Engineers turn to nickel-based superalloys when stainless steel can’t provide enough corrosion resistance or high temperature strength. The downside of alloys like Inconel^® though, is that they are hard to machine. This makes investment casting the most practical manufacturing route.

Superalloys and Their Properties

Steels lose strength long before reaching their melting point, which became a problem during development of the jet engine. This drove the creation of new alloys with superior high temperature strength. Three families of so-called superalloys emerged, based around iron, cobalt and nickel.

Today the nickel-based superalloys dominate, especially in aerospace applications, and Inconel, though a tradename of Special Metals Corporation, is becoming the generic term.

Incorporating iron, chromium and smaller proportions of other elements, nickel superalloys provide excellent corrosion resistance and strength at high temperatures. Special Metals lists 18 different grades of Inconel on their website, with varying compositions and corresponding properties. The best known, and most widely used of these is Inconel 718, more generally referred to as Alloy 718.

Why Inconel is Investment Cast

All the nickel superalloys are extremely difficult to machine. This is a consequence of:

• High hardness – Inconel ranges from 32 to 45 HRC
• Poor thermal conductivity – making chip breaking difficult
• A tendency to work harden

Minimizing machining requires a method of forming Inconel parts as close to net shape as possible. This is why investment casting is preferred.

Although more complicated than other casting processes, investment casting reproduces fine detail, thin walls and excellent surface finishes. Tolerances of +/-0.005” per linear inch (0.127 mm per 25mm) are achievable, along with surface roughness of 125RMS. (3.2μm Ra).

The Investment Casting Process for Superalloys

Investment casting begins with molding a wax pattern that reproduces the part to be made. Cores are incorporated into this pattern that will form cavities in the final part. If the part being made is small, multiple patterns will be arranged in a “tree” around a central wax feeding channel.

After tree assembly the wax is coated with a ceramic slurry. This is dried and the wax melted out to leave a hard ceramic shell mold. Molten metal is poured in and allowed to solidify, and the shell is broken apart to release the metal parts.

Inconel and other nickel superalloys are not straightforward to investment cast. Characteristics requiring particularly close attention are:

• Some of the elements used in these alloys are very reactive, so melting and pouring is usually done under vacuum
• The shell needs preheating to lessen the thermal shock of pouring
• Alloys tend to desegregate as they solidify, so heat treatment is always required after casting

Advantages of Investment Casting Inconel

The ability to reproduce fine detail, including internal features, reduces the amount of machining work needed. In addition, through careful control of solidification rates and direction, it’s possible to create metallurgical structures that increase the strength of the part.

While the process is more complicated than other casting processes, there is considerable scope for automation. This makes investment casting feasible for large as well as small production runs.

Applications

Aerospace applications, particularly for engine components, are the largest use of alloys like Inconel. Functionally similar, gas turbines require the same high temperature strength in their internal components.

Other applications take advantage of the excellent corrosion resistance of this metal, putting it to work in chemical processing equipment and marine environments.

Superalloy Investment Casting Capabilities at Impro

Impro can investment cast parts weighing up to 253 lbs in Inconel. Maximum size must be within an envelope of 43” (1,100mm) x 43” (1,100mm) x 47” (1,200mm).

Ask Impro About Investment Casting Inconel

Superalloys like Inconel provide capabilities not available from steel. However, machining these materials is challenging. This leaves investment casting as the best manufacturing route. If you need parts casting in Inconel, or another nickel-based superalloy, talk to us.