Aerospace materials and design

The past 15 years have been intriguing for aerospace, from a commercial and a technological standpoint. The world was introduced to the first predominately composite aircraft, the Eurofighter, in 2003. The United States followed shortly with arguably the world’s most advanced fighter, the F-22, its performance enabled by advanced materials and manufacturing methods. Meanwhile, commercial aerospace witnessed the entry-into-service of two revolutionary aircraft: Boeing’s 787 and Airbus’ A350. Engine original equipment manufacturers (OEMs) engaged in their own technology play. Advanced materials and modified architecture have recently delivered a 15% reduction in fuel burn in the form of CFM International’s LEAP 56 and Pratt & Whitney’s (P&W) geared turbofan (GTF). These innovative engines have prompted Boeing and Airbus to shelve plans for a clean-sheet narrow body replacement for a more immediate re-engined solution. What can the world expect for the next 10 to 15 years?
 

Raw material demand

The total material consumed in annual military and commercial aircraft production and maintenance, repair, and overhaul (MRO) is approximately 680,000 tons in 2014, according to ICF International. Two thirds is associated with Boeing and Airbus. Due to inherent inefficiencies in the production process, it takes an estimated 6 lb. of mill material for every 1 lb. of final material used on the aircraft. This is the buy-to-fly ratio per industry parlance. Each pre-form process (forging, casting, extrusion, machined plate) incurs a considerable loss of material. Most alloys have fairly high buy-to-fly ratios, in contrast to carbon fiber reinforced polymer (CFRP), which is closer to 1.5:1. The industry has placed increased emphasis on closed-loop revert programs and near-net shape manufacturing processes. Near net also has the advantage of minimizing the machining of difficult-to-machine superalloy and titanium alloys.

Aluminum is the most common material overall – 47% of industry material use. Steel alloy is second at 18%. Superalloy and titanium alloy constitute 15% and 11%, respectively. CFRP comprises just 4% of the total demand yet is the fastest growing. It is anticipated to increase 6.5% per annum, per ICF’s forecast. Titanium alloy will grow at 4.5% and superalloys at 2%. Aluminum and steel alloy growth rates are projected to be flat during the next decade.

Next generation

Given the variability of CFRP final mechanical properties, it is envisaged CFRP designs will continue to be optimized. The next clean-sheet design for air transport aircraft will likely combine a metal (possibly aluminum-lithium) airframe with a CFRP wing and empennage. The justification for using CFRP – a considerably more expensive material – for the wing includes weight, geometry, finish, and stiffness (for a high aspect ratio). A challenge of CFRP structures is damage detection and repair – reasons for metal fuselages that are subject to impact damage from ramp service vehicles. These aircraft will not appear until the end of next decade due to efforts to re-engine both Boeing (737 MAX) and Airbus (A320neo).

Next-generation materials include advanced aluminum-lithium and various derivatives of 2000 and 7000 series heat-treated aluminum alloy. The Federal Aviation Administration (FAA) has certified more alloys within the past decade than it has in the previous five; mainly these custom aluminum alloys. Fiber reinforced aluminum is also under investigation. Research emphasis for non-metals surrounds out-of-autoclave thermoset composites and thermoplastic extrusions, such as those for stringers. Curing composites requires enormous autoclaves and temperatures above 750°F. Researchers are studying carbon nanotube film that can produce high heat using only 1% of the total energy of the traditional process.
 

Ceramic matrix composites

Used in gas turbine hot sections, ceramic matrix composites (CMCs) – made from silicon carbide ceramic fibers and ceramic resin – have one-third the weight, twice the strength, and 20% greater thermal capacity than superalloy. Difficulty in machining and high cost of production are barriers for their wide-spread adoption.

GE Aviation has spent more than $1 billion on this technology during the past two decades, including $125 million for a manufacturing facility in Asheville, North Carolina. The material’s first entry-into-service will be the first stage high-pressure (HP) compressor shroud in the LEAP engine. The company is testing the CMCs in the HP turbine and combustor for GE9X. In February, GE successfully tested CMC rotating parts in the F414 military engine in the form of low-pressure (LP) turbine blades. This material has helped the company achieve the highest recorded combined compressor and turbine temperatures with the ADVENT military engine. Rolls Royce is testing CMCs for application in shrouds, then static structures, and eventually in rotating parts. P&W will focus its CMC efforts on turbine blades and combustors. The industry is considering applications for turbine disks.
 

Titanium-aluminide

Titanium-aluminide (TiAl) is as strong as many superalloys yet half the weight, but it is difficult to process due to low ductility. GE’s subsidiary Avio Aero is evaluating TiAl LP turbine blades produced via additive manufacturing. This process allows for complex internal geometry for cooling, and minimizes the material to be removed during machining. Rolls Royce is evaluating TiAl turbine blades for its next generation Trent engine.
 

Powdered metals

Powder metallurgy mitigates inhomogeneous microstructure that typically arises during the casting or wrought process. Historically, aerospace applications for gas atomized powders have been for coatings and isothermally forged disks, a technology developed in the 1980s. Each engine OEM has its own proprietary powder. The emphasis now is on additive manufacturing applications, mainly using titanium and superalloys, with parts in engines and aerostructure. New powders are being developed, with manufacturers focusing on cost reduction and qualify control for larger batch production.
 

Titanium under pressure

In the engine cold section, the primacy of titanium fan blades is being challenged. The next-generation engines by CFM International and P&W have CFRP and aluminum-lithium, respectively, for their narrow body engine fan blades and cases. The CFM LEAP and the P&W GTF are approximately 35% larger in diameter, so a lightweight fan is imperative. Rolls Royce has also announced plans to include a carbon-titanium composite fan for its next-generation engine. Less mass translates into less rotational kinetic energy. Moreover, the weight savings to an engine cantilevered on the wing means even greater weight savings for the engine pylon and wing structure.

In general, significant new material development is unlikely during the next decade. Most of the industry’s energy will be directed toward cost reduction (decreasing buy-to-fly), advanced monolithic machining, additive manufacturing, and automation. Given Boeing and Airbus’ 8-year production backlog and their aggressive production ramp up, it is logical the emphasis will remain on program execution and cost containment. The staggering cost overrun of the 787 development and execution and struggling profitability of the A380 program ensure both OEMs stay focused on incremental gain. Aviation is a conservative industry. The last decade was exceptional. OEMs will now harvest existing technologies – incumbents and emerging competitors alike.

Aerolytics LLC
www.aerolyticsllc.com

About the author: Bill Bihlman is the founder and president of Aerolytics LLC, a boutique management consultancy that specializes in aerospace. He can be reached at bihlman@aerolyticsllc.com.