How to Successfully MIG Weld Aluminum [Guide]

MIG welding aluminum: what you need to know

Aluminum, in its pure form, is a relatively soft metal that has many uses. However, it requires the use of alloys to increase its strength. When compared to working with steel, aluminum has unique properties and therefore presents unique challenges such as distortion and sensitivity to heat input. Despite these issues, the process of MIG welding aluminum can be straightforward with the right equipment and procedures. 

Here are the important factors to keep in mind when MIG welding aluminum for your application. 

Tips and considerations before you start welding aluminum

1. Consider material thickness: Aluminum thicknesses suitable for MIG welding are 14 gauge and heavier. The output capacity of your welder determines the thickness you can weld. For aluminum thinner than 14 gauge (.074 inch), specialized pulsed MIG or AC TIG welding equipment may be necessary.

2. Keep it clean: Aluminum should be thoroughly cleaned prior to welding, including the removal of any lubricants from the material. Remove oxides after degreasing using a stainless wire brush — either a hand wire brush or a cup wire brush will work. If using a power wire brush, keep the RPMs and pressures low to reduce smearing the surface of the material, which could entrap oxides and impurities under the surface. To avoid contaminating the base material, always clean with a wire brush dedicated to aluminum. 

3. Choose the right gas: Because aluminum is a non-ferrous metal, it requires a 100% argon shielding gas. Recommended flow rates are 20 to 30 cubic feet per hour. 

4. Select the appropriate process: When MIG welding aluminum, spray transfer is the desired mode. This process involves a smooth transfer of molten metal droplets from the end of the electrode to the molten pool, with no short-circuiting. The droplets crossing the arc are smaller in diameter than the electrode.  It offers high deposition rates and efficiency but generates significant heat, creating a large weld pool with good penetration that can be difficult to control. Avoid using this process on materials thinner than 14 gauge. 

5. Gun and wire feeding options: Choosing the right gun and wire feeding system is crucial for MIG welding aluminum. Aluminum wire is commonly fed using either a spool gun or a push-pull system.

   • Spool guns: These improve the feeding of soft wires by locating a small pool of wire on a pistol-grip-style gun. This setup eliminates the possibility of bird-nesting because the wire only feeds a few inches. 

   • Push-pull system: In this system, a motor in the gun pulls the wire through the liner, while a motor on the feeder acts as an assist motor. This option is ideal when welding far from the power source, and it can be more ergonomic and user-friendly. 

6. Select the right filler metal: It’s important to know the alloy of your base aluminum and the conditions the finished part will face. The two most readily available aluminum filler wires are ER4043 and ER5356. To get recommendations on wire alloys that fit your application, contact your local welding distributor or aluminum filler metal representative. 

MIG welding techniques to enhance welds

The operator’s skill level, joint types, fit-up, positions and the welding power supply will all have great influence on the weldability of aluminum. Consider these welding techniques to enhance your skills: 

• Use a 10- to 15-degree push travel angle with the tip and nozzle pointed in the direction of travel (See Figure 1). Avoid pulling or using a drag angle to prevent porous, dirty welds. 

   

Figure 1: Gun techniques

• Maintain the proper tip-to-work distance, with the contact tip recessed approximately 1/8 inch inside the nozzle if possible. (See Figure 2) 

Figure 2: Gun and weld area

• The reflective heat and weld puddle are very hot when MIG welding aluminum. Avoid holding the tip closer than recommended, as this can cause the wire to burn back to the contact tip and create feeding issues.

• Avoid large-weave beads on aluminum. For larger fillet welds, using multiple pass straight beads offers a better appearance and reduces the risk of cold lapping, burn-through and other weld defects.

• Increase the torch travel speed as the base material heats during the weld.

Common challenges when MIG welding aluminum and how to troubleshoot them

If you run into these common challenges when MIG welding aluminum, consider these steps to address them. 

1. Problem: Burn-Through (Melt-Through) Caused by Overheating the Base Material

   • Solutions:
     • Increase the travel speed and make shorter welds.
     • Move around on the part, spreading out the heat.
     • Use thicker material, change the joint design or switch the welding process to AC TIG. 
     • Eliminate/reduce gaps. 
       
       

2. Problem: Dirty Welds 

   • Solutions:
     • Use a push angle instead of a drag technique.
     • Increase the voltage to get into spray transfer.
     • Use the proper base metal cleaning techniques, such as a stainless steel brush.
     • Check for proper shielding gas and wire alloy type. 
       
       

3. Problem: Wrong Machine Settings

   • Solutions:
     • Refer to the inside cover of your Millermatic® welder for optimal starting setting or to fine tune for your application.
     • Order the Millermatic MIG Calculator for good starting settings.
        

4. Problem: Wire Burns Back to Contact Tip During or at the End of the Weld

   • Solutions:
     • Maintain proper tip-to-work distance. 
     • Ensure the contact tip size, drive rolls and gun liner match the wire diameter you are using.
       
       

5. Problem: Wire Birdnests (Piles Up) in Front of Inlet Guide on Gun

   • Solutions:
     • Check and adjust the drive roll tension.
     • Check to make sure drive rolls match the wire diameter. 
     • Replace contact tip if necessary. 
     • Check pressure adjustment on the aluminum spool gun hub. 

Preparation and practice are the keys to success

Successfully MIG welding aluminum requires understanding and applying the right techniques. By considering material thickness, maintaining cleanliness and choosing the right gas as you prepare for your project, you can then focus on employing the proper welding techniques to overcome the unique challenges aluminum presents. For more expert advice on setting up your MIG welding aluminum welding project for success, check out this article on best practices and equipment settings.