What would happen if aluminum was used in place of iron at the center of a sparkler wire?

It will give Bright White sparks instead of Orange sparks.

Explanation:

Sparkler wires are usually made up of a metal fuel at the center, an oxidizer and a binder. The burning of oxidizer also burns the powdered metal, resulting in sparkler's effect.

The metal used in sparkler wires can effect the color of the sparkles. Iron, which is mostly used in sparkler wires, produces orange sparks, while if we replace the iron with aluminium as a metal fuel, it will result in bright white sparks. Similarly, other metals can be used to get a variety of colors

the part of the wave he is travelling on is the crest