Effect of MQL on the microstructure and strength of friction stir welded 6061 Al alloy

In many applications, in the aerospace and other industries, acquiring a high-quality weld is challenging using conventional joining techniques especially for materials with low melting temperatures. Friction stir welding (FSW) is a high-quality solid-state joining technique for welding lightweight materials. However, there is a need to prevent the transfer of excessive heat into the weld zone and to have a steady-state heat flow at the FSW tool. Therefore, significant research is being conducted to find an effective coolant-lubricant for FSW. More recently, interest has increased in the possibility of replacing traditional lubrication methods with effective and more sustainable alternatives. The present study investigates the effect of minimum quantity lubrication (MQL) on the microstructure and mechanical properties of FSW aluminum alloy 6061-T651. Friction stir welding tool forces, tensile strength, and microstructure characterization were conducted on FSW Al 6061 samples produced using feed speeds from 100 to 260 mm/min and spindle speeds of 1000 to 2200 rpm. The results showed that MQL as a near-dry lubricant could reduce the measured forces during the FSW process. Most importantly, the results demonstrate that the average ultimate tensile strength (UTS) of the joints increased and the grain size area decreased with the use of MQL as compared to the dry process joining. The finest grain size area obtained was 6 μm2 using a spindle speed of 1600 rpm at a feed speed of 180 mm/min with MQL applied.