Hi Chuck,
What Mark said but I like using a 75%helium/25%argon gas mix for Mig or Tig welding Alumnium as the arc will alot hotter with helium gas. Although the 75%argon/25%Co2 will work your weld will have prositity included with-in the weld along with alot of AL splatter on the surface.You can use oxy/act to gas weld AL. You can use AL stick rod with the flux left on or use a scrap piece of AL with a flux made from borax with alcolhol added to make a paste and just dip the filler rod into the flux when needed. If you keep your filler rod with-in the gas flame zone then a flux is not needed. Be sure to remove all flux when done welding as AL will continue to oxidize if the flux is left on. There is a couple tricks to Gas or Tig welding Al. As your heating the base metal you will notice the weld puddle will start swrilling and that is the correct time to add filler rod and move backwards/forwards. I like moving away from my weld puddle as you can see how much filler rod needs to be added. A split second latter wrinkles appear with-in the weld puddle and if you don't quickly remove the heat or move forawrd then your weld puddle will end up on the floor. If you Tig weld SS you will also notice this swrilling action of the weld puddle. On SS that means you have just achived 100% penetration of the base metal and if your heat(amps) are set correct then you can hold this weld puddle and let it develope to the maxmium width wanted (1.25 times the thickness the base metal equals the weld puddle width your wanting) until your weld puddle drops to the inside then adding filler rod and moving forward at the same time thus achiving and inside weld puddle the same width as the outside weld puddle. The inside weld bead material is from the base metal dropping thru and you add the filler rod for the build-up of the outside weld bead. Thats how you achieve a weld that looks the same on the inside as it does on the out side.
So if you don't want 100% base metal penetration so it "sugars" the inside, don't let your weld puddle swrill.
Sugar means the inside weld bead oxidizes without using a purge gas usually argon at about 5cuft/min. I explained this process as it can be applied to any weld puddle control therory of any type of base metal and is the best example a welder will have to "see" the "controlled" process in "slow" motion. You can also use a chill bar clamped to the backside of your weld joint base meterial.
A chill bar is a thick piece of metal, usually 1/2" x 2" wide copper, used to draw the heat thru the base metal but hold the weld puddle from dropping thru and the weld puddle will not stick to the base metal. On welding AL you can use a piece of "clean" steel, no scalling, SS or CU. On welding Copper a SS chill bar works best. On welding carbon steel a CU, AL or SS chill bar will work. While welding AL you will notice a black oxide scale appear on the surface and the weld puddle becomes very difficult to control. Stop and remove this black oxide with a new SS wire brush or new grinding wheel. It's very important to use new wheels as if the wheels had beem used on Cu or Fe you can't get rid of the black oxides. I also like using tri-mix cover gas of 75%helium/24%argon/1%Co2 for my small 115v Mig welder as it allows me to weld a thicker base material for a given Amp setting. I use this tri-mix on every type of welding process Tig, Mig and as a purge gas. I use a Tee after the flow meter with a needle valve to control pruge gas flow. I only need one flow meter and one type of gas that way. Pure argon can be used for a cover or purge gas but is very expensive when welding Fe comparred to Co2. On 316 SS a tri-mix needs to be used. Although LP/air, Acet/air can be used to gas weld AL it's very slow and has a sever thickness limitation wheres oxy/acet does not. T_Bone
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