Defining a function using a preceding expression

How can I automatically define a function inside my Mathematica code using a result (expression) that preceded the point of definition.

For example:

tt = x + y
x + y     

f[x_, y_] := 2*tt     


f[1, 1]
2 (x + y)

doesn't give $f$ as 2($x+y$) with $x$ and $y$ being variables here. So, instead of giving f[1,1]=2(1+1)=4 it gave 2(x+y) not recognizing that $x$ and $y$ are meant to be variables.

How can I define $f$ here so that it picks up its definition from the expression of $tt$ automatically, with $x$ and $y$ as variables?

The reason I need this is that it would save me manually copying and pasting the expression x+y into the function defintion, and would make my code run automatically even when tt gives different expressions.

Answer

Another option is to use = instead of :=

tt = x + y

f[x_, y_] = 2*tt
f[1, 1]

f[x, 4]

f[x, y]