Frequently Asked Aluminum Welding Questions

By Frank Armao, Senior Application Engineer, The Lincoln Electric Company

Aluminum is being specified today in more and more applications because of its many advantages. Aluminum is lightweight (approximately1/3 the weight of steel), has excellent electrical conductivity and better corrosion-resistance properties than steel. As its growth accelerates, operators who are used to welding on steel will need to learn the "ins and outs" of welding aluminum.

Here are some of the most frequently asked questions that Lincoln Electric receives regarding the welding of aluminum, and their answers from an expert:

• How do I weld 7075?
• Why is my aluminum weld much weaker than the parent material?
• What type of shielding gas should I use for aluminum welding?
• For GTAW, what type of electrode is best for aluminum?
• How much preheat should I use when welding aluminum?
• What is the proper stress relieving practice for aluminum welds?
• How can I tell different aluminum alloys apart?
• How do I GTAW weld two dissimilar thicknesses of aluminum?

1. How do I weld 7075?

You can't, at least not for structural work. Most aluminum alloys are weldable, but there are a fair number of them that are not, including 7075 aluminum. The reason 7075 is singled out in this example is that it is one of the highest strength aluminum alloys. When designers and welders look for an aluminum alloy to use, many will start by reviewing a table that lists all of the aluminum alloys and their strengths. But what those newcomers don't realize is that few of the higher strength aluminum alloys are weldable - especially those in the 7000 and 2000 series - and they should not be used.

The one exception to the rule of never using 7075 for welding is in the injection molding industry. This industry will repair dies by welding 7075 - but it should never be used for structural work.

Here are some simple guidelines to follow when choosing aluminum alloys:

Lincoln suggests that if you have a need to design something of high-strength aluminum, look to a 5000 series high magnesium alloy instead of a 2000 or 7000 series. The 5000 series alloys are weldable and will produce the best results.

2. Why is my aluminum weld much weaker than the parent material?

In steels, a weld can be made as strong as the parent material, but this is not the case with aluminum. In almost all instances, the weld will be weaker than the parent material. To further understand why this occurs, let's look at the two classifications of aluminum alloys: heat-treatable and non-heat-treatable. The latter category is hardened only by cold working which causes physical changes in the metal. The more the alloy is cold-worked the stronger it gets. But, when you weld an alloy that has been cold-worked, you locally anneal the material around the weld so that it goes back to its 0 tempered (or annealed) condition and it becomes "soft". Therefore, the only time in the non-heat treatable alloys that you can make a weld as strong as the parent material is when you start with 0 tempered material.

With heat-treatable aluminum alloys, the last heat treatment step heats the metal to approximately 400° F. But when welding, the material around the weld becomes much hotter than 400° F so the material tends to lose some of its mechanical properties. Therefore, if the operator doesn't perform post-weld heat treatments after welding, the area around the weld will become significantly weaker than the rest of the aluminum -- by as much as 30 to 40 percent. If the operator does perform post-weld heat treatments, the proprieties of a heat-treatable aluminum alloy can be improved.

The following is a guide as to which series of aluminum alloys are heat-treatable and which are not:

Heat-treatable series: 2000, 6000, 7000.

Non-heat-treatable: 1000, 3000, 4000, 5000.

3. What type of shielding gas should I use for aluminum welding?

For both Gas Tungsten Arc Welding (GTAW or TIG) and Gas Metal Arc Welding (GMAW or MIG), use pure argon for aluminum materials up to ½" in thickness. Above ½" in thickness, operators may add anywhere between 25 to 75 percent helium to make the arc hotter and increase weld penetration. Argon is best because it provides more cleaning action for the arc than helium does and it is also less expensive than helium.

Never use any shielding gas that contains oxygen or carbon dioxide, as this will oxidize the aluminum.

4. For GTAW, what type of electrode is best for aluminum?

For most materials, including steel, a two-percent thoriated tungsten electrode is recommended, but since aluminum is welded with AC rather than DC, the electrical characteristics are different and the amount of energy put into the tungsten electrode is higher when AC welding. For these reasons, pure tungsten or zirconiated tungsten are recommended for aluminum welding. In addition, the electrode diameter for AC welding has to be significantly larger than when using DC. It is recommended to start with an electrode that is 1/8" and adjust as needed. Zirconiated tungsten can carry more current than pure tungsten electrodes. Another helpful hint for AC welding is to use a blunt tip -- the arc tends to wander around a pointed tip.

5. How much preheat should I use when welding aluminum?

While a little preheat is good, too much preheat can degrade the mechanical properties of the aluminum. As was discussed earlier, the last heat treatment for heat-treatable alloys is 400°F, so if the operator preheats the aluminum to 350°F and holds the temperature in that range while welding, the aluminum's mechanical properties are changed. For the non-heat-treatable alloys such as the 5000 series, if the operator holds the temperature even in the 200°F range, he or she can sensitize the material to stress corrosion cracking. In most cases, some preheat is acceptable to dry the moisture away from the piece, but preheat should be limited.

Because of aluminum's low melting point -- 1200°F compared to 2600°F to 2700°F for steel, many operators think they only need light-duty equipment to weld the aluminum. But, the thermal conductivity of aluminum is five times that of steel, which means that the heat dissipates very quickly. Therefore, welding currents and voltages for welding aluminum are higher than they are for steel, so operators actually need heavier-duty equipment for aluminum. Many inexperienced aluminum welders use preheat as a crutch. Since equipment for welding aluminum needs to operate at higher capacities, many feel that preheat helps eliminate equipment limitations, but this is not the case.

6. What is the proper stress-relieving practice for aluminum welds?

Post-weld stress relief is not recommended for aluminum. When welding, residual stresses are created around the vicinity of the weld because the molten material shrinks as it solidifies. Further, when you remove material by machining it tends to distort and create dimensional instability. To avoid this in steel, operators perform stress-relieving by heating the material hot enough to allow the atoms to move around.

For steel, the stress-relieving temperature is approximately 1050°F to 1100°F; but for aluminum, the proper stress-relieving temperature is 650°F. This means that in order for post-weld stress relief on aluminum to be effective, the material will have to be heated to a temperature where mechanical properties will be lost. For this reason, post-weld stress relief is not recommended for aluminum.

7. How can I tell different aluminum alloys apart?

There are quite a few different aluminum alloys and for proper and safe welding, you should know what alloy you're welding. If you don't, you can follow these general guidelines:

• * Extrusions are generally 6000 series alloys
• * Castings most often are a combination of aluminum/silicon cast -- some are weldable, others are not
• * Pieces of sheet, plate or bar are probably 5000 to 6000 series alloys

If you want to be precise, an alloy tester kit that will help you determine the exact makeup of your alloy.

8. How do I GTAW weld two dissimilar thicknesses of aluminum?

When an operator has two dissimilar thicknesses, he or she must set the parameters so that they are high enough to weld the thickest piece. When welding, favor the joint and put more of the heat on the thicker piece.