  We derive the metallicity profile of the Milky Way low-$\alpha$ disc
population from 2 to 20 kpc from the Galactic centre in 1 Gyr age bins using
the astroNN catalogue, and show that it is highly structured, with a plateau
between 4 and 7 kpc and a break at 10-12 kpc. We argue that these features
result from the two main bar resonances, the corotation and the Outer Lindblad
Resonance (OLR), respectively. We show that the break in the metallicity
profile is most visible in stars having 7-8 Gyr, reaching an amplitude of about
0.4 dex, and is the signpost of the position of the bar OLR. The bar formation
was accompanied by an episode of radial migration triggered by its slowing down
and is responsible for spreading old metal-rich stars up to the OLR. The data
show that the slowdown of the bar ended 6-7 Gyr ago. Based on numerical
simulations that reproduce well the break observed in the metallicity profile,
we argue that this implies that the bar formed in our Galaxy 8-10 Gyr ago.
Analysis of the metallicity distribution as a function of radius shows no
evidence of significant systematic outward radial migration after this first
episode. We argue that the variation of the metallicity dispersion as a
function of the guiding radius is dominated by the migration triggered by the
bar, but also that the libration of orbits around the bar resonances induces a
mixing that may have a significant impact on the observed metallicity
dispersion. In contrast, the absence of a break in the metallicity profile of
populations younger than about $\sim$6 Gyr and the flattening of the gradient
at younger ages is interpreted as evidence that the strength of the bar has
decreased, loosening its barrier effect and allowing the gas and metals on both
sides of the OLR to mix, erasing the break. Beyond the OLR, stars younger than
7 Gyr show very small metallicity dispersion, suggesting no or limited
migration.
