Composites and Superalloys Fill Aerospace Needs
It started with spruce and linen
By Robert Aronson
Aerospace requirements for lighter weight, greater strength, and more precision have spawned a new generation of materials, and the processes and machines needed to convert those materials into parts. With engines, higher-strength steel alloys and titanium dominate, while composites are the materials most used in fuselage design. Here's a look at how some major players are reacting to the new challenges.
What's Happening With Grinding? The aerospace industry has seen dramatic changes in the last 10 years in the ways it processes engine components. According to Technology Manager, Michael Hitchiner, St. Gobain Abrasives, Romulus, MI, the industry has moved away from grinding on large, complex machines to CBNbased grinding strategies for grinding nickel and cobalt-based superalloys on small, purpose-built three, four, and five-axis machines.
"Aerospace" is a catch-all that can cover many applications and industries from space exploration to aircraft engines to power generation. But for this article, the term aerospace refers to gas turbine technology for commercial and military aeroengines, as well as for land and marine-based power generators.
"Improvements in grinding-wheel technology have resulted in a shift away from machining and broaching to grinding for aircraft engine and even large land-based, power-generation components," says Hitchiner. "Grinding machines are more multifunctional, with milling, drilling, and deburring capabilities. At the same time, machining centers have acquired grinding capability.
"Heavy stock removal necessitates creep-feed grinding with conventional abrasive wheels, and new coolant-delivery strategies. These changes are not limited to the highest-volume components such as blades, vanes, shrouds, honeycomb, and buckets, but also chromed landing gear, gear coupling, and assembled-blade-tip grinding.
"For turbine engines, demands for ever higher performance has resulted in greater use of high-performance materials combined with more efficient processes," he explains.
The temperatures and pressures inside a gas turbine exceed the temperature limits of conventional materials. In the hottest stages, they can be as high as 1000°C. Demands for high-temperature strength and corrosion resistance have be combined with weight-reduction requirements. "Titanium alloys and heat-resistant superalloys based on iron, nickel and some cobalt are the dominant engine metals," Hitchiner continues. "Although there has been some interest in aluminides and metal-matrix composites made of Kevlar for low-pressure blades, along with ceramics for noncritical turbine blades, and HVOF carbide sprays to replace chrome in landing gears.
"Titanium alloys offer major advantages in terms of strength-to-weight for applications up to at least 540°C. However, these materials present problems when grinding. Titanium is chemically reactive with CBN. Some promising efforts have been made to develop diamond as an abrasive. However, porous vitrified SiC wheels are still dominant.
In addition, the machinability of titanium is significantly better than its grindability, with processing therefore moving from grinding to milling and turning, using primarily fine-grained, uncoated carbide tools and inserts.
"For aeroengines, precipitation-hardened stainless is most preferred for low-pressure compressor blades and vanes, with alloy steels used for hydraulic and gearbox systems.
"Some of these materials can withstand temperatures to 670°C or have very low thermal-expansion coefficients. As to grinding problems, most produce long chips and cause loading. These metals also work-harden, producing high grinding forces and wheel wear.
"Currently, most applications are ground using the more advanced ceramic-type grains such as SG or TG, or regular alox grains in CDCF grinding mode. CBN abrasive usage is growing on new machines for bore and curvic gear grinding.
"Nickel alloys remain the most widely used of all materials. They still represent over 50% of the weight of an aeroengine and are used extensively for blades, vanes, buckets, shrouds, shafts, nozzles, honeycomb, and rotors. The raw material may be supplied as a forging, casting, or bar stock, depending on the application. Blades and vanes, for example, are supplied as castings, usually hollow to reduce weight. Because the grindability is so poor, grinding costs are high. To reduce this cost, casting suppliers have dramatically improved their ability to produce near-net-shape parts. Some blades are even produced as directionally solidified single crystals to maximize the strength in a particular direction. The material may also be supplied after various heat treatments, including annealing, solution treatment, and aging," explains Hitchiner.
St. Gobain experience has shown that many basic material grades have several levels of heat treatment, each of which significantly affects its grindability. Nevertheless, all nickel-based alloys share the properties that make their grindability or machinability challenging:
- High strength, including dynamic shear strength, is retained at the high temperatures seen in grinding.
- Poor thermal conductivity.
- Material structure contains hard carbides for wear resistance and crack arrest.
- Work hardening during chip formation.
- Sensitivity to heat damage.
"The grindability of nickel-based alloys based on G ratio and surface quality of chip form is about a twentieth that of carbon steels or gray cast iron. Nevertheless, grinding still offers the most efficient way of generating the required profile tolerances and finish, and is said to be making major advances over processes such as milling, turning, broaching, and EDM," Hitchiner concludes.
How Do You Cut It? Cutting-tool researchers are looking at what new materials are available in aerospace, what's in the works, and what cutting challenges these materials represent.
The aerospace industry is using more special alloys to meet the requirement of high temperatures, strength, precision, and reliability. According to Cliff Mays, Sandvik Coromont, Fairlawn, NJ, "For aerospace engines, these include heat-resistant superalloys, Inconel, and Waspaloy. Plus the engine manufacturers are coming up with proprietary materials. For example, Rolls Royce has developed RR 1000, a nickel-based powder metal material that can be inertia welded.
"It's one of several new blends of powder-metal materials designed specifically for higher operating temperatures," he explains. "For example, engine disks and blades have to operate at temperatures exceeding 3000°C. An aggressive cooling system is needed for these components to survive."
Boeing is looking at a new 5553 titanium alloy for the 787, and is using the TMMC composite made of silicon carbon fiber and titanium power.
A low-alloy, high-strength steel using silicon, vanadium carbon, and molybdenum, designated 300 M, is also being used in landing gears, but these will move to titanium in the next generation of planes.
"In the 'cooler' forward sections, titanium is still the dominant material," says Mays. "It's used chiefly for weight reduction. In titanium research, a major goal is to reduce surface microcracks to lengthen component life. Turbine shafts are chiefly high alloy steel that can operate at 100°C."
To be ready for the next generation of materials, Sandvik maintains a four-point research program. For all emerging materials, they look at component geometry restrictions, finish, machinability, and surface integrity. With this multipart research, a database of properties and performance can be used to quickly fill a customer's needs.
Another issue is toolholding, chiefly when reaching into a recessed area or when creating thin-walled parts. Special-purpose holders may have to be designed. But Sandvik's goal is to have standard toolholders for typical features.
RP-Made Aircraft Parts. Among the newer processes being applied to aerospace part manufacture is laser sintering. It's a rapid prototyping process pioneered by 3-D Systems (Stone Hill, SC) that has been adapted for prototype and low-volume part manufacture.
In this process, powdered nylon with various fillers is laid up in layers. After a layer of powder is laid down, a CO2 laser selectively heats the powder. The laser path is computer controlled to create the shape of the part.
Solid Concepts (Valencia, CA) specializes in using this technique to make actual parts. "The major advantage is the ability to economically make widely different shapes in low volume," explains Vice President, Chuck Alexander. "One aerospace application is the manufacture of cooling tubes. These have to be installed in a limited space and must fit around other components, and so generally require a very complex shape. Parts made with this method are generally small, less than 16" (407 mm) in length. A big advantage is that no tooling is required, and a single laser-sintered part may replace four or five elements formerly needed."
"Eliminating parts brings a savings beyond part-cost reduction," he says. "With one part replacing four or five, the required traceability data is greatly reduced. Often, the cost of creating and handling trace data exceeds the cost of the part."
Manual layup is one of the more costly elements in composite utilization, particularly on complex surfaces. Automated systems are gaining popularity and have been available for some time. Under computer control they apply composite tapes in various widths to match contoured surfaces. For example, Goodrich Aerostructure (Los Angeles) is using new machine from Ingersoll (Rockford, IL) to replace the manual method for building major composite components for the 787 nacelle system. The Ingersoll machine applies composite filament tape to flat or contoured surfaces at speeds up to 1200 ipm ( 30 m/min). The machine combines numeric, pneumatic and adaptive controls to lay 1, 3 and 6" (25, 75, 150-mm) wide composite filament tape at contours of ±30° degrees.
Get Rid of Those Autoclaves!
Eliminating autoclaves by development of out of autoclave cure of materials and processes is the "holy grail" for composite manufactures and users. Because of their size, cost, and immobility, they are an obstruction to the wider use of composites. In most of the currently used processes for larger composite components, it is necessary to compress and cure the composite once it is formed. And the autoclave is used for that purpose.
(A further incentive to greater use of composites is that the price of aluminum has been skyrocketing.)
When composites were used chiefly on smaller parts, this was less of an important issue. But now that the entire fuselage of an aircraft may be made of composites, the autoclave in many cases can not handle the size of the components and production volume. It also limits the size of the part so large pieces have to be done in segments. That means joints will use fasteners that add to cost, weight, and reliability.
That's why much of the research on composites is devoted to eliminating the autoclave process step. Several ideas are being worked on. One effort is to ensure the composite layers are accurately laid up and there are no voids between the layers. If void elimination caused by moisture could be assured, the material could cure at atmospheric pressure. Then only an oven cure and bag vacuum would be needed.
A problem with the process is that the prepreg materials, the rolls of material that make up the large, composite sections/parts are kept in freezers until they are laid-up. The prepregs have to be removed from the freezer and there is a limited time before they begin to warm and harden. One solution is to change the prepregs compound so that it does not start to set for days if necessary.
A planned project for the near future is the follow-on to the space shuttle. A primary component of the next vehicle will be made up of a honey composite section sandwiched between two composite pieces of material, each 18' (5.5 m) in diameter and 18.5' (5.6-m) long. It is important the joints be eliminated so the shell is not failure sensitive, and that can be done with composites without the addition of joints or fasteners.
Another idea being tried is to encapsulate the prepregs to a temperature high enough to raise the boiling point but not the cure temperature. Once the moisture is driven out, the temperature is raised and prepregs can be cured.
A major weight benefit of composite use would be the bonding a composite fuselage to eliminate most fasteners. For example, the F-18 fighter currently in our inventory has over 100,000 fasteners, which accounts for a significant amount of the plane's weight and cost. Plus fastener placement is chiefly a manual job. The mechanic has to locate the hole position, drill the hole, insert and secure the fastener, then, where necessary, fill the fastener head.
Another advance in the works is reduction of composite thickness. Currently a large number of layers are used to ensure strength and resistance to impact detection.
To more accurately determine if the composite has been damaged by undetected impact, a sensor has been developed. If anything strikes the surface, like a wrench during maintenance or bullet in combat, the sensor sends a signal to a receiver that records the location and severity of the impact.
Currently 25–50% of the composite layup above minimum requirements is thicker than it needs to be so there is an ample safety factor to compensate for undetected impact through current visual inspection methods. With better understanding of composite performance, and sensors to evaluate damage, It may be possible to reduce the average thickness by up to 40%. This would mean up to a 6% reduction in total aircraft weight, and reduced cost for fabrication and assembly of an airframe.
Another major change in aircraft development is the increase in the use and number of unpiloted aircraft. Currently reconnaissance Uninhabited Aerial Vehicles (UAV) are common, but it is expected that it will be possible in the future to have fighter and attack aircraft flying autonomous missions.
With no pilot, there are a many opportunities for design change. First, the design does not have to consider keeping the pilot alive and functioning, so the g forces can be greatly increased without consideration to the pilot's ability to survive the stresses. This has opened a new set of requirements for titanium. Bulkheads are more numerous and have to be much stronger. Current casting methods are not sufficient to meet the increased loads at higher forces and therefore the components made from titanium have to be increased which adds weight and complexity. This increases the need for higher strength "super" strength titanium casting and machining methods that will provide the greater strength required by higher UAV performance without increase in weight and manufacturing complexity.
George N. Bullen
Principal Engineer, Production Engineering,
Los Angeles, CA
This article was first published in the March 2008 edition of Manufacturing Engineering magazine.