We study the collisional disruption of primordial black hole (PBH) clusters in the Milky Way halo. Encounters between clusters strip PBHs into a diffuse component, and the fraction of PBH mass in this smooth component along a sightline determines how microlensing constraints divide between isolated compact objects and extended cluster lenses. We combine an analytic Navarro-Frenk-White-based collision-rate model, 72 direct N-body binary collision simulations that calibrate the escaped-mass fraction as a universal function $\tilde{f}(\tilde{v},\tilde{b})$ of the relative velocity and impact parameter in units of the cluster velocity scale and half-mass radius, and cosmological N-body simulations of a Milky Way-like halo ($M_{200}\simeq 8\times {10}^{11}{M}_{\odot }$, $c_{200}\simeq 11$) that record the encounter history of every cluster from $z=9$ to $z=0$. For ${10}^6$ and ${10}^7{M}_{\odot }$ clusters, which lie on either side of the characteristic cluster mass at $z=9$, the local encounter rate at the solar circle is $2.9\times {10}^{-3}$ and $1.2\times {10}^{-2}{\mathrm{Myr}}^{-1}$, consistent with the simulations to within $\sim 40\%$. Because the peak-disruption velocity of Carr-radius clusters ($12–20\mathrm{km}{\mathrm{s}}^{-1}$) lies far below typical halo encounter velocities, the mass loss accumulates over many weak encounters, and most of it occurs early, during halo assembly, when the encounter velocities are still low: half of the total mass loss happens before $z\approx 2$, which analytic estimates made at $z=0$ do not account for. The surviving mass fraction at the solar circle is $S\simeq 0.50$ (${10}^6{M}_{\odot }$) and 0.04 (${10}^7{M}_{\odot }$), and the dark matter-mass-weighted smooth fraction toward the large Magellanic cloud and small Magellanic cloud is 0.49 and 0.92, respectively. Cluster-cluster disruption is therefore substantial over the lifetime of the Galaxy, and reanalyses of microlensing surveys should use the radially varying smooth fraction $f_{\mathrm{sm}}(r)$ obtained here.

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• Dark matter
• Galactic halos
• Primordial black holes
• Astrophysical & cosmological simulations