Aviation design and material selection had been relatively stagnant since the 1930s. With the advent of the Boeing 247 in 1933 – followed shortly after by the Douglas DC-3 – aircraft were essentially constructed with aluminum in a “semi-monocoque” configuration. This was a considerable departure from the historical design philosophy, which mimicked that of building and bridges, using internal frames for load bearing. Semi-monocoque designs effectively distribute flight loads between the metal skin and metal aerostructure (e.g. frame, stringers, ribs). And that was the paradigm for 75 years.
Aviation is a conservative industry. The prohibitive costs – both real and imaginary – of a catastrophic failure make it unlike any other industry except, perhaps, nuclear power. And the industry is unique with its obsession for light-weight structures. Fuel cost historically represents between 20 to 40% of the operating costs of an air transport category aircraft; thus, any weight savings directly translate into increased profitability for airline operators. The aircraft original equipment manufacturers (OEMs, e.g. Airbus and Boeing) painstakingly reduced weight by using impeccably qualified materials, coupled with judicious design practices.
Perhaps somewhat disquietingly, the industry actually has one of the lowest design margins of safety: 1.5 to 1. This helps minimize weight. In other words, to support a load of 1000 kg, for example, aviation components would be designed to carry an ultimate load of 1500 kg. The automotive industry, by contrast, would use a 3000-kg rated component. Rest assured, however: the inferior margin is safely offset by exacting quality control of materials and processes that have to conform to governmental requirements. Safety is furthermore upheld by an unparalleled preventative maintenance regiment. This, too, is strictly monitored by a governmental authority. As a consequence, you can board that British Airways A320 with peace of mind that your aircraft was well constructed and maintained.
Like any industry, cost is a primary design consideration. Increasingly, the emphasis however is shifting from acquisition cost to total life-cycle cost, as operators become more sophisticated in cost accounting and data management. For aircraft OEMs, the second design constraint for material selection is predicated upon a material’s strength versus weight. Aluminum had a monopoly for over 75 years; carbon fiber reinforced plastic (CFRP) composites have become the new material of choice with their applications on both the Boeing 787 and Airbus A350 within the past decade. Truly revolutionary aircraft. Part of the fundamental shift in material resulted from a third airframe design consideration: manufacturability. The manufacture of an aerostructure is still considerably labor intensive, as skilled craftsman scrupulously “buck” tens of thousands of rivets throughout the airframe, wing, and empennage.
CFRP helps to reduce labor content, as well as part count by 25% or more. Most CFRP designs still involve a semi-monocoque configuration and are therefore colloquially referred to as “black aluminum.” This is due to legacy tooling and assembly practices. Given the variability of CFRP final mechanical properties in comparison to a more isotropic material such as aluminum, it is envisaged that CFRP designs will continue to be optimized going forward. Indeed, the 787 and A350 have predominately CFRP aerostructures. Many experts believe that the next clean-sheet design for air transport category aircraft will likely combine a metal (aluminum lithium?) airframe with a CFRP wing and empennage. Nevertheless, these aircraft will not appear until the end of next decade due to the “re-engineing” efforts of both Boeing (737 MAX) and Airbus (A320 NEO). An ongoing challenge of CFRP structures is damage detection and repair.
The design criteria for gas turbines is rather different. Naturally, cost is still a principal concern. The second and third order design targets are operational efficiency and “maintainability.” Unlike the aerostructure, the predominant material (in this case, steel alloy) diminished markedly since the 1960s with the introduction of “superalloys” and titanium alloy. Superalloys are high-nickel (or cobalt) content alloys that have remarkable hot temperature performance capabilities which improve fuel burn and NOx emissions. They can withstand temperatures in excess of 800°C. These dense alloys include small, yet vital amounts of minor metals including chromium, molybdenum, niobium, zirconium, tungsten, vanadium, hafnium and the ever popular rhenium. Investment casted alloys – when coupled with advanced cooling-path design and thermal barrier coatings – can endure hostile environments that exceed twice their melting temperature. The caustic gas-path gases further compound the brutality of the environment.
Titanium is another relatively stable metal whose role has become increasingly important throughout the aircraft. The most ubiquitous variant is TI 6-4 (6% aluminum, 4% vanadium). Its strength is equivalent to most steel alloys yet it weighs 40% less. Titanium is used increasingly in the airframe for its strength to weight characteristics and compatibility with CFRP. In the gas turbine, its application is primarily the “cold” section (i.e. front portion) of the gas turbine, since the melting temperature is half that of superalloys. The inlet fan, in particular, is often titanium ,due to the grueling bird strike certification requirements. This is a test that requires a 2 to 3 kg bird to be hurled at the inlet of an engine running at maximum thrust. The shattered blades need to be safely contained.
As discussed, routine preventative maintenance is a primary factor to preserve the overall health of the aircraft. In terms of material consumption, engine maintenance, repair and overhaul (MRO) is significantly more material intensive. Engine MRO requires periodically replacing “life limited” parts (LLPs). These critical rotating parts, such as disks and shafts, must be scrapped after a certain number of operating cycles. This equates to every 8 to 10 years, on average. Extremely costly, LLPs are fabricated from expensive metals (note superalloy and titanium are roughly five times more expensive than aluminum and steel alloy) that undergo a highly scrutinized production process (all LLPs must be ultrasonically inspected, meticulously machined and the SKUs carefully tracked). Accordingly, materials constitute some 70% of the total engine MRO shop visit cost. This compares to only 20% for airframe MRO.
The total material consumed in annual aircraft production and MRO is approximately 680,000 tons in 2014, according to US-based ICF International. This is the mill material demand. Surprisingly, due to inherent inefficiencies in the production process, it takes an estimated 6 kg of virgin material for every 1 kg of final material used on the aircraft. This is known as the “buy-to-fly” ratio in industry parlance. Each material pre-form process (e.g. forging, casting, extrusion, machined plate) incurs a considerable loss of material. Most alloys have fairly high buy-to-fly ratios, in contrast to CRFP, which is closer to 1.5 to 1.
The industry has placed increased emphasis on closed-loop recycling/revert programs and “near-net” shape manufacturing processes. Near net also has the advantage of minimizing the machining of difficult-to-machine alloys, such as superalloy and titanium alloy.
Aluminum alloy is the most common material overall, accounting for 47% of the 680,000 tons. Steel alloy is the second most prevalent material at 18%. Superalloy and titanium alloy constitute 15% and 11%, respectively. As discussed, titanium is highly versatile. The engine accounts for 30%; the balance is consumed in the airframe (e.g. wing box, engine pylons, and other “hard” attach points) and the landing gear. Due in part to the low buy to fly, 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 flat over the next decade.
Next generation materials include advanced aluminum lithium and various derivatives of 7000 series heat-treated aluminum alloy. The US Federal Aviation Administration (FAA) has certified more alloys within the past decade than it has in the previous seven, the preponderance are custom aluminum alloys for niche airframe applications. Fiber reinforced aluminum is also under investigation. Research emphasis for non-metals surrounds out-of-autoclave thermoset composites and thermoplastic extrusions, for stringers, for example.
A potentially disruptive material for the gas turbine hot section is ceramic matrix composites (CMCs). CMCs are one-third the weight, twice the strength and have 20% greater thermal capacity than superalloy. Its difficulty to machine and high cost of production are stubborn barriers for wide-spread adoption. Titanium-aluminide is another material receiving attention. Powder metal is used with greater frequency for the forged disks. And “single crystal” casting is used increasingly for investment casted turbine blades. Its lattice structure is manipulated to optimize the longitudinal strength of the blade to stave off creep and low-cycle fatigue.
In terms of the engine cold section, the primacy of titanium fan blades is being challenged. The next generation engines by CFMI (a GE/ Snecma joint venture) and Pratt & Whitney have opted for 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, therefore a light-weight fan is imperative. Less mass translates into less rotational inertia and kinetic energy. These materials are part of the enabling technology package that has afforded an impressive 15% reduction in fuel burn.
In general, there has been only moderate metallurgical investigation for “disruptive” aerospace applications. Highly advanced new alloys cost “a few million” to develop, then one could easily triple that figure for an extensive business development campaign. It is a costly and slow endeavor. It is feasible the development cycle can take up to a decade. Consequently, many of the newer materials have marginal improvements in performance. Most of the industry’s energy is directed towards cost reduction (for instance, decreasing buy-to-fly) and advanced monolithic manufacturing. Furthermore, with eight years of production backlog and the aggressive ramp up for both Airbus and Boeing for narrow body production, it is logical that the emphasis will remain on program execution and cost containment. The staggering cost overrun of the 787 development and struggling profitability of the A380 program ensure that both OEMs stay focused on incremental gain. Indeed, aviation is a conservative industry. Chances are we will remain at the technological status quo with only marginal improvements for alloys during the next decade, save, perhaps, the forays into additive manufacturing. Sound business for incumbents. Time to harvest investment. A bit lackluster, however, for the rest of us technology enthusiasts…
Bill Bihlman, Aerolytics LLC, www.aerolyticsllc.com Speaker MMTA’s International Minor Metals Conference 2015