Etching of Aluminium alloys

Try Samuels' Metallographic Polishing by Mechanical Methods, whence comes the following information:

LIGHT METALS - Aluminum and Alloys
Composition Comments

1. 
   95 mL water, 2.5 mL HNO3, 1.5 mL HCI, 1.0 mL HF: Keller’s reagent, very popular general purpose reagent for Al and Al alloys, except high-Si alloys. Immerse sample 10-20 seconds, wash in warm water. Can follow with a dip in conc. HNO3. Outlines all common constituents, reveals grain structure in certain alloys when used by immersion.
2. 90-100 mL water, 0.1-10 mL HF: General-purpose reagent. Attacks FeAl3, other constituents outlined. The 0.5% concentration of HF is very popular.
3. 84 mL water,15.5 mL HNO3, 0.5 mL HF, 3 g CrO3: Graff and Sargent’s etchant, for grain size of 2XXX, 3XXX, 6XXX, and 7XXX wrought alloys. Immerse specimen 20-60 seconds with mild agitation.
4. 1.8% fluoboric acid in water Barker’s anodizing method for grain structure. Use 0.5-1.5 A/in2, 30-45 V dc. For most alloys and tempers, 20 seconds at 1 A/in2 and 30 V dc at 20 °C is sufficient. Stirring not needed. Rinse in warm water, dry. Use polarized light; sensitive tint helpful.

A fume hood and personal protective equipment is mandatory. Don't go near HF unless you fully understand the associated hazards, are fully protected, and have calcium gluconate on hand.