Aerospace Materials

Aerospace Materials
It is the continually evolving, new-generation superalloy materials that constitute the backbone of cutting edge aerospace technology. Gopi Hanumanthappa, Managing Director of ThyssenKrupp Aerospace Ltd.

What’s common between Captain America's shield, Thor’s hammer, the Samurai’s sword, King Arthur’s Excalibur, fighter jets, and battle tanks? Yes, they are all instruments of defence or attack but more importantly, they are made not from ordinary metals but from metals which have been designed to work under extraodinalry conditions. These special metals, designed for manufacture (DFM) under stringent conditions of material formation ensure that the required material charactersistics come together in regular and extreme conditions of performance.

The world of advanced materials is extensive, but making the jump from academic or R&D labs to commercial success is a tortuous and challenging path. Motorsport and sporting goods often provide initial market entries due to lower regulatory barriers, but in many cases, the first industry that allows capacity to scale-up and notable revenues to emerge is the aerospace and defense sector. Annually, close to two million pounds of the materials ( Aluminium , Steel, Titanium, Superalloys, Composities & other materials) are consumed by the industry.

It’s all about the integrity of materials - this aspect is usually taken for granted largely due to easy approach we have on materials used in the commercial space where conditions for performance is taken for granted. High performance critical applications require materials which have been proven rigorously over a period of time in all aspects of design, performance, manufacture, quality and provenance.

AVIATION PIONEERS

Typically, a unique set of materials which have evolved over the last 60 years hold the aircraft together. The ability of superalloys to operate at extreme temperatures is what makes catching a flight even remotely affordable.

The story began in the 1930s and 40s, when the first high performace aero engines were being built simultaneously by Frank Whittle (UK) and Hans von Ohain (Germany) , both on competing sides of an accelerating arms race. Steel engines developed serious shortcomings as they were unable to withstand operating temperatures beyond 500 degrees C.

Whittle worked with a nickel-chromium alloy that possessed greater heat tolerance and was at the same time light, inexpensive, corrosion –resistant and strong. Rolls Royce company adopted Whittles work in 1940s.

Most metals are made up of many tiny crystals, called grains, which are fused together. Since the grain boundaries are a source of weakness, engine makers build critical components using a single crystal, growing it from a melt using directional solidification. In effect, the turbine blade is like a gemstone, with a single atomic lattice all the way through! Rolls-Royce also spearheaded this invention.

CUSTOM MANUFACTURING

Material and component designs drive one another, and this symbiotic relationship between the two is an ongoing evolution to create new breeds of materials.

Precison components need a longer machining times resulting in expensive scrap. Compared to other industries, aerospace orders consist of short-run quantities and long-lead times, rendering scheduling for productivity, throughput, and profitability difficult.

Forty years ago, aluminium, then considered state-of-the art, dominated the aerospace industry due to its light weight and low cost. Now, a typical jet is built with less pure aluminium. Most of the non-critical structural material — panelling and aesthetic interiors — consist of lighter-weight carbon fibre reinforced polymers (CFRPs) and honeycomb materials.

There is a push for lower weight and higher temperature resistance for better fuel efficiency in engine parts and critical components, bringing new metals into the aerospace material mix.

Alloys have also been improved, and it is because of the additional alloying ingredients that the story of the jet engine has another chemical element in rhenium. It is one of the most scarce substances on earth. The entire annual worldwide production of rhenium is a mere 40 tonnes, and more than three-quarters of it goes into superalloys.

REPLACING THE UBIQUITOUS ALUMINIUM?

Heat-resistant superalloys (HRSAs), including titanium alloys and nickel alloys, are used to meet high-temperature demands, along with some nonmetal composite materials such as ceramics. Harder to machine than traditional aluminium, they increase manufacturing cost due to shorter tool life and less process security.

Standard aerospace aluminium — 6061, 7050, and 7075 — and traditional aerospace metals — nickel 718, titanium 6Al4V, and stainless 15-5PH — while still in use, are rapidly ceding territory to newer cost effective, high performance alloys.

One such new variant is AL-Li ispecifically designed to improve properties of the old type 7050 and 7075 aluminium. Overall, the addition of lithium strengthens aluminium at a lower density and weight. Airbus is currently using AA2050. Meanwhile, Alcoa is using AA2090 T83 and 2099 T8E67. The alloy can also be found in the fuel and oxidiser tanks in the SpaceX Falcon 9 launch vehicle and is used extensively in NASA rocket and shuttle projects.

Titanium 5553 (Ti-5553) is another metal that is reasonably new to aerospace, exhibiting high strength, lightweight, and good corrosion resistance. Major structural components that need to be stronger and lighter than the previously used stainless steel alloys are perfect application points for this titanium alloy.

COMPOSITES

Composites as an aerospace building block is increasing in usage. They are light weight thus improving fuel efficiency while being easy to handle, design, shape, and repair. Once only considered for light structural pieces or cabin components, the composites’ aerospace application range now reaches into true functional components — wing and fuselage skins, engines, and landing gear.

Moulded or shaped into complex shapes composite save production cost by avoiding machining and joinery. Pre-formed composite components aren’t just lightweight and strong, they reduce the number of heavy fasteners and joints, which are potential failure points, within the aircraft.

While CFRPs represent a large part of composite material in both cabin and functional components, and honeycomb materials provide effective and lightweight internal structural components, next-generation materials include ceramic-matrix composites (CMCs), which are emerging in practical use after decades of testing. These comprise of a ceramic matrix reinforced by a refractory fibre, such as silicon carbide. They offer low density/weight, high hardness, and most importantly, superior thermal and chemical resistance.

NEW MATERIALS

Material characteristics and component design must be in balance, and can’t really exist outside of each other.

Metallic and composite materials alike continue to be developed and improved to offer increased performance. The acceleration of this evolution of new materials gives manufacturers unprecedented access to materials previously impractical or too difficult to machine.

New material adoption is happening exceptionally quickly in aerospace. One-piece designs are continuing to reduce the number of components in overall assemblies. Still, difficulties exist in work holding, surface finish, and CAM tool paths. But designers, machinists, engineers, and machine tool/cutting tool partners are developing new solutions to keep evolution churning forward.

The mix of materials in aerospace will continue to change in the coming years with composites, freshly machineable metals, and new metals increasingly occupying the space of traditional materials. The industry marches towards components of lighter weights, increased strengths, and greater heat and corrosion resistance.

Ceramic Matrix Composites (CMC), Metal Matrix Composites (MMC), Polymer Aerogels, CNT-yarns will evolve over time to fully/partly substitute some metallic with 3D printing techniques .

Making the jump from academic or R&D labs to commercial success is a tortuous and challenging path due to the demanding conditions of performance, reliability and safety

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