I've used these...they are tricky to use for sure.
You have to heat both parent materials to 700 degrees and then place the aluminum rods on the material to be melted without the torch touching the rods.
If you melt just the rod, it doesn't adhere to the parent material. If you don't heat both parent materials to be welded, the aluminum doesn't adhere equally.
"For the High and Exalted One who lives forever, whose name is Holy says this: 'I live in a high and holy place, and with the oppressed and lowly of spirit, to revive the spirit of the lowly and revive the heart of the oppressed." - Isaiah57:15