shop air cutting alum

[Edwardo]

I have been reading up on some forums for alum boat building and see where some builders avoid using shop air plasma cut parts because they claim this changes the metallurgy of the alum along the cut line and it needs to be ground back before welding creating more time and build expense, they recommend using routers or hydef plasma instead.
I am hoping to someday build my own alum boat using my Dynatorch and PM 1250 so my questions are in using shop air how far back would i have to grind the edge, or can i use bottled gas and what kind/mixture rather than shop air with the 1250 to avoid this situation in the first place, and lastly is the issue of the changed metallurgy along the cut edge using shop air really that much of a problem to affect the structural integrity of the weld.

Thanks
EDD

[jimcolt]

When cutting aluminum with air as the plasma gas.....the cut edge of aluminum has a roughness and porosity that is caused by the oxygen in air. Sanding or grinding about .005" off the edege essentially eliminates the issues and returns the edge to the same condition as the base metal. You can use nitrogen as the plasma gas in a Hypertherm air plasma....and it will minimize the problem, but will not eliminate it....as ambient air that surround sthe cut area will have an effect on the cut edge as well.

The high definition plasma systems from Hypertherm and other companies use a dual gas torch that uses higher shield gas flow rates....often for cutting aluminum Nitrogen is used as the plasma gas with nitrogen as the shield gas....and the shield flow on these torches is designed to push the ambient air away from the cut edge. On thicker aluminum, a plasma gas using 35% hydrogen and 65% argon is used with a nitrogen shield....the hydrogen acts as a reducing gas....essentially reducing the amount of oxygen in the cut edge area, minimizing oxidation and porosity on the cut edge.

There is no perfect solution for an air plasma system to get ready to weld (tig) edges on aluminum.....although mig welding seems to work acceptably in most cases.

Jim colt

[Edwardo]

Thanks Jim

Just to follow up on this, i have cut some alum in the past and can see and feel the porosity along the edge, is this porosity the issue for welding or has the alum along edge been degraded in anyways, is it just that the texture has been changed from smooth to rough with the edge material remaining the same. All the welds will be Mig and in reality to remove .005 would not take much time, the weld areas need to be cleaned regardless and since this will be a personal project so time is not really a issue, i was more concerned about the cut edge affecting the weld bond.

Thanks again
EDD

[jimcolt]

Generally once the porosity is removed....you are back to base material. You are correct, aluminum needs cleaning before welding anyway....so air plasma cutting does not present a big problem in my experience.

Jim

[Edwardo]

Jim

As always, your knowledge and replies are very helpful and much appreciated, all the best in the New Year.

Regards
EDD