Science and Technology of Welding and Joining

Experiments and numerical models were applied to investigate the thermal and mechanical behaviours of materials adjacent to the weld pool during arc welding. In the experiment, a new high temperature strain measurement technique based on digital image correlation (DIC) was developed and applied to measure the in situ strain evolution. In contrast to the conventional DIC method that is vulnerable to the high temperature and intense arc light involved in fusion welding processes, the new technique utilised a special surface preparation method to produce high temperature sustaining speckle patterns required by the DIC algorithm as well as a unique optical illumination and filtering system to suppress the influence of the intense arc light. These efforts made it possible for the first time to measure in situ the strain field 1 mm away from the fusion line. The temperature evolution in the weld and the adjacent regions was simultaneously monitored by an infrared camera. Additionally, a thermal–mechanical finite element model was applied to substantiate the experimental measurement.

The Cold Metal Transfer process is investigated to join zinc coated steel with aluminium alloy by braze-welding. A 4043 filler metal is deposited on the surface of the coated steel, and the effect of the current waveform on the metal transfer and the heat transferred to the base metal is investigated. The reduction in the ‘boost phase’ duration of the current waveform decreases the volume of liquid drops at the wire tip and allows to increase the short-circuit frequency, inducing a similar or higher deposit rate. Regular deposits are obtained when the linear energy remains below ∼500 J mm⁻¹. The heat transferred to the base metal, and thus the thickness of the intermetallic layer formed at the Al/steel interface, is lower for an equivalent deposit rate when the short-circuit frequency is high.

The residual stress profiles in two different self-piercing riveted (SPR) joints were characterised using the neutron diffraction technique. The joints were produced using semi-tubular steel rivets and carbon steel sheets of different thicknesses and hardnesses. The residual stress in the sheet material inside the bore of the rivet was compressive at the centre and the stress became tensile away from the centre. The stresses found in the semi-tubular rivet were compressive, with a lower magnitude in the rivet head compared with those in the rivet leg. For the SPR joints examined, the compressive residual stress in the rivet leg was greater for the thin joint than the thick joint, and this was due to the higher force gradient encountered during the rivet flaring stage. The originality of this work was to relate the residual stress profile to the physical event that occurred during the SPR process.

In this study, we assess the effects of weld metal strength and transformation temperature on the residual stress in 2·25Cr–1·6W steel welded joints. We find that the stress levels in the weld and the peak residual stress position in the base metal can be controlled by the appropriate selection of filler metal. Low transformation temperature weld metals are capable of inducing compressive residual stresses in the weld bead. Meanwhile, for stresses and bending moment equilibrium, the high strength weld metal pushes the peak tensile stress outward to reside at the base metal, keeping off the weak region. A reasonable agreement is obtained between the numerical modelling considering phase transformation and the accurate local stresses measurement associated to weld microstructure regions.

Ultrasonically assisted friction stir welding (UAFSW) was explored. Metallographic structures and material flow prepared by UAFSW were compared with those prepared by conventional FSW. Hardness curves, tensile strength, and elongation of welds fabricated at different ultrasonic powers were characterised. Experimental results showed that hardness curves, tensile strength of welds increased with increased ultrasonic power when it was less than 50%. The elongations decreased with ultrasonic energy output increased. Scanning electron microscopy photographs in different positions of fracture by UAFSW show fracture pattern and origin.

In this work, friction stir welding was used to produce shear overlap joints between aluminium and a thermoplastic (AA 6111 to polyphenylene sulphide). The process uses the friction stir welding tool to create metallic chips which merge with the molten thermoplastic to form a joint. No special surface pretreatment is required before joining. Cross-sections show mechanical locking between the chipped polymer filled zone and the surrounding aluminium sheet. The effects of joining parameters such as rotational speed, translational speed and distance to backing were investigated in relation to the joint strength and failure mode. Optimum speeds and backing distances could be idetified. The joint strength is dominated by mechanical interlocking between the chip and polymer filled zone and the aluminium sheet.

In this study, the microstructures and mechanical properties of friction stir welded and post-weld heat treated joints of 32Mn–7Cr–1Mo–0·3N steel were investigated. Significant microstructural and mechanical property gradients were found along the transverse directions of the as welded joint. These gradients led to overmatching characteristic with higher strength but lower ductile than base material. The post-weld heat treatment effectively reduced the gradient characteristics in the as welded joint. Consequently, the mechanical property of the joint was nearly restored to the level of base metals. However, the abnormal grain growth was observed in the heat affected zone of the post-weld heat treated joint. The possible mechanism was discussed in term of the evolution of atom segregation at grain boundaries.

Nanostructured ferritic alloys, which have complex microstructures consisting of ultrafine ferritic grains with a dispersion of stable oxide particles and nanoclusters, are promising materials for fuel cladding and structural applications in the next generation nuclear reactor. This study evaluates microstructure of friction stir welded nanostructured ferritic alloys using electron microscopy and atom probe tomography techniques. Atom probe tomography results revealed that nanoclusters are coarsened and inhomogeneously distributed in the stir zone and thermomechanically affected zone. Three hypotheses on coarsening of nanoclusters are presented. The hardness difference in different regions of friction stir weld has been explained.

A solid state joining process, called friction bit joining, was used to spot weld aluminium alloy 7075-T6 to dual phase 980 steel. Lap shear failure loads for specimens without adhesive averaged ∼10 kN, while cross-tension specimens averaged 2·8 kN. Addition of adhesive with a thickness up to 500 μm provided a gain of ∼50% to lap shear failure loads, while a much thinner layer of adhesive increased cross-tension failure loads by 20%. Microstructures of the welds were martensitic, but the hardness of the joining bit portion was greater than that of the DP 980, owing to its higher alloy content. Softening in the heat affected zone of a welded joint appeared to be relatively small, though it was enough to cause nugget pullout failures in some lap shear tension specimens. Other failures in lap shear tension were interfacial, while all of the failures in cross-tension were interfacial.

Friction stir welding (FSW) is widely used in various industrial fields. However, high tiffness is required for FSW equipment which has to withstand high applied load and tool torque and therefore, the equipment becomes large sized. It is difficult to employ FSW for site welding such as repair welding and/or manual welding. The authors made a prototype of FSW apparatus equipped with a counterbalanced tool and a local heating device. The relationship between the applied load and welding parameters was investigated and it was found that the force in the welding direction, Fx is reduced to 70 N or less and that in the transverse direction Fy is to 50 N or less. The combined use of the prototype equipment with a counterbalanced tool and the local heating is very effective to downsize FSW equipment.

The phase transformation from austenite to acicular ferrite in the simulated coarse grained heat affected zone of a high strength low alloy steel was investigated by means of analytical characterisation techniques such as in situ microscopy, transmission electron microscopy and electron backscattered diffraction analysis. The acicular ferrite grains nucleated on inclusions (Zr–Ti oxides) in coarse austenite grain grew in different directions and effectively partitioned coarse austenite grain into several finer and separate regions. The crystallographic grain size became small for coarse austenite grains due to the effective partitioning by acicular ferrite laths or plates.

Dissimilar joints of magnesium alloys were obtained by friction stir welding. Detailed microstructure and texture examinations were performed on the joints. Significant difference in microtexture distribution and microstructural features is observed between crown and stir zones in the dissimilar joints. However, an overall effect of these factors on mechanical properties of layered joints is not obvious as both the up and middle samples present quite similar yield strength and strain hardening behaviour. Moreover, both samples fracture in the Mg–3Al–1Zn alloy side. The fracture of the middle sample starts at the boundary of transition and stir zones, while the up sample starts in the crown zone side.