Can You Weld Aluminum to Steel? All the Facts!
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Steel and aluminum are highly weldable metals on their own. It’s easy to weld steel to steel or aluminum to aluminum. The question that always arises is whether you can weld aluminum to steel. The answer is yes but welding these two metals together can be problematic and requires welding prowess.
Welding aluminum to steel is difficult because the two metals have different metallurgical and physical properties. They feature different melting points and thermal conductivity, making joining them hard. Besides, the two are virtually insoluble in each other. In a molten state, they react and form brittle intermetallic phases making them hard to join when welding. The resulting weld can be too brittle and inappropriate for most applications.
This article looks at why it is necessary to weld aluminum to steel, why it’s impossible to use traditional welding methods, and the best ways to weld aluminum to steel and get strong welds.
Why is it Necessary to Weld Aluminum to Steel?
Aluminum and its alloys are lighter compared to steel. Steel has a density of about 7.75 to 8.05 g/cm3, while aluminum and its alloys have a density of about 2.70 g/cm3. Therefore, steel is almost three times heavier than aluminum.
Most industries use steel for several structural applications because of its high strength. However, because of its high density, weight penalties are associated with using the metal for structural applications.
For some industries, you need to use lighter materials, making steel inappropriate for several applications. Some automobiles with steel structures are heavy and emit high greenhouse gasses, making them unsuitable.
Since it’s impossible to replace the whole structure with aluminum, mixing things and welding aluminum to steel is the best option. However, it is difficult to use conventional welding techniques such as arc welding, MIG welding, or TIG welding.
Why Can’t You Use Traditional Welding Methods to Join Aluminum to Steel?
The major differences between aluminum and steel make it hard to use arc welding, MIG welding, or TIG welding to join the two.
- Melting point: Steel has a melting point of 2,500°F, while aluminum melts at 1,221°F. Besides, aluminum has an oxide layer with a melting point of 3,700°F.
- Thermal conductivity: Aluminum has higher thermal conductivity than steel. This means more heat gets drawn away from the welding pool towards the cooler parts of the aluminum base. More heat produced during Arc, MIG, or TIG welding would make the steel warp.
- Type of current: As you TIG weld, you should have an alternating current. The electrodes should alternate between positive and negative charges. Positive electrodes blast away the oxide layer while the negative ones melt away the base aluminum. TIG welding needs a direct current and electrodes to be negatively charged. If you TIG weld with a direct current, you cannot break through the oxide layer. This combs the filler metal with a partially melted oxide layer leading to a contaminated weld bead.
- Brittle intermetallic compounds: The lack of fusion between aluminum and steel leads to the development of brittle intermetallic compounds. The compounds are weak, leading to weak points and breakages.
How to Weld Aluminum to Steel
Bimetallic transition inserts and dip coating are the two most effective methods to weld aluminum to steel. Let’s look at how to weld aluminum to steel using each method.
Bimetallic Transition Inserts
This is a popular method you can use to weld aluminum to steel. Bimetallic transition inserts usually produce high-quality welded joints. These sections feature the same parts of steel and aluminum already joined together.
The method helps in creating welds that are of high quality with similar strength like steel to steel or aluminum to aluminum joints. The size and shape of the bimetallic transition insert vary. The insert is usually made from one aluminum part with an equal part of steel bonded directly to the aluminum.
The insert is placed between the aluminum and steel you are welding. You can use TIG welding to complete the weld. The steel part is welded to the steel component, while the aluminum part is welded to the aluminum component.
The aluminum to aluminum part is welded first to ensure you don’t overheat the inserts. Once you have completed the initial welding, the aluminum components absorb heat and ensure the inserts don’t overheat as you weld the steel to the steel part. This method is applied when welding aluminum steel pipes, tubing on heat exchanges, and similar applications.
Here are the steps you need to follow as you weld aluminum to steel using bimetallic transition inserts.
- Start by TIG welding the aluminum to the insert to prevent overheating because of the difference in melting points. Weld the other part of the insert to the steel. Here are steps on how to TIG weld.
- Check the right gas flow depending on the selected steel and aluminum metal size and nozzle.
- Adjust your welding machine to the correct current type and amperage.
- Turn your welding machine on and check the foot control. Place the control in a comfortable area.
- Press foot control and then strike the welding arc
- Add the filler rod to the leading edge
- Clean the aluminum and steel you want to join with the inserts by grinding or brushing using a stainless-steel brush.
- Now weld the two parts together.
- Allow the metals to cool down. Now your aluminum and steel metals are joined together.
Dip coating is the other effective method you can weld aluminum to steel. Also referred to as hot-dip aluminizing, the process involves dipping the steel component into the aluminum to coat it completely. This ensures that intermetallic compounds don’t form as you weld.
The molten aluminum will contact the aluminum-coated steel part during the welding process. Since it doesn’t get directly in touch with the steel, the weld is relatively strong. In this case, you should use arc welding to join the two metals.
However, as you weld, ensure that the arc does not get into contact with steel since it can burn through the aluminum coating. The arc should only be directed at the aluminum part. The molten aluminum part in the weld pool should then be directed to the steel-coated aluminum.
Dip coating is used to weld aluminum to steel used for seam sealing but not for structural appliances. They have less application than those welded with bimetallic transition inserts.
Here are steps to follow when the dip-coating process.
- Start by dipping your steel into melted aluminum for coating
- Arc weld the aluminum to the dipped steel, following these steps:
- Prepare your workspace and ensure there is ample ventilation
- Wear protective gears
- Prepare the aluminum part to weld by scraping off the oxidized film or coating for better welding.
- Preheat the aluminum up to 400°F before striking the arc since it quickly dissipates the heat. When the soot disappears, start to weld.
- Start welding the aluminum by setting your machine at 85 amps, and then use DC reverse polarity.
- Strike an arc and apply brisk travel speed to complete the welding. Direct the molten aluminum to the steel-coated aluminum while welding to join the two metals.
- Allow the metals to cool down to get a strong bond.
Safety Tips When Welding Aluminum to Steel
Welding aluminum is different from other metals because of its unique properties. You need to take extra precautions to ensure a safe working environment.
- Aluminum appearance doesn’t change when heated: When preheating or welding aluminum, it’s hard to differentiate when it’s hot or cold since it doesn’t change in appearance. Besides, aluminum has a higher thermal conductivity compared to steel. So, as you weld aluminum to steel, wear leather gloves to reduce the risk of injuries.
- Aluminum to steel welding can lead to electrical shock: Aluminum and steel welds can cause electrical shock. So, you need a well-insulated welding system, and proper grounding is required to protect you from electrical shock risk.
- Aluminum has high reflectivity: When welding steel, radiated light is a common problem but is also an issue in aluminum welding. With this high reflectivity, there is a high risk of light-related injuries when welding aluminum to steel. Long-sleeved clothing or light-blocking curtains can help reduce this exposure.
It is possible to weld aluminum to steel, though the process is challenging and requires high-level professionalism. The two metals differ in various aspects, including thermal conductivity, the melting point, and having different metallurgical properties, making it hard to weld the two together.
Despite the challenges, welding aluminum to steel is still necessary to make steel more applicable for structural and other applications that require lighter metals. Traditional welding methods such as MIG, TIG, or arc welding cannot be used to weld the two because of the different properties.
Bimetallic transition inserts welding in which metal inserts with both aluminum and steel are used to join the two metals. You can also use the dip-coating process in which steel is dipped into the aluminum to coat it completely. This makes the welding process possible because molten aluminum doesn’t get in contact with steel during the welding process leading to a strong weld.
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