Forging Materials: Titanium Alloys
Titanium is the main constituent in titanium alloys, of course, but they can contain a significant amount of other elements, which are added for a variety of metallurgical reasons. The strength of titanium alloys can often be comparable to steel, but they have the advantage of having only about 60% of the weight. The lower density of titanium allows these alloys to be used in applications where lower weight is advantageous.
Like iron, pure titanium has two solid crystalline forms. At low temperatures, the crystalline phase is called alpha, and it has a hexagonal closed packed (HCP) structure. At high temperatures, the solid phase is called beta and has a body centered cubic (BCC) structure. The temperature at which the solid becomes fully beta is called the beta-transus temperature. With the additions of alloying elements to titanium, the beta-transus temperature can vary from about 1250°F to about 1925°F, depending on the specific composition of the alloy.
Chemistry and Grades
Titanium comprises 70-100% of the composition of the titanium forging alloys. The prime elements that are added to titanium are: aluminum (0-6%), which stabilizes the alpha phase; tin (0-6%), which also stabilizes the alpha phase; vanadium (0-13%), which is a beta-phase stabilizer; molybdenum (0-11%), which also stabilizes the beta phase; and chromium (0-11%), which stabilizes the beta phase. Of all of these alloying elements, only aluminum contributes to lowering the density of the alloy. All the other common alloying elements cause the density to increase, and a small price is paid for their usage in terms of added weight. The beta transus for pure titanium is 1675°F. The alpha-stabilizing elements cause an increase in the beta-transus temperature, whereas the beta-stabilizing elements cause the beta transus to decrease.
Titanium alloys are often classified into three major categories: alpha/near alpha, beta/near beta and alpha plus beta. Each of these categories requires special forging temperatures and considerations.
The most common titanium alloys are commercially pure titanium (Ti-CP), which is in the alpha classification, and titanium with 6% aluminum and 4% vanadium (Ti-6Al-4V or just Ti-6-4), which is an alpha-plus-beta alloy. Unlike steels and aluminum alloys, there is no systematic grade designation for titanium alloys, which are often referenced as Ti- followed by a string of numbers. The numbers indicate the weight percent of various alloying elements. For example, Ti-13-11-3 is titanium with 13% vanadium, 11% chromium and 3% aluminum, which is a beta alloy with a beta-transus temperature of 1250°F.
Figure 1. Ti 8Al-1Mo-1V (near alpha) alloy forged at 1650°F (below normal forging temperature). White is alpha phase, black is beta phase. Used with permission from ASM Handbook, Volume 2, 1990, pp. 592-633.
Figure 2. Ti 8Al-1Mo-1V (near alpha) alloy forged at 1840°F (normal forging temperature). White is alpha phase, black is beta phase. Used with permission from ASM Handbook, Volume 2, 1990, pp. 592-633.
The microstructure of many titanium alloys is a mixture of alpha and beta phases. The forging temperature and the cooling after forging have a strong influence on the morphology of the two phases within the microstructure. For example, Figure 1 shows a Ti-8Al-1Mo-1V alloy that was forged at 1650°F (below the normal forging temperature for this alloy) and cooled in air. The beta transus for this alloy is 1900°F. The white phase is alpha, and the black phase is beta. Note that the alpha is dominant under these conditions. Figure 2 shows the same alloy forged at 1840°F and air cooled. Note that the major phase under these conditions is beta. Figure 3 shows the same alloy forged at 2000°F with very rapid air cooling displaying the major beta phase, but the alpha phase is elongated into what is often called a basket-weave structure. The forging temperature in this case was above the beta transus, and the alpha phase occurred in the structure during the rapid cooling. Each forging condition produces different microstructures and hence different properties in the forged component.
|Figure 4. Titanium alloys are used for a wide range of critical-service and high-performance components. High strength and low weight are critical for aircraft structures. The ability to withstand relatively high temperatures makes titanium ideal for the fan and compressor sections of aircraft turbine engines. Corrosion resistance is important for energy and biomedical applications. High-end sporting goods are an excellent application.|
Forging of Titanium Alloys
In general, beta alloys are easier to forge than the alpha-plus-beta alloys and alpha alloys. Initial forging temperatures for ingot breakdown are higher than the intermediate forging temperatures, which are higher than the finish forging temperatures. What is referred to as conventional forging occurs at temperatures below the beta transus, usually in the alpha-plus-beta region of the alloy. Beta forging occurs at temperatures above the beta transus.
Titanium alloys are more challenging to forge than most steel and aluminum alloys due to the required process control. Control of the forging temperature is essential to achieve good forgeablity and to produce the microstructure required to achieve mechanical properties in service. Temperature is not limited to the furnace setpoint but must account for adiabatic heating, die chill and heat loss to the environment. Titanium can rapidly soften at high temperatures and strain rates, resulting in (sometimes severe) flow localization. This type of defect can appear to be a lap or even a forging crack. Thus, it is common to forge titanium at lower strain rates on hydraulic or screw presses. Hammer forging titanium takes extreme care and is uncommon for the most critical applications. Figure 5 shows the range of temperatures for the forging of titanium alloys.
Figure 5. Typical temperature ranges for titanium alloys are shown for service life, forging and heat treatment.
Titanium will lose heat to cold dies and the environment faster than steel due to the lower heat capacity and lower density. Thus, best practice in titanium forging requires heated dies or isothermal forging. These forging process are more costly but can provide a better forged product of this high-value material. Hot-die forging and isothermal forging allow a component to be forged closer to its final shape, resulting in lower machining cost and waste of this expensive material. Figure 6 shows the flow stress of a typical titanium alloy.
Dies may wear quickly when forging titanium, especially when near-net-shape features are forged. When forging titanium above 1100°F in air, a surface-oxide scale can form, and the oxygen can diffuse into the component. This surface condition is called alpha case, and it should be removed before the component can be put into service. In many alloys, alpha case has lower ductility and is subject to moderate to severe surface cracking. A range of coatings are commercially available to assist in lubrication and to create an oxygen barrier. Control of the coating process, including preheating and surface cleaning, is one more intricacy to be considered, or the coating may be rendered useless.
Processing of Titanium Alloys
Figure 6. Typical flow stress for titanium alloys exhibits significant softening and strain-rate sensitivity in the hot forging range. The dashed lines represent higher strain rates. Lower ductility is observed near or at room temperature.
In this second article on nonferrous forging materials, we have explored titanium alloys. Like aluminum, these types of alloys are used in components where high strength and low weight is important, frequently with corrosion resistance and tolerance to high operating temperatures. Titanium is a high-performance and versatile material used in numerous critical-service applications. Commercially available alloys of titanium must be forged with care. We will explore the forging of superalloys in our next article.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure the nation’s forging industry is positioned for challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the FIA Theory & Applications of Forging and Die Design course by Scientific Forming Technologies Corporation.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or email@example.com.