portion 47 as well as a central longitudinal through bore 48 ferrule head 46 may be of any predetermined size or shape such as for example having a domed head as shown in fig2 and 4 or a flat head as shown in fig3 ferrule head 46 extends at a minimum both axially and radially beyond the annular end surface 37 of tube end 36 and over a portion of the top surface 58 of braze foil 55 preferably for a distance greater than the wall thickness of core tube 35 the outside diameter of shank portion 47 is approximately equal to the inside diameter of core tube 35 so as to permit a slip fit thereof into core tube bore 38 ferrule or rivet 45 is made of a rigid malleable non ferrous metallic material such as of an aluminum alloy for example referring to fig2 and 3 the ferrule 45 is radially expanded by means of an expansion mandrel tool 65 this tool 65 includes a connector 66 such as a wire or rod having a teardrop shaped or ball shaped expander portion 67 affixed at one end expander portion 67 has an outside diameter slightly greater than the diameter of ferrule bore 48 tool 65 also includes a collet 68 having an axial through bore 69 extending therethrough connector 66 is adapted to slidably move within collet 68 through bore 69 in order to radially expand ferrule 45 expander portion 67 is placed inside core tube 35 with connector.