Process for making tube in sheet heat exchangers

A process for forming an opening in a composite metal panel comprising at least two sheets of metal having a bond therebetween which can be broken by inflation, said opening extending between said sheets and inwardly of said panel from an edge thereof, said composite metal panel being free of stop weld material sandwiched between said sheets and being adapted for forming a heat exchanger therefrom having a desired pattern of inflated tubular passageways therein, said process comprising:

• providing a hammering member having a blunt surface;
• hammering with said blunt surface said edge of said panel at a position wherein said opening is to be formed for delineating a bond interface between said sheets of said composite metal panel, said hammering being adapted to upset said metal at said position of said edge of said panel so that said bond interface is delineated by a crack formed between said sheets;
• providing a tool having a sharp end adapted to be positioned at said crack formed between said sheets;
• and
• forming said opening at said edge of said panel by inserting said sharp end of said tool into said crack.