  High irradiance lasers incident on metal surfaces create a complex, dynamic
process through which the metal can rapidly change from highly reflective to
strongly absorbing. Absolute knowledge of this process underpins important
industrial laser processes like laser welding, cutting, and metal additive
manufacturing. Determining the time-dependent absorptance of the laser light by
a material is important, not only for gaining a fundamental understanding of
the light-matter interaction, but also for improving process design in
manufacturing. Measurements of the dynamic optical absorptance are notoriously
difficult due to the rapidly changing nature of the absorbing medium. This data
is also of vital importance to process modelers whose complex simulations need
reliable, accurate input data; yet, there is very little available. In this
work, we measure the time-dependent, reflected light during a 10 ms laser spot
weld using an integrating sphere apparatus. From this, we calculate the dynamic
absorptance for 1070 nm wavelength light incident on 316L stainless steel. The
time resolution of our experiment (< 1 us) allows for the determination of the
precise conditions under which several important physical phenomena occur, such
as melt and keyhole formation. The average absorptances determined optically
were compared to calorimetrically-determined values, and it was found that the
calorimeter severely underestimated the absorbed energy due to mass lost during
the spot weld. Weld nugget cross-sections are also presented in order to verify
our interpretation of the optical results, as well as provide experimental data
for weld model validation.
