What You Need to Know About Cleaning and Preparing Aluminum Filler and Base Metals Before Welding
November 6, 2012
Dirty aluminum sets you off on the wrong foot before you ever strike an arc. Take these cleaning/prep steps for the optimal welding of aluminum.
A welding solutions specialist with a local welding supply, when recently asked what the most important factor was for a successful aluminum weld, replied “clean, clean, clean, clean… and clean.” Welding aluminum poses its own unique set of challenges – trying to weld with dirty base and filler metals adds unnecessary difficulty and can lead to poor weld quality. You also have to deal with the natural oxidation that takes place with aluminum. Suitable preparation prior to welding is important - especially when fabrications are required to meet the weld quality requirements of industry codes, such as AWS D1.2 that governs the structural welding of aluminum.
There are key steps you can take in the treatment and preparation of filler and base metals to ensure that you don’t face problems associated with dirt, grease and airborne shop dust – as well as the natural oxidation process that takes place when aluminum is exposed to the air. We’ll cover these steps and present you with best practices to make sure you’re not sabotaging your aluminum welding processes before you even begin.
The Natural Challenges of Aluminum
Aluminum presents immediate challenges versus steel. Aluminum has a greater potential to develop problems such as lack of fusion due to the tough oxide layer, lack of penetration due to high thermal conductivity, and porosity due to the high solubility of hydrogen in molten aluminum. As such, all moisture and hydrocarbons must be eliminated, and the thickness of the oxide film must be controlled and prevented from hydrating due to the presence of excessive moisture.
Proper Storage and Handling of Aluminum Filler and Base Metals
Oxides on aluminum TIG filler metal can be introduced directly into the weld puddle and lead to porosity. Oxides on MIG filler metals can lead to a number of problems, including an erratic arc, added resistance, sooting and poor feeding. And thicker oxide layers on aluminum base metals must either be removed mechanically or face the risk of contaminating the weld. Here are a few things you can do in storage to prevent this from happening:
- Store all welding filler metals and base metals in a dry location with minimum temperature fluctuation (to minimize condensation). Welding filler metals should be stored in a dry and heated room or cabinet.
- Store aluminum pieces vertically to minimize condensation and absorption of water contamination between layers.
- When possible, bring all filler and base metals into the welding area 24 hours prior to welding to allow them to reach room temperature (minimizing condensation).
- Keep filler metals covered at all times prior to welding. This would include a sealed case for TIG rods and a spool cover for MIG wire.
Optimal Joint Preparation for Welding Aluminum
There are two key steps in preparing the joint for welding: mechanically removing the oxide layer to make it manageable (the oxide layer has a higher melting point than the base metal) and removing any oil, grease, dirt or moisture that could contaminate the weld.
- Do not use cutting methods that leave a ground or smeared surface. For example, a high-speed circular saw is suitable for cutting aluminum whereas a band-sawed surface can lead to smearing. Grinders should be avoided when possible, but use a coarse disc grinder over a wheel grinder if it’s your only option. The goal is to use as fast and sharp of a method as possible that quickly cuts away the material rather than smearing it. A wheel grinder, for instance, can embed pieces of stone into the aluminum that may become a contaminant or inclusion. A smeared surface may cause lack of fusion and should be filed to remove any smeared metal prior to welding.
- Do not use oxy-fuel gas cutting, carbon arc cutting or gouging processes, or oxy-fuel flames to preheat. These processes damage the heat affected areas and promote the growth and hydration of the oxide film present on the surface.
- Use plasma arc cutting and gouging, and laser cutting, as these processes are less likely to introduce hydrogen and moisture to the oxide layer.
- Mechanically remove the plasma arc and laser cut edges from 2xxx, 6xxx and 7xxx series alloys. The melted edges of these alloys contain detrimental solidification cracks and heat affected zone conditions. Remove a minimum of 1/8-inch of metal from the cut edge. Use mechanical methods, such as milling or a coarse burr grinder, to cut and remove metal chips.
- When possible, do not use lubricants in aluminum cutting applications. Any petroleum-based lubricant contains hydrocarbons, which will break down during the welding process and introduce hydrogen into the joint, leading to porosity and cracking.
- Most aluminum will come from the factory with oil and grease on it. To clean the joint prior to assembly, first degrease the surfaces with a solvent, and then use a cheesecloth or paper towel to clean and dry the welding joint (these are preferred because they are porous and will absorb more oils/moisture). Also wipe the opposite side of the joint clean so that not impurities are pulled through the aluminum and into the weld puddle.
- Look for a solvent that leaves behind no residue, and do not use chlorinated solvents in the welding area because these substances may form toxic gases in the presence of a welding arc.
- Do not use shop rags to clean welding joints as these rags can easily transfer oil and dirt to the welding surface.
- Do not use compressed air to blow off the joint. Compressed air contains moisture and oil contaminants.
- Clean the joint with a stainless wire brush only after solvent cleaning. Wire brushing prior to cleaning embeds hydrocarbons and other contaminants in the metal surface, as well as transfers these undesirable elements to the brush – which then makes the brush unsuitable for cleaning.
- Use a stainless steel wire brush to clean all metal that has been etched. The by-product residuals from etching must be removed prior to welding.
- Clean all wire brushes and cutting tools frequently to prevent the transfer of contaminants to the weld joint and, if possible, keep dedicated brushes solely for welding aluminum to minimize opportunities for contamination.