Science and Technology of Welding and Joining

Tests were performed to assess the feasibility of bonding single crystal silicon to copper and aluminium nitride. The application of the bonded component is in a cryocooled monochromator for synchrotron X-ray optics. Aluminium, gold and indium–silver interlayers were tried. Some bonded components were tested by immersion in liquid nitrogen to simulate cooling of the component in service. Although bonds could be formed, they were of insufficient integrity for use in a challenging cryogenic application.

Two kinds of austenite grow from δ-ferrite during the cooling of the duplex stainless steel weld deposits studied here, Widmanstätten plates and allotriomorphs which precipitate at δ–δ grain boundaries. It is found using microtexture measurements that the preferred crystallographic orientation of the Widmanstätten austenite can be estimated using established theory if it is assumed that there is an interaction between external stress and the growing plates. It is also demonstrated that the Widmanstätten and allotriomorphic forms of austenite may not be identically oriented even when the former appears to originate from the latter; this may be a consequence of differences in the transformation mechanisms of these two forms of austenite.

Joining of components of cylindrical shape made of two materials chosen from Fe2Ni, 316L and M2 steels and manufactured by powder injection moulding (PIM) are studied in this paper. In general, joining of PIM cylindrical components requires that the to be joined outer part has a bigger shrinkage than the inner part during sintering. The sintered composite part consisting of Fe2Ni as outer layer and 316L as inner layer is a good example satisfying the above condition. Although the shrinkage of 316L and M2 are less than that of Fe2Ni, the composite parts with Fe2Ni as inner layer and 316L or M2 as outer layer can be successfully produced when different pretreatments are applied to Fe2Ni. Perfect diffusion joining without detectable pores in the vicinity of the bonding area is formed in Fe2Ni-1(or Fe2Ni-2)/316L(or M2) components. Diffusion of Cr and Ni elements has been verified by EDX analysis and it is shown that the diffusion accounts for the hardness of the transition zone.

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at ‘high load’ and ‘low load’ levels.

A multibody coupling finite element model, in which worktable, clamping strip, welding plate and fixture are included, is built to simulate the welding transient distortions under restraining forces. Overlaying welding experiments are also carried out to measure transient welding distortions and dynamic restraining forces to validate the model. The effects of the restraining force location and magnitude, release of restraining force and restraining moment on welding distortion are discussed. Results show that the variation of the restraining force is similar to that of the welding angular distortion during welding, and the minimum initial restraining force for a specified restraining location and the appropriate restraining distance from welding centreline for a specified restraining force to restrain the welding angular distortion can be optimised using the model. However, the welding plate would spring back to some extent after the restraining force release. The restraining moment plays an important role in controlling the welding angular distortion under the same welding parameters, the angular distortion almost the same with the same restraining moment no matter the location and magnitude of the restraining force, the angular distortion decreases rapidly with increasing restraining moment, the distortion before restraining force release can be fully restrained and the values of spring back keep almost the same when the restraining moment is larger than a threshold.

The kinetics of dissolution and isothermal solidification at the bonding temperature during diffusion brazing SS304/BNi-2/SS304 has been studied through a combination of analytical modelling and experimental investigations. The modelling is based on the diffusion theory and the consideration of transient motion of liquid/solid interface. A set of coupled finite differential equations has been programmed to track the motion of liquid/solid interface during the isothermal solidification of liquid filler. Four parameters can be mathematically determined from the analytical modelling including the evolution of solute concentration profile, the maximum diffusion distance, and the maximum liquid thickness as well as the time to complete the isothermal solidification. These analyses are helpful to understanding the joining mechanism during diffusion brazing. The temperature dependent diffusion coefficient used in the modelling is derived together with the experimental data from brazing the wedge shaped joint specimen of SS304/BNi-2/SS304. The effects of bonding temperature and initial joint thickness on the joining process have also been investigated.

Gas metal arc (GMA) welding is one of the most important processes for joining metals. However, this process is characterised by a relatively large number of variables and high sensitivity to changes in operational conditions. Furthermore, GMA welding stability can be degraded by several factors. Although some of these instability problems have been studied extensively a few of them, particularly those linked to the arc transition regions, are still poorly known and studied. The present paper studies a strong process instability characterised by repulsive metal transfer, large variations in welding current and voltage, and the formation of large quantity of spatter. It was observed previously in the welding of low carbon steel with gas shielding of low oxidation potential, and was linked to alterations in electron emission mechanisms on the cathode surface. Welding current and voltage oscillograms, arc light intensity measurements, and high speed filming were used in this study. It was shown that this instability can also occur in the welding of austenitic stainless steels. Indications of its occurrence were also observed in Al. Changes in metal transfer, arc light intensity and welding current and voltage occur simultaneously during the instability. Unstable operation was also characterised by alterations in arc position and shape and by the lack of removal of the oxide layer on the base metal surface. The onset of instability was linked to the occurrence of a rapid short circuit between the electrode and the weld pool.

Applications of advanced high strength steels in tailor welded blanks are a major focus to reduce vehicle weight and manufacturing cost, and to improve part performance. In the present work, formability of two types of laser welded steels in biaxial stretch forming has been studied. Laser welded blanks of the dual phase steel showed larger reduction of formability than those of high strength low alloy steel. This was due to formation of soft zones in the outer heat affected zone of dual phase steel where strain localisation and fracture occurred. Finite element simulations of dome testing incorporating local weld zone properties were able to accurately predict experimentally observed behaviour including deformation patterns and failure locations. In particular, the simulations have shown that under matching of the softened heat affected zone leads to reduced formability by localisation of strain; and that the fracture location changes from radial to circumferential plane when the flow curve of soft zone is evenly matched with the base metal. It was also concluded that the width of the soft zone has minor influence on formability during biaxial stretch forming.

Tensile shear fatigue tests have been performed using resistance spot welded (RSW) joint and friction stir spot welded (FSSW) joint of aluminium alloy sheets. When fatigue strength was characterised in terms of applied load, RSW joint showed higher fatigue strength than FSSW joint in the entire load range, while the fatigue strength normalised with respect to the nugget area was higher in FSSW joint than in RSW joint. Friction stir spot welded joint exhibited a very small scatter of fatigue life compared with RSW joint, indicating that FSSW is a very stable and reliable joining method. Macroscopic fatigue fracture modes were similar in both joints: nugget shear fracture in high applied load region and parent sheet fracture in low applied load region. It was indicated that geometrical factors around the weld zone played a dominant role in determining the fatigue strength and fracture mechanism of the joints.

A new spot joining technology relying on a consumable joining bit has been developed and evaluated on dual phase (DP) 980 steel and a dissimilar combination of aluminium alloy 5754-O and DP 980. This new process, called friction bit joining (FBJ), uses a consumable bit to create a solid state joint in sheet materials by the action of cutting and frictional bonding. FBJ lap shear fracture loads were 14˙7 kN for 1˙4 mm DP 980 compared to 16˙6 kN for RSW, for the same spot diameters. FBJ of a dissimilar combination of aluminium alloy 5754-O and DP 980 produced joints with average lap shear fracture load of 6˙3 kN. This strength is greater than lap shear fracture load obtained by self-piercing riveting (SPR) in HSLA 350 bonded to AA 5754-O.

Laser welding is a very attractive technique to join different alloys at the industrial level, due to its low heat input, high flexibility, high weld quality and high production rate. In this work, the weldability of the aluminium alloy AA 5083 with a high power diode laser has been tested. Concisely, samples were subjected to lineal treatments of laser radiation, with the objective of studying the properties of the bead on plate welds generated. The main objective of the present work has been to study the influence of both the processing rate and the superficial treatment of the AA 5083 samples, on the morphological, microstructural and corrosion properties of the laser weld beads. The sizes of the welds were higher as the processing rate was decreased. The weld beads were seen to have better behaviour against corrosion than the base metal due to the microstructural refinement. It was also verified that a blasting process before processing gave beads with lower size but better corrosion resistance than the application of a black layer, due to the minimisation of the magnesium evaporation in this former superficial treatment.

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.

This paper describes a technique for determining the position of a friction stir welding (FSW) tool with respect to the weld seam during welding. Forces are used as a feedback signal, and a general regression neural network is trained to predict offset position given weld forces. Experimental results demonstrate the accuracy of the developed position predictor. This technique is proposed for online misalignment detection or as a position estimator for in-process tracking of the weld seam for FSW and robotic FSW.