Conundrum of strongly coupled supercurrent flow in both under- and overdoped Bi-2212 round wires

${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1}{\mathrm{Cu}}_{2}{\mathrm{O}}_{x}$ (Bi-2212) is the only high-temperature superconductor (HTS) available as a round wire with high critical current density ${J}_{c}$, which makes it a very compelling candidate for ultrahigh-field magnet applications. By contrast, other copper oxide HTS conductors like $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (where $R$ stands for rare earth) and (Bi,${\mathrm{Pb})}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ (Bi-2223) must be made in tape form to minimize the density of current blocking high-angle grain boundaries. Understanding the mechanism enabling high ${J}_{c}$ in round wire Bi-2212 is important intellectually because it breaks the paradigm that forces HTS conductors into tape geometries that reproduce their strong crystalline anisotropy. The biaxial growth texture of Bi-2212 developed during a partial melt heat treatment should favor high ${J}_{c}$, even though its $\ensuremath{\sim}{15}^{\ensuremath{\circ}}$ full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range of $\ensuremath{\le}{5}^{\ensuremath{\circ}}$ misorientation. Its ability to be strongly overdoped should be valuable too since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While ${J}_{c}$ and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature [a hysteretic ${J}_{c}(H)$ characteristic] evident in even the very best Bi-2223 tapes with an $\ensuremath{\sim}{15}^{\ensuremath{\circ}}$ FWHM uniaxial texture. We attribute the high ${J}_{c}$ and lack of weak-link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212. These features enable $c$-axis brick wall current flow when ab-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.