Materials Science Forum Vols. 539-543

Abstract: Commercial Waspaloy plates with two different initial microstructures (annealed and aged) were joined by Friction Stir Welding (FSW). This study presents the evolution of the grain structure, and precipitation, dissolution and reprecipitation of the γ' phase during FSW using Optical Microscopy, Scanning Electron Microscopy and Transmission Electron Microscopy. The relationship between microstructure and microhardness is also discussed.

Abstract: The present paper deals with the control of microstructure of friction stir processed aluminum alloys focusing on grain refinement, thermal stability at elevated temperature and texture development in some aluminum alloys such as 5083, 6061 and 7075 commercial aluminum alloys. 3mm thickness plates of 5083, 6061 and 7075 Al alloys were friction stir processed/welded with several rotation speeds and travelling speeds. Optical microscopy revealed the grain refinement in the stirred zone of each alloy and the average grain size decreased with decreasing rotation speed under various travelling speeds. Annealing of the joints brought about abnormal grain growth at temperatures higher than 773K for 5083 alloy. Critical temperature of the abnormal grain growth tended to decrease as the rotation speed decreased for the fixed travelling speed. Dissimilar joining of 5083 Al alloy to 6061 Al alloy also showed abnormal grain growth when annealed at 773K. A peculiar texture development of 7075 Al joint showing (111)//ND-oriented grains existing throughout the nugget was revealed by EBSP analysis.

Abstract: Mg alloy has a poor formability at room temperature because of lack of the active slip systems, but the grain refinement improves its ductility. Friction stir processing (FSP) can create homogeneous microstructure consisting of fine grains in Mg alloys, thus it would be expected that FSP enhances the formability of Mg alloys. In this study, multi-pass FSP was applied to Mg alloy AZ31B, and then formability of FSPed alloy was evaluated. Multi-pass FSP produced the fine recrystallized grains in Mg alloy. The stir zone exhibited larger fracture limit major strains than the base material under uniaxial tension and plane strain deformation, and these values increased with decreasing grain size. The stir zone having grain size of 2.9 μm showed the fracture limit major strains which are roughly as same as those of an annealed pure Al. The present study suggests that FSP is an effective method to enhance the formability of Mg alloys.

Abstract: Friction stir welding (FSW) is rapidly becoming accepted as a viable manufacturing process for aerospace applications. One potential area of concern, however, is the corrosion resistance of some FSW joints. While the corrosion resistance of friction stir welded 7075 aluminum has been investigated, and attempts made to enhance its corrosion resistance through different combinations of starting temper and post-weld artificial aging (PWAA) treatments, a clear approach for selecting pre-temper conditions and PWAA processing has not emerged. For this investigation, FSW butt-welds in sheets of 0.125-inch 7075-T73 and 7075-T6 were given a variety of PWAA treatments, some of which were developed previously by other authors. Thermal treatments were evaluated using optical microscopy, exfoliation, electrical conductivity, microhardness, tensile, and fatigue crack propagation testing. An important conclusion drawn from this work is that 7075-T73 can be friction stir welded and post-weld aged in a manner that produces superior exfoliation resistance and good mechanical properties without invalidating the bulk parent material temper specified in AMS 2770G.

Abstract: Residual strain profiles in friction-stir processed (FSP) AZ31B magnesium-alloy plates were measured using neutron diffraction. Two different specimens were prepared to investigate the influences of the tool shoulder and the tool pin on the residual-strain profiles: (Case 1) a plate processed with both the stirring pin and tool shoulder, i.e., a regular FSP plate subjected to both the plastic deformation and frictional heating, and (Case 2) a plate processed only with the tool shoulder, i.e., subjected mainly to the frictional heating. The results show that the strain profiles of both cases are qualitatively quite similar. The longitudinal strain is mainly tensile with its maximum near the bead of the FSP plate. On the other hand, the transverse and normal strains are mildly compressive in both Cases 1 and 2.

Abstract: Despite the large number of studies that are being conducted to advance the friction stir processing (FSP) technology, the effects of FSP on various mechanical and microstructural properties are still in need for further investigations. In addition, correlations between FSP parameters, mechanical properties and microstructural characteristics are not yet well understood. Accurate correlations are needed for successful modeling and process optimization. It is established that the temperature generated during FSP plays an important role in determining the microstructure and properties of the processed sheet and defining the tool life. Process parameters must be carefully chosen to allow the generation of enough heat to soften the material while limiting significant grain growth. Accurate measurement of the temperature distributions during processing are essential to understand the complicated deformation and associated mechanisms and to allow for effective process optimization. In this work, a dual-band thermography approach is used to measure the temperature distributions of AA5052 sheet during FSP. The setup utilizes two infrared detectors, to neutralize the emissivity and the facial effects, with 30 Hz acquisition rate. The variation of temperature with process parameters and their correlation to the resulting microstructure are discussed.

Abstract: Friction stir welding (FSW) allows the joining of aluminum alloys in ways previously unattainable offering new manufacturing technology. Friction stir processing (FSP) of cast alloys such as Ni-Al bronze eliminates casting voids and improves the properties to that of wrought material. However, the local heating produced by both FSW and FSP can leave a fusion zone with reduced mechanical properties and a heat-affected zone with tensile residual stresses that can be deleterious to fatigue performance. Controlled plasticity burnishing (CPB) is an established surface treatment technology that has been investigated and described extensively for the improvement of damage tolerance, corrosion fatigue, and stress corrosion cracking performance in a variety of alloys. Mechanical CPB processing in conventional CNC machine tools or with robotic tool positioning is readily adapted to industrial FSW and FSP fabrication of components, either simultaneously or as a post process. CPB was applied to FSP Ni-Al Bronze to produce a depth of compression of 2.5 mm and a maximum subsurface magnitude of –150 ksi. The effect of FSP on the fatigue performance in a saltwater marine environment and in the presence of foreign object damage (FOD) was documented with and without CPB processing. FSP was found to increase the fatigue strength of the Ni-Al Bronze by 70% without affecting the corrosion behavior of neutral salt solution. FSW actually produced a more noble material in the acidic salt solution. CPB after FSP mitigated damage 1 mm deep.

Abstract: The effect of friction stir welding (FSW) parameters on microstructure and properties of 15vol.%SiCp/2009Al composite welds was investigated at tool rotation rates of 400-1000 rpm for a constant welding speed of 50 mm/min. 8mm thick defect-free welds were produced at higher tool rotation rates of 600-1000 rpm, whereas an unwelded seam was distinctly visible at the root of plates at a lower tool rotation rate of 400 rpm. FSW resulted in generation of fine and equiaxed recrystallized grains of ~6 μm and significant improvement of SiC particle distribution in the weld nugget. Under as-FSW condition, the hardness of the weld nugget was significantly higher than that of as-extruded parent material. Furthermore, tensile and yield strengths of as-FSW composite welds in both the longitudinal and transverse directions were superior to those of as-extruded parent material. Post-weld T4 temper resulted in limited grain growth due to the pinning of SiC particles. Under the T4 condition, the tensile strength of the welds along the longitudinal and transverse directions reached 82 and 95% of the parent material, respectively.

Abstract: In order to understand the microstructure changes during welding processes , kinetic information about the phase transformation is essential. In our research group, in-situ observation systems consisting of undulator beam and imaging plate and two dimensional pixel detector have recently been used. These make it possible that phase transformation can be identified in real-time under the condition of directional-oriented solidification . In the present work, a combination of analyzing method: the in-situ observation system by X-ray diffraction technique using intense synchrotron radiation, and morphological observation by high-temperature laser scanning confocal microscopy is suggested to analyze the phase transformation during the welding process. Using the results acquired by these analysis methods, phase evolution of hypereutectoid carbon steel, during fusion welding was analyzed. The primary phase was directly identified as an austenite phase. Precipitation of pearlite phase was observed followed by the martensitic transformation.