Quality Components in Aerospace Parts Manufacturing

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Quality in aerospace manufacturing is a critical component.

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Quality in Aerospace Within the U.S.

If you have ever worked in aerospace or a related field you know the high standards that these types of companies must diligently meet. Airplane flights are still considered one of the safest means of travel as a result of strict regulations imposed by the Federal Aviation Administration (FAA).

Even with high regulation in this industry, airplane failures cannot be completely avoided. In 2010 in the U.S., 1,331 planes were involved in crashes. Not all incidents in this statistic were a direct result of a specific part malfunction. There are other elements like the weather, the skill of the pilot, and how the aircraft was assembled that could have contributed to airplane failures. Let us just say it is better to keep crashes to a minimum for the fact that life is precious. When there is a faulty part and the result is a crash, the consequences are fierce. These results usually end up in huge lawsuits.

While working in aerospace manufacturing, I observed and even participated in quality activities relating to internal and external quality failures, quality assurance, and prevention. All accumulated costs associated with these items were considered normal, except for external failure costs. With a six-sigma agenda, there is no room for failure. My employer used a vast amount of the quality techniques associated with Six Sigma, ISO 9001, and lean manufacturing. Just to be clear, this organization was not a builder of aircraft, they were a supplier of pre-assembled parts. Parts included airplane safety equipment, water tanks, water valves, hydraulic valves, and many more items.

External Parts Diagram of an Airplane

This diagram shows an overhead view of the exterior parts of a jet.

Quality Assurance Costs

To keep the quality of manufacturing up to par, my former employer had plenty of quality assurance costs associated with running the business. The development of aerospace parts takes a great deal of time, patience, and among other things precautionary tactics.

During production, there were plenty of people surrounding each value stream who complete quality assurance activities. These individuals or Quality Assurance (QA) had to hold an internal certification to engage in such activities. It was tough to get certified because of the heavy training that was involved. Activities that were associated with these certified QA employees took away from production and included in-process inspections, tracking and reporting measurement comparisons to product specifications, and assuring damaged material requisitions.

There were also special assignments where "audit groups" would convene to conduct an internal audit for a department. These internal audits were scheduled for every value stream in the company. The higher the frequency of mistakes the more the audits would occur.

Due to the adherence to ISO 9001 standards, a large part of assurance costs in this company comes from administering a quality management system (QMS). One person from each division of quality assists the QMS manager in making updates to the QMS. The upkeep of the QMS takes a lot of time to update the vast number of documents stored on it. The QMS is run on the company intranet. The software displays web pages that display quality, safety, and business process data. You will find anything on the system from work instructions, controlled documents, and forms used directly in the quality assurance process. Every individual in the company is linked to the system somehow.

Another aspect of quality assurance that was imperative was the administering of all the measuring tools used to check the dimensions of product components. During most of the assembly processes, measurements need to be made to match against measurements in technical drawings. There were plenty of types of measurements that were taken. I lumped them into 3 categories length measurements, electrical test measurements, and machined dimension measurements. Length measurements were made with a steel ruler or with the use of an automatic cutting machine. The rulers were calibrated in the lab periodically, but the machines needed to be calibrated annually by the machines' manufacturer. Wire and shrink tubes are examples of a few materials that were cut on these machines.

The electrical measurements were made by amperage meters and dielectric meters and any timers used in the production process. These electrical devices also required calibration services from outside parties. It was the job of one person to purchase and certify all the measuring equipment in a temperature-controlled lab. The person in this role managed over 1000 different tools at once. At the beginning of each shift, any person using these instruments would have to check the certification dates per company policy.

Electrical Testing Equipment

This dielectric tester is a popular testing device for electronic equipment in aerospace. It can give multiple readings including readings for a high-pot test.

Internal Failure Cost

As for internal failure costs, this company has a lot. This is not to say that their manufacturing is not up to par. Most items that have any quality defects are rejected before they would leave the building for good cause.

Internal failures could be considered defective inventory that is received and failed by quality control who must inspect before the inventory can be added to the ERP system. When products advance through the system of work in progress, not only did quality assurance reject work-in-progress inventory during in-process inspections, but if you did not feel comfortable with the quality of the previous process as an assembly worker you had the responsibility to consider sending the product back to the previous process. Rework would have to be completed because of the physical or operational aspects of the product.

When parts needed to be returned to the supplier there were tons of shipping costs and piles of paper to be completed. Each part that was ordered was already pre-specified for an order due to lean manufacturing requirements, so it was a lot better when they were able to find supplier defects earlier in the process so returns could be made immediately. Finding these defects was not always easy due to random sampling during receiving. When too many defects from a supplier are found, sampling is then increased for those problem suppliers or suppliers are replaced.

Plane Landing at Tampa International Airport

Millions of people fly daily without knowing how much effort was put into the quality of the aircraft that they took off in. With an inspection process so rigorous you wonder how the suppliers of the part or airlines make any money.

Photo by Joshua Crowder

External Failure Costs

This employer as I noted before, is a six-sigma company. Some external failures seemed so ridiculous, but in aerospace, the customer requirement needs to be implemented into products exactly as prescribed. These external failure costs were avoided as much as possible due to following six-sigma quality standards. If rejection did occur, the product would have to go through the same quality assurance system again.

In addition, engineers, quality managers, the supervisor, and the production manager of the line would all be involved in creating reports for the incident. They would also have to explain the incident at a meeting with upper management. Depending on the product, external failure costs for one product could cost thousands of dollars.

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Prevention Costs

Prevention costs were consistent. Most employees were trained in quality, continuous improvement, lean production, and blueprint reading. At least 3 hours a month of training was typical for any employee in production. Committees were set up specifically to discuss continuous improvement, lean production, and safety all with the intention of keeping quality and other related costs to a minimum.

Some activities that may be considered assurance costs but may be considered prevention costs are peer audits and internal audits. The audits were conducted to prepare employees in the case that an ISO audit or customer audit would occur. These audits were completed to make sure production lines were following work instructions to build the product exactly to print, but also to check the organization of processes. Internal audits consisted of a group of diversely selected employees. These internal auditors were given a check sheet to go through and try to spot issues throughout the whole production process.

The ISO 9001 requirement is a QMS that companies use to help them consistently provide quality products and services that meet high standards.

After seeing how many obstacles one aerospace parts manufacturer goes through, it is no wonder a round trip from LAX to NYC can cost upwards of $900! Each supplier of each part that an aircraft uses for assembly has similar manufacturing techniques as described above.

Whether you are experienced in production, management, or even quality, I would recommend a career in this sector. It could be a very challenging and rewarding experience.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2018 Joshua Crowder

New alloys and tempers across aluminum, titanium, and ferrous systems have been continuously evolving since the start of the use of metal structures in aircraft. Although some tailoring of alloys has been achieved to improve performance, especially corrosion resistance, and to reduce cost, this has abated to some extent because of the loss of design acreage to CFRP. CFRP for major structures first played a significant structural role on commercial aircraft on the Boeing 777 (B777). (The first major use was on the Lockheed Martin B-2 bomber.) The skins and some of the internal structures on the horizontal and vertical stabilizers were fabricated from CFRP and have performed well, with low maintenance. That success convinced Boeing to fabricate virtually the entire fuselage, wing, and empennage skins and much of the support structure of the successor B787 aircraft from CFRP.

This evolution is illustrated by the change in the materials distribution of major structures on Boeing aircraft, with the amount of composite structure increasing with each new model, taking a major step with the B777 (Figure 3). With increasing composite utilization, use of titanium has also increased because of its galvanic, stiffness, and thermal-expansion compatibility with graphite composite and the development of high-strength alloys to compete with steel in landing-gear structures. Titanium accounted for 3–5% of the structural weight on earlier aircraft, but accounts for approximately 15% for new composite-intensive designs. Aluminum alloys have experienced the largest reduction in use, from approximately 80% of the structural weight on earlier aircraft to about 25% on the 787 15 (see Figure 3).

Distributions of structural materials used on selected Boeing commercial aircraft.

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In the past, many aerospace alloys were developed by empirical methods. In contrast, integrated computational materials engineering (ICME) allows researchers to optimize alloy compositions and thermal processing to achieve novel materials more quickly and at lower cost. Thus, ICME is being extensively pursued in research and manufacturing facilities worldwide. (See the article in this issue by Xiong and Olson for an example of the use of ICME in materials design.)

Aluminum alloy development

The primary structural aluminum alloys have been the copper-containing 2XXX alloys (starting with 2024) and the zinc-containing 7XXX alloys (starting with 7075). These alloys are still used today. Although these alloys have been modified to improve their strength and toughness, the development of newer alloys such as 7150 and 7055 along with improved tempers has resulted in higher strengths and improved corrosion resistance.

Figure 4 illustrates improvements in the properties of 2XXX- and 7XXX-series alloys. Significant strides have been made in improving both the static and fracture properties of each alloy. Many of these goals were achieved by reducing the permissible levels of impurities, in particular iron and silicon, which reduces the volume fraction of coarse second-phase particles. Because these secondary phases are often the nucleation sites for fatigue damage and fracture, improved purity levels led to more damage-tolerant variants of the well-known alloys, for example, alloy 2024 progressed to 2124, 2224, and ultimately 2524.

Evolution of properties improvements in conventional 2XXX- and 7XXX- series alloys. Note: ksi, kilopounds per square inch (1 ksi = 6.9 MPa, 1 ksi in. 1/2 = 1.1 MPa m 1/2).

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Strength improvements were accomplished through improvements in thermomechanical processing, including all elevated-temperature processing from ingot breakdown, rolling of plate, forging, extrusion, and so on, plus the final heat treatment. However, the increased use of composites, which have replaced many of the aluminum applications, has driven the industry to make more significant properties improvements, leading to the development of more competitive third-generation aluminum– lithium alloys.

First- and second-generation alloys had higher lithium contents, which was beneficial in terms of reducing density. Some additional potential benefits of lithium were improved strength, modulus, corrosion resistance, and fatigue and damage tolerance. However, not all of these potential benefits were realized, and some of the more significant issues with these alloys included low short-transverse fracture toughness, high anisotropy, and casting challenges.

These issues were largely overcome by third-generation airframe alloys, primarily based on the aluminum–copper– lithium system with lower lithium contents, targeting strength improvements with modest reductions in density. Incorporating minor levels of elements such as silver 16 and zinc improves both the strength and corrosion resistance of these alloys. This effort has resulted in improvements in microstructure control through thermomechanical processing and heat treatment to provide the improvements required. Advances continue in this alloy class in terms of increased strength, damage tolerance, corrosion resistance, and thermal stability with reduced density.

Titanium alloy development

Titanium and titanium alloys did not become production materials until the 1950s, under significant government support. Similarly to 2024 aluminum, Ti-6Al-4V was one of the first titanium alloys developed and remains the predominant titanium alloy in the aerospace industry, because of its balanced and robust property set. (Numbers in the alloy name indicate the weight percentages of each alloying addition.) In addition, numerous other titanium alloys have been developed over the years 17 that offer a wide range of properties. Ti-6Al-4V has an ultimate strength level of ~ 900 MPa with toughness ranging from ~55 MPa m1/2 to well over 100 MPa m1/2, depending on the annealing temperature. Ti-6Al-2Sn-2Zr-2Mo-2Cr used at a strength level of about 1100 MPa has a toughness of about 100 MPa m1/2, and Ti-10V-2Fe-3Al at about ~1240 MPa has a typical toughness of ~55 MPa m1/2.

At present, most alloy development for airframe materials is focused on cost reduction, with relatively few dollars going toward performance improvements. An effort that has been pursued successfully at Boeing is the development of fine-grain Ti-6Al-4V to enable a reduction of the superplastic-forming (SPF) temperature by about 110°C to about 775°C, and a reduction of the SPF/diffusion-bond temperature as well. The resulting reductions of the allowable processing temperatures has several significant advantages, such as a large increase in die life, a decrease in surface contamination, and much greater comfort for the operators who must transfer the sheets into and out of the press upon completion of forming. 18 Titanium is the only structural material with an alloy such as Ti-6Al-4V that is superplastically formable in sheets and, to a lesser extent, plates using standard production methods. Other alloy systems require special chemistries or special processing, increasing costs, and do not have the formability of Ti-6Al-4V.

Another area being studied is additive manufacturing, again to reduce component costs. 19 (See the articles in this issue by Babu et al. and Bandyopadhyay et al. for more information on additive manufacturing.) Both powder and wire input stocks are being evaluated utilizing laser-beam, electron-beam, and plasma-transferred-arc energy sources. Because input stock is significantly more expensive than wrought forms, the key savings would result from reducing the buy-to-fly ratio.

Some suppliers have estimated that quite significant cost savings could be achieved using this technology. However, one serious challenge is the nondestructive testing (NDT) of additively manufactured shapes. Initial applications will likely be for components with large fatigue and crack growth design margins. These would not be flight-critical and would provide the opportunity for suppliers to demonstrate that they can provide a product of consistent quality with on-time deliveries. As development proceeds, suppliers could develop sufficient fatigue and NDT data to provide customers the confidence they need to consider this technology for more critical applications. Current studies on additively manufactured parts are primarily focused on Ti-6Al-4V.

Another potential benefit of additive manufacturing is the opportunity to vary the material composition at different locations within a part. If higher strength is required in a given location, for example, but is not desirable over the entire part because of a corresponding loss in fracture toughness, one could modestly increase the oxygen or iron content in that location without changing the properties through the rest of the part.

Powder metallurgy also offers the opportunity to develop materials of much higher strengths than are possible using ingot metallurgy. Many of the most potent alloying additions to improve strength are difficult to melt because of segregation issues. This might not be an issue with powder products, however, as powder particles cool quite rapidly.

In the United States, performance improvements are being pursued through Air Force Research Laboratory-sponsored Materials Affordability Initiative (MAI) programs. These are research collaborations with industry through which each company commits funding to pursue common goals. One such initiative is alloy additions to alloys such as Ti-6Al-2Sn-4Zr-2Mo and β-21S (Ti-15Mo-3Al-2.7Nb-0.25Si) to improve the elevated-temperature and creep strengths with a concomitant increase in oxidation resistance.

Ferrous alloy development

In general, steels offer the highest strengths for commercial metallic structures and span a limited number of applications in aircraft such as landing gear, flap tracks, actuation components, and systems. The highest-tonnage ferrous alloy used for airframes is the 4340M (or 300M) alloy, also referred to as a high-strength low-alloy (HSLA) steel. This alloy is used at a minimum tensile strength of 1930 MPa with a toughness of ~60 MPa m1/2. This chromium–molybdenum steel alloy was used for most of the landing-gear structures prior to the 1990s. For new commercial aircraft designs, β-titanium alloys have replaced steels in many of these applications.

Since about 2000, landing-gear structures for US Navy aircraft have had to meet a minimum fracture toughness of 110 MPa m 1/2. This requirement resulted in the development of AerMet 100 by Carpenter Technology Corporation (Carpenter), which meets the 1930 MPa ultimate strength requirement with a minimum toughness of 110 MPa m 1/2. This is not a stainless steel; it has corrosion characteristics similar to those of 300M, but with a minimum toughness about twice that of 300M. 20 It is used for applications such as the main landing gear on F-18, F-22, and F-35 fighters and the arrestor hook on the F-35. Carpenter also developed AerMet 310, which has the capability of being heat-treated to over 2000 MPa, still with a toughness superior to that of 300M. Although the improvements in mechanical properties significantly improved performance, the lack of stainless corrosion properties limited applicability because of customers’ desires to reduce maintenance.

Corrosion is a significant issue for steel landing-gear structures. About every 7–10 years, the landing gear must be removed from the aircraft and cleaned. Specifically, cadmium and chromium plating must be chemically removed and the landing gear refurbished to remove any rust or pits, after which the part is reassembled. This takes considerable time, effort, and expense, compounded by the loss of aircraft service during maintenance.

Stainless steels are also used on airframes, and their usage has been increasing since about 2000 with the development of higher-strength grades. A driving force for their development is an interest in extending the time required between refurbishments of the landing gear. These alloys have high nickel and chromium contents, providing good corrosion resistance. Alloys such as 15-5PH (precipitation hardening) and PH13-8 stainless steel alloys provide corrosion resistance, but their strength until recently was limited to approximately the 1035– 1520 MPa range. Carpenter developed Custom 465 as part of an effort to achieve higher-strength stainless steels that can be heat-treated up to the 1930 MPa strength level as a direct replacement for 4340M. This would mitigate the corrosion issue and eliminate the need to use undesirable cadmium for corrosion protection. At this stage, Carpenter’s Custom 465 has been heat-treated to ultimate tensile strengths of ~1655–1795 MPa with the performance of a “true” stainless steel. The resulting parts have been used by major aircraft manufacturers worldwide for applications such as torque tubes, pneumatic cylinders, braces, struts, fuse pins, and flap tracks. Carpenter also reports making progress on a new stainless steel alloy, Custom 565, that can be heat-treated to very close to the 1930 MPa target. 21

Using an ICME approach (see the article in this issue by Xiong and Olson), QuesTek Innovations developed two new stainless-steel-type alloys: (1) Ferrium S53 (AMS 5992) has a minimum tensile strength of 1930 MPa, matching that of 300M, with better corrosion resistance than the latter alloy. This was the first ICME-designed and qualified alloy to fly (in December 2010), when it was used on landing gear on the Northrop T-38 aircraft. (2) Ferrium M54 steel (AMS 5616) was designed as an ultrahigh-strength and high-fracture-toughness steel (minimum value of 110 MPa m 1/2) with high resistance to stress–corrosion cracking. M54 has been qualified by the US Navy for safety-critical hook shanks on the T-45 trainer and is in production for future spares. 22

Two ultrahigh-performance carburizable steels have also been designed to replace AISI 9310 and Pyrowear alloy 53 because of their higher strength, toughness, surface hardness, and fatigue and temperature resistance. Ferrium C61 (AMS 6517) has a typical ultimate strength of 1655 MPa and is being qualified for the transmission shafts of Boeing’s Chinook helicopter upgrade, allowing for increased power density with the existing geometry. Ferrium C64 (AMS 6509) is a higher-hardness alloy that is being qualified for future helicopter transmission-gear steels across the US Army and Navy. 22

Utilization of nickel-based alloys on commercial airframes has been minimal; they are included in the miscellaneous category in Figure 3. Inconel 625 is used, mostly as a sheet-type product, in the annealed condition at a minimum tensile strength of 827 MPa at temperatures of about 700°C and sometimes slightly higher, primarily for plug and nozzle applications in the engine exhaust area. (This section of the aircraft is separate from the engine propulsion unit and contains, shapes, and directs the engine exhaust plume.) It is also used for brackets and high-temperature ducts. Inconel 718 is used in the solution-treated and aged condition at a tensile strength of 1240 MPa in the nacelle area at temperatures up to 650°C. It is also used for high-strength fasteners at minimum tensile strengths of 1520 MPa. The primary product forms are sheet, high-pressure ducts, and bar.

Composites development

All of the potential benefits of using structural composites as an alternative to metallic structures have been attained in varying degrees, based on experience with composite materials in aircraft. However, there are two open issues affecting composite material selection: (1) overall cost trends and (2) long-term maintainability and repairability.

The first composites used were “wet-layup” composites that impregnated dry fiber with polyester resin (much like for boats). These wet layups required high skill levels and, once the resin was mixed, a short-fuse process. The Boeing Stratocruiser long-range airliner achieved a 20% weight savings over metal ducting by using a fiberglass composite. Supplier pre-impregnated fabrics (called prepregs) that provided consistent resin content and eliminated the messy process of wet layup were first used in 1961. The B727 aircraft utilized a first-generation fiberglass-reinforced cured epoxy composite for radomes and fairing panels. The B737 aircraft used both a first-generation fiberglass-reinforced 175°C-cure epoxy in the hot areas and a second-generation fiberglass-reinforced 120°C-cure epoxy (rubber-toughened/self-adhesive) on radomes, fairings, and control-surface cover panels, mainly with a honeycomb core. The B747 used similar materials in similar applications on a much larger scale. The progression of composite development at Boeing is shown in Figure 5.

Timeline of composites development on Boeing aircraft. Note: CFRP, carbon-fiber-reinforced polymer; GRP, glass-reinforced plastic; ACEE, Aircraft Energy Efficiency; B, Boeing; DC, Douglas Commercial; MD, McDonnell Douglas; NG, Next Generation.

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The introduction of carbon fibers in commercial aircraft came about as a result of a NASA program (1975–1985) in collaboration with Boeing, McDonnell Douglas, and Lockheed, called the Aircraft Energy Efficiency (ACEE) Program, to design and fabricate CFRP parts. Among the parts manufactured through this program were B737 spoilers; a B727 elevator; and a B737 horizontal stabilizer torque box, where the latter was the first primary structure made from CFRP by Boeing. The service experience for these parts was excellent, with the horizontal stabilizer torque boxes still in commercial service.

The success of this effort led Boeing to employ CFRP on the B767 aircraft using the concepts developed through the NASA program. The inboard ailerons, elevators, and rudders used the same material and design as the ACEE B727 elevator, which used a standard-modulus carbon fiber with an untoughened 175°C resin cocured with aramid paper honeycomb core to make panelized skins, spars, and ribs that were bolted together. The B737 spoilers and outboard ailerons fabricated within the NASA program were made from polyacrylonitrile-based standard-modulus (220 GPa) carbon fibers reinforced with 120°C- and 175°C-cure epoxy matrixes. This yielded a full-depth aluminum honeycomb core with precured skins bonded secondarily.

The B777 empennage and floor beams were fabricated using intermediate-modulus (290 GPa) carbon-fiber prepregs for primary structure. In addition to the higher modulus, these prepregs had significantly better impact resistance. The success here then led to utilization of CFRP for wing, empennage, and fuselage skins for the B787.

Other types of composites are also being evaluated. Titanium–graphite is a combination of titanium foil (Ti-15V-3Cr-3Al-3Sn) and carbon-fiber epoxy, which improves the impact resistance and bearing strength of the laminate. Another fiber-reinforced polymer–metal composite is a combination of aluminum sheets and glass fiber/epoxy. The fiberglass improves the crack-growth (damage-tolerance) performance of the aluminum.

As a general rule, composite parts are lighter than their aluminum counterparts, but their costs have historically been significantly higher. One way to offset this disadvantage is to reduce the part count. Composites provide the capability to bond many smaller parts into a more monolithic structure, which reduces the number of fasteners. If done properly, the cost of the resulting monolithic part is less than that of fabricating and assembling multiple parts to form a metallic structure.

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