Weldcraft -

TIG welding anodized aluminum is a bit like trying to poke a hole in a sheet of ice without disturbing the water underneath. In this case, the ice is the hard layer of oxide created by the anodization process and the water is the soft aluminum underneath.

Fishing rod holders, being welded here with a water-cooled torch, are one of the many products made using anodized aluminum.

Anodized aluminum is simply standard aluminum that has been treated to produce a thick layer of oxides on its surface. The aluminum base is a very soft material and melts at approximately 1100 degrees F, but the oxide layer is extremely hard (some types approach the hardness of diamonds) and melts around 3600 degrees F. The difficulty in welding anodized aluminum lies in removing the oxide layer without also burning through the aluminum base.

Interestingly, it is these properties of the oxide layer that make anodized aluminum a useful material in the first place. Because it is relatively inexpensive, visually appealing, very lightweight and extremely resistant to corrosion — particularly in high salinity environments — anodized aluminum is a common material in coastal areas. It is used for tuna towers, rod holders, chairs and t-tops on sport fishing boats, piers, bow rails, wakeboard towers and more.

Without any treatment, aluminum naturally forms a very thin layer of oxides. The process of anodizing aluminum uses an electrolytic chemical application, usually with Sulfuric acid, to create a layer of oxides several times thicker than would naturally form — ranging from .0002 to .001-in. thick.

There are four types of anodized aluminum: standard, bright finish, colored and hardened.

Bright finish anodized aluminum can be visually distinguished from standard anodized aluminum by its shiny, chrome-like finish. This type of material contains a thicker layer of oxides than standard anodized aluminum, making it more difficult to GTAW weld, and is used primarily for cosmetic reasons.

Colored anodized aluminum is also used for cosmetic purposes. This material uses dyes in the anodization process, which allow the material to take on different hues but also introduce potential contaminants into the weld.

Hardened anodized aluminum is almost as hard as a diamond and is very difficult to weld. This type of material is usually used only in highly specialized industrial applications and will not be discussed in detail here.

Bright finish, colored and standard anodized aluminum all use much the same equipment, preparation and technique as standard aluminum, but they also require some unique considerations in order to be welded successfully.

Equipment selection
The proper equipment is the first step to successfully weld anodized aluminum. Materials unique to anodized aluminum include a torch with a finger tip control and a 5356 class filler metal. An air-cooled torch is suitable for welding under 200 amps, but a water-cooled torch should be used for applications requiring more amperage.

Properly TIG welded anodized aluminum, as shown here, can result in products of excellent cosmetic quality.

Like standard aluminum, anodized aluminum also requires a 100 percent Argon shielding gas or an Argon/Helium mixture, and a 2-percent Ceriated (orange stripe) or Thoriated (red stripe) tungsten. [Note: Thorium is radioactive; always follow manufacture’s warnings, instructions and the MSDS (Material Safety Data Sheet) for its use.] Pure tungsten (green stripe) can also be used, but only up to about 70 percent the amperage of Ceriated or Thoriated tungsten.

To prepare the tungsten, grind it to the same type of point used for steel or stainless steel. Re-grind the tip when it gradually rounds off and the arc becomes unstable, repeating this process as necessary.
A power source capable of alternating current (AC) is also necessary because it is the electrode positive portion of the current cycle that breaks apart the oxide layer.

Although there are traditional, transformer-based TIG power sources that are capable of successfully welding anodized aluminum, an inverter is highly recommended for its balance and frequency controls.

Balance control allows the user to adjust how long the current spends in each part of the AC cycle. For example, a balance control set to 30/70 means that the current spends 30 percent of its cycle as electrode positive, cleaning the oxide layer from the base material, and 70 percent on electrode negative, directing the electrical energy into the weld joint and joining the two pieces of material.The actual balance control setting will be determined by the experience and skill of the operator, with a more skilled operator often using a high percentage of electrode negative (80 to 90 percent) in order to work faster.

The frequency control function available with inverter units allows the user to determine the length of time that it takes the unit to complete one full current cycle, (the combined time spent on electrode positive and electrode negative.)

Transformer-based power sources produce an output of 60 Hz (50 Hz in Canada), which is the same frequency that comes from the wall power receptacle. Inverters, however, are able to adjust the frequency from 20 Hz — 400 Hz. For anodized aluminum, a frequency around 160 — 200 Hz generally produces the best results. The increased frequency produces a narrower arc cone and, consequently, a narrower weld bead and heat affected zone. The strength of the weld increases by reducing the area of the base material exposed to the heat of the arc. A narrower weld bead reduces the time and filler metal needed to make the weld.

An inverter machine also eliminates the need for a backhanding technique. Backhanding, also known as backing around the weld, involves making two passes on each weld joint — one pass going forward to clean off the oxide layer and then reversing direction to add filler metal to the area just cleaned. In addition to doubling the time it takes to weld the joint, backhanding requires the work piece to be heated twice, and can reduce the strength of the weld.

Material Preparation
Properly preparing the material is critical to ensuring strong and cosmetically appealing TIG welds on anodized aluminum.
The material should be at room temperature for at least an hour before welding. Otherwise, condensation can form within the aluminum and react with the argon shielding gas to create porosity and a black, sooty appearance on the weld.

Cleaning any residues, dirt or other foreign material from the base metal and filler metal with a clean cloth prior to welding helps avoid weld defects, such as porosity, inclusions and lack of fusion.

Although the electrode positive portion of the AC cycle helps remove the oxide layer that builds up on aluminum, you may want to use a dedicated stainless steel brush to mechanically clean the material prior to welding. Using this brush for other purposes will introduce foreign contaminants to the weld joint and compromise the weld’s structural integrity. An oxide layer begins to form immediately after brushing, so it is also necessary to re-brush a piece that sits for an extended amount of time without being welded.

Technique
Anodized aluminum requires a technique known as bumping, which is a way to remove the thick oxide layer without putting too much heat into the weld pool. Because it uses a finger-tip control switch, a foot pedal is unnecessary for this technique.

Bumping involves very briefly starting an arc using the finger control switch, adding the filler metal, extinguishing the arc, moving down the weld joint about 1/16-in. and repeating the process. Turning the finger-tip switch off gives the weld pool a moment to cool and solidify before reintroducing the heat with a new arc.

Although easily described, bumping is one of the more difficult TIG techniques to master. A proficient operator can typically reach a travel speed of about an inch every 30-40 seconds.

The amperage range will depend on the skill of the operator, with someone at a beginner to intermediate skill level using around 180 — 190 amps, and a highly skilled operator welding at around 230 amps.

The thickness of the material will require the welding operator to adjust the travel speed — thinner material requires faster travels speeds than thicker material — but the amperage range typically remains the same regardless of material thickness. At those amperage levels, a filler metal of 1/8-in. diameter should be used. Anything thinner will melt off before it even gets to the weld pool. It is also important when adding the filler metal to insert it at the leading edge of the weld pool while the arc is live and to remove it before extinguishing the arc, so that the filler metal doesn’t become trapped in the weld pool as it cools.

The end of the filler metal should be kept no further than the diameter of the torch cup away from the weld to avoid a loss of shielding gas and possible weld pool contamination.

Although anodized aluminum certainly does pose some unique challenges, its weight, inexpensive costs and resistance to corrosion make it the best option for many applications and environments. Following the advice above should help overcome the difficulties associated with this material and realize its potential benefits.