<p>AZ91 is one of the most widely used Mg alloys in structural applications. The casting process directly influences the initial microstructure, which consequently governs the mechanical and deformation responses of AZ91. To overcome limitations associated with conventional casting, two advanced manufacturing routes, strain-integrated gas infusion (SIGI) and acoustic gas infusion casting (AGIC), have been developed, which demonstrate improved mechanical performance. This study demonstrates that casting-route-induced microstructural modification directly governs hot deformation stability and processing window expansion in AZ91. Therefore, this study investigates the deformation response of AZ91 produced through three processing routes. Processing maps were developed using the dynamic materials model (DMM) over the temperature range of 250&#xa0;°C to 450&#xa0;°C and strain rates of 0.001 to 10&#xa0;s⁻¹. The conventional alloy exhibited ~ 70 pct stability and 30 pct instability, whereas AGIC and SIGI displayed higher stability of 85 and 93 pct, with instability reduced to 15 and 7 pct, respectively. Experimental, theoretical, and artificial neural network (ANN)-based machine learning (ML) approaches were used to establish and predict the flow behavior and processing maps of different routes. The developed ANN-ML model effectively captured the strain-dependent transitions between softening and hardening, demonstrating superior agreement with the experimental curves. Microstructural observations revealed that deformation is governed by dynamic recrystallization (DRX) and its interaction with second-phase distribution. The improved stability in AGIC and SIGI conditions is attributed to grain refinement, uniform β-Mg₁₇Al₁₂ phase distribution, and enhanced DRX kinetics, which suppress flow localization and promote uniform deformation. These results demonstrate that casting-route-induced microstructural engineering directly controls deformation mechanisms in AZ91 alloy.</p>