Go to the hardware store, buy ~10 lbs of lead and a nice wooden handle.
Drill a large hole thru the handle on the side you want to be the face.
Build a wood mold the size you want your hammer to be.
Wearing leather gloves, protective clothing, a face shield, and boots:
Melt the lead in a steel can with a propane torch.
Adding flux will make for a better pour...read directions carefully.
Heat the mold and handle with the torch, making sure there is no water on either.
Put handle in mold, pour lead around handle, let cool.
The drill hole is full of lead, and will keep the head from coming off.
You will have to rework it every few years; cut it all off the handle, remelt it, and repour it.
Don't pour molten anything into a wet mold; you won't like it, and you'll make babies cry when you walk by...
Don't use lead free alloys; they're too soft.