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Navigating Challenges: Welding in Aerospace Materials

Welding aerospace parts and structural elements requires a lot of skill due to stringent industry standards and challenging specialized materials. Aircraft and spacecraft rely on materials like titanium, nickel superalloys, special aluminum types, beryllium, and specialty steels. Almost every aerospace material requires a specialized approach because the welding process can negatively alter the properties of these metals and lead to many weld discontinuities and defects.

What Makes A Good Aerospace Material?

Aerospace materials must tick several critical boxes, including but not limited to:

• High strength. All materials used in aircraft must be able to withstand the forces they are subjected to. Tensile, compression, shear, and torsional forces can be extreme in aircraft, requiring the application of exceptionally strong materials.
• Long-term reliability and high resistance to corrosion and fatigue cracking.
• Low weight. This is essential for aircraft fuel economy. Every aerospace manufacturer prioritizes reducing weight as it’s the most critical aspect of an aircraft’s cost-effectiveness.
• Low thermal expansion. Cooling and heating cause materials to shrink and expand, leading to cyclical thermal stresses in critical aircraft and spacecraft parts.
• Good weldability. Metals used in the aerospace industry often require welding with specialized processes. The easier the material is for welding, the better. But, this is hardly ever the case with aerospace materials. 

 

Aerospace materials directly influence the initial and ongoing costs of an aircraft, fuel economy, design options, speed and range of an aircraft, safety, recycling, and even the passenger’s comfort. 

Just to name one example of how material selection can alter the characteristics of an airplane, we can look at how aluminum use influences passengers’ experience. Humidity must be intentionally lowered during flight to prevent aluminum oxidation if the airplane uses aluminum for structural materials. As a result, passengers may get dehydrated on long flights and have a less convenient flight. So, seemingly unrelated phenomena like using aluminum in the airplane can have unexpected outcomes. The importance of alloy selection for any aircraft cannot be overstated, as all metals have pros and cons that must be carefully weighed.

When engineers design aircraft, they must consider a vast array of problems and choose materials to meet the most critical demands of the aircraft. However, welding technologies and the weldability of the materials play a crucial role in alloy selection.

Welding Aerospace Materials – Challenges And Solutions

Welding codes and aerospace industry standards require exceptional weld quality in almost all welding applications, especially critical parts. Landing gear, structural elements, fuel tank, exhaust system, and fuel and hydraulic lines must be welded with utmost quality. There is no room for error. Meticulous non-destructive testing (NDT) can spot all weld flaws, usually prompting rework. 

Such stringent requirements paired with highly exotic alloys that do not respond well to high heat make welding in the aerospace industry very challenging. However, every problem has a solution. There is always at least one welding process that can be configured to meet the demands of almost any alloy in the aerospace industry. 

Aluminum

Aluminum has been the top choice for most aerospace applications ever since it superseded wood in the 1920s/1930s. The first aluminum alloy used for aircraft was Duralumin. Today, modern aircraft rely on far more advanced and highly specialized aluminum alloys for excellent structural integrity, recyclability, and low cost and weight. 

While aluminum does pose challenges like oxidation, as we mentioned earlier, it’s still a highly relevant material for commercial, military, and private aircraft. Aluminum has excellent machineability, long fatigue life, high damage tolerance and facture toughness, and a good corrosion resistance.

Welding aluminum is challenging, especially when you need to meet AWS D17.1 specification for fusion welding for aerospace applications. 

There are many aluminum alloys in the aerospace industry. Some alloys can be fusion welded, while others require friction welding and other specialized welding processes. For example, the most commonly used aluminum alloys for aerospace structural components are the 2000 and 7000 series aluminum, which usually require friction stir welding. 

Aluminum alloys like 1000, 3000, and 5000 are also frequently used in aerospace construction for wing ribs, stiffeners, tanks, framework, ducting, flaring, wheel pants, nose bowls, cowlings, fillets, oil tanks, and other important aircraft parts. These alloys can be fusion welded using the MIG and TIG welding processes.

TIG welding aluminum produces the best fusion welding results. The TIG welding process allows precise heat input control, filler metal addition, and weld accuracy. This welding process is also exceptionally clean, with a minimal chance of inclusions.

Aluminum has a surface oxide layer that melts at a significantly higher temperature than the base metal underneath. This is one of the biggest challenges when welding aluminum. However, this problem is eliminated when using advanced AC TIG power sources. The positive side of the alternating current breaks the oxide layer, while the negative side melts the base material. High-end power sources give you precise control over the AC balance and individual amperage values for cleaning and penetration sides of AC TIG. The best example is the Miller Dynasty 400 AC/DC TIG welder with its extensive array of highly customizable functions.

Welding aluminum for aerospace applications also benefits from pulsed AC TIG and multiple waveform selection. Aluminum can easily warp due to excessive heat input, but applying a correct pulsed waveform can make distortion less likely to occur. This is critical for the aerospace industry, where distortion tolerances are minimal.

Magnesium

Magnesium is 33% lighter than aluminum, making it one of the lightest structural metals in use today. However, magnesium isn’t used in significant quantities in aircraft due to its high cost and lower stiffness and strength compared to aluminum. It used to be one of the most commonly used materials for aircraft in 1940s and 1950s, but aluminum has superseded it in many applications.

Today, magnesium is still used for gearboxes and gearbox housings of aircraft, covers of components, electronic housings, flight control systems, aircraft wheels, and the transmission housing of helicopters. While aluminum is less costly and can have higher strength, magnesium’s low weight and excellent machineability make it an irreplaceable material for certain commercial and military aircraft parts. 

Welding magnesium for aerospace applications often requires the friction welding process. However, less critical aerospace parts can be fusion welded with TIG and MIG. This is especially the case when repairing aircraft parts and engine castings. 

Welding castings from magnesium requires high expertise. Magnesium is flammable, castings can soak in oil (which can significantly impact the weld quality), and magnesium alloys can contain zinc (which evaporates and can cause porosity). 

Like aluminum, magnesium has oxides on its surface that must be removed before welding. But, AC TIG can significantly reduce the chances of oxide inclusion when set correctly. 

The MIG welding process can often be a better choice for magnesium, depending on the welded part. When repairing a thick magnesium piece, MIG’s high heat and deposition rate is beneficial. Since magnesium conducts heat rapidly like aluminum, you need more heat to keep the puddle going, and this is especially the case with thick casting walls.

However, like aluminum, magnesium filler metal wire is much softer than steel wire, which requires using a specialized wire feeding system. This is where advanced aluminum and magnesium MIG welding systems come in. 

The Miller AlumaFeed Synergic aluminum MIG welder coupled with the Miller XR-Aluma-Pro Push-Pull gun, offer a seamless MIG welding experience with soft filler materials with advanced arc output control. With up to 900 inches per minute wire feeding speed and up to 425A output, you can weld most aluminum and magnesium thicknesses with high efficiency. In addition, Miller’s Profile Pulse allows you to achieve a TIG-like weld appearance using a pulsed MIG welding process.

Titanium

Titanium is the holy grail of metals. It’s the most critical material in the aerospace industry thanks to its incredible strength, low weight, excellent corrosion resistance, and ability to retain its mechanical properties under high temperatures. Titanium ticks all boxes except price, so its applications are limited to critical aircraft parts like airframe structures and jet engine components.

Titanium aircraft can withstand supersonic speeds higher than Mach 2. Aluminum becomes soft when exceeding Mach 1.5 due to friction between the airplane skin and the air. Titanium was the material USAF used to develop and construct the SR-71 Blackbird in the 1960s, allowing this engineering marvel to reach speeds above Mach 3. No other material could provide the necessary strength and heat resistance for the most sophisticated and fastest aircraft of all time. On its last flight in 1990, the SR-71 Blackbird set a speed record by flying from Los Angeles to Washington, D.C. in 64 minutes and 20 seconds. Titanium’s contribution to the speed of modern aircraft cannot be overstated. 

Today, military aircraft, like the famous F-22 Raptor, still utilize titanium alloys in far greater quantities than commercial aircraft. That’s because titanium structural members can withstand extreme loads generated by air maneuvers that jets must be able to handle. But, titanium is an irreplaceable material for all aircraft and spacecraft. Commercial airliners, helicopters, military jets, drones, and other aircraft rely on titanium’s strength for structural members and low weight to improve their range and fuel economy.

Welding titanium is challenging because this material quickly oxidizes at high temperatures and requires exceptional weld cleanliness and purity. High weld accuracy and impeccable arc stability are non-negotiable when working with titanium in the aerospace industry.

Pure commercial titanium, alpha and near alpha, and some alpha-beta alloys are weldable, while some alpha-beta titanium alloys are very challenging to weld. However, the extremely strong alpha-beta titanium alloy Ti-6Al-4V is weldable, which is one of the reasons it is commonly used for aerospace structural components. 

Welding titanium requires meticulous care in ensuring the shielding gas coverage. Not only does the weld pool need to be protected with argon gas, but you also need a trailing shielding gas coverage to protect the weld as you move along the joint. Since titanium is highly reactive with oxygen at elevated temperatures, it’s critical to ensure that the welded joint is covered with a shielding gas as it cools. One of the best indications of an overly oxidized titanium weld is discoloration, which is why AWS D17.1 requires discolored titanium welds in a range from violet to white to be rejected. Overly oxidated titanium welds become brittle and cannot withstand the forces exerted on the aircraft’s structural members or engine components.

Pulsed TIG can help prevent oxidation by reducing the heat input. The lower the heat, the lesser the chance of titanium to react with the oxygen in the atmosphere. This is again where advanced TIG welding machines, like the Miller Dynasty 400 AC/DC, can save the day. The Dynasty can output up to 5000 pulses per second in DC TIG, giving you better control over the heat input and reducing the chances of titanium oxidation. Likewise, using such extreme pulse speeds concentrates the arc, allowing higher travel speed, which again reduces excessive heat input. 

One particular benefit of high pulses per second when welding sluggish materials like titanium is breaking surface tension through arc pulses. This can sometimes help eliminate porosity and improve puddle wetting. 

Superalloys

Titanium can operate at high temperatures and retain its properties, but its limit is at about 1100°F (600°C). On the other hand, the temperatures in jet and rocket engines reach far higher temperatures of up to 3100°F (1700°C). The need for materials that can withstand these extreme temperatures led to the development of nickel, iron-nickel, and cobalt superalloys.

Superalloys are some of the greatest human inventions yet. Nothing like them exists in nature. These materials retain all of their mechanical properties in brutal conditions at extreme temperatures where jet fuel meets air, pressure, and fire, resulting in combustion that would melt or negatively influence pretty much any material known to us. The development of these alloys has allowed engineers to dramatically raise the thrust power of jet-powered engines, resulting in far greater speed and significantly lower fuel consumption. 

Superalloys are usually named by the company that developed them. The most critical nickel superalloy used in jet engines is the Inconel 718. Other examples include Hastelloy X, Inconel 625, Inconel 901, Rene 95, and Discaloy. 

All superalloys contain a very carefully selected mix of alloying elements that can include niobium, yttrium, cerium, ruthenium, platinum, iridium, zirconium, titanium, tantalum, tungsten, and many other rare-earth materials.

Welding superalloys is challenging because it’s extremely important not to negatively alter the material in the weld zone and heat affected zone (HAZ). Some of the biggest welding challenges are:

• Alloying elements with a low melting point can cause embrittlement. Lead and sulfur are a good example.
• Cracking due to stress concentration in the HAZ.
• Incomplete fusion.
• Carbide precipitation and subsequent crack development.
• Successfully manipulating the weld metal. The challenge is the sluggishness of nickel alloys, which makes welding accuracy difficult.
• Excessive heat input can cause harmful metallurgical changes, resulting in weld cracking and loss of corrosion resistance.

 

The TIG welding process can produce excellent results when welding many aerospace superalloys, particularly with the Inconel 718 and similar superalloys with excellent weldability. Of course, like with all aerospace alloys, the welding process selection depends on many variables, not limited to the application.

Advanced TIG welding power sources can make welding superalloys less challenging thanks to high-end features that allow maximum arc control. For example, pulsed DC TIG can help reduce excessive heat input and improve the solidification structure and tensile properties of the weld. Nickel and cobalt superalloys are very sensitive to the welding approach, which is why it’s critical to use a TIG power source that has an extensive array of settings. 

Sometimes, the difference between an inspection-passing weld and a flawed weld is just about micro adjustments on the power source. This is particularly true when using pulsed TIG, as high pulses per second can make it far easier to weld highly sluggish nickel alloys. Nickel alloys aren’t as fluid as steel when in the molten state. So, high arc pulses can help break their surface tension and settle the weld better by agitating the puddle. But, to maximize this effect, the pulse settings must be set according to your travel speed, the alloy type, thickness, and desired weld profile. 

Specialty Steels

The aerospace industry wouldn’t be complete without steel. But, not all steels can meet the criteria for aircraft and spacecraft. The steels used are usually about two times the strength of titanium and three times stronger than aluminum. However, as strong as specialty steels can be, there is no getting around their excessive weight. So, high-strength steels are only used for about 5-10% of aircraft’s structural elements. Steel application is usually limited to critical elements like landing gear and wing box components. 

The three most commonly used steel types in the aerospace industry are:

• Maraging Steel – This is a unique steel type with exceptionally low carbon content but incredibly high strength, ductility, and fracture toughness. Maraging steel is one of the strongest metals ever discovered, and it’s often used for heavily loaded aerospace components.
• Precipitation Hardening Stainless Steel – PH stainless steel is corrosion resistant and very strong, making it an excellent choice for applications where these properties are critical.
• Medium Carbon Low-Alloy Steels – These steel types contain between 0.25% and 0.5% and various alloying elements to increase hardness and high-temperature strength. They are typically used for undercarriage parts.

 

These specialty steels are best welded with minimal heat input and high travel speed. For example, Maraging steel should not be held at elevated temperatures for a long time. While these materials can be MIG welded, TIG welding again produces a better result due to its clean nature. Still, pulsed MIG can be more favorable in some applications for productivity gain.

Specialty steels may require post-welding heat treatment (PWHT) and specific interpass temperatures to be maintained, depending on the alloy and application. Whenever the work requires precise temperature control, we recommend the Miller ProHeat 35 induction heating system. Induction heating heats the material from within, resulting in high efficiency, ease of setup, maximum temperature control, and uniform heat treatment. 

Red-D-Arc – Your Source For Specialized Welding Equipment

The number of materials in the aerospace industry is enormous, and each requires a different welding approach. Even composite materials rely on welding for mold fabrication and repair. Our team of experts can help you choose and optimize the most suitable arc welding process for fusion welding of aerospace parts, whether you are a small repair shop or a large contractor in the aerospace industry.

We have a massive fleet of specialized welding equipment for all industries, and aerospace is no exception. Contact us today to learn more about our advanced MIG and TIG power sources and how we can help you meet your client’s expectations.