Ti and its Alloys & their Heat Treatments Presented by Professor Ali H. ATAIWI 1

Ti and its Alloys 2

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Classification of Ti Alloys 5

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Heat Treating of Titanium and Titanium Alloys Ti and Ti alloys are heat treated for the following purposes: ● To reduce residual stresses developed during fabrication (stress relieving ) ● To produce an optimum combination of ductility, machinability, and dimensional and structural stability (annealing) ● To increase strength (solution treating and aging ) ● To optimize special properties, such as fracture toughness, fatigue strength, and high creep strength. 13

*Various types of annealing treatments (single, duplex, beta, and recrystallization annealing, for example) and solution-treating- and-aging treatments are imposed to achieve selected property combinations. *Stress relieving and annealing may be employed to prevent preferential chemical attack in some corrosive environments, to prevent distortion (a stabilization treatment), and to condition the metal for subsequent forming and fabricating operations. 14

Stress Relieving Titanium and titanium alloys may be stress relieved without adversely affecting strength or ductility, depending on the alloy and specific heat treatment. *A stress relief of mill- or beta annealed Ti-6Al-4 V would not affect the properties, whereas a full stress relief solution treated and aged Ti-6Al-4V would result in a strength reduction. *On the other hand, beta alloys, such as Ti-10V-2Fe-3Al can be stress relieved at lower temperatures, and the aging temperature may be sufficient to also impart a stress relief 15

* Stress-relieving treatments decrease the undesirable residual stresses that result from no uniform hot forging deformation from cold forming and straightening, forging, welding, and asymmetric cooling following heat treatment (more severe with faster cooling rates). * Removal of such stresses helps maintain shape stability and eliminate unfavorable conditions, such as the loss of compressive yield strength commonly known as the Bauschinger effect ( The phenomenon by which plastic deformation increases yield strength in the direction of plastic flow and decreases it in other directions). * Table 18 presents combinations of time and mperature that are used for stress relieving titanium and titanium alloys., 16

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Annealing *The annealing of titanium and titanium alloys serves primarily to increase: - fracture toughness. - ductility at room temperature. -dimensional and thermal stability. - and creep resistance. * Many titanium alloys are placed in service in the annealed state. * Common annealing treatments include: - mill annealing. - duplex annealing. - recrystallization annealing. - and beta annealing. 18

* Recommended annealing treatments for several alloys arc given in Table 19. * Either air or furnace cooling may be used, but the two methods may result in different levels of tensile properties. * If distortion is a problem, the cooling rate should be uniform down to 315°C (600 o F). It may be difficult to prevent distortion of close-tolerance thin sections during annealing. 19

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A* Mill annealing is a general-purpose treatment given to all mill products. It is not a full anneal and may leave traces of cold or warm working in the microstructures of heavily worked products, particularly sheet. * Duplex annealing alters the shapes, sizes, and distributions of phases to those required for improved creep resistance or fracture toughness. * Recrystallization annealing and beta annealing are used to improve fracture toughness. * In recrystallization annealing, the alloy is heated into the upper end of the alpha-beta, held for a time, and then cooled very slowly. 21

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23 * Like recrystallization annealing, beta annealing improves fracture toughness. Beta annealing is done at temperatures above the beta transus of the alloy being annealed. To prevent excessive grain growth, the temperature for beta annealing should be only slightly higher than the beta transus. * Annealing times are dependent on section thickness and should be sufficient for complete transformation. Time at temperature after transformation should be held to a minimum to control beta grain growth. * Larger sections should be fan cooled or water quenched to prevent the formation of alpha phase at the beta grain boundaries

Solution Treating and Aging * Time/temperature combinations for solution treating are given in Table 20. * A load may be charged directly into a furnace operating at the solution-treating temperature. Although preheating is not essential, it may be used to minimize distortion of complex parts Quenching * The rate of cooling from the solution-treating temperature has an important effect on strength. If the rate is too low, appreciable diffusion may occur during cooling, and decomposition of the altered beta phase during aging may not provide effective strengthening.. 24

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Aging * The final step in heat treating titanium alloys to high strength consists of reheating to an aging temperature between 425 and 650 o C(800 and 1200 o F). * Aging causes decomposition of the supersaturated beta phase retained on quenching. * A summary of aging times and temperatures is presented in Table 20. The time/temperature combination selected depends on required strength. 26

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