Welding aluminum is challenging. Welding it for aerospace? Even more so. Meeting quality requirements and specifications is more difficult than in most other industries. The aerospace industry only resorts to welding when other options are unfeasible. Welding heat can weaken aluminum, making engineers even more reluctant to apply it. This is only the tip of the iceberg of the issues manufacturers face in aerospace.
Challenges of Welding Aluminum in Aerospace
Aerospace welding challenges with aluminum can be divided into three categories, as presented below. They are intertwined in the real world, but we can isolate them for a clearer understanding.
Most aerospace welding challenges come down to one core problem. The industry requires parts that are lightweight yet strong enough to withstand repeated stress cycles. This means working with specialty alloys. And such aluminum alloys are notoriously difficult, and sometimes nearly impossible, to weld. On top of that, every weld must meet strict industry standards. The more of these challenges that stack up on a single part, the thinner the line gets between a good weld and a failed one.
In addition, restoring material properties through postweld heat treatment compounds the challenge. Aerospace aluminum welding becomes a multimodal engineering problem. As a result, it requires exceptional care in process selection. Likewise, choosing the correct manual or automated execution steps and equipment is critical.
Welding Challenges
Aluminum’s high heat conductivity demands high heat input, but its low melting point risks melt-through.
Aluminum forms an oxide layer in open air, which can compromise the weld unless removed.
The oxide layer (melting point 3700°F) acts as an insulator over the base metal (melting point 900°–1200°F). Welding aluminum requires a two-step process: oxide removal, then metal fusion.
Aluminum is highly susceptible to porosity, inclusions, and other discontinuities, particularly those caused by hydrogen and hydrocarbons.
Cross contamination is a serious risk. The welding area and tools must be clean and dedicated solely to aluminum work.
Welding behavior varies greatly between alloys.
Some aluminum alloys are prone to thermal cracking during welding.
A mismatch between the wire alloy and the base metal can cause cosmetic issues during postprocessing, such as discoloration after anodization from elements like silicon, chromium, or magnesium.
Both base materials and welding wire require demanding storage conditions.
Specialty gas mixing (e.g., argon and helium) may be required, which can lead to reliance on infrequently used or inadequately maintained equipment.
Procedural Challenges
Welders must adhere strictly to quality procedures.
Personnel must thoroughly understand the stringent requirements and limitations associated with legacy processes.
Customer requirements can be difficult to change due to requalification costs and complexity.
Rigid legacy requirements prevent or slow down the adoption of more suitable processes.
Sometimes, requalifying procedures and adopting more recent welding technology are a nonstarter.
Flight hardware carries even stricter requirements, requiring additional quality procedures.
Suppliers often lack expertise in exotic aluminum-alloy consumables.
Many suppliers and stakeholders misunderstand the characteristics and behaviors of aluminum alloys.
Automation Challenges
Pushing the soft aluminum through a robotic liner is like pushing a rope through a tube. It’s incredibly difficult to get right without the push-pull equipment for gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW).
Large sheet metal assemblies are more difficult to weld with automation.
Introducing automation can be slow due to stringent requirements.
Fixturing and upstream quality can make or break automation efforts.
Selecting the appropriate automated systems for a given application is critical.
Improper problem definition can result in replacing one issue with another when automation is implemented without fully understanding the underlying challenge.
Numerous technical factors must be addressed, including robotic access to the joint and the configuration of positioners to avoid creating new bottlenecks.
Automation efforts require robust software and system integration.
Management must be persuaded to justify the investment required for automation projects.
Process Selection and Industry Norms
The welding process for aluminum in the aerospace industry is determined by industry standards, legacy requirements, existing procedure qualification records (PQRs), and customer specifications.
However, GTAW remains dominant in the aerospace industry, especially for aluminum. Manufacturers won’t necessarily seek to qualify GMAW or other contemporary processes. Achieving higher welding speeds isn’t always worthwhile. Qualifying a new process in an environment built around GTAW can require substantial time and resources. As a result, manufacturers must weigh the substantial costs of process qualification against the potential productivity increases that newer processes might offer.
GMAW may not always be desirable either. Yes, it can produce exceptional welds on aluminum. However, the alloy, the joint, and aluminum’s susceptibility to discontinuities often require the precision and control of GTAW. One of the primary advantages of GTAW is decoupling the filler material from the welding process. This separation, despite reducing the welding speed, makes fusion and discontinuities easier to identify. Likewise, GTAW can break down the aluminum oxide layer ahead of the weld pool. This is a key element in evacuating the impurities before they get trapped.
Modern GTAW equipment allows precise control over the waveform, including the alternating current (AC) portions. During AC welding, the direct-current electrode-positive (EP) phase removes the aluminum oxide layer, while the direct-current electrode-negative (EN) phase melts the underlying aluminum. Advanced power sources can control the current in both EP and EN to optimize the trade-off between cleaning and joint penetration. For example, some Fronius welding power sources allow adjusting the EP and EN offset in amperage, in addition to AC balance control.
According to the stringent requirements of AWS D17.1, Specification for Fusion Welding for Aerospace Applications, porosity acceptance limits become more restrictive as material thickness decreases. The aerospace industry constantly strives to reduce aircraft weight, so thin sections are the norm. In that context, GTAW is often the process of choice because it excels at avoiding porosity and detecting it during testing.
How to Think of Automation as an Aerospace Manufacturer
Every manufacturing sector has a labor gap. Nothing is different in aerospace, especially for highly skilled welders who can perform GTAW. However, that shouldn’t be the sole focus for automation.
Consider the following before attempting to automate:
1. What is the primary issue you can identify with manual welding?
2. How repeatably can you present the parts to the robot?
3. Can your downstream handle a boost with robotic welding output?
4. What kind of user input do you rely on now that might be lost if you try to automate, and can you work around that problem?
5. What are your limiting beliefs on the needed volume? Aerospace typically relies on traditional automation for high volume. Collaborative robot applications are changing that while offering highly capable automation. For example, some systems can include advanced GTAW/GMAW welding processes, positioners, coordinated motion, tracks, and other additional hardware in human-to-cobot welding cells.
Ending Thoughts
Start with the right equipment, especially in aerospace. Many issues are avoidable with solid technology foundations. Conversely, plenty of problems you face with poor equipment selection cannot be resolved. Your robot should never collect dust; it should actively deliver return on investment. Finally, find an automation supplier that stays committed in the after-sales process. You don’t want to go through all the trouble of automating only for the system to prove inefficient once your processes evolve — or to find that your supplier is unwilling or unable to help adapt the system when you need support.
More automation providers need to think along the lines of supporting a company’s full range of needs, from integration to future capacity planning. Buying an automated system is not the end of the process; you don’t know what you don’t know, and the future brings plenty of unknowns. Working with a vendor that keeps the door open for system expansion is essential for staying competitive, especially in demanding industries such as aerospace.