You clean the metal because the rust, and any imperfections such grease, millscale, paint, whatever, because otherwise, when you start welding, the contaminants burn, bubble, outgas and do all kinds of crazy things because you have all these foreign bodies mixed into the molten puddle.
The worst thing it does is weaken the weld. Any stress, flex or pull could cause the weld to fail and pop apart. If your welds have to be certified, such as in the food or nuclear industries where they check the welds with X-ray machines, they’ll probably find a pocket in the weld. Then you have to grind it out and start from the beginning.
Kevin grabs two rusty railroad spikes. He has his vise clamped to the table and his ground attached to the table. He puts one spike in the vise, wiggling it around a little to get an electrical contact. He’s set his Everlast PowerMTS 251Si on 170 amps and is using a freshly ground, 1/8″ tungsten.
He lowers his welding helmet and TIG welds the railroad spikes together. Sparks are flying.
How many other things went wrong? The end of the tungsten melted from all of the contaminants bubbling up. The weld itself started OK, but then you see porosity, and the weld ended with a crater. “Pretty darn ugly looking,” admits Kevin. He suspects that, if he cut the weld open and etched it, you wouldn’t find much penetration.
Just to show you the difference, Kevin takes two spikes with shiny spots that he has cleaned up on the grinder, removing enough surface to get rid of rust and any other contamination. He cleaned up the sides of one spike, too, so he could get a good electrical contact. He sets up the clean spikes just like the earlier, dirty spikes and TIG welds them together.
This second weld is much quieter and cleaner, without the sparks. Then he shows the two welds, which are obviously different. Kevin hopes this answers your questions about whether you need to clean metal before welding, and SOMEONE owes him a six pack!
Don’t miss Kevin being, well, kinda disrespectful (OK, REALLY disrespectful!) ….