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Materials that Enabled the ‘Jet Age’
Inconel alloys are often referred to as ‘superalloys’ due to their unique mechanical properties that include heat resistance, corrosion resistance, surface stability and mechanical strength. Developed in the 1960’s, this class of high-nickel alloys made the advent of the jet engine possible. Where steel alloys would begin to fail at around 900°F (480°C), these new Inconel alloys could maintain their basic mechanical properties up to around 1400°F (760°C). Specialized coatings have pushed this temperature threshold even further.
Inconel and Aerospace Manufacturing
This unique set of characteristics has made nickel-based alloys an indispensable category of material for aerospace manufacturing. In fact, these alloys constitute roughly 50% of modern aircraft engines by weight. Their excellent high-temperature properties make them the material of choice for the hottest sections of gas turbine aircraft engines. Two of the most common alloys for these demanding applications are Inconel alloy 625 and Inconel alloy 718.
Challenges of Forming Inconel
Inconel alloys are among the most compositionally complex metallic alloys ever deployed for commercial use. They typically combine nickel with chromium, aluminum and titanium in very precise ratios. In addition to aircraft engine applications, they are used in other demanding environments such as nuclear reactors, rocket engines, industrial furnaces and heat exchangers. Recently a new use case has emerged – hypersonic missiles and aircraft.
A typical sheet metal forming operation starts with a trimmed sheet blank and plastically deforms the blank into a desired three-dimensional shape with a mechanical press. The tool set usually consists of a punch and a die. Sheet metal forming may be performed as a cold working process or at elevated temperatures. Many types of alloys, including aluminum, titanium and Inconel, can be hot formed to improve ductility and reduce required forming loads. The forming parameters for each type of alloy are unique. The varying behavior of these alloys under specific temperatures and strain rates is complex.
Forming Inconel sheet metal components can be uniquely challenging. The high strength and work hardening characteristics of these nickel-based alloys result in high forming loads and other manufacturing difficulties. These forming challenges combined with part geometries that are often very complex have traditionally resulted in very high manufacturing costs.
Inconel Forming and Controlling Springback
Every sheet metal forming process will begin with elastic deformation that transitions into permanent plastic deformation. The deviation of the final part geometry from the nominal contour of the tool depends on the elasticity of the metal sheet after the forming loads are removed. This is known as the springback phenomenon, and it can be particularly problematic when forming nickel-based alloys.
Minimizing springback can be accomplished by using a several different techniques:
- Modifying die surfaces to account for the springback effect
- Adjusting the forming temperature
- Adjusting the dwell time at the end of the forming cycle
- Controlling the blank holder force within the tooling system
Traditionally, the development of new products stemming from sheet metal forming was very expensive and time consuming, requiring many rounds of trial and error and physically modifying tools and forming parameters with each iteration. Finite element analysis (FEA) and modern forming simulation software have helped to streamline the new product development process. However, as valuable as simulation is to the development cycle, there is no replacement for hands-on experience. OMADA International utilizes state-of-the-art software tools but also supplements that with decades of experience forming ‘superalloys’ for the most demanding applications in the aerospace industry. OMADA continues to push the envelope in the fabrication of Inconel and other ‘superalloys’ for use in emerging hypersonic applications.
References:
http://ltu.diva-portal.org/smash/get/diva2:1087064/FULLTEXT01.pdf
https://www.tms.org/superalloys/10.7449/1997/Superalloys_1997_315_329.pdf