Science and Technology of Welding and Joining

Process parameters and base material thermal properties strongly affect peak temperature evolution in friction stir welding (FSW). This work develops a semi-empirical model for peak temperature estimation during FSW of aluminium alloys. Several bead-on-plate runs were conducted on AA5083-H111, AA6082-T6 and AA7075-T6. Different rotational and welding speeds were tested while measuring the temperature with a thermocouple embedded in the tool. A polynomial model was proposed with the model's coefficients strongly correlating to the material thermal properties. Additionally, a threshold peak temperature was defined to avoid cold defects by combining temperature measurements with joint quality information obtained from X-ray scans. The efficiency of the model and the defined threshold are proven by applying them to the literature.

Interfacial texture and bonding strength of Cu/Al laminate metal composite fabricated by ultrasonic additive manufacturing (UAM) were investigated by electron backscattered diffraction, transmission electron microscopy and peeling tests. Results show that after the UAM process, Al foils have obvious shear texture or recrystallisation texture components in the region near the interface of Cu/Al, indicating that Al foils are prone to shear deformation during consolidation, which plays an essential role in the bonding formation of the two dissimilar metals. The rougher Cu foil surface produced by direct contact with sonotrode promotes more remarkable shear deformation of the mating Al foil near the interface during the subsequent consolidation pass, which obviously enhances the interfacial bonding strength.

Gas metal arc-directed energy deposition is a metal additive manufacturing technique that involves the melting of a filler wire under an electric arc resulting in a high rate of deposition of molten metal and fast building of large scale parts in a layer-by-layer manner. A novel attempt is presented here for real-time simultaneous monitoring of the arc current, voltage and power, and the nature of surface temperature field and filler wire deposition during the process. The recorded temperature fields are analysed to trace the layer-wise variation in melt pool dimensions and solidification cooling rates. Together these process signatures demonstrated a synchronised assessment and potential to support an integrated feedback control system for gas metal arc-directed energy deposition.

The effects of welding hold time on weld quality of aluminium alloy to dual-phase steel resistance spot welds were studied. Results demonstrated that there existed an optimum range of hold time (350∼500 ms), within which the largest weld nugget diameter (7.6 mm), thinnest intermetallic compound (IMC) layer (1∼5 µm) and maximum tensile shear peak load (4.7 kN) were obtained. Too short hold time led to shrinkage, small weld nugget and thick IMC layer due to reduced heat dissipation after removing electrodes, thus resulting in low tensile peak load and brittle fracture along the IMC layer. While too long hold time, i.e. 750 ms, produced a smaller weld nugget diameter due to more rapid heat dissipation.

The friction stir alloying (FSA) procedure successfully produced dissimilar joints between AZ61 magnesium alloy and mild steel with Al-CNT additions. Before welding, Al-CNT additives were placed in a gap between the AZ61 and steel specimens. The experimentation was performed using different welding speeds while tool rotational speed was kept constant for varying weight percentages of carbon nanotubes (CNT) in Al powder. The influence of additives and welding speed on tensile strength and characterisation across the welding interface was studied. Tensile strength of joint with 3% CNT improved up to 222 MPa. Al-rich intermetallic compounds were found at the welding contact, while the formation of Al₂MgC₂ carbide was detected, which contributed to CNT strengthening effects all to improved welding strength.

In vacuum laser welding, a laser beam is irradiated into a chamber through a coupling glass exposed to contamination. A conical protective system for the coupling glass is suggested, and contamination is analysed via simulation and experiments based on the conical component height and the gas flow rate. Stable descending flow is a key to suppressing the contamination, which affects the particle ascending velocity, horizontal deflection of particles and number of residual particles. The laser transmission area can be protected using a gas flow rate of 1.0 L min⁻¹ or a cone height of 90 mm for the conical component. The conical protective system demonstrates excellent contamination suppression capability. The simulation-based approach can provide an effective design for the protective system.

Microstructure, inclusions and cryogenic impact toughness of Fe-22.03Mn-0.45C austenitic weld metal were investigated. The results show that microstructure of weld metal consisted of straight columnar dendrites with different orientations. Microsegregation of Mn was observed in the interdendritic regions and Mn depletion thus occurred in the dendrite core, which is suggested to lead to the different morphologies of MnS oxides (Mn–Al–Si–O) depending on their location. MnS patch-type oxides were found at dendrite boundary and MnS shell-type oxides were found at dendrite core. Inclusions as crack sources were distributed in the dimple of cryogenic impact fractography, it was found that MnS patch-type oxides were more ready to act as a crack source and cause the formation of dimples than MnS shell-type oxides.

The Cu/Sn–49Bi–1Ag/Sn–3.0Ag–0.5Cu/Ni–P hybrid solder joints for customer electronics were assembled by low-temperature soldering at 170–200°C. The relationship between microstructural evolution and board-level drop failure mechanism of these solder joints during aging at 110°C was investigated. The Bi-rich zone increased from 24.98to 78.85% with increasing soldering temperature from 170 to 200°C. The growth rate of intermetallic compound (IMC) in 170-SAC/SB solder joints was approximately 3 times faster than that in 200-SAC/SB solder joints. The drop failure mechanism was explained that the drop-induced crack propagation path shifted from Bi phases in the as-soldered joints to the solder/Cu₆Sn₅ interface in the aged joints, since the interfacial Cu₆Sn₅ IMC grew thicker with increasing aging time.

Low-temperature Pb-free solders are in demand in the electronic industry for reducing processing emissions and enhancing product reliability. Sn–Bi-based alloys have drawn great interests. We performed high-throughput CALculation of PHAse Diagram modelling and systematically screened 35,343 alloy compositions among the Sn–Bi–Ag–In and Sn–Bi–Ag–Zn quaternary systems. An optimal alloy of each system was proposed, experimentally characterised based on microstructural, thermal, and mechanical analyses, and their phase equilibria and solidification reactions are elaborated. The designed Sn–Bi–Ag–In alloy with solidus and liquidus temperatures of 137.6 and 157.1°C, respectively, shows superior mechanical properties, e.g. the ultimate tensile strength of 57 MPa, elongation of 50%, and toughness of 20.6 × 10⁶ J m⁻³, which is a promising material for low-temperature interconnection.

Self-piercing riveting (SPR) is a high-speed mechanical joining technique for sheet metal materials. In this study, the mechanical properties of DC04 steel and AA5182 aluminium alloy in triple combinations joined by the SPR method were investigated. For this purpose, triple-joining processes were performed in different combinations, and macro-structural analysis and tensile shear tests were carried out by comparing their mechanical properties. As a result of the study, the highest shear forces were obtained in FFA (DC04-DC04-AA5182) and AFF (AA5182-DC04-DC04) combinations. As the number of aluminium layers increases, the probability of tearing increases in triple joining. Higher spread ratios were generally measured under conditions with hard materials in the upper layers and soft materials in the lower layers.

Commercial 6061 Al alloy and SiCp reinforced 6092 Al matrix composite (AMC) were joined by friction-stir welding (FSW) with different tool offsets. Results show offsetting the pin to Al alloy side could significantly reduce tool wear, while maintaining ultimate tensile strength of the joint around 180 MPa. As tool offset increased from zero to 3 mm, pin and shoulder wear length reduced from 0.17 mm to almost zero and from 0.28 to 0.02 mm, respectively. The corresponding estimated maximum safe welding length increased from 568 to 3125 mm. Fe debris were generated from the steel pin by the sharp SiC particles, then transferred into the nugget zone, eventually formed Al8Fe2Si particles during the FSW process.