An advanced numerical model of friction stir welding of DH36 steel

A numerical model of Friction Stir Welding (FSW) of DH36 steel plate (6mm thickness) has been developed using a CFD technique. Two welding speed conditions were used, a low welding speed of 200 RPM - 100mm/min, and a high welding speed of 550RPM- 400 mm/min. The heat generation, material flow and strain rate were calculated based on plastic deformation and frictional contact between the tool and workpiece. A CFD-based model has been produced to represent the asymmetry in temperature distribution between the advancing and retreating side, the material flow and the strain rate. The geometry of the model includes the tool plunged into the plate. The cooling system was also included in the simulation by calculating the heat flux lost for each part of the tool. The heat generated by viscous dissipation away from the tool was also taken into account. The total heat generated was divided into the individual tool parts (shoulder, probe side and probe end) and was found to be in good agreement with the experimental results for the areas affected by these parts. The maximum temperature obtained for the slow welding speed was 1012oC and for the high welding speed was 1250oC. Experimental metallographic examination has also been carried out on DH36 FSW steel plates to validate the CFD model. SEM analysis showed the formation of a fine microstructure of bainite, acicular ferrite and ferrite/cementite aggregate in the welded zone as compared to the ferrite/pearlite morphology in the base metal. It is found from the CFD and experimental results that the high speed welding conditions can produce defects such as wormholes and cracks in the welds associated with the probe side and probe end due to the lack of material flow especially on the advancing side. Tensile and fatigue testing were carried out for both slow and high welding speed samples, which broke outside the welded region in the tensile test, however, slow welding speed samples show more resistance to fatigue test and survived 644128 cycles, the high speed welding samples failed after 111,736 cycles under the same load.