Trouble getting puddles to bridge gap

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I am just getting started with TIG welding aluminum. I get nice puddles of aluminum forming on both parts but have difficulty getting the two puddles to join. Using a bit of filler rod to make a ball of metal and then adding more heat is the only way it seems to work for me. Once they do join, it is easy to keep it going down the joint. I have watched a few videos where the operator is tacking spots along the seam without any difficulty at all and that is why I am wondering if I am doing something wrong. My welder is a Lincoln 175 pro.
Reply:Taz, often a problem with aluminum welding is keeping the filler TOO close to the puddle when you are welding. It will become the blob, and it oxidizes on the outside. It can be tough to get the blob to merge into your puddle, and tougher still to get the blob to close a gap. Try and keep the filler tip 1/2" away, and kinda zip it into the puddle edge, and use it to bridge across the puddle. If the gap is a big one, then concentrate the puddle on one piece, and add a few dabs of filler to build up one edge. Then, after you have 3 or 4 dabs of filler to close the gap, go back to your start point, and you can close the gap easily.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Without going into a long discussion about molecular cohesion vs adhesion, thermodynamic stability, the influence of oxide coatings, etc.; if working with small/thin material, when you melt the aluminum, it becomes a small attached bead of liquid which tends to form a semi-spherical shape rather than flatten out and spread sideways. Until metal from the two sides actually touch, they have essentially no attraction for  each other and won't pull together as two magnets might. If the pieces are originally very (seemingly touching) close, you can use the gas flow and arc force to move the small puddles around and get them to touch, or you can use the filler to bridge any small gap. Something like playing with water drops on a slick oily counter surface. If you work with thick material and can have a large contained puddle (as a close-fitting butt joint), the weight of molten metal will overcome the cohesion, so join by itself. In brazing, the flux often helps by reducing the surface tension of the filler, allowing it to flow better. Its primary purpose, however, is to chemically clean the base metal surfaces to allow better adhesion of the filler.
Reply:Oldiron2:DING, and a light goes on in my head. Thanks. 9-11-2001......We Will Never ForgetRetired desk jockey. Hobby weldor with a little training. Craftsman O/A---Flat, Vert, Ovhd, Horz. Miller Syncrowave 250
Reply:encountered the same in ms-- get a double puddle that wont coalesce... i learned here from th pros that my arc length was too long... try that...not sure if alumionum works the same...