ummmm....are you welding on cast pieces? You might know about this maybe not..... It could be cast magnesium which looks very much like cast aluminum and you can run an aluminum bead on it it will stay for a few seconds till it starts to cool then it will basically crack out into pieces. A simple test for this is to file some of your base metal then place the filings on a table and expose them to a flame from say a bic lighter......if it ignites into bright white sparks (think sparkler) you have magnesium and not aluminum. If it is Aluminum then you should identify the base metal as Lawerence asked and also provide what filler material you are using, machine settings, gas etc.