This study aimed to investigate the effect of the welding heat input on the heat affected zone (HAZ) of AW 6005-T6 aluminium alloy for a butt-welded joint using gas metal arc welding (GMAW). The study involved using a welding experiment, numerical simulation, physical simulation, and mechanical tests. The welding was carried out using the pulsed GMAW transfer, and type J thermocouples were used to develop the thermal cycles in the HAZ. Simufact® Welding was utilized for the numerical simulation. An optical microscope was used to evaluate the microstructures and the Vickers microhardness test was done along the weld cross-section. The HAZ was located on the weld cross-section with a mean hardness of 63.7 HV, which is considerably lower when compared with the base metal (BM) which has a hardness of 100 HV. This indicates thermal softening occurred due to the heat input to the material. There is a match in the hardness values of the Gleeble samples and the locations on the weld cross section suggested by the model showing the validity of the simulation. It is important to note the fact that there is an influence of heat input into aluminum AW 6005-T6 weld joints and its mechanical properties in the design of welding process parameters for automotive parts. The welding parameters can be optimized to decrease the heat input into the weld, as this can directly affect the mechanical properties in the HAZ.