In an elastic collision both momentum and kinetic energy are conserved. For a head-on elastic collision with one body initially at rest, the final velocities are $v_1' = \frac{m_1-m_2}{m_1+m_2}v_1$ and $v_2' = \frac{2m_1}{m_1+m_2}v_1$. Once block B moves alone, its kinetic energy converts fully into spring potential energy at maximum compression.