that's weld contamination. either you haven't been sterilizing the tungsten on a piece of copper, or you haven't been scrubbing the aluminum weld area w/ a stainless steel brush. when you sterilize the tungsten, you strike the arc on a thick piece of copper (1/4" thick or so) and watch the arc...when the arc stops wandering around, the tungsten is clean. and anytime you touch the tungsten to anything while the arc is struck, you will contaminate the tip and you'll either have to sterilize it over the copper again, or if it's contaminated enough, you'll have to re-grind the tip and then sterilize it. a good TIG bead should be totally clean no matter what the welded material is.

Cambo,

Some Aluminum alloys are very weldable while others aren't, not knowing exactly the alloy designation of your *Hardware Store* aluminum practice pieces we can't speak with absolute authority. Here is the best bet as to what your seeing in your weld.

The grainy appearance (like glass beads) that you see upon the surface of your completed weld beads is most likely a manifestation of hydrogen porosity or possibly lack of cleaning action. An aluminum TIG weld that shines on top is often the sign of a *Cold* weld that may lack sufficient penetration. While on the other hand the etched appearance may well be a very good thing. When that etch becomes slightly bumpy or grainy that is usually a sign of porosity (very small bubbles). The porosity can be caused by technique (Too slow travel speed/too much heat input). Aluminum is a great thermal conductor, heat travels quickly thru it, but if the heat has no place to go problems invariably follow.

Weld porosity in aluminum is caused principally by bubbles of hydrogen that form in the solidifying weld pool. The hydrogen solubility of aluminum increases almost twenty times as the material makes the transition from solid to liquid state and continues to increase as the temperature increases. Hydrogen absorbed during its molten state is forced out of solution as the aluminum cools and changes to its solid state. The hydrogen can be trapped in bubbles that cannot effectively be removed from the weld.

What does this mean and what can you do about it? Here are some rules of thumb.

1. Make sure your using readily weldable alloys and fillers.

2. Surface prep must be excellent.

3. A/C Balance control must be set properly

4. Heat input must be controlled by means of travel speed or heat sinks

Mike Sammons of Miller electric has posted some technical information regarding GTA welding of aluminum, most of which seems agreeable to me http://millerwelds.com/pdf/techart/Tigalum.html

Also I suggest the textbook "Modern Welding Technology" by Howard B. Cary

Finally, Tungsten electrodes are not made sanitary by an arc strike on a piece of copper. If your tungsten electrode becomes contaminated by contact with the work or the filler or has melted out of shape, it will not perform properly until it is mechanically cleaned. There are several posts in this forum addressing proper tip prep for electrodes that GTA weld Aluminum.

Lawrence Bower
Welding Instructor
United Airlines