Brazing engineer Ed Arata has a simple answer when asked where brazing technologies are used within the structure of an aircraft: “Brazing is used just about everywhere—it’s difficult to classify.” Arata, an engineer at Morgan Advanced Materials (also known as Wesgo) in Hayward, California, which specializes in developing and manufacturing brazing alloys, says the company’s alloys are used for everything from repairing aeroengine turbine blades to making the rocket nozzles for spacecraft to constructing hydraulic assemblies.
Nick Ludford, principal project leader at The Welding Institute (TWI) in Cambridge, England, concurs that brazing is widely used in aerospace and says manufacturing and repairing turbine blades is one of the most important applications. But the technology is also used in the subassembly of sensors for pressure, temperature and speed; the construction of aluminum and steel heat exchangers; and also for manufacturing hydraulic lines on older aircraft. With an eye on the future, TWI is investigating the brazing of aerospace components produced via additive manufacturing (3D printing), he says.
How Brazing is Unique
But what is brazing, and why is it used so extensively in manufacturing aerospace components? Vacuum brazing, the process typically used in the factory at Morgan Advanced Materials, enables the creation of complex components by joining parts using a braze metal or alloy. The brazing alloy has a lower melting point than the parts being joined and when heated flows by capillary action—the ability of a liquid to flow in narrow spaces without the assistance of, or in opposition to, external forces like gravity—into the space between the parts, creating an exceptionally strong, sealed joint. In fact, anyone who has ever soldered copper pipes in their home or circuits in an electronics assembly has carried out a process similar to brazing.
Brazing is distinguished from soldering because it takes place at temperatures greater than 450C (842F), explains Arata. “When you join pipework, you don’t melt the pipe; you’re melting the filler, that is, the solder material. It flows in and fills the joint,” he explains. This distinguishes both brazing and soldering from welding, in which the base materials to be joined are heated to their melting point and united by pressing or hammering them together. Brazing is also distinct from soldering not only because of the higher temperatures used but also because of the tight fit and strength of the joint.
Advantages of vacuum brazing, according to aerospace heat treatment and vacuum-brazing specialist Wallwork Group of Bury, England, are that it is a clean and flux-free process. Wallwork says it also produces “high-strength components with excellent dimensional stability, free of voids and inclusions, resistant to shock and vibration and able to withstand high pressures.” Brazing is suited to the assembly of multi-part, complex components and to joining dissimilar materials, including some non-metals. The parts to be joined may be produced by a variety of methods including cold stamping and forming, casting, forging, extrusion, fabrication or machining and could be made from almost any metal, alloy or even ceramics, Wallwork explains. A major advantage of brazing is its ability to join the same or different metals in a way that makes them very strong.
Brazing’s role in Aerospace
Brazing is widely used within the aerospace industry for a number of reasons. These include: cost savings because multiple parts can be processed together; the fact that multiple joints can be formed simultaneously; minimum distortion to parts, as the whole assembly is heated up and cooled uniformly; and the ability to “join dissimilar materials with different coefficient thermal expansion rates, including ceramics,” says Ludford.
The technology’s evolution has been considerable. “In the early days, brazing development [at TWI] was more experimental work. Nowadays, companies are looking for process control and improvement and auditing of the process, systems and quality and process development,” he says.
Like Morgan Advanced Materials and Wallwork, TWI uses vacuum-furnace technology for brazing, as well as some other techniques.
The company has recently invested in a new industrial-scale vacuum furnace to support its aerospace activities. “This facility, along with other associated equipment, is specifically aligned with the requirements through process, quality controls and documentation that are required in the aerospace industry today,” says Ludford.
For example, aerospace standards that brazed components must meet include ISO9001, AS9100, NADCAP, AMS2750 Rev E and internationally recognized standards for brazing as well. “A properly designed and brazed joint will usually never fail. If it fails, something has gone wrong. Usually, once you qualify a braze process, you never think about it again. It’s the whole assembly that will fail first, and the braze joint is good for the lifetime of the assembly or component,” says Arata.
Arata and his team are designing brazes for ceramic-ceramic, ceramic-metal and metal-metal joints, including mechanical and thermal parameters as well as alloy selection for both vacuum- and hydrogen-brazing environments at Morgan Advanced Materials’ 220,000-ft.2 facility in Hayward, which serves as the company’s global center of excellence for joining technologies.
Braze alloy materials produced on- site are typically made from precious metals such as gold and silver. As well as manufacturing and supplying braze alloys—which can come in wire, powder, sheet, foil and paste forms—the company also provides brazing on-site as a service for customers.
Arata explains that Wesgo’s (in the metals industry, the company is commonly known by its former name) background was originally as a precious metals recycler and reseller. “Our expertise is in gold, silver and platinum. We focus on high-purity materials. Gold and silver are relatively high-temperature and high-strength, and they are corrosion-resistant, too. Aerospace requires high-temperature materials with high-strength and corrosion resistance. So we can fill that niche with braze alloys.”
At TWI in the UK, nickel, silver and other precious metals are used as braze filler metals, says Ludford. Other metals used to make braze alloys at Morgan/Wesgo include palladium and titanium.
As aeroengines run hotter, engineers are interested in the potential of ceramics to allow turbine blades to operate at higher temperatures, which is key to improving an aeroengine’s efficiency. That is because less air needs to be used to cool the turbine vane and blades and can instead be used for propulsion. According to the engineering department at Cambridge University in England, most engineering ceramics can be used at temperatures greater than 1,000C—but suffer from poor toughness and can be difficult to join. “We are interested in bonding these ceramic materials to structures within the engine,” says Arata. “Not all traditional braze alloys will stick to ceramic materials, but our active braze alloys bond metal to a ceramic in one step. Brazing means uniform heating, and we can go slow so that we do not crack the material during heating up or cooling down in the brazing cycle.”
As aerospace engine designers and engineers move toward higher temperatures within engines, new active braze alloys are being introduced that can bond metals and ceramics and allow metal to be bonded directly to ceramic without metallization. Brazing will continue to play a key role because it is difficult to mechanically bond ceramics to metal or weld ceramics to metal, Arata points out.
According to Cambridge University, coatings such as zirconia are among those being used to help increase aeroengine operating temperatures. These coatings can operate at much higher temperatures and protect the metal from chemical attack. Combined, the effects of air cooling and ceramic coatings mean that the combusting gases can reach temperatures of more than 1600C—higher temperatures than any metal can withstand.
Brazing For Repair
In terms of MRO, inside the turbine, products known as pre-sintered preforms (PSP) from Morgan are also being used to repair turbine vanes. If a vane breaks down due to excessive heat and wear, a PSP featuring a small amount of braze alloy mixed with the parent metal is used, primarily in the turbine section, to repair vane cracks and areas of wear.
“That means there’s an alternative to creating a new casting or a new forging of the housing for the engine, for example,” explains Arata. “We can build up material, and the customer can machine back to create a new part.”
PSPs are a customized blend of superalloy and braze powders in either a plate form or specific shape that allows selective buildup of worn surfaces to be achieved efficiently, providing time and cost savings. Extrudable paste also allows for the application of PSPs in difficult-to-reach areas, while a brushable paint form is used for sealing deep or narrow cracks or micro-cracks. PSP components also require minimal post-braze machining to restore a part to its original dimensions, Morgan says. “We’re allowing some very expensive components to achieve a much longer life,” says Arata.
The flexibility of vacuum-furnace brazing is seeing the technology find uses in other advanced aerospace applications. Britain’s Reaction Engines is one company relying on brazing technology to develop high-tech aerospace systems. Reaction Engines announced late last year that it had developed and installed a giant, bespoke vacuum furnace for brazing the pre-cooler system for its hypersonic propulsion system, the Synergetic Air-Breathing Rocket Engine (SABRE). Intended to combine the fuel efficiency of a jet engine with the high speed of a rocket, SABRE sounds like the stuff of science fiction. It promises to power hypersonic flights that would cut a 20-hr. journey in a Boeing 747 to 4 hr., traveling at speeds in excess of Mach 5.
Central to the SABRE design are its ultra-lightweight heat exchangers (pre-coolers), which prevent engine components from overheating at high flight speeds. Reaction Engines’ engineers needed to identify ultra-lightweight but strong metals and materials as well as how to join them together. They eventually determined that manufacturing the pre-cooler—effectively a complex system of ultra-fine but very robust metal tubes—would require a specialized furnace that could precisely control a brazing process to produce a design with “unique form, geometry and metallurgy,” the company says. The pre-cooler, composed of thousands of thin-walled tubes that carry coolant, features approximately 2,000 km of microfine tubing, with wall thicknesses half the width of a human hair. Every joint must be hermetically sealed, enabling the system to reduce air temperature from 1,000C to -150C in a fraction of a second.
Working with engineers at Reaction Engines, Consarc Corp. in the U.S., which makes furnaces and related systems for vacuum-induction melting, vacuum-arc remelting, and vacuum precision investment casting, as well as specialty furnaces for proprietary processes, delivered a custom vacuum-brazing furnace to Reaction Engines for manufacturing the pre-cooler. Everything about the furnace—from vacuum capability to its temperature control—has been calculated to ensure exact, repeatable results.
The result is a unique furnace that can quickly and accurately heat and cool modular assemblies containing large quantities of delicate tubing within controlled and repeatable conditions. The scale is part of what makes the furnace unique: Its hot-zone charge size is one of the largest of its kind in the world, Reaction Engines says, measuring 2.5 m (8.2 ft.) in diameter and 1.5 m high. “The sheer size ensures that the furnace will accommodate not only the pre-cooler’s test component but also the sections that will ultimately comprise the full-scale production version of SABRE,” Reaction Engines says.
Arata of Morgan notes that brazing is an economical technology for very complex assemblies such as Reaction Engines’ pre-cooler. “We have some customers that have 1,000 joints in one assembly. You put it in a vacuum furnace, and 4 hr. later it is complete.”
Brazing is also very good at joining different thicknesses of material as well as dissimilar materials such as metal to ceramics or steel to nickel alloys in aerospace applications. Arata agrees with TWI that the brazing of additively manufactured components is only going to increase in importance in the future.
“The big thing everybody is talking about is 3D printing. When you look at some of the materials that have emerged in the last five years, they are only going to get better and better. And there is going to be a need to bond those materials, whatever blend they are,” Arata says.
Brazing technology is also supporting the development of honeycomb metals for aerospace and the increased introduction of composite materials, he concludes.