Friction Stir Welding of DH-36 Steel

摘要

The objective of this work was to determine the feasibility of FSW for joining DH-36 steel for shipbuilding applications. Welds were produced on 0.18 inch-thick plates of DH-36 steel using tools of W-25 percent Re with a shoulder diameter of 3/4 inches. The tool was rotated at 400 or 500 rpm and travel speeds of either 8 or 10 ipm were employed. Subsequent to welding the welds were evaluated using optical microscopy, microhardness and tensile testing. No measurable changes in tool dimensions were found after welding suggesting that no appreciable tool wear or deformation occurred. Examination of metallographic samples revealed microstructurally distinct regions including the stir zone, heat-affected zone (HAZ) and base metal. Many of the welds contained small defects near the bottom of the stir zone resulting from incomplete bonding. The base metal microstructure was composed of equiaxed grains of ferrite approximately 15 to 20 mu m in size with pearlite colonies. The microstructure of the base metal also showed evidence of banding. The stir zone microstructure contained grains of polygonal ferrite about 5 um in size. No regions of martensite were observed in the stir zone. The HAZ contained regions of partially-spherodized pearlite (fuzzy pearlite) but no grain-coarsened or grain-refined regions. The banding extended from the base metal into the stir zone. The microhardness of the stir zone averaged approximately 215 VHN, while that of the base metal was about 190 to 195 VHN. Consistent with the lack of martensite, no evidence of hardening was found in the microhardness results. The weld samples showed acceptable strain to failure with yield and tensile strengths exceeding those of the base metal. With one exception, all samples failed through the stir zone owing to the presence of lack-of-bonding defects. These defects can be eliminated with proper tool design. FSW of DH-36 appears feasible, and further work is warranted.