In a fifth example of an assembly of the invention, the metal piece of Ni-based alloy used in the first example described above is replaced by a metal piece 1 of an alloy based on Co, e.g. the alloy known under the trademark ???Haynes 188??? and typically comprising, in percentages by weight: 20% to 24% Ni; 20% to 24% Cr; 13% to 15% W; not more than 3% Fe; 0.2% to 0.5% Si; not more than 1.25% Mn;