I have used the alumaweld stuff with probably 85-90% success.I have never had something break after welding it just didn't work to start with. The secret to a good weld is to clean and scuff parts,then clean with a stainless steel brush. Get a good tight fit if possible and put parts on a cast iron skillet,place in oven at about 350-400 degrees.This allows you a lot more control since now you only need another 300-400 degrees to melt rod.VERY IMPORTANT heat part hot enough so that rod will melt when touched to part.Drag the rod in the seam keeping part hot always allowing contact with part to melt rod,
I have tried to just heat part and sometimes part will distort before rod will melt. I have added bosses to mufflers,patched up motorcycle parts,fixed an old XKE door handle,and a high pressure line on an air conditioner. You will know immediately if it is going to work because it will flow smoothly into gaps. THE ROD MUST BE MELTED BY DRAGGING ON THE PART.
Take time to set up parts where they won't move when you try to weld because you won't be able to touch them.
Try to make good fill as you go because it doesn't work well going over what you have done. It takes a little playing with but I have had pretty good luck with it. I just use a propane torch with a round flame.