>>36476195In Japan Lucario's "aura" powers are actually refered to with the word "wave". It's category is "wave pokemon" rather than the "aura pokemon", and Riolu is the "ripple pokemon" instead of the "emanation pokemon". Its trademark move "Aura Sphere" is called "Wave Bomb" in japanese. In the manga it's compared to sound.
This leads me to believe that Lucario gains a steel type upon evolving because it masters the art of controling waves, as metal is often used to guide waves irl like with the apropiately called waveguides and some musical instruments like this one:
https://www.youtube.com/watch?v=rEFbgeFtgc4 