Robotic pulsed gas metal arc welding (GMAW) of thin aluminum sheets requires precise thermal control because excessive heat input promotes heat-affected-zone (HAZ) softening, grain coarsening, and porosity formation, whereas insufficient heat input may compromise fusion stability. This study develops a heat-input-governed process–structure–property framework for 1.8 mm AA5754-H0 sheets using a 3 × 3 welding matrix defined by current (81–89 A), travel speed (12–18 mm/s), voltage (15–17 V), and linear heat input
Q
= η
UI
/
v
(η = 0.7 ; 53–88 J/mm). The observations were interpreted using Rosenthal moving heat-source analysis, diffusion-length scaling, cooling-rate-dependent solidification, and molten-pool residence-time concepts to link heat input with HAZ development, grain evolution, porosity, and fusion stability. Weld geometry, HAZ development, grain morphology, Fe–Mn–Si-rich particles, porosity, microhardness, and tensile behavior were evaluated by macrostructural examination, microscopy, ASTM E112 grain-size analysis, SEM, EDS, Vickers microhardness, tensile testing, and radiographic inspection. Increasing
Q
from 53 to 88 J/mm enlarged HAZ width from 0.7 to 1.2 mm and HAZ grain size from 43 to 112 μm. Porosity was minimized in the optimal regime at 0.9%, whereas rapid solidification increased pore entrapment to 3.2%. Welded joints retained ultimate tensile strengths of 198–206 MPa and joint efficiencies of 92%–95%, but ductility and fracture stability were governed by grain coarsening, pore formation, intermetallic morphology, and fusion stability. The optimal interval of 56–70 J/mm balanced controlled HAZ grain growth, reduced porosity, hardness uniformity, and strength–ductility response, with HAZ grain size limited to 43–77 μm and elongation up to 13%. HAZ width increased systematically with √
Q
, confirming diffusion-controlled thermal penetration as the governing weld-zone development mechanism. The proposed framework reformulates heat input as a thermal-severity descriptor linking heat transfer, microstructural evolution, defect formation, and mechanical performance, and defines a thickness-normalized process window,
Q
/
t
, of 31–39 J/mm
2
for robotic welding of thin Al–Mg alloy sheets.