Science and Technology of Welding and Joining

The joints of aluminium matrix composites with high SiC particle content were achieved by vacuum brazing with Al–12Si–1·5Mg–4Ti foil as active filler. The effect of Ni coating and Ti on the wetting behaviour of filler metal on the composite was investigated. Owing to the presence of Ni coating, the wettability of liquid filler metal on the composite was enhanced obviously, and Ti(AlSi)₃ intermetallic is in situ strengthening phase in the seam. Sound joints with well appearance were obtained, and the optimised shear strength of the joint reached 109.3 MPa. Capable welding parameters were broadened by the presence of Ni coating and Al–12Si–1.5Mg–4Ti foil as filler for brazing of aluminium matrix composites with high SiC particle content.

Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.

Friction stir welding was successfully applied to joint Mg–3Al–1Zn alloy sheets. Serious strain localisation was observed during transverse tensile tests of the welds. Fracture was initiated from the interface of thermal–mechanical affected zone and stir zone in retreating side. The reasons for such localised fracture behaviour were explained by analysing the texture, Schmid factor, grain size and strain localisation characteristics in retreating side and advancing side. The results suggest that the coarser grain size and higher Schmid factor resulted in the easier activation of basal slip in retreating side than advancing side, which caused more serious strain localisation and stress concentration, and hence promoted fracture initiation from the interface of thermal–mechanical affected zone and stir zone in retreating side.

Resistance spot welding of commercial pure titanium and aluminium alloy 6061-T6 is carried out. The microstructure and tensile shear properties of the weld joints are studied. There is a critical range of welding current at which the reaction between liquid aluminium and solid titanium changes to a reaction between liquid aluminium and liquid titanium. The microstructure in the aluminium side is a type of transition structure, which is considered to be the same microstructure that connects the columnar dendrite and equiaxed dendrite in similar aluminium alloy resistance spot welding. The welding current has the largest influence upon the tensile shear properties of the Al/Ti weld joint, while the welding time has a small effect and the electrode force has virtually no effect.

In order to explore the reasons for coarse grained embrittlement, post-weld heat treatment (PWHT) with different times was conducted on the coarse grained heat affected zone (CGHAZ) of A533B steel. The results reveal that the impact toughness of the CGHAZ at room temperature decreases with the prolongation of PWHT time from 0.5 to 4 h. As the PWHT time increases to 4 h, the fracture feature of the CGHAZ presents intergranular cleavage ruptures, and the impact toughness exhibits the minimum value. After PWHT for 4 h, the area of the intergranular cleavage fractures decreases, and the impact toughness gradually increases. Further research reveals that there is phosphorus grain boundary segregation in the CGHAZ. The theory of non-equilibrium grain boundary segregation (NGS) is proposed to explain the phenomenon. It is found that the PWHT time with minimum impact toughness is consistent with critical time of phosphorus NGS.

Controlling the Fe–Al intermetallic layer thickness along the joint interface has remained a critical challenge in gas metal arc welding of galvanised steel and aluminium alloys. An attempt is presented here to join galvannealed steel and aluminium alloy sheets using a novel gas metal arc welding process that allows controlled short circuiting to reduce the rate of heat input significantly. The real time current and voltage transients during the process are monitored to estimate the rate of heat input and its influence on the formation of intermetallic phases and layer thickness. The results show that the intermetallic layer thickness can be controlled and good joint strengths can be achieved when the rate of heat input remains lesser than 130 J mm^− 1 for the typical lap joint configuration considered here.

In the present investigation, a microstructural characterisation of welds performed by resistance seam welding was carried out, with special focus on weld defect analysis. In order to perform a comparative weldability analysis, the welds were performed using similar welding procedures. Similar welds in aluminium (5754-H22) and zinc (Zintek) alloys, as well as dissimilar welds between galvanised steel and zinc, were studied. The defective aluminium welds were found to be characterised by important grain growth inside an inhomogeneous nugget and by the presence of important voids and cracks. The zinc welds showed a well defined nugget, but with porosities and some cracks. In the dissimilar steel–zinc welds, important macroscopic defects were observed. Microstructural analysis evidenced the occurrence of melting at the zinc side of the welds; meanwhile, no microstructural modifications could be observed for the steel side. Defect formation, as well as weld morphologies, was related to the variation in welding parameters.

This paper investigates the effect of Ce on inclusions, microstructure and mechanical properties of the weld metal in flux copper backing submerged arc welding. The results show, with the addition of Ce in weld metal, the inclusions are mainly composed of Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS and Ce₂O₂S compound, and the proportion of the inclusions of < 1.0 μm in size increased. In competitive nucleation, acicular ferrite nucleates on the oxysulphides of Ce and inhibits the formation of proentectoid ferrite and ferrite side plate. This led to a high proportion of acicular ferrite, good mechanical properties of weld metal. A small lattice disregistry between CeS, Ce₃S₄, Ce₂O₂S and acicular ferrite contributed to low energy for nucleation of acicular ferrite.

In this paper, the effect of pin features and orientation/placement of the materials on advancing side were investigated for friction stir welding (FSW) of dissimilar aluminum alloys AA2050 and AA6061. Pins for FSW were produced with a 2·12 mm pitch thread having three flats/flutes. Three sets of rotational speed/welding speed were used to perform a series of welds in a butt joint arrangement. The results show that, joint quality, process response variables and welding temperature are highly affected by pin features and material orientation in FSW. Defect free joints with effective material transportation in the weld nugget zone were obtained when welding was performed with AA2050 on the advancing side. The tool also encounters less in plane reaction force for welding with 2050 on the advancing side. Pin with thread+3 flats produces quality welds at low rotational and travel speed regardless of the location of alloys on advancing or retreating side.

A 1000 MPa galvanised dual phase steel was fibre laser lap welded by different welding parameters. The surface appearance, cross-section macrostructure and tensile shear properties of laser lap welds were evaluated. The different methods for improving the laser lap weldability of galvanised steel were analysed. The results showed that the increasing laser power resulted in the formation of the keyhole, which could be used as the escape route for the vapourised zinc formed at the faying surface. Therefore, the weld defects, such as spatter, cavity and blow hole, were eliminated, and the full penetrated laser lap welds with smooth surface appearances were acquired. A proper joint gap also worked as the escape route for the zinc vapour and led to the formation of sound welds. Adding proper preheating procedures improved the mechanical properties of the welds to some degree, but weld defects still existed owing to the residual zinc.

This paper reports a study on the effect of Al₂O₃, SiO₂ and TiO₂ nanoparticles on the adhesion strength of steel joints bonded with two-part structural acrylic adhesives. Introduction of the nanoparticles in the two-part acrylic adhesive led to a remarkable enhancement in both the shear and the tensile strength of the steel joints. Addition of the nanoparticles caused an increase in the shear and the tensile strength of the joints; however, a reduction was observed in the peel strength of the joints. Differential scanning calorimeter analysis revealed that glass transition temperature values of the adhesives rose with the addition of the nanofillers. The water contact angle was decreased for adhesives containing nanoparticles. Scanning electron microscope images revealed that the addition of the nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.