Matthew J Peel

Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds

Friction stir welding (FSW), like other friction welding techniques, has the advantage that many of the welding parameters, e.g. tool design, rotation speed and translation speed, can be controlled in a precise manner, thus controlling the energy input into the system. However, the effect of different welding speeds on the weld properties remains an area of uncertainty. In this paper, we report the results of microstructural, mechanical property and residual stress investigations of four aluminium AA5083 friction stir welds produced under varying conditions. It was found that the weld properties were dominated by the thermal input rather than the mechanical deformation by the tool.

Dissimilar friction stir welds in AA5083-AA6082. Part I: Process parameter effects on thermal history and weld properties

The aim of this study was to explore the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082. To that end a systematic set of nine instrumented welds were made using rotation speeds of 280, 560, and 840 rpm and traverse speeds of 100, 200, and 300 mm/min with AA5083 on the advancing side and another nine with the materials reversed. For comparison a smaller series of AA5083-AA5083 and AA6082-AA6082 welds were also made. Thermocouple measurements, tool torque, extent of material mixing, and macrostructural observations all indicate that the temperature under the tool is more strongly dependent on the rotation than the traverse speed. It was found that in the current case, the power (energy/s) and heat input (energy/mm) do not correlate simply with the weld temperature. As a result, such metrics may not be suitable for characterizing the conditions under which welds are produced.

Dissimilar friction stir welds in AA5083-AA6082. Part II: Process parameter effects on microstructure

The aim of this study is to explore the bounds of the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082 using a systematic set of rotation and traverse speeds. The first paper in this series examined the thermal and macroscopic aspects. In this paper, several microstructurally related characteristics, including hardness, grain size, and precipitate distribution, have been examined in greater detail. The observed variations are correlated and contrasted with the observed and predicted thermal distributions. In addition, the thermal model developed in part I has been coupled to hardness models based on classical isothermal aging studies for each alloy to predict the hardness variations across the welds.

The extent of relaxation of weld residual stresses on cutting out cross-weld test-pieces

Weld residual stress (RS) measurements are often undertaken on test-pieces which have been Cut Out from large components, yet it remains unclear to what extent the RSs in test-pieces are representative of those present in the original component. Similarly weld mechanical performance tests are frequently undertaken on cross-weld test-pieces without a proper understanding of the level or influence of retained RS. We present a systematic study of the relaxation of longitudinal RS in thin-plate butt welds produced using different materials and welding methods (FSW, laser-MIG, and pulsed-MIG). In each case the RSs were measured repeatedly in the same location as the welds were progressively and symmetrically cut down. Although cutting inevitably leads to stress redistribution, significant relaxation of the longitudinal RS was only observed when the weld length or width was reduced to below a certain value. This critical value appears to correlate with the lateral width of the tensile zone local to the weld-line and may be considered to be the characteristic length as defined in St. Venants principle. Further, it was found that the level of stress relaxation as a function of weld length for all the welds studied could be collapsed onto a single empirical curve using a simple approach based on the characteristic length scales of the weld. Given the range of materials and welding methods used, this relation appears to be of general use for thin-plate welds although further work is required to test the limits of its applicability

Synchrotron Energy-Dispersive X-Ray Diffraction Analysis of Residual Strains around Friction Welds between Dissimilar Aluminium and Nickel Alloys

Friction welding processes, such as friction stir welding (FSW) and inertia friction welding (IFW) are popular candidate procedures for joining engineering materials (including dissimilar pairs) for advanced applications. The advantages of friction welding include lack of large scale material melting, ability to join dissimilar materials, and relatively low propensity to introduce defects into the weld joint. For these reasons FSW and IFW have become the subjects of a number of studies aimed at optimising the joining operations to obtain improved joint strength and reduce distortion and residual stress. In the present study we used the diffraction of high energy polychromatic synchrotron X-rays to measure interplanar lattice spacings and deduce nominal elastic strains in friction stir welds between dissimilar aluminium alloys AA5083 and AA6082, and in coupons from inertia friction welds between dissimilar nickel-base superalloys IN718 and RR1000. Energy-dispersive diffraction profiles were collected by two detectors mounted in the horizontal and vertical diffraction planes, providing information about lattice strains in two nearly perpendicular directions lying almost in the plane of the plate samples mounted perpendicularly to the incident beam. Two-dimensional maps of residual stresses in friction-welded joints were constructed. Apart from the 2D mapping technique, the sin2ψ method (transmission) was also used in the case of inertia friction-welded joint between nickel alloys. Comparison between the two results allowed the variation of the lattice parameter with the distance from the bond line to be deduced. It was found that friction welding of two dissimilar materials with significant strength mismatch may lead to the creation of a region of compressive stress in the vicinity of the bond line, in contrast with the behaviour observed for joints between similar materials.

Comparison of methods to determine variations in unstrained unit cell parameter across welds

Many alloys undergo complex changes in local chemistry in the vicinity of weldments due to the thermal excursion during welding. The resulting changes in solute concentrations can lead to significant local variations in the unstrained unit cell parameter which, if not accounted for, can lead to serious error when determining residual stress by diffraction methods. Age-hardening aluminium alloys are particularly susceptible to such effects. The present paper compares three methods (plane stress assumption, sin2ψ method, and comb correction method) for evaluating the stress-free unit cell parameter variation for friction stir welds in AA7449-W51 plates of two different thicknesses. All three methods gave comparable results for thin (5u2009mm) sheet, but for the thicker (12.2u2009mm) plate the results calculated on the basis of the plane stress assumption diverged from the other two, largely because in this case the other methods indicate there to be a significant triaxiality of stress. In the example cases, hardness and unstrained unit cell parameter variations were found to be strongly correlated across the welds. The advantages and disadvantages of the three methods are compared.

Measurement and prediction of residual stresses in aluminium friction stir welds

Friction stir welding (FSW) is a solid-state process that offers the possibility of creating high quality joints between dissimilar aluminium alloys. However, to date little is known about the distribution of residual stresses within such joints. In this study, a combined modelling and experimental approach has been undertaken in an attempt to clarify this situation and develop a practical predictive tool. The model appears to have captured the asymmetry of the welding process but the peak residual strain is typically 30% higher than measured by synchrotron X-ray diffraction indicating that some optimisation is required.

Determination of residual stresses around blisters in Zr-2.5%Nb pressure tubes

We have used synchrotron X-ray diffraction experiments to measure the strain field introduced by a hydride blister grown on a section of a pressure tube from a CANDU nuclear reactor. After charging the tube section with a homogeneous hydrogen concentration of 300 wt ppm, the blister was produced by creating a small cold spot on its surface (similar to 200 degrees C), while the bulk was kept at a temperature of 338 degrees C over a period of 1008 h. The blister studied here is ellipsoidal in shape, with its long axis along the tube axial direction. The experiments were performed on the wiggler beam line ID15 at the European Synchrotron Radiation Facility (ESRF) using a polychromatic beam of high-energy X-rays (60 to 300 keV). Unlike conventional X-ray diffraction, in this mode the scattering angle is fixed and the diffracted beam is discriminated on the basis of the photon energy. The results show that the blister is composed by two crystallographic phases (delta-ZrH and alpha-Zr), with volume fractions varying with position. The maximum stresses appear at the blister-matrix interfaces. Near the tube outer surface, we found large compressive stresses of (-450 +/- 90) MPa along the blister long axis, and tensile stresses (+320 +/- 90) MPa along the tube hoop direction. The main uncertainty in these stresses results from the uncertainty in the elastic constants of the hydride phase. Large strains and broad peaks were observed for this phase, which were explained by a rather low Youngs modulus (35 GPa) for the hydride. The results are compared with finite element simulations found in the literature