Types of Titanium Alloy Used in Investment Casting
Though light and strong, titanium is difficult to machine, weld, and form. That makes investment casting the best way of producing parts in this metal.
In this blog post we’ll explain:
- Why titanium is often the preferred material for parts for demanding environments
- What sets it apart from other metals
- Why it should be investment cast
- Titanium alloys for investment casting
Strong, Light, and Corrosion-Resistant
Although titanium was discovered in the 18th Century, a high melting point, (3,020° F or 1,660° C) and a tendency to react with oxygen made processing difficult. Only in the 1960s did it start being used in the most challenging aerospace applications.
The aerospace industry likes titanium because it’s only a little heavier than aluminum yet more than twice as strong. It also resists cracking and fatigue and exhibits very little creep, all of which make it ideal for airframe components.
Two other notable characteristics are corrosion resistance and biocompatibility. A self-healing oxide layer on the skin, which behaves like the one on aluminum, helps resist attack by saltwater and most other chemicals, (although not strong acids.) This makes it the preferred metal for use in desalination plants and especially in desalination heat exchangers.
In addition, bio-compatibility means it’s a metal that can be implanted into humans without adverse effects.
Besides the high melting point, titanium is hard to work with because it reacts readily with oxygen and it’s difficult to machine.
Reactivity is addressed by melting and pouring under vacuum. However, welding is a significant problem as it’s very hard to exclude air completely.
Machinability challenges stem from the “gummy” nature of the metal. Rather than sliding smoothly across the cutting tool, it tends to stick. This causes galling that shortens tool life dramatically.
Investment Casting: The Best Way to Form Titanium Components
Investment casting is a near-net shape process that can be performed under vacuum. This avoids reactions with oxygen, eliminates the need for welding, and minimizes machining.
Investment casting starts with a wax replica of the part to be cast. The wax is coated with a ceramic slurry which dries to a hard shell. The wax is then melted out, leaving a cavity into which molten titanium is poured under vacuum. Once it solidifies the shell is broken away to reveal the cast part.
The investment casting process is capable of high geometric accuracy and faithfully reproduces fine surface detail. Undercuts, thin walls and recessed features are possible and machining allowances can be very small. This ability to produce complex shapes avoids the need to weld smaller pieces together.
Titanium Alloys For Investment Casting
Titanium is available in a number of commercially pure grades and four main types of alloy. Alloying, primarily with aluminum, molybdenum, vanadium and niobium, is done to improve corrosion resistance, creep resistance, and weldability.
The four alloys are:
- 6AL-4V ELI
- 3AL 2.5
Of these, 6AL-4V is the most widely cast. In addition to extensive use in aerospace, saltwater corrosion resistance makes it a favorite for marine propeller shafts and desalination heat exchangers.
6AL-4V ELI is considered surgical grade titanium alloy as it has exceptionally good bio-compatibility along with fatigue resistance.
3AL 2.5 is the most weldable grade of titanium. It’s also very corrosion-resistant and is often chosen for parts used in high temperature chemical processes such as heat exchangers.
Airframe applications require very high creep resistance, which is where 5AL-2.5Sn excels. A high strength-to-weight ratio is another reason for using titanium.
Titanium Investment Casting From Impro
Titanium offers a host of advantages over other metals but is difficult to machine, weld and form. The best way of producing parts in titanium is investment casting, which yields accurate, near net shape geometries with thin walls and fine detail.
Commercially pure grades are used for some applications but it’s more common to use titanium alloyed with aluminum and other elements. Depending on the exact composition, this improves casting behavior and creep and corrosion resistance.
The majority of titanium parts are cast from the 6AL-4V alloy but in some applications other alloys may work better. As one of the leading investment casting companies, Impro can advise on which alloy will best meet the needs of your products. Contact us to learn more.