I main ashe, there is no specific build for ashe, your job is to kill and do crazy damage. Your item build have to adjust according to who is in the other team and how much you are earning in a specific game.
The famous Infinity Edge + Last Whisper is a good one against mage and heros with few hp, but in higher elo, even mage will have like 2600+ hp with the 2 +hp ap item. So with those weapon you are doing around 380 dmg / sec(inc crit), taking armor in to calculation, you need around 10 sec to kill a mage with 2600 hp.
So good items for ashe are:
Last Whisper - Is a must for any game, everyone has base around 80 armor, that's like 35-40% damage reduce
Black Cleaver - For extra armor reduction, combine with last whisper, it does amazing damage, just try it if you dont believe so.
IE - For crazy crit. A mid game item, not recommand for first item.
Burtalizer - Is a must against high HP target. Anytime there is a stable tank on the other team, you need one of this.
Stinger - If you dont have a CDR and mana regen page, highly recommand. With the lizard buff, you can spam volley every 2.X sec, it almost feels like instant! Good for pushing and excellent good for tower defending.
Stark, Bloodthrister - Good life steal items. Malady only if you dun have BC and already has a stark, otherwise Stark always better.
Sword of Divine - When there is a JAX, this item is a must unless you want to run (which you might probably fail 70% of the time) when you see one.
NO Frozen mallet, you already have slow, the 700 hp is alot, but with less money, you can get a Banshee Veil or Warmog which both is a better defence item and as an ashe player, you are not suppose to take damage anyway.
Phantom Dancer - Get this if your build has an IE, it's a good item but very expensive.
Core Items -
BC, LW, IE - Reduce armor build
Burtal, LW, IE - Against heavy tank build
IE, LW, PD - Crit Build
PLUS 1 Life steal item and 1 defence item, or both life steal item or both defence item.