Bullseye tubular cranks are .035" wall 4130 sheet. They are welded with 4130 wire and heat treated. To reach a certain weight goal you would need to heat treat the structure which would require heat-treating and 100% 4130 material.

Material engineered for forging may not be ideal for welding. It may also be fine. Constructing from tube would require a larger arm than solid but not too much. The square-taper interface is a bit narrow for directly attaching a tubular arm. People use fork legs for arms sometimes. I have pedal bosses if you need some. I have made many cranks from steel.