For friction stir welding (FSW) of advanced high strength steel (AHSS) sheets with tensile strength grades between 590 and 1180 N mm 22 , the appropriate welding condition range and the influence of welding conditions on microstructures and mechanical properties of the welds were investigated. The appropriate welding conditions to avoid defects such as the incomplete consolidation at the bottom of the weld were obtained for the steel sheets up to 1180 N mm 22 grade. The higher tool rotation speed evidently resulted in the larger volume fraction of martensite and higher hardness in the stir zone (SZ), attributed to an increase in the peak temperature of its thermal cycle. The tensile strength of the weld joint was as high as that of the base metal for the steels up to 980 N mm 22 grade, but slightly lower than that of the base metal for the steel of 1180 N mm 22 grade due to the heat affected zone (HAZ) softening.

Five types of ferrite-pearlite structure carbon steels with different carbon contents (IF steel, S12C, S20C, S35C, S50C) were friction stir welded under various welding conditions, and the mechanical properties and microstructures of the FSW carbon steel joints were evaluated. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. When the carbon content is less than or equal to 0.12 mass%, the welding produces ferrite-pearlite structures, and the strength slightly increases compared to the base metal due to the refined microstructure; when the carbon content is above 0.2 mass%, the welding produces ferrite-pearlite plus harder phases like the martensite and bainite microstructures, resulting in a significantly increased strength of the joints. These are dependent on each of the thermal-mechanical cycles.

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Friction stir welding (FSW) was performed with single-sided one-pass butt welding using 12 mm thick structural steel plates with 400 N mm -2 grade of tensile strength. The microstructures and mechanical properties such as the Vickers hardness, tensile strength, and Charpy V-notch toughness of the weld joint were investigated, which verified the inhomogeneous features of the microstructure and toughness within the stir zone (SZ). Thermal cycle examinations with or without deformation at the peak temperature were undertaken using a thermomechanical testing apparatus in order to study the predominant factors on the microstructural evolution and resulting toughness within the SZ. It is indicated that the variation in the thermal cycle, especially the peak temperature, within the SZ plays a predominant role in defining the inhomogeneous features within the SZ.

Synopsis : Materialflowofsteelswithdifferentcompositioninfrictionstirwelding(FSW)wasdirectlyinvestigatedbyx-rayradiography.Thethree-di-mensionalflowpatternsinvariousFSWconditionswereobtainedusingtwopairsofx-raytransmissionreal-timeimagingsystems.Itrevealed thatthematerialflowpatternandtheflowrateweretightlyconnectedtothedeformationresistanceofthesteelatprocesstemperature.The materialflowzoneofthesteelwithhighercarboncontent,whichhashigherdeformationresistance,wasnarrower,anditsflowratewaslower thanthoseofthesteelwithlowercarboncontent.Theshapeofthematerialflowzonearoundtherotatingtoolwaschangedfromconcentric toellipseduetothestagnationofthematerialflowonadvancingside(AS)instirzone(SZ)undertheinsufficientheat-inputFSWcondition. Furtherdecrementoftheheat-inputinFSWledtothedefectonASinSZ.TheobtainedrelationshipbetweentheFSWconditionsandtheflow pattern showed that the lower travel speed of the tool is effective to keep the concentric shape so as not to form the defect for the steel with highercarboncontent.