Tip #1 -- Clean the aluminum. This is the most important tip I have. I read this in several places before I began to practice welding, but it didnt seem so sink in and I wasted a lot of metal by trying to weld two pieces of dirty aluminum together. ALUMINUM THAT LOOKS BRAND NEW AND CLEAN IS ACTUALLY DIRTY. ITS NOT LIKE STEEL.Here are some of the signs that your aluminum is dirty. A wandering arc -You cant get a puddle started without burning through or distorting the metal Your filler wont blend into the puddle, instead it rolls into a difficult to re-melt ball. The aluminum seems to have surface tension, like beads of water on a waxed surface. When trying to join 2 pieces the edges curl away from each other and form an even bigger gap.Here's what's happening: Aluminum quickly forms an more or less invisible coating of aluminum oxide. Aluminum oxide melts at three times the temperature of aluminum. When you try to weld uncleaned aluminum, the aluminum under the aluminum oxide coating will melt but the aluminum oxide coating will stay solid and act as a membrane, much like a water balloon. When you finally succeed in penetrating the coating, the very runny aluminum inside will flow out all at once, much like a bursting water balloon.Heres how I prep the aluminum for welding. First, I spray the aluminum with acetone. Don't use brake cleaner unless it's 100% acetone. If you need convincing, check this out -- Then, I rinse the aluminum in water, just in case there's any nasty residue.The aluminum should be completely dry before welding, including all of the nooks and crannies. Then, I use a stainless steel brush (make sure the brush is stainless, I've read this is important) to scrub the aluminum shiny clean around the area to be welded.Some articles I've read suggest that the aluminum should be scrubbed in one direction only to avoid working contaminants into the aluminum. I don't always follow this rule and I haven't noticed any problems stemming from it, but I'm not working on anything too critical and I'm far from an expert. I've also read that 3M Scotchbrite pads are a good way to prep aluminum. If you do not weld on the aluminum immediately after cleaning, you should give it a touch up cleaning before you start to weld. I've read eight hours exposure after cleaning is the maximum acceptable without re-cleaning.Tip #2 -- Clamp your work to a heatsink made of copper or aluminum whenever possible. Aluminum transmits heat very well. Once the area you trying to weld gets hot enough to melt, the rest of the work is likely to be so hot that it's shrinking and warping. Using a heat sink under the area being welded will absorb some of the heat and help keep the work from warping.Tip #3 -- Preheat before welding. This makes it a LOT easier to weld aluminum. This is not a subject that is without controversy. The issue is that some aluminum is heat treated, and by heating and cooling heat treated aluminum it will get softer. I've read opinions ranging from "heat treated aluminum should never be preheated" and "preheating is a crutch for inexperienced welders", to the opposite extreme, "aluminum should always be preheated to prevent cracking". Recommended preheating temperatures range from 275 deg. F, to 500 deg. F. I suspect that many of these opinions are correct in their own contexts. The proper procedure likely varies for a welding a space shuttle door in a vacuum chamber and welding a cracked cylinder head. One thing I know for sure is welding thicker pieces of aluminum with our 165A welder without preheating is impossible. I once tried to weld two pieces of 8 mm thick aluminum together without preheating The result were a very shallow and weak weld, a circuit breaker that tripped twice and the welder overheated and shut down after every two inches welded. I don't have an oven handy, so I use a propane torch aimed at the heat sink I clamp the work to and an infrared thermometer to tell when it's hot enough. I usually can't get the work any hotter than 350 deg., so that's the temperature I use. I've considered getting a cheap used electric oven or an electric hot plate but haven't yet. I don't use the torch directly on the work. I don't know if it would cause a problem or not, I feel more comfortable heating the heat sink instead.Tip #4 -- If the tungsten gets contaminated, stop welding and fix it. When the tungsten gets touches the weld pool or the filler, the arc becomes unstable and the weld quality goes way down. This happens to me a lot, unfortunately. The best method for fixing this is to remove the tungsten, lay it on a flat surface with the contaminated part hanging over the edge, hit the contaminated part of the tungsten (it will snap right off), reinstall the tungsten, change the polarity to DCEP (direct current electrode positive), strike an arc on some scrap metal to re-ball the tungsten, switch back to AC high, and you're ready to weld again. By the way, KEEP YOUR GLOVES ON WHILE YOU DO THIS! Otherwise you gonna have a nasty burn. This only takes about 30 seconds once you've done it a few times.Tip #5 -- Fit the parts together as tightly as possible leaving no gaps. When using a MIG welder, I've found it fairly easy to fill gaps between the two pieces of steel being welded. However, I've found it very hard to do this while welding aluminum with the TIG. The heat from the TIG is very localized. When there is a gap, and heat is applied to the metal on one side, the metal pools on that side, but metal on the other side stays solid. You can alternate from side to side to get both sides to pool, but when I do this, I usually end up with a wider gap than when I started. The only way I've found to fill a gap is to "slop" a glob of filler across the gap, then work on the filler until it melts into both sides, then build off of the filler. This is hard to do. You can save a lot of time by using a file or die grinder and making the part fit together better before you start welding. The tighter the pieces are pressed together and the fewer the gaps, the easier the welding is.Tip #6 -- A few "rules of thumb" for base settings Use one amp per .001" of material thickness. Set the amperage a higher than the maximum you expect to use and use the foot pedal to back it down. Use pure tungsten for aluminum if you have a cheaper (non-square wave) welder like me. Use a 2% ceriated tungsten ground to a point if you have a more expensive (square wave) welder Use a 1/16" pure tungsten for 30 to 80 amps Use a 3/32" pure tungsten for 60 to 130 amps Use a 1/8" pure tungsten for 100 to 180 ampsUse 15 to 20 CFH Argon flow Use a filler rod size equal to the tungsten size. Adjust the tungsten to project from the hood a distance roughly equal to the diameter of the tungsten. The arc length should be roughly equal to the diameter of the tungsten.The first 3 tips were my major breakthroughs while learning to weld. They were the things I wished somebody had told me before I began my frustrating experimentation. There are, of course, many other things to know about welding aluminum, but there are many more complete articles available on the Internet. Follow the links below for some of my favorites.