That is not true. They are not the same concepts. Rational means based on reason. Valid means logically correct. An argument is either logically correct or it is not. Because of the rules of logic, if any single step is not logically correct the entire argument is invalid. In fact, every valid argument is also a tautologies, it may be wonderfully interesting but, in the end, it is empty.
But an argument can be based on reason to varying degree. As I suggested, visit a DPW thread and it is clear.
Also, rationalize is not what I mean. People do that too certainly, but I am talking about the real process by which they choose.
Tell you what though, arguing over the meaning of words is wasteful and beside the point. What word do you want me to use for a choice made partly based on reason and partly not?
You seem to be claiming that an argument can be "based on reason," while at the same time "logically incorrect." That's a contradiction in terms. The relevant definition is, "Consistent with or based on reason; logical: rational behavior. See synonyms at logical."
The word "rational" can also mean "sane," which isn't relevant here, and it can also mean "capable of rational thought," which also isn't relevant.
What word do you want me to use for a choice made partly based on reason and partly not?
Choice is utterly subjective. Attempts to rationally justify a choice are inherently post-hoc rationalizations. So "choice" or "preference" simply shouldn't be used in the same sentence as "rational." As for arguments, anything less than a fully rational argument is "irrational," or "incorrect," or "invalid," or perhaps simply "bad."
What fuzzifies matters is that people constantly interject their preferences as premises. Doing so always invalidates the argument. The argument can be rescued by clearly identifying those assumptions, at which point the focus will shift to the validity or invalidity of those assumptions. In general, if your assumption is one of preference, then it's fine for you to act on it--but it's not fine for you to consider me to be bound by your preferences.