Double-sided friction stir welding of 40 mm thick low carbon steel plates using a pcBN rotating tool

Abstract

Double-sided friction stir welding was conducted on 40 mm thick low carbon steel plates using a pcBN rotating tool with a probe length of 20 mm. During the welding, the tool rotation speed and travelling speed were maintained at 150 rpm and 25 mm/min, respectively, and the maximum applied load reached about 15 tons. The final welded area showed a vertically symmetric bowl-like shape with the overlapped area having a width of about 21.3 mm. The welded areas formed in the first pass and second pass welding process were very similar, which contained a large stir zone and wide thermal-mechanically affected zone. The microstructure of the stir zone consisted of a coarse acicular ferrite and bainite structure, compared with the banded ferrite and pearlite structure of the base metal. While the thermal-mechanically affected zone showed two types of microstructures, one contained fine ferrite and bainite structure and another one contained mixed ferrite and pearlite structure. The hardness distribution and tensile tests showed that the welded area had improved mechanical properties than that of the base metal. In addition, high density of BN particles caused by the tool wear was confirmed along the border of the stir zone on the advancing side. Inside the stir zone, BN particles were also observed, however, with a much lower density.