Feasibility study of the friction surfacing process

Friction surfacing is a solid state cladding process based on the plastic deformation of a translating and rotating metallic consumable rod pressed against a stationary substrate. It is mostly used on mild and stainless steel and on aluminium. Thanks to the solid state nature of the process, it allows to join dissimilar metal combinations, e.g. aluminium to steel or to ceramics or several combinations of non-ferrous metals. Moreover, a continuous and fine-grained deposition is formed. Most research has been focussed on the feasibility of certain material combinations and on correlating the deposited layer quality to input parameters. In this work, a methodical approach to evaluate clad layers and to assess their properties is discussed. This approach consists of a visual assessment, a macrographic examination and a performance analysis and has shown to be apt to compare the clad layer quality

Metallographic evaluation of the weldability of high strength aluminium alloys using friction spot welding

Friction spot welding is a recent solid-state welding technique well suited for spot-joining lightweight materials in overlap condition. Aerospace and transport industries show great interest in this technique to join high-strength aluminium alloys, but published research is still limited. In this project, the link between process parameters and weld quality is investigated for EN AW-7075-T6 material. Techniques used are metallographic qualification, measurement of hardness reduction and lap shear strength. This paper focusses on the metallographic investigation of the weld region and its imperfections. Increasing joining time and heat input creates an easier material flow resulting in fewer imperfections. Limited plunge depths lead to typical interface imperfections. Variation in the rotational speed shows distinctive stir zone shapes as a consequence of severe stirring and frictional heat

Investigation of the weldability of copper to steel tubes using the electromagnetic welding process

Magnetic pulse welding is an innovative joining method which allows joining of dissimilar metal combinations. However, much remains unknown about the process and its parameters. In this paper, the weldability of copper tubes to steel rods and tubes is discussed, with the goal of examining the influence of the wall thickness of the supporting steel tube on the weld and the deformation of the components. Large deformations were observed, causing an undesirable decrease in diameter of the tubes. The quality of the obtained welds was shown to decrease with decreasing inner tube thickness as well, most likely due to the deformation of the workpieces in radial direction. Because of this, it is advisable to use an internal support to prevent deformation of the support tubes. To gain more insight in the precise mechanisms of weld formation and failure, numerical simulations are advised

Experimental investigation of the weldability of tubular dissimilar materials using the electromagnetic welding process

This paper describes the magnetic pulse welding process (MPW) for tubes. Material combinations of aluminium to steel and copper to aluminium were experimentally evaluated. The first major goal of this work is to experimentally obtain the optimal input parameters like the discharge energy, the stand-off distance and the tool overlap for MPW of the material combinations. Welding windows with all possible input parameters are created for both material combinations. Furthermore, a comparison is done between three coil systems; a single turn coil with field shaper, a single turn coil with a field shaper and transformer and a multi-turn coil and field shaper. Metallographic investigation of the samples, hardness tests and leak tests were executed to determine the most suitable machine set-up and the optimal input parameters for each set-up. A second major goal is to determine the influence of the target tube wall thickness on the deformation of tube-tube welds when no internal support is used

The influence of material anisotropy and spiral welding on tensile strain capacity of spiral welded pipes

The longitudinal strain capacity of spiral welded pipelines displays to some extents unexplained behaviour. Therefore, they are not (yet) used extensively in offshore applications and harsh conditions, demanding a strain based design. An important factor that influences the tensile strain capacity is the quantity of anisotropy in terms of strength and toughness. Starting from an anisotropic hot rolled highstrength steel skelp, the process of helical forming and post-treating of the pipe adds heterogeneity and changes the level of anisotropy of the product. A parameter that should be examined with respect to anisotropy is the crack driving force, a measure for the toughness of the pipeline steel. Additional to the mode I loading (opening of the crack), the mode III component drives the in-plane shear motion of a crack in the spiral weld when the pipe is subjected to longitudinal deformation. This action, not present in longitudinal welded pipes, shows a decreasing contribution with increasing plasticity. FE simulations have demonstrated a rise of crack driving force in anisotropic cases with respect to an isotropic reference. However, exact data and variation of various parameters, along with experimental testing need to be conducted. The outcome analysis of such simulations and tests can validate existing models, or help create a better understanding of anisotropic and heterogenic influences on the tensile strain capacity of spiral welded pipes

Low temperature tensile properties of line pipe steels

Given the expected increase in Arctic oil and gas exploitation, there is a demand for high-strength line pipe steels able to cope with the Arctic climate. The state-of-the-art of the tensile properties of API 5L steels at low temperatures is reviewed and discussed. Well-known characteristics such as an increase in strength and Young’s modulus with decreasing temperatures are confirmed. The Y/T ratio is fairly unaffected by changes in temperature. Lüders elongation manifests itself at low temperatures where the Lüders plateau tends to increase. Conflicting statements about the relation between ductility and temperature were found. Altogether, quantifiable test results are scarce, especially for the high strength grades from API 5L X90 grade onwards. The urgent need for more tensile strength and ductility data of these steels at low temperatures is stated and defended

Sensitivity study of crack driving force predictions in heterogeneous welds using Vickers hardness maps

Weld flaws often require an engineering critical assessment (ECA) to judge on the necessity for weld repair. ECA is a fracture mechanics based prediction of the integrity of welds under operating conditions. Adding to the complexity of an ECA is the occurrence of local constitutive property variations in the weldment (‘weld heterogeneity’). Their quantification is important to allow for an accurate assessment. Hereto, hardness measurements are widely adopted given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of different hardness transfer functions and additionally recognize substantial scatter in predictions of strength. Within this context, this paper investigates the suitability of hardness mapping to perform an accurate weld ECA. A finite element analysis has been conducted on welds originating from steel pipelines to simulate their crack driving force response using single-edge notched tension (SE(T)) specimens. Vickers hardness maps and hardness transfer functions are combined to assign element-specific constitutive properties to the model. The resulting crack driving force curves are probed against experimental results. The variable agreement between simulations and experiments highlights the need for further research into the characterization of local constitutive properties of heterogeneous welds. A hardness transfer procedure based on all weld metal tensile testing appears to be particularly promising

Strain based design considerations for spiral welded pipelines

Pipelines are constructed in hostile environments where the occurrence of imposed plastic deformations can necessitate a strain based design approach. Under such conditions not only the strength and toughness properties have to be considered; also the strain capacity of pipe and weld metal become crucial. Considering the use of spirally welded linepipe sections, the helical seam weld and anisotropic material properties pose real challenges to pipeline designers. In our work, the tensile strain capacity and defect tolerance of high strength, high toughness spiral pipes will be investigated. This paper briefly discusses the different steps in the spiral pipe manufacturing process and their influence on the mechanical properties of the pipe. The forming angle is a key parameter as it determines (a) the anisotropy in strength and toughness of the pipe steel, and (b) the orientation of possible seam weld defects. Each mechanical operation (forming, expansion) and each thermal operation (welding, coating) will affect local or global strength, toughness and ductility properties of the pipe metal. A thorough material characterization at each process step is needed for a qualitative and quantitative understanding of these effects

Crack growth around stress concentrations in pipes and tubes

Fatigue crack growth behaviour in pipes fundamentally differs from fatigue growth in shafts and flat plates. The aim of this paper is to give a better understanding of this phenomenon. In a first part of the paper, the general principles of the fracture mechanics are concisely described. The energy approach as well as the stress intensity factor (SIF) approach are explained. An analytical method, a numeric method as well as an experimental method to determine the SIF are discussed. Special attention is given to the experimental method. A theoretical model predicting the deflection of a pipe tested in a resonant bending test setup is evaluated and compared to experimental measured deflections. Several methods to measure the crack growth in a pipe during and after a fatigue bending test are discussed. In addition, an overview is given of results obtained by other authors in the field of fatigue crack growth behaviour of pipes