Jinglong Li

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.

Numerical Analysis of Joint Temperature Evolution During Friction Stir Welding Based on Sticking Contact

A three-dimensional numerical model for friction stir welding was developed by using the ABAQUS software based on a fully sticking friction. The temperature measurement was performed to validate the reliability of the model. The simulated thermal histories are in good agreement with the experiments. Simulated results show that the rotation speed has no influence on the time to reach the peak temperature in the workpiece, while the welding speed has significant effect on the time to reach the peak temperature at points away from the plunging center. The value of this peak temperature also changes somewhat. Moreover, the peak temperature in the workpiece tends to reach a quasi-steady state at the beginning of the moving stage; but the temperature at some distance away from the weld does not reach the quasi-steady state during the welding.

Effect of post-weld heat treatment on microstructure and property of linear friction welded Ti17 titanium alloy joint

Abstract In this study, post-weld heat treatment of linear friction welded Ti17 (Ti–5Al–2Sn–2Zr–4Mo–4Cr) titanium alloy joints was performed at 530, 610 and 670°C for 4 h followed by air cooling. Results show that with increasing treatment temperature, the recrystallisation extent of the α and β phases in the weld and deformation zones increases significantly. The overall property of the joint is remarkably improved, and the fracture behaviour of the tensile and impact samples changes from brittle failure to a ductile one. After treatment at 670°C, the impact toughness of the joint is 93·3% of the parent metal, and the failure of the tensile samples occurs in the parent metal far away from the weld. According to these findings, a treatment temperature slightly lower than 670°C but higher than 610°C would be a good value for linear friction welding Ti17 joints.

Modelling of entire friction stir welding process by explicit finite element method

Abstract A coupled thermomechanical three-dimensional finite element model was developed for friction stir welding in the ABAQUS environment using Johnson–Cook material law and Johnson–Cook failure model. The temperature evolution during the plunge, dwell and moving stages of a friction stir welded 7050 aluminium alloy and the effect of heat conduction by the back plate were investigated. Results show that the temperature almost symmetrically distributes across the plate cross-section, and the temperature contour in the weld nugget zone presents a V type shape after the plunge stage. In the dwell stage, the frictional heat conducts around to preheat the plate. While in the moving stage, the heat gradually accumulates until a quasi-stable temperature field is formed. Moreover, it is shown that the heat conduction through the back plate has a significant effect on the temperature field. With the increasing heat convective coefficient of the back plate, the temperature field remarkably shrinks.

Analysis of Nugget Formation During Resistance Spot Welding on Dissimilar Metal Sheets of Aluminum and Magnesium Alloys

The nugget formation of resistance spot welding (RSW) on dissimilar material sheets of aluminum and magnesium alloys was studied, and the element distribution, microstructure, and microhardness distribution near the joint interface were analyzed. It was found that the staggered high regions at the contact interface of aluminum and magnesium alloy sheets, where the dissimilar metal melted together, tended to be the preferred nucleation regions of nugget. The main technical problem of RSW on dissimilar metal sheets of aluminum and magnesium alloys was the brittle-hard Al12Mg17 intermetallic compounds distributed in the nugget, with hardness much higher than either side of the base materials. Microcracks tended to generate at the interface of the nugget and base materials, which affected weld quality and strength.

Nugget quality prediction of resistance spot welding on aluminium alloy based on structureborne acoustic emission signals

Abstract The structureborne acoustic emission signals during resistance spot welding on 2024 aluminium alloy were detected in real time and analysed to find the characteristics of signals corresponding to the physical phase of welding process. The curve fitting models were developed based on the acoustic emission count and positive peak of nugget nucleation event. These mathematical models were used to predict the tensile–shear strength of spot weld. The results showed that the physical phases of welding process can be characterised by the acoustic emission signals detected during the resistance spot welding process. The acoustic emission count and the positive peak of nugget nucleation event of resistance spot welding on 2024 aluminium alloy have good relevance to the tensile–shear strength of spot weld, and these correlations were fitted to comply with some functional relationships, which can be used to realise the prediction of the strength of spot weld. It can be concluded that the prediction of weld strength based on the acoustic emission count of nugget nucleation event has good performance, but the prediction of weld strength based on the positive peak of nugget nucleation event is more susceptible to the expulsions and has poor performance when there are expulsions.

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.

The Fabrication and Properties of the Squeeze-Cast TiN/Al Composites

Co-continuous TiN/Al composites with different volume fractions of Al phase have been fabricated by the squeeze-casting method. TiN porous ceramics with different porosities were fabricated through carbothermal reduction by changing the content of TiN and were used as preforms. The outstanding mechanical properties were attributed to the absence of excessive interface reaction between TiN and Al for the co-continuous TiN/Al composites. With the increase of Al content in the composites, the flexural strength and the microhardness decreased, and the fracture toughness increased. The strengthening and toughening mechanism of composites included dislocation strengthening, ductile rupture, crack deflection, and secondary cracks.