Jiangtao Xiong

High strength lap joint of aluminium and stainless steels fabricated by friction stir welding with cutting pin

Abstract Dissimilar lap joints of aluminium and stainless steel were first friction stir welded by the tool with a cutting pin. The results showed that sound joints could be obtained by this method. When the pin was inserted into the lower steel sheet, macrointerlocks were formed by the steel flashes plugging into the upper aluminium at both sides of the nugget bottom. At the aluminium/steel interface, a thin intermetallic compound (IMC) layer and the mechanical bonding of microinterlocks were formed. In addition, the aluminium near the interface was also strengthened by grain refinement and IMC particles. Therefore, the beneficial effect of the macrointerlocks provided by the steel flashes was removed, the shear strength of the joint reached 89·7 MPa, which was even higher than that of the base metal of aluminium.

Periodic variation of torque and its relations to interfacial sticking and slipping during friction stir welding

Abstract The fraction of slipping between the tool and the workpiece during friction stir welding was calculated based on the measured torque and axial force with a sampling frequency of 1000 Hz. The results showed that the torque and fraction of slipping present a variation with the same periodicity that is equal to the time of one tool rotation. A model was proposed to present the average slipping fraction under different process parameters. The variation of the contact condition between the tool and the workpiece in one rotation was explained.

Optimisation of interpass temperature and heat input for wire and arc additive manufacturing 5A06 aluminium alloy

ABSTRACT Undulated surface appearance and inconsistent layer cross-sectional dimensions are two common types of appearance defects, which are formed owing to insufficient heat input and nonlinear time-varying thermal boundary condition of molten pool. Appropriate interpass temperature control and heat input regulations are effective ways to realise and maintain the consistent thermal boundary condition during bottom-up additive manufacturing process. Equivalent heat dissipation factor is used to synthetically characterise the variation of heat dissipation in process of additive manufacturing. Combing with Rosenthals analytical solution, a theoretical model is developed to optimise interpass temperature and heat input for each layer deposition, so as to realise stable additive manufacturing. Finally a specification for the wire and arc additive manufacture procedure was designed according to the developed model which was verified effectively by manufacturing a wall part. It could also be a process design reference for other kinds of metal additive manufacturing.

A study of the mechanisms involved in initial friction process of continuous drive friction welding

The effects of the welding conditions on the initial friction process of continuous drive friction welding are investigated using 20-Mild steel. The results show that the friction interface will sequentially experience three typical friction behaviours of abrasion, slide and stick before the initial peak torque. The torque in slide stage shows a platform feature, which transforms to stick stage with a rapidly increasing feature of the torque. As the friction pressure increases from 15 to over 60 MPa, the initial joining location (i.e. corona bond) shifts from one-fourth of the specimen radius (1/4R) towards 1/2R. The maximum surface temperatures show initial slow rising, platform and rapid rising features before the highest temperature appears, which is corresponding to the interface friction status of abrasion, slide or stick. The initial joining location virtually coincides with the location where the maximum temperature is attained.

Characterisation of periodic variation in torque occurred in friction stir welding process

Abstract Friction stir welding was physically simulated to study torque variation by a rotational friction process under constant axial pressing speed. The results indicated that the torque accurately reproduce the same periodic characteristics as the friction stir welding torque, i.e. the torque cycle is equal to the time of one tool rotation. The periodic variation intensity of torque, which was obtained at a constant rotational speed ω and a constant axial pressing speed v, was characterised by non-dimensional amplitude of the torque Γ*. Moreover, a semianalytical model, which describes Γ* as a function of ω2/v in Arrhenius form, was established. In this model, Γ* exponentially increases with ω2/v, and low ω2/v suppresses the variation intensity of torque.

Microstructure and enhanced atomic diffusion of friction stir welding aluminium/steel joints

ABSTRACT Lap joints of friction stir welding between aluminium and stainless steel sheets were conducted using a welding tool with a cutting pin. The atomic diffusion of Fe–Al system during the severe plastic deformation was investigated. The interfacial microstructure and metallurgical reaction was analysed. The metallurgical reaction layers were identified as a compounds containing a phase of Al3Fe, partial solid solution of Fe and Al, and amorphous with a thickness of 0.9–3.3 μm which depending on the process parameters. The interdiffusion coefficient between Al and Fe atoms is about 4 orders of magnitude compared with that under thermal equilibrium state. The nanocrystalline and partial amorphous were formed near the interface which may caused by the enhanced atomic diffusion.

Asymmetric microstructure and fracture behaviour of friction stir welded joints of 2024 aluminium alloy under cyclical load

In this study, the effect of asymmetric microstructure on fracture behaviour of friction stir welded joints of 2024-O aluminium alloy under cyclical load was investigated via optical microscopes, electron backscattering diffraction, microhardness and infrared thermography, and digital imaging correlation (DIC). The results demonstrate that the thermo-mechanical affected zone (TMAZ) at retreating side possesses a relatively higher fraction of low-angle grain boundaries and coincidence site lattice boundaries in comparison to that at the advancing side (AS), which results in the strain localisation and fracture path along the region between nugget zone and TMAZ at the AS under cyclical load. The profile of microhardness presents the asymmetric characteristic as well. The temperature evolution of the joint obtained by infrared camera is in good agreement with strain distribution measured by DIC. The thermography proves to be generally sufficient to predict the fracture path before failure under cyclical load. GRAPHICAL ABSTRACT

Equal-strength precision diffusion bonding of AA6063 aluminum alloy with the surface passivated by a self-assembled monolayer

Abstract Precision diffusion bonding of aluminum alloy requires good mechanical properties and very low deformation. However, oxidation lowers the quality of the joint. This paper used a self-assembled monolayer, 1-hexyl mercaptan, on the aluminum surface to prevent the secondary oxidation after pre-bonding treatment. An AA6063/Ni/AA6063 sandwich structure joint was intentionally designed and bonded to characterize the effect of the self-assembled monolayer. AA6063/AA6063 joints were diffusion bonded, in which three different pre-bonding surface treatments were used that were as-cleaned, ethanol adsorbed and self-assembled monolayer passivated. With the surface passivated by self-assembled monolayer, there were few voids at the bonding interface. Ductile fracture was observed during tensile testing. The joint had a bond ratio of 97.4% and elongation of 20.4%, which is very close to the base metal and is considered as equal-strength bonding.

Study on microstructure evolution of AISI 304 stainless steel joined by rotary friction welding

The object of the present work is to research the microstructure evolution of AISI 304 stainless steel (304SS) joined by rotary friction welding. The dynamic recrystallization (DRX) and texture formation process were analyzed by optical microscope and electron back-scattered diffraction (EBSD) technology. The results showed that DRX, which emerges on the torque rising stage, mainly occurs in the welding zone, and completes on the torque quasi-steady stage. Furthermore, obvious shear type texture ({110} <112>) forms in the joint of 304SS.

Formation and improvement of surface waviness for additive manufacturing 5A06 aluminium alloy component with GTAW system

Purpose As known, the wire and arc additive manufacture technique can achieve stable process control, which is represented with periodic surface waviness, when using empirical methods or feedback control system. But it is usually a tedious work to further reduce it using trial and error method. The purpose of this paper is to unveil the formation mechanism of surface waviness and develop a method to diminish it. Design/methodology/approach Two forming mechanisms, wetting and spreading and remelting, are unveiled by cross-section observation. A discriminant is established to differentiate which mechanism is valid to dominate the forming process under the given process parameters. Findings Finally, a theoretical method is developed to optimize surface waviness, even forming a smooth surface by establishing a matching relation between heat input (line energy) and materials input (the ratio of wire feed speed to travel speed). Originality/value Formation mechanisms are revealed by observing cross-section morphology. A discriminant is established to differentiate which mechanism is valid to dominate the forming process under the given process parameters. A mathematical model is developed to optimize surface waviness, even forming a smooth surface through establishing a matching relation between heat input (line energy) and materials input (the ratio of wire feed speed to travel speed).