Powdering aluminum chemically?
There is no simple and affordable way to obtain fine aluminium powder chemically. As permeakra said, in the absence of a ball mill, it's just easier to buy some.

Note that a finely dispersed aluminum powder obtained commercially is always going to have low level of impurities such as $\ce{Fe, Mn, Si, Mn}$ (from the grinder's balls) and traces of wax/fats on the surface (added by manufacturer on purpose to prevent adhesion and separate particles from each other).

As for the chemical/physicochemical methods, there is a great overview included in the US5885321A patent, among which there are the following:


exploding of aluminum wire in a vacuum by a high electric current (particles' size varies a lot);
condensation of vaporized aluminum in a current of inert gas (very energy-inefficient);
decomposition of aluminium carbonyls (there is no experimental evidence for any of these to exist);
reduction of halides in solution (issue with separation of the particles from the salts);
CVD of aluminium thin films by decomposition of tertiary amine complex of aluminum hydride (complex multistep process with low yield and expensive and volatile precursors);
decomposition of the dialkylaluminum hydride in $\ce{H2}$ atmosphere at elevated temperatures (up to $\pu{260 °C}$) with a catalyst ($\ce{Ti, Zr, Hf, Ln}$) and a tertiary amine (no particle size control, questionable efficiency);
decomposition of diethylhydridoaluminum or diisobutylhydridoaluminum in diisopropyl ether or triethylamine at $\pu{90 °C}$ to $\pu{185 °C}$ with titanium isopropoxide catalyst.


The patent itself proposes


  decomposing alane-adducts in organic solvents under an inert atmosphere to provide highly uniform particles selectably sized from about $\pu{65 nm}$ to about $\pu{500 nm}$


where adducts are


  trialkyl amines and tetramethylethylenediamine, ethers and other aromatic amines;


and conditions such as


  atmospheric pressure and at temperatures as low as $\pu{50 °C}$ with xylene solvent, ... , toluene, dioxane, and tetramethylethylenediamine.


As a catalyst, titanium halide, amide and alkoxide are used. Authors also provide a complete table of the systems tested:


  


References


Higa, K. T.; Johnson, C. E.; Hollins, R. A. Preparation of Fine Aluminum Powders by Solution Methods. US5885321A, March 23, 1999. (PDF)