Engraving metal is not possible with the equipment you have. The definition of engraving is to “cut or carve (a text or design) on the surface of a hard object”. When engraving, you are removing material, which in this case, you are wanting to remove metal.
You can, however, create works on the surface of this material using a chemical bonding agent. Paint or spray on, laser to bond to material, wash remaining off surface to reveal final results. The power / speed settings required to bond will be based on the bonding agent manufactures recommendations.
The power required to engrave metal is significantly higher than the output from these DIY diode jewels. 