#melt through

[haaaaaaaaaaaave-you-met-ted]

Melt Through by Piotr Foksowicz

[weldingtheway]

Burn through and melt through occur when a welder melts through the metal they are trying to weld. Burn through creates an unsightly hole or gap. Melt through causes a raised area along the back of the material being welded. Both defects are particularly troublesome with sheet metal and thin plates. It is generally a result of too much penetration caused by having too much heat input in the weld zone. This problem can be corrected, with MIG, by reducing the wire-feed speed, which, in turn lowers the welding amperage. Travel speed may also be increased. Excessive penetration often causes burn through. While melt through is not necessarily bad, it is unsightly and the welded part may be rejected.

[peacycles-blog]

I have been practicing TIG welding thin wall tubing in anticipation  : of making a bicycle but I am finding it hard to get a pretty bead.  : Also I am wondering if melt through is acceptable.  I know that the  : head tube and seat tube are reamed before the painting operation so is  : it OK if there is some melt through in these places?  :     If I weld slow and cold then the HAZ is wide and the bead kind of  : ugly and "cold" looking.  :     If I weld hot and fast and hold a slightly longer arc length then  : things look better but I get some melt through. 

Melt through will kill you frame finishing tools.  Try to avoid it.  With  a 1/16 2% tungsten (the most common for framebuilding) you should keep a  1 mm arc length at all times.  Holding a constant arc length is one of  most difficult skills for a beginner to master.  Remember that the energy  in the arc goes UP as you pull away, so pulling back is a bad habit.  Use  the foot pedal for your heat control. 

:         Which is preferred?  Should I use copper back up bars in the  : head tube and seat tube?  :     The tubing is CrMo (4130) and wall thickness is around .032 for the  : thinner tubes, my machine is set at around 65 amps. 

The way to control your HAZ is to move quickly.  For .8mm tubing I use 95  amps (on a Maxstar 151) and a 1 mm wire.  If you can afford one, a pulser  will help you travel even faster and still control your penetration.  An  experienced welder can do high quality work without using backup bars.    They are kinda like training wheels...someday you're going to have to  take them off. 

If you are really rolling in the dough, you might want to try an inverter  type power source.  These machines will respond to your foot control far  quicker than conventional machines, and they are very energy efficient.