MIND OF METAL: INCONEL®

If you’ve searched the periodic table and found no reference to ‘Inconel’, you aren’t alone.  Inconel® is a “superalloy”, very deserving of the descriptor and ideally suited to 3D printing!  

Please note: the length of this article could have been exhaustive, so an attempt was made to stick to the essentials. If you have any questions, let us know. We will be happy to answer whatever questions you may have.


Frank Whittle’s W2/700, used to power the Gloster Meteor in the 1940’s
The Backbird SR-71 is powered by two Pratt&Whitney J58 engines

ABOUT INCONEL®

Inconel is a group of roughly 16 austenitic nickel-chromium based super-alloys with a colorful history, and of growing importance in modern industry. The Inconel registered trademark may have been acquired by Special Metals Corporation in 1998, but it was actually originally developed in the 1930’s by the International Nickel Company and began to be noticed in the 1940’s when it was used in the development of Frank Whittle’s Jet engine, which is credited with being the first turbojet engine.

Since that time, this nickel-chromium-molybdenum superalloy has found consistent use in hot section components of turbine blades, ducting systems, and engine exhaust systems. In fact, 50% of the materials used in aircraft and rocket engines are nickel-based superalloys.

While there are more than a dozen Inconel alloy composition, the primary focus of this article will be on the two predominant members of the group – Inconel 625 and 718.

INCONEL 625

Typical Chemical Composition of Inconel 625. Multiple sources.

The workhorse of the Inconel group of superalloys is Inconel – or alloy – 625 developed in the 1950’s and ’60’s in response to industrial needs for an alloy that would be able to better withstand the heat, pressure and corrosion for steam-line piping. Given it’s intended application, it was also important for the alloy to have good weldability and high creep resistance.

INCONEL 718

Typical Chemical Composition of Inconel 718. Multiple sources.

Developed almost in sync with 625, Inconel 718 or alloy 718 received patent protection in 1962. branched away from 625 – also in the 1960’s – with the addition of aluminum and titanium becoming what could be considered the first ‘modern’ superalloy. Alloy 718’s composition makes it more heat resistant and stronger, more suited to the rigors of the aerospace industry and resource extraction.

Notably, one of the earliest uses of Inconel 718 was in the diffuser casing for the Pratt and Whitney J58 turbo jet engine. The diffuser was the point of highest pressure, and casings using the previous the previous alloy cracked leading to the introduction of Inconel 718. The J58 engine was used in Lockheed’s A-12 and the subsequent development of the Blackbird SR-71 reconnaissance air craft.


Courtesy of Extreme Bolt
Courtesy of Extreme Bolt

Courtesy of Extreme Bolt

CHARACTERISTICS OF INCONEL®

As you might have guessed from the introduction, Inconel’s three major benefits include strength, temperature resistance and corrosion resistance.

STRENGTH
One of Inconel’s hallmark features is its high tensile strength. This is a direct result of the inclusion of molybdenum and niobium and their stiffening effect on the nickel-chromium. While it is not always easy or even practical to compare metal to metal, the metal strength comparison chart offers a look at the UTS of various metals at room temperature.

RESISTANCE TO EXTREME TEMPERATURES
There are a number of materials that feature high tensile strength at room temperatures (like 17-4 stainless), so strength alone is not enough.

Resistance to temperature extremes begins to help set Inconel apart. Inconel’s unique composition allows it to benefit from – among other things – very low rate of thermolinear expansion, giving it a wide effective operational temperature range from cryogenic temperatures (less than −238°F) to 1800°F. One ancillary benefit is creep resistance which allows Inconel to resist the plastic deformation introduced by elevated temperatures but below the metal’s yield strength.

The tensile data charts provided to the left displays the effect of elevated temperatures on Inconel alloys.

CORROSION RESISTANCE
Inconel forms a passivation layer when heated which grants it a high corrosion resistance. (The result is similar to what happens when stainless steel is subjected to nitric or citric acid.)

While Stainless Steel and other materials have a higher melting temperature than nickel, their mechanical properties (e.g. tensile strength) and ability to resist corrosion begin to deteriorate at higher temperatures.  But Inconel has excellent strength, and the chromium in the alloy gives it superb corrosion resistance at room temperature.

Inconel has a list of other benefits but the three listed above are common to the entire Inconel family. Again, if you would like any more information – general or about a specific alloy – please let us know.

BUT….

MACHINING
Inconel is not the easiest material to work. Inconel metals maintain their tensile strength at temperatures that would render plain steel pliable. As a result, Inconel machining requires high cutting forces. Proper training in how to tool Inconel is important in order to maintain tool life and create even surface finishing.

For instance, Inconel 718 is one of the most difficult-to-cut materials due to its toughness, lower thermal conductivity and easy work hardening properties. Furthermore, as a kind of easy work hardening material, Inconel 718 suffers several plastic deformations during the machining process.

COST
When compared to other metals like stainless steel, Inconel can be very expensive and impractical for certain mass applications like continuous use in undersee piping.

As you will note in the section dealing with 3D printing Inconel, the additive process offers a means to circumvent some of the difficulties and limitations of traditional means of working with Inconel.


COOL, BUT WHAT’S IT FOR?

Any application that requires high strength and high corrosion resistance in an elevated temperature environment is typically a good candidate for Inconel use. We see it used, for example, in manufacturing equipment, tools and firearms, and in the automotive and oil & gas industries. The list is extensive, but here are a few key areas:

Inconel clad oil extraction bore

OIL AND GAS EXTRACTION

Inconel is ideally used in oil and gas extraction equipment due to its high temperature resistance and oxidation-resistant properties. The oil and gas industries need a superalloy metal that can withstand extreme environments and volatile, corrosive gases.

The superalloy Inconel 625 is especially useful in the processing systems required for natural gas production. Due to Inconel 625’s particularly strong thermal fatigue strength and oxidation resistance, it is often used for the separation of extracted fluids or in line steel transfer piping. 

SALTWATER APPLICATIONS

Inconel is also often used in marine applications — offshore drilling wells, propeller blades, undersea piping, oceanside treatment and processing facilities are some examples — because of its extraordinary resistance to sodium chloride at a variety of temperatures.

Other alloys – like stainless steel 316 – also feature excellent resistance to saltwater. But if there is any concern about an environment becoming one of high pressure or of temperatures potentially exceeding 1,000°F, Inconel is preferable. It retains oxidation resistance much better at higher temperatures than most other alloys.

JET & ROCKET ENGINES

Another common application for Inconel is in the aerospace industry. Jet engines must withstand extreme temperatures. Even at a fairly low cruising altitude of 36,000 ft, the average air temperature outside your passenger jet is a very chilly -56.3°C (-69.3°F). But the combustion process within the engine can exceed temperatures of 1,150°C (2120°F). That’s quite a range!

Cooling technologies are used to rapidly reduce high engine temperatures, but the remaining heat can still easily exceed the tolerances of many metals. Inconel 600 can tolerate such high heat. But even more brilliantly, it maintains high tensile strength even when subjected to rapid changes in temperature from the combustion and cooling processes.

Fuel nozzles, afterburner rings, turbines and other engine components are thus commonly made out of Inconel. This choice of materials allows the parts to perform well in the high temperature environment within an operating jet engine. The parts will also resist the risk of corrosion presented by jet fuel and other liquids.

For the same reasons, Inconel is also often used in rockets and space exploration vessels. Common alloys in the aerospace industry include Inconel 625 and Inconel 718.

NUCLEAR INDUSTRY

Another common use of Inconel superalloys is in the nuclear industry. Nuclear reactors require the high strength, high corrosion resistance, and excellent elevated temperature performance that Inconel offers. Common alloys in the nuclear industry include Inconel 600 and Inconel 690.

MANUFACTURING: RAPID TEMPERATURE CHANGES

Some manufacturing processes may combine high and low temperature processes in rapid succession. Most Inconel alloys retain excellent oxidation resistance at high and low temperatures. A single basket made from Inconel can, therefore, be used in processes where temperatures vary between near-cryogenic lows and heat treatment highs.

MANUFACTURING: HEAT TREATING BASKETS

Heat treating baskets are used to hold materials at higher (extreme) temperature environments.

Inconel is famously resistant to extreme temperatures; it retains sufficient tensile strength to hold moderate loads (For example, Inconel 625 retains 13.3 ksi tensile strength at 2,000°F). This makes Inconel® the ideal basket material for heat treat applications.

Unlike most stainless steel alloys, a basket made from a superalloy like Inconel won’t lose easily shape when holding parts through a rigorous heat treat application.


These aviation intake manifold components were printed using the Eplus EP-M260
Microstructure of INC625 printed on Eplus3D EP-M260
Gas turbine component printed using the Eplus3D EP-M260
Microstructure of INC718 printed on Eplus3D EP-M260
The LaserDyne 811 reparing a Inconel625 gas turbine blade

ADDITIVE MANUFACTURING AND INCONEL®

It isn’t a stretch to claim that Inconel and other nickel-based superalloys are made for additive.

If you are relatively new to additive manufacturing and could use a primer on its merits of this technology, please CLICK HERE.

As noted, Inconel offers huge number of industrial benefits. But those benefits come at the cost of being more difficult to work and higher material costs. Component performance can suffer when improper traditional manufacturing practices and methods are used. They can compromise some of the hallmark features of the material like high temperature corrosion and creep resistance.

Interestingly, these two limitations of Inconel® (cost and machining difficulty) – are actually two of the main reasons why it is especially suited to additive processes.

Reduced Machining
The incremental, layered approach of 3D printing grants the user design latitude not easily replicated (if at all) using traditional processes and with possible exception of any post processing, reduces the amount of machining required to achieve a final part.

Tests of parts produced by way of additive manufacturing Inconel 718 have shown that mechanical properties are not sacrificed, and in some cases can even exceed properties of cast or wrought parts.

Material Efficiency – Reduced Waste
Inconel offers material efficiency and overall reduced waste.

While the cost of Inconel powder is correspondingly as expensive as the milled Inconel you might use in traditional subtractive manufacturing, the additive process is inherently material-efficient. Little waste material is generated compared to what we see in traditional forms of production.

In addition, the incremental layered production process offers the user the ability to mitigate the amount of Inconel used. That is to say, the user may adapt the design of the internal structure of the component so as to optimize material usage.

Currently, the most available Inconel alloys available for additive manufacturing are 625 and 718. Industry watchers expect more of these alloys to become available as demand grows.

Selective Laser Melting
For a quick review of Selective Laser Melting, please click here.

SLM is arguably the most trusted means of 3D printing Inconel because properly printed, it produces parts with near net dimensional accuracy and at densities approaching 100%.

The prints to the left were printed using the EPlus3D EP-M260 intelligent SLM printer and includes a metallographic microstructure for both Inconel 625 and Inconel 718.

Direct Energy Deposition
For a quick review of Directed Energy Deposition, please click here.

3D printing with Inconel can also be achieved with DED technology. One of the key advantages of DED over SLM technology is that DED systems are larger and ‘open’. (Meaning that: you can add to, print on or repair existing parts, or cladding.) Please refer to the picture to the left

The DED microstructure for printed components is similar to that of SLM. However, because of the enhanced speed and larger particle size, if finish and dimensional accuracy are considerations, milling may be required to attain required tolerances and finish.

Binder Jetting
For a quick review of binder jetting, please click here.

Binder Jetting can quickly print Inconel parts, but a word of caution is warranted with this method. Binder Jetting Inconel alloys can achieve near cast strengths. However, it is essential that the user account for the geometric changes and dimensional variances that result in both the de-binding and subsequent sintering steps.


How detailed can INC718 prints be? Above are pictures of the Arial font printed to different and corresponding sizes – embossed and and engraved. Courtesy of Materialize.
Minimizing support structures is a great way to reduce post processing time and material used.
The University of Pittsburg offers these examples of thermal stresses caused by improper settings and design considerations.

AM DESIGN CONSIDERATIONS FOR INCONEL

THICKNESS
As a rule of thumb, wall thickness of any Inconel part should be a bare minimum of 1mm.

However, in practice we generally recommend at least a 2mm wall thickness. Wall thickness should account for part geometry and structural requirements. It must also contemplate the possible need for post-processing. (For example, if a part will be milled or blasted before use, of course this will decrease wall thickness). While there is theoretically no maximum wall thickness, consider that thicker areas may increase stresses in your part and thus cause deformations and part failure.

DETAIL
Inconel lends well to printing very fine detail.

This is a great benefit if the printed components need to be identified with a serial number. Fine detailing like engraved or embossed text will print very nicely if you outline the letters with a corresponding line thickness.  The photos to the left illustrate a 40µm line thickness for Arial 20pt. 

Some SLM printers like the Eplus3D’s EPM line have a wide laser spot size range.

GEOMETRY
Design geometry is always an important consideration when 3D printing.

Design geometry does beyond the end function of a part and incorporates aspects like such as aesthetics, part strength, surface adhesion, and support use. Proper consideration of these aspects can minimize the amount of post-processing and optimize material usage.

A NOTE ABOUT THERMALLY INDUCED STRESSES
This is something you want to avoid!

Layer wall thickness and geometry can introduce thermally induced stresses to the 3D printing process

Layered powder melting and its solidification naturally leads to thermally induced stresses as the powder cools. An unsuitable design can lead to build failures and/or part deformations.

It is therefore essential that product designers consider the restrictions specific to their choice of 3D printing process while designing the part. We recommend rounding off or filleting design edges with a minimum radius of 3 mm. Avoid creating sharp edges for the same reason. Try to avoid large material accumulations and, in general, favor organic shapes over edged designs.


For more information about Metal 3D Printing, feel free to check out our Mind of Metal blog articles:

We will be adding incrementally to the Mind of Metal series, so email us to Join our Newsletter and type “Join” in the subject bar to stay apprised.


If that wasn’t more than you wanted to know about Inconel and Inconel 3D printing, we’d love to the chance to answer your questions! 

Contact us.

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