Good afternoon, i have some question regarding the expired ssl certificate of the node :

1. Can we join that node to cluster ?
2. Can we migrate that node vm to another node ?
3. Can cluster remain healthy ?
4. Can we create VM in a node (expired ssl certificate) ?

If answers to the above questions is "yes", then i need to know why are we using certificate then ? If its expired, node communication is happening in encrypted format, I can login to that node easily and encrypted traffic is going on.
Please tell me i am waiting for your response.

Yes, the answer is Yes.
Its needed for trust, because other wise Man in middle attack could occurs. It's also needed for compliance and regulations. But indeed if your environment is isolated then self signed or even expired certificate might be okay