THEDANIMAL 
Some aluminum tubing is welded, some is rolled sheet and crimped, and some is extruded. Crimped is obvious....example: old style tv and radio antennas. Welded tubing is usually a lot less obvious. Ever look at those old style aluminum patio chairs? There's no sign of welds, but all the ends of the tubes are capped and welded. Pretty safe bet that the tubing is too. Extruded aluminum tubing usually has tiny, hair thin lines, running the length of the tubing. You have to look extremely close to notice this.
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