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Though aluminum can be joined to most other metals relatively easily, when aluminum is arc welded to steel, copper, magnesium, or titanium, very brittle intermetallic compounds are formed that weaken the strength of the join. When steel and aluminum must be joined via arc welding, two special techniques have been developed to isolate the metals from each other during the arc welding process.
How do you TIG weld aluminum to stainless steel? Two methods have been developed to arc weld aluminum and stainless steel components without causing the formation of very brittle intermetallic compounds.
- The first is to use bimetallic transitions, in which aluminum and stainless steel have been joined by methods that do not create the compounds, which allows the joining of the two metals by only welding aluminum to aluminum and stainless steel to stainless steel.
- The second is to coat the stainless steel with aluminum or silver solder and arc weld the aluminum to that instead.
We’ll take a closer look at both methods, so you know when to use it and how to do it, but first let’s explore why you can’t TIG weld aluminum directly to stainless steel.
Why You Can’t TIG Weld Aluminum to Stainless Steel
When welding, it is always easiest if the metals are as similar as possible. If you were spot welding two sheets of the same thickness of the same metal, it would be simple to create a good join because as the arc passed through both work pieces it would create the same amount of resistance, and therefore the same amount of heat. The same amount of metal would liquefy and harden together, forming a perfectly even join.
Each variable that is introduced, however, complicates the process. Even something as simple as a difference in thickness between the two work pieces changes the equation because the heat will dissipate differently through the different thicknesses of the same metal.
Even more drastic of a variable is welding different metals together because each metal has different properties. Sometimes these variables can be controlled for by adjusting amperage, exposure time, electrode material, and other factors, but certain metals, such as aluminum and stainless steel, are simply too different.
Differences Between Aluminum and Stainless Steel
Aluminum and stainless steel have drastically different properties, which make them incompatible for TIG welding.
- Melting point. Aluminum’s melting point is 1,221 degrees Fahrenheit, which is much lower than the melting point of steel, which is 2,500 degrees Fahrenheit. To further complicate the situation, aluminum has an oxide layer which has a melting point of about 3,700 degrees Fahrenheit.
- Service temperature. As the service temperature decreases, the strength of aluminum increases. This is the opposite of steel, which becomes more brittle as service temperature decreases.
- Thermal conductivity. Aluminum is 5 times more thermally conductive than steel, meaning that more heat is drawn away from the site of the weld pool to cooler parts of the base aluminum. More heat input is required to penetrate the aluminum during the weld. More heat, however, increases stainless steel’s tendency to warp. The aluminum oxide layer also acts and an insulator, causing further complications during the weld.
- Current type. Aluminum is TIG welded with alternating current (AC). The electrode alternates between being positively and negatively charged. When the electrode is positive it blasts away the oxide layer, and when it is negative it melts the base aluminum.
Steel, on the other hand, is TIG welded with direct current (DC) with the electrode always negatively charged. If aluminum is TIG welded using direct current (DC), the weld will not break through the aluminum oxide layer. The filler metal will combine with the partially melted oxide layer, creating a contaminated bead.
- Hydrogen reactivity. The presence of hydrogen causes cracking in steel welds. In aluminum welds, hydrogen is drawn from the atmosphere into the liquid aluminum, in which it is very soluble, and held in solution. As the liquid aluminum cools and solidifies, the hydrogen gas forms bubbles that become trapped, causing porosity.
TIG welding offers the option of protecting the weld from hydrogen in the atmosphere by blowing a helium or argon shielding gas mixture, but doing so will require an increase of voltage to overcome the higher ionization potential of the gas (especially when helium is used), resulting in an increased heat input, leading to further complications mentioned above.
As a result of all of these differences, when you TIG weld aluminum to stainless steel, very brittle intermetallic compounds are formed which weakens the strength of the join. Two techniques have been developed to replace the arc welding of aluminum to stainless steel.
Bimetallic Transition Inserts
A bimetallic transition insert is a component that is made of two metals pre-bonded via a method other than arc welding. In this case, it would be a part that is aluminum on one side and stainless steel on the other. By using this insert, the welder can simply TIG weld the aluminum part to the aluminum side of the insert and the stainless steel part to the stainless steel side of the insert.
When doing so, you must be careful not to overheat the insert too much, because doing so will essentially weld the aluminum and stainless steel where they have been pre-bonded, creating the very brittle intermetallic compounds you’re trying to avoid.
It is recommended that you TIG weld the aluminum to aluminum side first because doing so will create a larger heat-sink for when you are TIG welding the stainless steel to stainless steel side. As discussed above, aluminum melts at a much lower temperature than steel, so if you were to TIG weld the stainless steel part to the stainless steel side of the insert first, this could easily overheat the aluminum side of the insert.
By TIG welding the aluminum to aluminum side first, on the other hand, you effectively increase the amount of aluminum the heat can be dissipated to (keeping in mind that aluminum is 5 times more thermally conductive than steel) in order to prevent overheating.
Methods of Pre-bonding Aluminum to Stainless Steel Inserts
You may be wondering, knowing that aluminum cannot be TIG welded to stainless steel without creating very brittle intermetallic compounds, how these bimetallic transition inserts are created. There are several processes that can bond aluminum to stainless steel, but they are not practical for assembly purposes.
- Roll bonding. One or more sheets of aluminum and one or more sheets of stainless are passed through a pair of flat rollers at sufficient pressure to bond them together. The touching surfaces are first scratched, cleaned, and de-greased to increase their friction coefficient and remove any oxide layers. The metals are sometimes heated prior to rolling in increase their ductility, but can also be bonded cold.
- Explosion welding. Precision explosions are used to bond aluminum to stainless steel while retaining the mechanical, electrical, and corrosion properties of both metals. Because the explosion lasts only milliseconds, no bulk heating occurs, and the metals experience no dilution.
- Friction welding. A piece of aluminum and a piece of stainless steel are rubbed against each other so fast (the process takes only a few seconds) that the friction creates enough heat to bond the metals. No melting occurs, however, which mitigates grain growth and the production of very brittle intermetallic compounds. The process creates a flash of metal that carries away and dirt and debris present on the surfaces of the metals.
- Flash welding. The pieces of aluminum and stainless steel to be welded are set apart at a predetermined distance based on material thickness, material composition, and the desired properties of the finished weld. Current is applied to the metals, and the gap between them creates enough resistance to melt them. They are then pressed together, forging them together. By melting the metals separately, no very brittle intermetallic compounds are formed.
- Hot pressure welding. This process is similar to friction and flash welding, except that the source of heat is the flames of oxy fuel torches or eddy currents caused by electrical induction from a suitable inductor coil. Once the aluminum and stainless steel components are heated, they are pressed together. Fusion temperature is never reached.
By bonding the bimetallic transition inserts in these ways, the very brittle intermetallic compounds that are formed when arc welding aluminum to stainless steel are not formed. The inserts can then be used in the assembly process to join the two metals via TIG welding.
This process is often used for producing welded connections of excellent quality within structural applications, such as attaching aluminum deck-houses to steel decks on ships, for tube sheets in heat exchangers that have aluminum tubing with steel or stainless steel tube sheets, and for producing arc welded joints between aluminum and steel pipelines.
Coating the Stainless Steel
The other technique developed to replace the arc welding of aluminum to stainless steel is to coat the stainless steel in another substance that aluminum can be welded to. There are 2 processes by which this is done:
- Hot dip aluminizing, in which the stainless steel is dipped into molten aluminum, and
- Brazing, in which the surface of the stainless steel is covered with silver solder.
In either method, care must be taken to ensure that when TIG welding the aluminum to the aluminum (or silver solder) coating, the arc does not impinge upon the steel. If the heat burns through the protective aluminum (or silver solder) coating and interacts with the stainless steel, the weld will result in the formation of very brittle intermetallic compounds.
Neither of these coating methods are typically provide full mechanical strength and are usually used for sealing purposes only.
Before the development of these bonding techniques, aluminum and stainless would be bonded mechanically by rivets. In this process, holes are drilled through both the aluminum and stainless steel components. A rivet, which comprises a smooth cylindrical shaft with a head on one end, is passed through the holes. The tail end of the rivet’s cylinder is then smashed, forming a head on the opposite end, resulting in a barbell shaped piece of metal holding the sheets together.
Because there is now a head on both ends of the hole, rivets can be used to support tension loads, in which the force is exerted along the axis of the rivet’s shaft, but it is far more effective at supporting shear loads, in which the force is exerted perpendicular to the axis of the rivet’s shaft.
The more rivets used, the more force the mechanical bond can hold, but because each rivet takes up a certain amount of space and can only support a certain amount of force, this method is not as strong as intermetallic bonding, which has the supporting power of the entire joined surface.
How to TIG Weld
As noted above, the presence of hydrogen can cause cracking in stainless steel welds, and porosity in aluminum welds. Therefore, whether you are using bimetallic transition inserts or a method of coating the stainless steel in molten aluminum (or brazing with silver solder), you need to use a type of welding that protects the weld from hydrogen and other potential contaminants in the atmosphere.
Tungsten inert gas (TIG) welding, also known as gas tungsten arc welding (GTAW), is best suited for welding aluminum and stainless steel. In TIG welding, a non-consumable tungsten electrode is used because it has the highest melting temperature among pure metals, at 3,422 °C (6,192 °F), and therefore doesn’t contaminate the weld by introducing traces of the electrode’s element. The purity of the weld is so dependent on the absence of contamination that if the tungsten electrode ever touches the aluminum weld puddle, it must be reshaped (preferably in a grinder used only for grinding tungsten) before the next weld.
The weld area is further protected from oxidation or other atmospheric contamination by an inert shielding gas. The type of inert gas used varies depending on the joint design and desired final weld appearance. Typically argon is used because it helps prevent defects due to a varying arc length. In heliarc welding, helium is used as the inert shielding gas in order to increase the weld penetration in a joint, increase the welding speed, and to weld metals with high heat conductivity, such as aluminum.
The work lead (often referred to as the ground cable) is attached to the work piece or the metal surface that the work piece is on. One hand holds the electric torch. The torch contains the tungsten electrode protruding about 1/8th of an inch from a ceramic cup through which an inert gas is blown.
The torch must be held close enough to the work piece to ensure that the arc is small and contained within the inert gas, but not so close that the electrode ever touches the work piece or aluminum weld puddle. You can tell if this has happened because it will result in a different sound and color. The other hand holds filler metal which the welder feeds into the weld area as needed.
When the torch and filler metal are in place, the arc is struck by engaging the TIG welder with a foot pedal. Once the maximum amperage has been set, the foot pedal can be used to gradually initiate and reduce the amperage and resulting heat. This soft starting and soft stopping prevent temperature shocking of the metal, which can result in brittle welds.
When the arc is struck, the torch is first moved in a small circle to create a weld puddle of aluminum. Then the torch is tilted back 10 to 15 degrees from vertical and moved along the seem, with filler metal added to the front end of the weld pool as needed. The filler metal is removed from the weld puddle whenever the torch advances but is not removed from the cone of inert gas so as to avoid contamination and oxidation. Again, it is essential that the tungsten electrode is kept about 1.5–3 mm (0.06–0.12 in) from the work piece at all times.
By employing TIG welding, you can keep both your stainless steel and aluminum welds free from contamination by hydrogen or other contaminants in the atmosphere. Another benefit of TIG welding is that it uses lower amperage than other welding methods, reducing the risk of overheating your bimetallic transition insert or burning through the aluminum (or silver solder) coating covering the stainless steel, which could result in the production of very brittle intermetallic compounds that weaken the join.
Because TIG welding aluminum to stainless results in the formation of very brittle intermetallic compounds that weakens the strength of the join, two techniques have been developed to join the dissimilar metals. By using bimetallic transition inserts or coating the stainless steel in aluminum or silver solder, you can TIG weld aluminum and stainless steel components to each other in a more effective way then mechanically joining them with rivets.