha precipitates from the retained beta. STA is used with both alpha-beta and beta alloys to achieve higher strength levels than can be obtained by annealing. Solution treating consists of heating the part to high in the two-phase alpha + beta field followed by quenching. Solution treatments for alpha-beta alloys are conducted by heating to slightly below

Table 4.3 Effects of Different Anneal Cycles on Titanium Properties

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