Friction Stir Welding (FSW) is an innovative welding technique with the potential for use for lightweight materials. However, defects can occur in the welding zone due to the excessive heat generated in FS welded similar or dissimilar lightweight materials. Therefore, FSW requires an effective cooling technique to moderate temperatures and avoid defects, that is more sustainable than the traditional techniques. The present study investigates the effect of the Minimum Quantity Lubrication (MQL) coolant approach on the microstructure and mechanical properties of Friction Stir welded aluminum alloys 6061-T651 and 5052-H32 (similar and dissimilar).

Analysis of variance (ANOVA) was employed to study and analyze the effects of MQL design variables (i.e. flow rate, nozzle orientation, and nozzle diameter) on the welding force, power consumption, and surface roughness for both similar and dissimilar friction stir welded aluminum alloys. A multi-objective model was also established to provide the optimal levels for the studied design variables by considering all the mentioned outputs. A comparison between the predicted outcomes and the results of the optimal response (i.e. surface roughness and welding force) was established to validate the effectiveness of the utilized assessment model. An integrated sustainability assessment model was implemented to investigate the sustainability of the FSW process with/without MQL. Four sustainability indicators were considered in this assessment:​ power consumption, costs, waste management, and environmental impact.

Higher levels of flow rate resulted in lower welding forces, power consumption and a better surface quality. The multi-objective model achieved a balance between all the studied outputs. When compared, the optimal experimental values and the model predicted values showed a good agreement. Using the MQL system as a cooling and lubrication technique reduced the required welding forces during FSW to join aluminum alloys. It significantly improved the microstructure through making the grains finer at the welding zone and increases the average ultimate tensile strength (UTS) compared to dry FSW. This research presents the first attempt in the literature to discuss and investigate the MQL-FSW interactive.