  The thermodynamic properties of {\alpha}-Al and other phases (GP zones,
{\theta}'', {\theta}' and {\theta}) in the Al-rich part of the Al-Cu system
have been obtained by means of the cluster expansion formalism in combination
with statistical mechanics. This information was used to build the Al-rich part
of the Al-Cu phase-diagram taking into account vibrational entropic
contributions for {\theta}', as those of the other phases were negligible. The
simulation predictions of the phase boundaries between {\alpha}-Al and either
{\theta}'', {\theta}' or {\theta} phases as a function of temperature are in
good agreement with experimental data and extend the phase boundaries to a
wider temperature range. The DFT calculations reveal the presence of a number
of metastable Guinier-Preston-zone type configurations that may coexist with
{\alpha}-Al and {\theta}'' at low temperatures. They also demonstrate that
{\theta}' is the stable phase below 550K but it is replaced by {\theta} above
this temperature due to the vibrational entropic contribution to the Gibbs
energy of {\theta}'. This work shows how the combination of cluster expansion
and statistical mechanics can be used to expand our knowledge of the phase
diagram of metallic alloys and to provide Gibbs free energies of different
phases that can be used as input in mesoscale simulations of precipitation.
