A slight tangent: hollow vs solid: there is very little difference in strength of a tube (hollow) vs a solid. The bending strength of a solid axle is proportional to the diameter raised to the fourth power and inversely proportional to the length. For a hollow axle it is the outer diameter raised to the fourth power minus the inner diameter raise to the fourth power.

Lets look at some numbers:

10mm solid axle, the strength is proportional to 10^4 = 10,000

10mm axle with a 5mm hole through it: 10^4 - 5^4 = 9375

less than 7% difference, which is easily made up be a slight alloy or heat treat change.