Generating fully spin-polarized currents is a key issue in spintronics. Theoretical studies have shown that $d$-wave altermagnets with perfect orthogonal Fermi surfaces in the first Brillouin zone (BZ) can generate 100% spin-polarized currents. However, such ideal orthogonal Fermi surfaces are rarely realized in materials. Two situations are commonly observed: (1) The orthogonal Fermi surfaces are accompanied by additional Fermi pockets at the BZ boundaries, and (2) the Fermi surfaces are no longer perfectly orthogonal. Both scenarios reduce the spin polarization. Using tight-binding models and first-principles calculations, we demonstrate that strain engineering effectively tunes the Fermi surfaces of $d$-wave altermagnets. In the tight-binding models, we show that biaxial strain transforms nonideal Fermi surfaces into perfectly orthogonal ones, and uniaxial strain tunes the transport states of spin-polarized currents. First-principles calculations provide concrete material examples. In two-dimensional (2D) ${\mathrm{Pd}}_2{\mathrm{I}}_2$, biaxial strain eliminates the extra Fermi pockets originating from the $d_{xz}$ and $d_{yz}$ orbitals, yielding orthogonal Fermi surfaces and increasing the spin polarization from 54.5% to 99.5%. In the three-dimensional (3D) altermagnet ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$, in-plane biaxial tensile strain modulates the $d_{xy}$ states and eliminates the extra Fermi pockets near the X and Y points, thereby realizing orthogonal Fermi surfaces in a 3D system. In 2D ${\mathrm{Pd}}_2\mathrm{BrI}$ under hole doping, reversing the uniaxial strain direction enables deterministic on/off control of the fully spin-polarized current, accompanied by a piezomagnetic moment for transport-state readout. These results demonstrate that biaxial strain provides a route to realizing perfect orthogonal Fermi surfaces and fully spin-polarized currents in altermagnets across both 2D and 3D systems, while uniaxial strain provides a feasible approach for designing mechanically tunable spintronic devices.

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• Altermagnetism
• Magnetism
• Spintronics
• Altermagnets
• Boltzmann theory
• Density functional calculations
• Strain engineering
• Tight-binding model