What Are Cast Titanium Alloy Types, Properties, and Defects?

Cast titanium alloys are grouped by annealed microstructure into α, α+β, and β types. Their useful properties include low density, high specific strength, and corrosion resistance, while casting behavior is governed by melt fluidity, solidification shrinkage, and reactivity. This guide compares the material groups, composition and mechanical properties for the listed grades, and common casting defects.

Properties of Cast Titanium Alloys

Titanium combines low density, high strength, good toughness, nonmagnetic behavior, a high melting point, a low coefficient of thermal expansion, and strong resistance to corrosion and biological attack. Cast titanium alloys generally have the following ten characteristics.

1. Low Density, High Strength, and High Specific Strength

Titanium has a density of 4.51 g/cm³, about 57% that of steel. It is less than twice as heavy as aluminum but can be about three times as strong. In the comparison presented here, titanium alloys have specific strengths about 3.5 times that of stainless steel, 1.3 times that of aluminum alloys, and 1.7 times that of magnesium alloys. This makes them important structural materials for aerospace applications.

2. Excellent Corrosion Resistance

Titanium passivity depends on its oxide film. Its corrosion resistance is much better in oxidizing media than in reducing media, where rapid corrosion can occur. Titanium resists attack in media including seawater, wet chlorine, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chlorides, sulfides, and organic acids.

3. Good Heat Resistance

Aluminum generally loses its original performance around 150°C and stainless steel around 310°C, while titanium alloys can retain useful mechanical properties near 500°C. At about Mach 2.7, an aircraft surface may reach 230°C, where aluminum and magnesium alloys can become unsuitable. Titanium alloys are used in aero-engine compressor disks and blades and in aft-fuselage skins.

4. Good Low-Temperature Performance

For some titanium alloys, such as Ti-5Al-2.5Sn ELI, strength increases as temperature falls while ductility decreases only modestly. Their low-temperature ductility and toughness support use in liquid-hydrogen and liquid-oxygen rocket engines and in cryogenic vessels and tanks for crewed spacecraft.

5. Nonmagnetic Behavior

Titanium is nonmagnetic. In submarine hull applications, it does not trigger magnetic mines.

6. Low Thermal Conductivity

Titanium thermal conductivity is about one-fifth that of steel, one-thirteenth that of aluminum, and one-twenty-fifth that of copper. Poor heat conduction is a disadvantage in some applications.

7. Low Elastic Modulus

Titanium has an elastic modulus of about 55% that of steel. For structural applications, this relatively low modulus can be a disadvantage.

8. Tensile and Yield Strengths Are Close

In the cited Ti-6Al-4V example, tensile strength is 960 MPa and yield strength is 892 MPa, a difference of only 68 MPa.

9. Titanium Oxidizes Readily at High Temperature

Titanium has a strong affinity for hydrogen and oxygen, so oxidation and hydrogen absorption must be controlled. Welding requires argon shielding. Titanium tube and sheet are heat treated under vacuum, while titanium forgings require careful control of a mildly oxidizing furnace atmosphere.

10. Low Damping Capacity

If bells of identical shape and size are made from titanium, copper, and steel and struck with the same force, the titanium bell rings longer. The impact energy dissipates slowly, indicating low damping capacity.

Classification of Cast Titanium Alloys

The 12 cast titanium grades shown below are grouped by annealed microstructure. The ZT designation identifies cast titanium in the Chinese designation system. The cited Chinese national standards are GB/T 6614-2014, Titanium and Titanium Alloy Castings, and GB/T 15073-2014, Cast Titanium and Titanium Alloys.

Chemical Composition of Cast Titanium and Titanium Alloys

The following composition table is attributed to GB/T 6614-2014; its limits apply to the casting grades and conditions represented here.

Notes: Other elements are trace elements inherently present during titanium-casting production, generally including Al, V, Sn, Mo, Cr, Mn, Zr, Ni, Cu, Si, Nb, and Y, excluding intentional alloying elements in the grade. Individual and total limits for other elements are analyzed only when required by the purchaser.

Mechanical Properties of Cast Titanium and Titanium Alloys

The following composition and mechanical-property table is attributed to GB/T 15073-2014; its values are not presented as universal limits for all cast titanium products.

1. α titanium alloys. The annealed structure is entirely α phase. They have good structural stability at high temperature and form the basis of heat-resistant titanium alloys, although room-temperature strength is not high. Commercially pure titanium is also an α material.
2. α+β titanium alloys. The annealed structure contains α and β phases. These alloys can be strengthened by heat treatment and have relatively high strength and good heat resistance at 400-450°C. At higher temperatures, structural stability and weldability are poorer than for α alloys.
3. β titanium alloys. The annealed structure is β phase. Quenching and tempering can produce high mechanical properties.

α Cast Titanium Alloys

Commercially Pure Titanium Castings

Commercially pure titanium castings include ZTA1 (Chinese CP titanium casting grade), ZTA2 (Chinese CP titanium casting grade), and ZTA3 (Chinese CP titanium casting grade). The first of these grades has the lowest impurity content and therefore higher ductility but lower strength. The third contains more impurities and therefore has higher strength but lower ductility. The as-cast structure is entirely α phase with coarse Widmanstätten grains; annealing produces an equiaxed structure.

Casting behavior resembles that of a pure metal. Fluidity is good, dispersed shrinkage is limited, concentrated shrinkage cavities can form, and hot cracking is unlikely. CP titanium castings have moderate strength, good ductility, and good castability, corrosion resistance, and weldability. They can operate for extended periods at 300°C.

Ti-5Al-2.5Sn Cast Titanium Alloy

Ti-5Al-2.5Sn Cast Titanium Alloy is a medium-strength, single-phase α alloy containing 5% aluminum as an α stabilizer and 2.5% tin as a neutral element. It has good elevated-temperature strength and fusion weldability. It cannot be strengthened by heat treatment and is normally used in the annealed condition. Process ductility is relatively low, so sheet forming requires heating.

Applications include casing shells and panels. The reported operating temperatures for this alloy are up to 500°C long term and up to 800°C briefly.

Ti-0.2Pd Cast Titanium

Ti-0.2Pd Cast Titanium is a corrosion-resistant α material produced by adding a small amount of palladium to commercially pure titanium. Palladium improves corrosion resistance in reducing media and also in oxidizing media. In hot, concentrated chloride media, this cast material is described as having particularly strong crevice-corrosion resistance.

Ti-0.2Pd Cast Titanium has applications similar to CP titanium castings but costs more because of the palladium addition, so it is selected when commercially pure titanium cannot meet service requirements. Applications include dilute hydrochloric, sulfuric, and phosphoric acid environments; crevice-corrosion-resistant titanium equipment; shell-and-tube heat exchangers; textile spinnerets; flange liners in acetaldehyde plants; and flanges, joints, and other crevice-bearing components.

Ti-0.3Mo-0.8Ni Cast Titanium Alloy

Ti-0.3Mo-0.8Ni Cast Titanium Alloy is a near-α corrosion-resistant alloy developed to improve the crevice-corrosion performance of pure titanium. Its 0.3% Mo and 0.8% Ni strengthen the alloy and improve resistance to hot, low-pH chloride media and mildly reducing acids. Its reported corrosion resistance is substantially better than that of pure titanium and close to Ti-0.2Pd Cast Titanium. It also has good process ductility and weldability and is widely used in the chemical industry.

Ti-6Al-2Zr-1Mo-1V Cast Titanium Alloy

The cited composition for this high-aluminum near-α alloy is 6.58% Al and 1.46% Mo. It combines the elevated-temperature strength and weldability of α alloys with process ductility approaching that of α+β alloys. The reported long-term operating temperature is 500°C for 3,000 h, with momentary exposure up to 800°C for no more than 5 min. At 450°C, reported life reaches 6,000 h.

Applications include structural and welded load-bearing parts operating below 500°C, engine blades and casings, aircraft sheet-metal parts, beams, joints, large panels, welded load-bearing frames, other welded structures, and castings.

Ti-4Al-2V Cast Titanium Alloy

Ti-4Al-2V Cast Titanium Alloy has good room- and high-temperature mechanical properties, fatigue resistance, crack-growth resistance, impact resistance, and corrosion resistance in various media. It is weldable and can be formed hot or cold.

Applications include missile pressure vessels and rocket-engine casings; aircraft compressor disks, blades, spacers, shields, skins, ribs, webs, wings, beams, horizontal stabilizers, rotor hubs, landing gear, support beams, and wheel hubs; marine hydrofoils, propulsion equipment, and ordnance; and steam-turbine blades, axial- and radial-compressor disks, corrosion-resistant springs, and internal-combustion-engine connecting rods.

α+β Cast Titanium Alloys

Ti-6Al-4V Cast Titanium Alloy

Ti-6Al-4V Cast Titanium Alloy is described as one of the earliest and most widely used titanium alloys. Aluminum strengthens the α phase by solid solution and improves room- and high-temperature strength. Vanadium increases strength and improves ductility. It also suppresses formation of the α₂ ordered phase, reducing long-term embrittlement risk.

The alloy combines broad mechanical performance with good processing characteristics and superplastic formability. It can be formed by several pressure-working methods and welded by multiple processes. Common semifinished forms include bar, forgings, thin and thick plate, profiles, and wire. It is mainly used annealed but can also be solution treated and aged.

Aerospace uses include aircraft beams, frames, rails, landing-gear beams, fan and compressor disks and blades, rocket shells, pressure vessels, and fasteners. Replacing 30CrMnSiA structural steel can reduce component weight by about 30%. Civil applications include gas-turbine blades, marine propellers, offshore drilling platforms, corrosion-resistant pumps, medical implants, and sporting goods. Non-cast Ti-6Al-4V is also used for automotive frames, crankshafts, connecting rods, bolts, intake valves, and suspension springs.

Ti-6Al-4.5Sn-2Nb-1.5Mo Cast Titanium Alloy

This alloy combines high strength, high toughness, damage tolerance, and weldability and can operate for extended periods below 500°C. The applications described include structural components for aerospace and civil use; bar, forgings, and thick plate are also listed as principal semifinished forms. Those wrought product forms are distinct from the cast material discussed in this section.

β Cast Titanium Alloys

Ti-32Mo Beta Cast Titanium Alloy offers very high corrosion resistance and can be welded by various methods, but it is not described as heat-treatable for strengthening. Its high molybdenum content complicates melting, raises density to 5.69 g/cm³, lowers elastic modulus, and causes severe oxidation when heated in air above 500°C. It is therefore used mainly for cast pumps, valves, and similar components exposed to strong acids.

Casting Behavior and Common Defects

Alloying elements generally widen the solidification temperature range and reduce fluidity. Increasing aluminum content, however, raises the heat of crystallization and can improve fluidity. Adding 10% Al by mass raises the heat of crystallization from 327 J/g to 435 J/g. Ti-32Mo Beta Cast Titanium Alloy contains 31%-35% Mo, has a wide solidification interval and poor fluidity, and is unsuitable for thin-wall castings.

Concentrated shrinkage cavities occupy about 1% by volume in commercially pure titanium. At an alloying-element content of 10% by mass, the concentrated-shrinkage volume fraction is about 0.5%-1.5%. In alloys with a wide solidification range, shrinkage cavities formed during solidification can be filled by gas from the remaining liquid, producing gas-shrinkage cavities. Titanium alloy castings have a strong tendency to form this defect. Dispersed shrinkage porosity also increases as the solidification range widens.

Commercially pure titanium has a linear contraction of 1.0%-1.1%. Titanium-alloy contraction increases with aluminum content. The low elastic modulus and coefficient of thermal expansion, together with relatively high strength at elevated temperature, give titanium alloys good resistance to hot cracking. Surface cold cracks are associated with reactions between molten titanium and the mold during pouring or reactions with gaseous impurities at the casting surface and in gaps, which form a brittle oxygen-enriched α-case. This brittle surface layer readily develops cold cracks.