Anthony P. Reynolds

Structure, Properties, and Residual Stress of 304L Stainless Steel Friction Stir Welds

Alloy 304L stainless steel sheets, 3.2 mm thick, were joined by friction stir welding at a single welding speed using two different tool rotation rates. The resulting welds were overmatched relative to the base metal. Longitudinal residual stresses (tensile) were near the base metal yield strength.

Microstructural studies of friction stir welds in 2024-T3 aluminum

Friction stir welds in 7 mm thick, 2024-T351 aluminum rolled sheet material have been completed. Metallurgical, hardness and quantitative energy dispersive X-ray measurements have been performed which demonstrate that a segregated, banded, microstructure consisting of alternating hard particle rich and hard particle poor regions is developed. Mixed-mode I/II monotonic fracture experiments confirm that the observed banded microstructure affects the macroscopic fracture process. Since the band spacing is directly correlated with the welding tool advance per revolution, our results indicated that the opportunity exists to manipulate the friction stir weld process parameters in order to modify the weld microstructure and improve a range of material properties, including fracture resistance.

Prediction of Temperature Distribution and Thermal History during Friction Stir Welding: Input Torque Based Model

AbstractA novel three-dimensional thermal model is proposed to study the transient temperature distributions during the friction stir welding of aluminium alloys. The moving heat source engendered by the rotation and linear traverse of the pin tool has been correlated with the actual machine power input. This power, obtained from experimental investigation, has been distributed to the different interfaces formed between the tool and the weldpiece based on the torques generated at different tool surfaces. Temperature dependent properties of the weld material have been used for the finite element based numerical modelling. Good agreement between the simulated temperature profiles and experimental data has been demonstrated. The effects of various heat transfer conditions at the bottom surface of the workpiece, thermal contact conductances at the interface between the workpiece and backing plate and different backing plate materials on the thermal profile in the weld material have also been investigated nume...

Visualisation of material flow in autogenous friction stir welds

Abstract Material flow in friction stir welds has been visualised using embedded marker materials. The fidelity of the visualisation technique has also been demonstrated. Results from the flow visualisation show that the friction stir welding process can be roughly described as an in situ extrusion process wherein the tool shoulder, the weld backing plate, and the cold base metal outside the weld zone form an ‘extrusion chamber’ which moves relative to the workpiece. Deviation from this description occurs primarily at the top surface of the weld where significant material transport occurs owing to the action of the rotating tool shoulder. The transport caused by the shoulder leads to a small amount of circulation about the longitudinal axis of the weld with material moving primarily from trailing to leading sides at the top of the weld and primarily from leading to trailing near the middle and bottom of the weld.

Nanomechanical Characterization of Single-Walled Carbon Nanotube Reinforced Epoxy Composites

Nanomechanical properties of single-walled carbon nanotube (SWNT) reinforced epoxy composites with varying weight percentage (0, 1, 3, and 5 wt%) of nanotubes were measured by nanoindentation and nanoscratch techniques. Hardness and elastic modulus were measured using a nanoindenter. Scratch resistance and scratch damage were studied using the AFM tip sliding against the SWNT reinforced sample surfaces. Nanoindentation/nanoscratch deformation and fracture behaviour was studied by in situ imaging of the indentation impressions/scratch tracks. Viscoelastic properties of the nanocomposites were measured using nanoindentation dynamic mechanical analysis tests. The reinforcing mechanisms are discussed with reference to the nanotube dispersion, interfacial bonding, and load transfer in the SWNT reinforced polymer composites.

A Study of Residual Stresses and Microstructure in 2024-T3 Aluminum Friction Stir Butt Welds

Three-dimensional residual stress mapping of an aluminum 2024-T3 arcan specimen, butt-welded by the friction stir technique, was performed by neutron diffraction. Results indicate that the residual stress distribution profiles across the weld region are asymmetric with respect to the weld centerline, with the largest gradients in the measured residual stress components occurring on the advancing side of the weld, with the longitudinal stress, σ L , oriented along the weld line, as the largest stress. Within the region inside the shoulder diameter, the through-thickness stress, σ Z , is entirely compressive, with large gradients occurring along the transverse direction just beyond the shoulder region. In addition, results indicate a significant reduction in the observed residual stresses for a transverse section that was somewhat closer to the free edge of an Arcan specimen. Microstructural studies indicate that the grain size in the weld nugget, is approximately 6.4 microns, with the maximum extent of the recrystallized zone extending to 6 mm on each side of the weld centerline. Outside of this region, the plate material has an unrecrystallized grain structure that consists of pancake shaped grains ranging up to several mm in size in two dimensions and 10 microns in through-thickness dimension.

Banded microstructure in AA2024-T351 and AA2524-T351 aluminum friction stir welds: Part I. Metallurgical studies

Abstract Results from careful investigations of the banded microstructure observed on horizontal transverse cross-sections in all AA2024-T351 and AA2524-T351 aluminum friction stir weld (FSW) joints indicate the presence of periodic variations in (a) the size of equiaxed grains, (b) micro-hardness, and (c) concentration of base metal impurity particles (i.e. constituent particles) that correlate with the observed band spacing. The latter trend is more distinct in AA2024-T351, which has a higher volume fraction of constituent particles resulting in easily recognizable particle rich regions on horizontal cross-sections near the mid-thickness of the joint and well-defined variations in hardness. In AA2524, the trends are more muted but clearly visible. Results from recent numerical simulations of the FSW process enable interpretation of the trends in grain size along the weld centerline in terms of the time–temperature cycle experienced by the material. Specifically, the AA2524 FSW joints having low power and high input energy (i.e. the slow FSW), exhibit micron-size grain structure across both bands. Conversely, the fast and medium FSW in AA2524 have higher maximum temperatures, and a corresponding six-fold increase in grain size.

Experimental Investigation of E/M Impedance Health Monitoring for Spot-Welded Structural Joints:

Health monitoring results obtained during fatigue testing of a spot-welded lap-shear structural-joint specimen using the electromechanical (E/M) impedance technique are presented. The test specimens were instrumented with piezoelectric wafer transducers, and the base E/M impedance signature was recorded in the 200-1,100 kHz frequency range. Fatigue testing was applied to initiate and propagate crack damage of controlled magnitude. Calibration tests using plain specimens were first performed to correlate stiffness decrease with damage progression and remaining life. During the subsequent health-monitoring tests, the decrease in structural stiffness was used to assess damage progression in the specimen. As damage progressed, the E/M impedance signatures were recorded at predetermined intervals. Signature data were processed, and the RMS impedance change was calculated. Damage index values were observed to increase as crack damage increases. The initiation and propagation of damage was successfully correlated with the E/M impedance measurements. Sensing and the localization principles of E/M impedance method were confirmed, and the rejection of spurious information was verified. These experiments demonstrated that the E/M impedance technique is a potentially powerful tool for damage detection, health monitoring, and NDE of spot-welded structural joints.

Mechanical response of friction stir welded AA2024: experiment and modeling

Abstract The mechanical response of heterogeneous structures, such as weldments, is largely governed by the response of the local constituents. In the present paper, the global and local mechanical response of friction stir welded AA2024 is examined experimentally and numerically. Full field strain measurements are obtained on transversely loaded tensile specimens via the digital image correlation technique. Assuming an iso-stress configuration, local constitutive data were determined for the various weld regions and used as input for a 2-D finite element model. The simulation results were compared with the experimental results to assess the viability of the modeling approach and the validity of the iso-stress assumption

Friction Stir Welding of DH36 Steel

Abstract Hot rolled DH36 carbon steel, 6.4 mm in thickness, was friction stir welded at speeds of 3.4 mm s-1 (8 in min-1), 5.1 mm s-1 (12 in min-1), and 7.6 mm s-1 (18 in min-1). Single pass welds free of volumetric defects were produced at each speed. The relationships between welding parameters and weld properties are discussed. Optical microscopy, microhardness testing, and transverse and longitudinal tensile tests have been performed. Bainite and martensite are found in the nugget region of the friction stir welds whereas the base material is comprised of ferrite and pearlite. The maximum hardness is observed in the weld nugget, and the hardness decreases gradually from the weld nugget, through the heat affected zone, to the base metal. Tensile testing also indicates overmatching of the weld metal relative to the base metal. Maximum hardness and longitudinal (all weld metal) tensile strengths increase with increasing welding speeds. Weld transverse tensile strengths are governed by the base metal properties, as all transverse tensile bars fail in the base metal.