How to TIG Weld Aluminum to Stainless Steel: The Complete Guide

Key Takeaways:

• TIG welding aluminum directly to stainless steel results in brittle intermetallic compounds that weaken the joint.
• Use bimetallic inserts pre-bonded by methods like roll bonding to weld the metals separately.
• Coat the stainless steel with aluminum or silver solder to weld the aluminum to that instead.
• TIG welding with argon or helium shielding gas prevents contamination while welding.
• Move the TIG torch in circles to create a weld pool before moving along the seam.
• Keep the tungsten electrode 1.5-3 mm from the workpiece to prevent contamination.

Joining aluminum and stainless steel together directly through TIG welding results in a brittle, weak joint due to intermetallic compounds formed when the dissimilar metals are fused. To TIG weld aluminum to stainless steel, use a bimetallic insert pre-bonded through roll bonding, explosion welding or other methods. This allows you to weld the aluminum and stainless steel components separately. Alternatively, coat the stainless steel with aluminum or silver solder and weld the aluminum to that coating instead, taking care not to burn through to the steel. Use TIG welding with argon or helium shielding gas to prevent contamination of the welds.

Welding different metals together can be a difficult task, but it is often necessary in industries like shipbuilding, aerospace, and construction. Few metal combinations cause as many headaches for welders as joining aluminum and stainless steel. The differences in their properties and welding behavior make them essentially incompatible for direct fusion through traditional welding methods.

Attempting to weld aluminum and stainless steel together typically results in the formation of extremely brittle intermetallic compounds in the weld joint. These compounds severely weaken the connection and can lead to weld failure under load. Fortunately, experienced welders have developed techniques to allow these metals to be joined successfully through intermediate materials or barriers between the incompatible metals.

This guide will explore the challenges of welding aluminum to stainless steel and proven methods to create strong, lasting joints between them. With an understanding of why direct welding fails, and how proper joint preparation and welding technique allows these metals to be joined, you’ll be able to take on aluminum to stainless steel welding jobs with confidence.

Challenges of Welding Aluminum and Stainless Steel

Aluminum and stainless steel have very different physical, chemical, and electrical properties that cause problems when trying to weld them together directly. Here are some of the major differences that interfere with welding:

• Melting points – Aluminum melts at 1220°F while stainless steel melts around 2500°F. It’s difficult to generate enough heat to adequately penetrate the steel without overheating the aluminum.
• Oxide layer – Aluminum forms an oxide coating that has a melting point over 3700°F. This further complicates welding and requires much higher heat input.
• Thermal conductivity – Aluminum conducts heat 5 times faster than steel, making it harder to maintain a stable weld pool. More heat gets drawn away from the weld joint into the aluminum base metal.
• Thermal expansion – Aluminum expands more than steel when heated. This can lead to distortions and weaknesses in the joint as it cools.
• Electrical conductivity – Aluminum conducts electricity much better than stainless steel. This affects how much current flows through each metal during welding, resulting in an imbalanced weld pool.

The most significant problem, however, is that welding aluminum and steel together directly causes intermetallic compounds to form in the fusion zone and heat affected zones around the weld.

Dangers of Intermetallic Compounds in Welds

When aluminum and stainless steel are brought together in a molten weld pool, their atoms intermingle and combine to form what are called intermetallics. These compounds contain distinct crystalline structures that have vastly different properties than either base metal. Some key facts about intermetallic compounds:

• They form through diffusion bonding between dissimilar metals at elevated temperatures.
• They are extremely hard and brittle – hardness rating over 500 HV (much higher than either stainless steel or aluminum).
• They lack ductility and crack easily under stress, causing weld failure.

-Regions with a high concentration of intermetallics become weak spots in the joint.

• Thicker intermetallic zones form at higher temperatures during welding.
• Faster weld cooling rates help minimize thickness and concentration.

Intermetallic compounds in an aluminum-to-steel weld are hard, brittle, and prone to cracking under stress.

You can visually identify regions of intermetallic compounds by their very dark etched appearance compared to the base metals, when looking at a cross-section of the weld joint under a microscope. There can be multiple distinct intermetallic phases present, each with their own compromising effects on the weld integrity.

The formation of these extremely hard, brittle intermetallic compounds is what causes aluminum to stainless steel welds to have inadequate strength, ductility, and shock resistance for most applications. Cracks initiate in the intermetallic zones and propagate through the joint.

Editor’s Note: Welders have developed two primary methods for joining aluminum to stainless steel while preventing the formation of detrimental intermetallic compounds in the joint: 1) Using bimetallic transition inserts allows welding the metals separately, and 2) Coating the stainless steel surface enables welding aluminum directly to the coating.

Method 1: Welding with Bimetallic Inserts

One proven technique for successfully welding aluminum to stainless steel is to use a multi-layered insert material between the two metals, known as a bimetallic transition joint.

The insert features a gradient composition from 100% aluminum on one side to 100% stainless steel on the other side. This insert can be welded in place with aluminum fused only to aluminum, and steel fused only to steel. The metals are thus kept isolated during welding to avoid intermetallic compound formation.

Bimetallic inserts allow isolating dissimilar metals during welding to avoid mixing and intermetallic compound formation.

There are a few different bonding methods used in industry to manufacture these transition insert materials:

• Roll bonding – Stainless steel and aluminum materials are rolled together under extremely high pressure to cold weld them into a layered composite.
• Explosion welding – Controlled detonations slam the metals together to fuse them. The ultra-fast bonding minimizes intermetallic growth.
• Friction welding – The metals are rubbed together at high speeds, generating friction heat to bond them without melting.
• Diffusion bonding – The materials are pressed together and heated just below melting temperature in a controlled environment. This gradually fuses the materials.

Pro Tip: Roll bonding is one of the most common and cost-effective methods for producing quality bimetallic transition inserts on a large scale. The rolls bond the metals through extremely high pressure without melting or intermetallic formation.

The gradual composition of the insert from pure aluminum to pure stainless steel provides a combination of strength, corrosion resistance, and fracture toughness in the joint. When welding, care must be taken to avoid overheating the area and encouraging intermetallic growth through diffusion between the layered metals.

Bimetallic inserts allow you to utilize the standard welding parameters recommended for welding aluminum to aluminum, and for welding stainless steel to stainless steel. Following proper procedures, you can create high-integrity joints between the dissimilar base metals.

TIG Welding Aluminum and Stainless Steel with Inserts

For most applications, TIG welding provides the best precision and control for welding aluminum components to stainless steel when using a bimetallic transition piece. Here are some recommendations for successful TIG welding with inserts:

• Weld the aluminum side first to minimize heating of the stainless side. Aluminum conducts heat away from the joint faster.
• Use alternating current (AC) output for the aluminum welds to penetrate the oxide coating. Use direct current (DC) for the steel side.
• Keep heat input low on both sides to limit intermetallic growth. Watch heat tinting around welds.
• Use 100% argon shielding gas; optionally add 1-5% helium to increase penetration on aluminum side.
• Target a concave weld profile to resist cracking from thermal stresses. Peen welds while cooling.
• Grind any silicone grease off filler rods to prevent weld contamination on aluminum side.

Pro Tip: Pay close attention to interpass temperature when making multi-pass welds on bimetallic joints – allow the joint to cool below 500°F between passes to limit intermetallic formation.

Editor’s Note: The other popular method for joining aluminum and stainless steel while avoiding mixing of the metals is to introduce a compatible pure aluminum or brazing alloy coating on the steel surface prior to welding the aluminum component.

Method 2: Welding Aluminum to Coated Stainless Steel

Rather than inserting a buffer material, some situations allow for coating the surface of the stainless steel so that aluminum can be welded directly to the coating material instead of the steel itself. This serves as a barrier preventing the stainless steel and aluminum from mixing during welding.

There are two common coating techniques used:

1. Hot Dip Aluminizing

• Stainless component is dipped into a molten bath of pure aluminum.
• Creates a thin aluminum coating alloyed to the steel surface.
• Allows TIG welding aluminum directly to the aluminized steel surface.

2. Brazing

• Silver-based brazing alloys are melted across the steel surface.
• Aluminum can then be TIG welded to the silver solder coating.
• Brazing filler materials available for joining aluminum to steel.

With both methods, the weld heat must be controlled to avoid burning through the coating and contacting the underlying stainless steel. This takes careful technique, as too much heat will still allow intermetallic compounds to form beneath the aluminum or braze filler metal.

Editor’s Note: Coating techniques provide an alternative to bimetallic inserts when the geometry or design doesn’t allow for an insert. The thickness of the coating material must be enough to act as a reliable diffusion barrier.

TIG Welding Aluminum to Coated Stainless Steel

Similar precautions must be taken when TIG welding aluminum to coated steel as you would with bimetallic inserts. The objective is still to avoid direct mixing of the molten aluminum and stainless steel. Follow these tips:

• Maintain consistent arc length and avoid contact of filler metal with base metal.
• Position torch at 10-15° push angle pointing in direction of travel.
• Clean surfaces thoroughly – any impurities can cause weaknesses.
• Use alternating current (AC) output for aluminum welds.
• Keep heat input low to prevent burning through the coating.
• Target a flat or slightly convex weld profile.
• Peen welds while cooling to relieve stresses.

Pro Tip: When TIG welding aluminum to aluminized steel, wire brush the coating first to expose some of the stainless steel. This helps the aluminum bind to the coating for maximum strength.

Editor’s Note: Proper welding technique is critical to prevent the two incompatible metals from mixing when welding aluminum to coated stainless steel surfaces. Keeping heat input low, shielding the weld pool from oxidation, and preventing filler contamination are imperative.

Best Practices for Strong Welds

To achieve high weld quality and prevent defects when welding aluminum to stainless steel via inserts or coatings, following fundamental welding best practices is key:

• Clean Surfaces – Remove all dirt, oil, oxide layers, and other contaminants from joint surfaces. This helps maximize weld strength.
• Proper Fit-Up – Components must fit together evenly with optimal joint gap for welding process. Misalignments create weaknesses.
• Shield from Oxygen – Shield welds from air exposure using inert gas flow to prevent porous, oxidized welds.
• Control Heat Input – Too much heat compounds problems of overheating, distortion, and intermetallic growth.
• Filler Selection – Match filler metal to base metal alloy type to ensure proper melting, flow characteristics, and strength.
• Prevent Contamination – Molten aluminum readily absorbs hydrogen and moisture that lead to cracking and porosity in the solidified weld.

Following these essential guidelines will help you achieve aluminum to stainless steel welds that are free of defects and avoid the major problems encountered when welding these metals. Implementing the methods outlined in this guide will allow you to create welds that maintain the desired qualities of both base materials for a given application.

Applications and Industries

While joining aluminum and stainless steel can prove difficult, the combination of corrosion resistance, strength, and low weight makes it an ideal solution for many products. Some examples of where aluminum-stainless steel welds are commonly used:

• Aerospace – aircraft structures, rockets, space vehicles.
• Transportation – railway cars, truck/trailer bodies, boat hulls.
• Architecture – roofing, walls, structural supports.
• Manufacturing – robotics, equipment, tooling.
• Electrical – power transmission, computer components.
• Chemical Processing – tanks, pipelines, reactions vessels.

The techniques for successfully welding aluminum to steel have made these material combinations possible in critical applications across industries where high weld integrity and longevity are needed.

Pro Tip: Bimetallic inserts are often used in boat hull construction to join aluminum superstructures to stainless steel deck plating. The inserts provide a smooth transition between the dissimilar metals.

Quality Control

To ensure welds between aluminum and stainless steel meet necessary standards, there are a few methods of testing the weld quality:

• Visual inspection – Look for surface defects and inconsistencies. Check for cracking or evidence of burn-through.
• Liquid penetrant testing – Detects subsurface cracks and porosity not visible to the eye.
• Radiographic testing – X-rays or gamma rays reveal internal defects in thicker welds.
• Ultrasonic testing – High-frequency sound waves identify flaws like slag inclusions.
• Destructive testing – Cross-section samples are made to evaluate joint penetration, fusion, and internal anomalies.
• Hardness testing – Measures hardness across the weld to detect regions of brittle intermetallic compounds.

A combination of nondestructive and destructive testing methods is recommended to thoroughly evaluate weld joint integrity. This helps ensure the joint will perform safely and reliably throughout its service life.

Editor’s Note: Visually inspecting every weld is the first step in quality control. More thorough nondestructive and destructive testing methods should be used periodically and on critical welds or when visual inspection reveals defects or questionable areas.

Safety Tips

Working with molten metal carries inherent hazards. Follow these safety measures when welding aluminum to stainless steel:

• Wear ANSI-approved safety glasses and welding helmet to protect eyes from sparks and UV/IR radiation.
• Use welding gloves and protective clothing to prevent burns from hot metal and sparks.
• Work in well-ventilated areas to avoid buildup of welding fumes. Use respirators when adequate ventilation is not possible.
• Isolate welding area from flammable materials. Have fire extinguishers nearby.
• Pull welding curtains around work area to protect nearby workers from ultraviolet arc rays.
• Shut off shielding gas supply and electrical power before leaving workstation.

Understanding the risks involved and taking appropriate precautions is necessary when welding and working with metal components. Follow all equipment manufacturer safety guidelines as well to operate welding equipment safely.

Pro Tip: Position welding screens around the workspace when TIG welding bimetallic joints. The lower amperages used can make the reflected arc rays less visible but still a hazard to eyes.

Troubleshooting Common Issues

Despite the best preparations, issues can still arise when welding aluminum to stainless steel. Here are some potential problems and solutions:

Weld cracks:

• Reduce heat input to minimize cracking from thermal stresses.
• Ensure joint fit-up is properly aligned – misalignment causes cracking.
• Remove any contaminated filler rod that may contain moisture.
• Check shielding gas flow is adequate around entire weld area.

Lack of fusion:

• Increase welding amperage to improve penetration into base metals.
• Regrind tungsten to sharpen electrode if worn.
• Ensure metals are clean prior to welding – remove ALL surface coatings.

Burn-through:

• Reduce amperage levels and travel speed to prevent excess heat input.
• Watch heat tinting and cool work between passes – don’t overheat.

Porosity:

• Eliminate any source of moisture contamination in filler metal.
• Maintain steady travel speed and arc characteristics.
• Prevent drafts or low shielding gas flow which can induce turbulence in weld area.

Careful control of heat input, joint preparation, filler metal handling, and shielding gas coverage when welding can eliminate most defects in aluminum to stainless steel welds.

Pro Tip: When troubleshooting welding issues, review each variable in the process. Improper technique, joint fit-up, filler metal, shielding gas, or base material contamination can all cause weld defects.

Conclusion

Welding aluminum and stainless steel together has always presented challenges due to their incompatible properties and tendency to form extremely brittle intermetallic compounds when mixed in a molten weld pool. However, through the use of layered bimetallic transition inserts or coating the stainless steel surface, welders can successfully join these metals without mixing them directly.

Careful preparation, precise welding technique, and controlling heat input gives the best results when welding aluminum to stainless steel. TIG welding provides the most precision and control for managing the heat and isolating the base metals from direct contact during welding.

With practice and an understanding of how to prevent contamination between the materials, high-quality welded joints can be