Bolting a steel ring terminal to a steel tab on IGBT?
The tabs are usually tin plated copper as these need to be solder to the substrate.

I would not use a star washer. These are great for earth bonding points to break the insulative oxide layers (lower contact resistance), they will not offer that great of a contact surface area and thus your current density will be higher and overall resistance higher.

I would personally advocate using copper washer with the designated tightening torque