Hi Bill,
The most common defect in welding aluminum is porosity. Aluminum welds are more prone to exhibit porosity than steel welds because molten aluminum has a very high solubility and affinity for hydrogen. While the weld is being made, it will absorb any hydrogen in the area. However, solid aluminum has almost no solubility for hydrogen, so as the weld solidifies, it tries to reject any dissolved hydrogen. If it can, there is no problem. If there is too much dissolved or if the weld solidifies too quickly, it forms porosity in the weld. There are two primary sources responsible for the presence of hydrogen. One is a breakdown of hydrocarbons in the arc, and the other is a breakdown of water vapor in the arc. In order to minimize porosity, the following precautions should be taken: preweld cleaning to remove oil and grease, don't allow excess drafts in the welding area, make sure you're using a welding grade gas with a dewpoint no higher than -70 degrees Fahrenheit, make sure the welding wire doesn't have residual drawing lubricant on its surface, use a slight leading angle of 10-15 degrees, but don't let the lead angle become excessive, and if you're welding vertical, weld vertical up because vertical down causes increased weld porosity.
Since you didn't mention cleaning, I'll further address the need for clean surfaces, as it's importance cannot be over emphasized. The surfaces must be clean to obtain good wetting between the filler metal and base metal. This means that they must be free of relatively thick oxide, moisture, greases, oils, paints, or any other substance. Many contaminates break down at elevated temperatures and produce hydrogen, which causes porosity. Degreasing may be done by wiping, spraying, or dipping in a suitable, safe solvent or by steam cleaning. Surface oxides can be removed by action of arc welding, as the welding progresses. However, it is best to remove the oxide from the surface by appropriate chemical or mechanical means. Chemical removal is accomplished with alkaline, acid, or proprietary cleaning solutions. Mechanical oxide removal such as wire brushing, scraping, filing, sanding, and scrubbing with steel wool are usually satisfactory if performed properly, but are generally not as consistent as chemical methods. Just curious. Why not ER4043 rather than ER5356 for 6061 to 6061?
Surfaces are cleaned and the process has already been certified so he can't change to the helium mix that was also my first thought, the probly was that he didn't preheat his base enough and I suggested that he switch to 4043 wire but he will have to go through the re cert process. The best advise that I could give him at the time is to try and pre-heat his base metal to a higher temp.
Bill
Hi Bill,
The main cause of porosity in aluminum welds is entrapped gas in the weld puddle. Porosity occurs when the metal freezes before the gas has a chance to escape. Gases may originate from contaminants in the shielding gas, from air and water, from contaminants in the shielding gas, from contaminants that get into the weld puddle from dirty base or filler metal, from too long an arc, or as a result of violent arc action. Since you said the surfaces are clean, I'd take a hard look the other possible causes.
Shielding gas, air, or gaseous contaminants can be entrapped in the weld puddle as a result of violent arc action. The weld porosity produced as a result is similar to the air bubbles that are entrapped in a rapidly frozen ice cube. Turbulence in a weld pool is related to droplet transfer. If too low a welding current is used, so that large globules of metal transfer across the arc, more turbulent puddle reaction occurs than if small, well formed droplets transfer in a fine spray pattern. Excessive welding currents can entrain gas by depositing metal over a gas bubble in the weld pool. Such metal freezes before the entire bubble has escaped. This type of porosity is generally irregular in shape. In addition to air entrapment, the generation of hydrogen from contaminants in the weld is another cause of porosity. Molten aluminum has a high affinity for atomic hydrogen. On the other hand, solid aluminum can contain very little hydrogen. Therefore, hydrogen gas is emitted as the weld puddle freezes. If the cooling rate of the weld puddle is too fast, the gas cannot rise to the surface, but remains within the metal, causing porosity. Any compound containing hydrogen and contaminating the filler wire or work surface can be the source of hydrogen gas. For example, if a film of drawing compound (containing hydrocarbons) or hydrated aluminum oxide remains on the filler wire, extreme porosity will occur in the welds made with that wire. Other possible sources of hydrogen forming contaminants include the base material and the shielding gas. A bottle of wet gas will cause contaminated welds, as will moisture leaks in the welding torch or hose line. In my original post, I indicated to make sure you're using a welding grade gas with a dewpoint no higher than -70 degrees Fahrenheit, but I have read elsewhere that gas having a dewpoint higher than -40 degrees Fahrenheit is unsuitable for mig welding aluminum.
Problem solved, thanks guys. not pre-heating enough
I'll second that cleanliness is essential (that goes for the joint and the filler wire). Use of dedicated tools for prep work is not a bad idea either. Oxides can and will form rather quickly, so clean and weld as soon as possible.
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