stack away vs pile up

stack away and pile up both are verbs.

Word	Noun	Adjective	Verb	Adverb
stack away	No	No	Yes	No
pile up	No	No	Yes	No

As verbs, pile up is a hyponym of stack away; that is, pile up is a word with a more specific, narrower meaning than stack away:

Other hyponyms of stack away include bin, computerise, computerize, victual, accumulate, amass, collect, compile, hoard, roll up, hive.

stack away (verb)	pile up (verb)
keep or lay aside for future use	increase in amount by collecting or gathering
	arrange into piles or stacks
	get or gather together