How am I welding my exhast wrong?

Hi there, I'm not too experienced a welder and I am sure this is probably very simply answered. One side of my exahast (it's a cat-back) is disconnected at the muffler, and I have repeatedly tried welding it back up (very poorly; can't seem to get a good, solid weld on) only to have it disconnect again a few days later. I think that either I am somehow dealing with two different types of metal here (which I don't know how that could be, because it's a flowmaster kit) or maybe I am using the wrong wire to weld with. Any suggestions or ideas?

 

make sure the surface is clean and the temp is not too hot to burn the metal. what kinda welder?

 

oh and make sure there is a good ground

 

you got a picture? sometimes if it real dirty or rusted the clean metal will catch the flux and bond good, but the dirty part it will just sit on top and cool off and not make any penetration. +1 on not welding too hot, but also make sure it's not too cool either or you won't get any good penetration. I suggest you now take it off and grind a good exposed spot of shiny metal on the muffler and then try to do the same on the pipe.