Friction welding

Dear all.

Two metalical piece of al and cu standing opposite of one another around the x axial. al rotates around x circle and cu approches to al with minimum axial force of x . while al is approching it is expected that on the impact of the friction between al and x tempreature goes up and defussion occures but while it undergoes under cu condition can not keeps its structure and brokes up . What is ths reasen of cu broking up? I have attached additional information. I would appreciate it if you could please help me.

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