  The role of major mergers in galaxy evolution is investigated through a
detailed characterization of the stellar populations, ionized gas properties,
and star formation rates (SFR) in the early-stage merger LIRGs IC 1623 W and
NGC 6090, by analysing optical Integral Field Spectroscopy (IFS) and high
resolution HST imaging. The spectra were processed with the Starlight full
spectral fitting code, and the emission lines measured in the residual spectra.
The results are compared with control non-interacting spiral galaxies from the
CALIFA survey. Merger-induced star formation is extended and recent, as
revealed by the young ages (50-80 Myr) and high contributions to light of young
stellar populations (50-90$\%$), in agreement with merger simulations in the
literature. These early-stage mergers have positive central gradients of the
stellar metallicity, with an average $\sim$0.6 Z$_{\odot}$. Compared to
non-interacting spirals, they have lower central nebular metallicity, and
flatter profiles, in agreement with the gas inflow scenario. We find that they
are dominated by star formation, although shock excitation cannot be discarded
in some regions, where high velocity dispersion is found (170-200 km s$^{-1}$).
The average SFR in these early-stage mergers ($\sim$23-32 M$_{\odot}$
yr$^{-1}$) is enhanced with respect to main-sequence Sbc galaxies by factors of
6-9, slightly above the predictions from classical merger simulations, but
still possible in about 15$\%$ of major galaxy mergers, where U/LIRGs belong.
