Science and Technology of Welding and Joining

In welding, the concept of optimisation had been used, till date, to serve two specific purposes. To obtain any desired weld attributes, a set of required optimal input parameters could be determined utilising the principle of optimisation. To establish an input–output relationship of a welding process accurately, which might be essential to automate it, the tools for optimisation had been used extensively. These optimisation problems were formulated as either a single-objective or multi-objectives or many-objectives ones. Both traditional and nature-inspired optimisation tools were used for the said purposes. This paper provides an extensive review on the role of optimisation in welding. Some research areas have also been identified for future study.

In this study, the influence of the microstructural evolution of galvannealed (GA) coating and the thermomechanical characteristics of liquid metal embrittlement (LME) cracking during resistance spot welding (RSW) of steels were investigated. The analysis of LME cracks, coating microstructure and thermomechanical characteristics at various weld regions revealed a synergistic effect of liquid Zn, α-Fe(Zn), and tensile stress on LME cracking during RSW. A continuous α-Fe(Zn) layer inhibits liquid Zn from contacting the steel substrate; hence, LME was suppressed. However, a long-duration, high-magnitude tensile stress can fragment the α-Fe(Zn) layer and enable LME cracking. Conversely, α-Fe(Zn) particles allow the easy infiltration of liquid into the steel and make the crack on tensile stress development.

The effects of ultrasonic-assisted soldering parameters on the interfacial microstructural evolution and shear strength of designed Al/Sn–9.77Zn–11.36Bi–1.46Ag/Cu solder joints were investigated. The ultrasonic-assisted soldering process promoted the dissolution of Al substrate and roughed Al/solder interface, which improved the pinning effect of Sn–9.77Zn–11.36Bi–1.46Ag on Al substrate. With an increasing ultrasonic soldering temperature, time and power, more Al atoms dissolved from Al substrate and migrated across solder to the opposite Cu substrate to form Al-rich (Zn) solid solutions. Consequently, Al_4.2Cu_3.2Zn_0.7 formed between ϵ-AgZn₃ and Cu₅Zn₈ layers at Cu substrate, resulting in thinner Cu₅Zn₈ and higher shear strength of solder joints. The highest shear strength of solder joints (38.6 MPa) was achieved after ultrasonic-assisted soldering at 300°C for 10 s under 9 W.

Successful joining of heat conducting materials, to high-temperature components is of significant importance for heat management in nuclear power plants and liquid propellant launch vehicles. Solid-state processes are considered for joining such dissimilar materials. Diffusion bonding of a copper alloy (C18150) to a Ni-based superalloy (GH4169) was investigated in this work. The bonding trials at 900°C for 60 min under a constant 10 MPa pressure led to formation of sound joints free from intermetallic, pores, voids and discontinuities. Conventional tensile testing led to the failure within the copper alloy and away from the joint. Due to the softening of the copper alloy during the bonding process, the maximum tensile strength of the as-bonded copper alloy was only 48% of its as-received strength. Post-bonding solution treatment at 960°C for 60 min followed by ageing at 450°C for 3.5 h, restored the tensile strength of the copper alloy up to 77%.

The microstructural evolution, mechanical and electrochemical properties of the rotary friction welding joint of 1MnCrMoNi maraging steel were investigated after post-weld heat treatment (PWHT). The joint could be divided into two zones: weld zone and thermo-mechanically affected zone. After PWHT, the parallel alignment of the laths turned ‘blurred’; sub-grain and cell structure appeared; M23C6 and -Cu precipitated, respectively; the joint was softened. But the microhardness was still higher than that of base materials and distributed unevenly. The closer it was to the weld centre, the higher the microhardness was. No deformation occurred in the joints during tensile test, indicating the excellent bonding strength. PWHT process slightly reduced the impact toughness and corrosion resistance of the joint.

Components of the offshore industry have to withstand high mechanical loads under extreme conditions. Thus, noble metals have been used for manufacturing of pipes, with materials as corrosion resistant alloys (CRA). Nevertheless, fusion welding for CRA still lacks of reproducibility, looking for more effective processes. In this context, friction stir welding is suitable for CRA. Therefore, this study focuses on mechanical and corrosion properties of friction stir welds in UNS N06625 and UNS S32760. At first, the weld microstructure and its corresponding heat input were analysed. Besides, microhardness measurements were performed. Moreover, the pitting resistance was studied according ASTM G48A. The results showed that friction-stir-welded alloy 625 had greater pitting resistance than friction-stir-welded super duplex.

Magnesium-lithium alloys are among the lightest commercially-available structural alloys. However, the reactive nature of Mg and Li makes fusion welding of this family of alloys very difficult, if not impossible. The solid-state joining processes are considered suitable alternatives since the joining can be carried out at temperatures well below the melting of these alloys. Refill Friction Stir Spot Welding (Refill FSSW) of 2.2 mm thick Mg-Li plates, using air and argon as the cooling gas, was investigated. Initial examinations revealed formation of adherent oxides-based residues on the tool shoulder. The microstructural examination of the air-cooled samples showed the presence of oxide inclusions within the weldment. As expected, the argon-cooled welding process reduced oxide formation and additionally resulted in flat and smooth welded surfaces. Nevertheless, and somehow unexpectedly, the presence of oxide inclusions in the air-cooled samples had no noticeable effect on the shear strength or microhardness of the joints.

The solidification cracking susceptibility was calculated for quaternary Al–Si–Mg–Cu, Al–Zn–Mg–Cu and Al–Li–Mg–Cu alloy systems. It was calculated based on the crack susceptibility index |dT/d(f_s) ^1/2 |, where T is temperature and f_s the fraction solid. Three different levels of back diffusion were considered: no diffusion, diffusion under a cooling rate of 100 ^oC/s and diffusion under 20 ^oC/s. The calculated results were first validated by experimental data previously reported, and then extended to the composition ranges where no experimental data are available. For each alloy system, the crack susceptibility was calculated for 132651 Al alloys, with six crack susceptibility maps covering 15606 Al alloys at each diffusion level. These calculated results can be a database useful for welding, casting and additive manufacturing.

Tungsten electrode contamination caused by metal vapour from a base metal during a tungsten inert gas welding was investigated. The metal ions transported from the weld pool surface were deposited on the tungsten electrode surface during welding. The tungsten fraction of the electrode surface layer was decreased because the deposited metal vapour diffused inside the tungsten electrode. The melting point of the electrode surface layer was lowered, decreasing to the electrode operating temperature. Results show that the electrode melted and deformed. This electrode contamination was promoted when a larger amount of metal ions was transported to the electrode surface.

Dissimilar Ti-based amorphous alloy sheets were butt welded by a pulsed laser and metallurgical joints were obtained. The β-Ti crystal grains in nanometre scale unevenly distribute on the amorphous background in the fusion zone. With the increase of laser pulse energy and pulse duration, the crystalline β-Ti grains grow up from spherical to dendritical profile, but the crystalline size reaches its maximum of about 1 µm, which is much smaller than the grains in base metal. The refinement of the grains in fusion zone results from the fast cooling speed during welding. The microhardness in the welded joints ranges from 470 to 525 HV. Average tensile strength of the welded joint is about 1510 MPa (86% of pure Ti-based amorphous alloy BM and 91% of β-Ti dendrite reinforced Ti-based amorphous alloy BM).