Science and Technology of Welding and Joining

The liquid infiltrated powder interlayer bonding (LIPIB) process is proposed to make large gap, transient liquid phase joints. Some basic aspects of the LIPIB process are discussed. In the LIPIB process, a joint with high mechanical properties requires a low volume fraction of porosity in the powder interlayer and low concentrations of melting point depressants in the joint. To achieve the fully infiltrated large gap joint, it is essential to maintain an open pore structure at the initial stage of infiltration, while allowing complete elimination of pores behind the infiltration front. Approximate analytical solutions are applied to describe dissolution of capillaries and isothermal diffusional solidification during infiltration. Calculations based on the Cu–Ag binary system show that infiltration kinetics are strongly affected by the geometry of the capillaries. When the effective capillary size is smaller than the critical effective capillary radius at a certain infiltration temperature, the isothermal solidification time will be less than the time required for full infiltration, leading to an incompletely infiltrated powder interlayer. Full infiltration of powder compacts requires tight control of the rearrangement rate of the particles such that a relatively large capillary radius can be maintained to prevent diffusional solidification before full infiltration. Meanwhile, the rearrangement should be able to close the pores behind the infiltration front. Small amounts of chromium or nickel powder addition into spherical copper powders were found to affect the rearrangement significantly, whereas little effect was observed for irregular copper powders. Copper–copper square joints with clearance up to 22 mm have been successfully made.

The yield strength and ultimate tensile strength of ferritic steel weld metal have been expressed as functions of chemical composition, the heat input during welding, and the heat treatment given after welding is completed. The method involved a neural network analysis of a vast and fairly general database assembled from publications on weld metal properties. The outputs of the model have been assessed in a variety of ways, including specific studies of model predictions for the so called C–Mn and 2.25Cr–1Mo systems. Where possible, comparisons have also been made with corresponding methods which use simple physical metallurgical principles. The models created are believed to have been trained on the largest weld metal database to date, and are shown to capture vital metallurgical trends. The computer programs associated with the work have been made freely available on the World Wide Web.

Previous work presented models which can be used to estimate the yield and ultimate tensile strengths of ferritic steel welds. The present paper deals with properties that are much more difficult to predict: the elongation and Charpy impact toughness. While the models are found to be useful and emulate expectations from current physical metallurgy principles, it is clear that much more systematic experimental data are needed before the predictability becomes as good as the strength models of Part 1.

This study is concerned with correlating the metallurgical factors and the toughness of the intercritically reheated coarse grained heat affected zone (ICCGHAZ) of multipass welded SA508–cl. 3 reactor pressure vessel steel. The influence on toughness was investigated by means of a simulated heat affected zone test. Toughness and other properties of the ICCGHAZ were strongly influenced by metallurgical factors such as high concentration of carbon martensitic islands (secondary phase, martensite–austenite constituents) formed along the prior austenite grain boundaries and martensite lath interfaces, and a softened matrix (tempered martensite or bainite). The characteristics (amount, hardness, and size) of the islands were found to be strongly correlated with both peak temperature and the cooling time of the previous pass. The toughness of the ICCGHAZ deteriorated with increasing amounts of martensite–austenite constituents, which were made more abundant by increasing the final peak temperature within the intercritical temperature range. Meanwhile, for the same intercritical peak temperature, toughness decreased with increased cooling time. When the cooling time was short, the dominant factor influencing toughness of the ICCGHAZ was the amount of martensitic islands. However, when the cooling time was long, the hardness difference between the martensitic islands and the softened matrix (tempered martensite) was found to be the dominant factor.

Microstructure and fracture properties relationships have been investigated in the heat affected zones (HAZs) of a high strength low alloy steel used for offshore applications. Metallographic examinations of simulated HAZ microstructures were conducted to investigate the detailed microstructure of the martensite–austenite (M–A) constituents. Using scanning and transmission electron microscopy, various microstructures were found for the M–A constituents. In mixed particles, retained austenite was located at the periphery of the islands. Chemical and/or mechanical effects could possibly account for the stabilisation of this austenite phase. In situ cooling experiments in the transmission electron microscope showed that stacking faults play an important role in the thermal stability of austenite. Impact properties of simulated HAZ microstructures are strongly affected by both the bainitic microstructure and M–A constituents. In particular, freshly transformed high carbon martensite was shown to be much more deleterious than retained austenite.

In phase thermomechanical fatigue (TMF) in the temperature range 573–973 K (300–700°C) and isothermal fatigue behaviour at 973 K in air were studied for type 316 stainless steel using smooth cylindrical specimens machined from base metal, weld metal, and the weld joint (cross-weld). In all joint specimens, fatigue failure occurred in the weld metal region. The lifetimes of weld metal and joint specimens were almost equal and were always inferior to those of base metal specimens. In the base metal, the effect of strain rate on the isothermal fatigue life was not very significant. Although TMF lifetimes were always a little shorter than the isothermal fatigue lifetimes in base metal, the difference was small for the same mechanical strain range and similar strain rate. This may be because the fracture mode for both types of loading was of a similar mixed type. Conversely, a dramatic reduction in lifetime was observed in weld metal and joint specimens under TMF in comparison with isothermal fatigue. This was attributed to the additional damage caused by many independent subsurface cracks at σ phase boundaries and linkage of these cracks with the surface crack, leading to rapid crack propagation. The δ ferrite in the weld metal completely transformed to σ phase in both the isothermal fatigue and TMF tests.

The influence of spot welding on the microstructure and fracture behaviour of an Al–6.6Mg alloy has been investigated. Results showed that dendrites were formed with porosity and cracks in the nugget. Grain boundary melting occurred in the heat affected zone and wide grain boundaries appeared. Spot welding increased the tendency towards intergranular fracture. Tensile shear fracture was mixed with dimples and intergranular fracture. Fatigue fracture was transgranular with striations at the surface. However, the alloy sheet without welding showed dimpled tensile fracture and fatigue fracture mixed with intergranular and transgranular fracture.

Mechanisms by which active fluxes increase the penetration of conventional tungsten inert gas (TIG) welds (so called A-TIG welds) are reviewed. The most dominant mechanism for increased penetration is considered to be arc constriction rather than a change in the surface tension of the molten pool. An experimental programme of work was carried out using A-TIG flux in combination with a number of welding processes. The plasma process was investigated as it gives greater penetration than conventional TIG welding by increasing current density. The CO₂ laser and electron beam processes which do not rely on a current carrying arc as the heat source for welding were also investigated. Macrosections taken from the welds made by these processes showed that the A-TIG flux was only effective when the weld pool was produced by an arc or plasma. Where there was no arc or plasma present, the flux had little effect.

Real time detection of defects in laser welds has been the subject of substantial research and development efforts. In this review, the state of progress in developing so called emission based monitoring systems is described. These monitors collect and analyse various forms of energy radiated from the laser–material interaction site as the weld is being formed. The most common emissions that have been used for monitoring purposes include optical and near optical wavelength electromagnetic radiation, audible and ultrasonic airborne acoustic waves, and integrated charged particle flux. The various system configurations and the weld quality characteristics that have been measured with these emissions are summarised. As researchers continue to refine these systems, there has been increasing interest in the mechanisms by which the signals are produced and their relationship to the various physical processes occurring in the laser weld keyhole. A review of the current literature pertinent to the sources of electromagnetic, acoustic, and plasma charge emissions is given. Finally, results illustrating the relationships between the various emissions signals are presented.