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The torsional section coefficients are the same, and Wp is only related to the cross-sectional shape.

The maximum shear stress is the same, shear stress = torque/Wp.

The torsion angle is different, angle = (torque*length)/(G*Ip). Among them, G is the shear modulus, which is different for steel and copper. Ip is the polar moment of inertia, which is the same for axes with the same shape.