Sun joe mower: how to fix blade
The central bolt and washers in the correct order are used to hold the blade to the mounting plate.  The pins on the mounting plate, some of which may be broken in your case, fit into the off center holes on the blade so that the bolt need only keep the blade pinned to the plate and the pins support the torque.  Inspect carefully.  If you have missing or broken pins, your mounting plate or mower need to be repaired or replaced.