It is to put some brazing powder and flux among the sandwich bonding surfaces, melt at high heat to bond together. For this kind of bottom, the aluminium could be very thin. However, the bonding at the capsule edge is not so good that there will be gap after cooking for several times where water and other dirts could be easily hided in. Besides, if the pan overheats for an extended period, the brazing powder might melt, causing disc separation.