Development of a process envelope for friction stir welding of DH36 steel : a step change

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. However, the fundamental knowledge of the friction stir welding process in relation to steel remains relatively limited. A microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in ship and marine applications has been undertaken. In this comprehensive study, plates of 2000 x 200 x 6 mm were butt welded together at varying rotational and traverse speeds. Samples were examined microscopically and by transverse tensile tests. In addition, the work was complemented by Charpy impact testing and micro-hardness testing in various regions of the weld. The study examined a wide range of process parameters; from this, a preliminary process parameter envelope has been developed and initial process parameter sets established that produce commercially attractive excellent quality welds through a substantial increase in the conventionally recognised weld traverse speed

Defect tolerance of friction stir welds in DH36 steel

Friction stir welding of steel is in the early stages of development. The aim to commercialise this process creates a trade-off between welding time, cost and quality of the joint produced. Therefore, it becomes critical to analyse the lower quality bound of steel friction stir welds in conventional square edge butt welding configuration. Work has been undertaken to evaluate the microstructure and fatigue performance of 6 mm thick DH36 steel plates friction stir welded with sub-optimal process conditions, resulting in the development of embedded and surface breaking flaws. The defective weldments were characterised to understand the nature of the flaws and a programme of mechanical testing was undertaken (including fatigue assessment) to determine the relationship between the flaw geometry, location and weld quality. A number of characteristic flaws were identified and seen to interact with the samples’ fatigue fracture mechanisms. Samples with wormholes at the weld root produced the lowest fatigue performance. Fracture from incomplete fusion paths at the retreating side of the welds’ top surface was seen to correspond to the highest recorded fatigue lives. The work provides an insight into the complex nature of characteristic flaws in steel friction stir welds and their interaction with fatigue behaviour

Fatigue and bending behaviour of friction stir welded DH36 steel

Friction stir welding presents many advantages over conventional welding techniques; however, there is limited published data with regards to the fatigue and bending performance of friction stir welded steels. Hence, this investigation aims to evaluate friction stir welded DH36 steel subjected to these loading conditions. A comprehensive fatigue and bending programme has been implemented to assess the impact of process related features, such as weld root flaws, on the welds’ performance. Strain gauges located on the top and bottom surfaces of fatigue samples allowed the secondary bending stresses to be quantified when clamped in the fatigue test machine. Bend test samples were completed to a 180° U-bend for as-welded and ground samples. The bend testing programme demonstrated satisfactory performance of friction stir welded DH36 steel. Despite the presence of surface flaws, cracks did not propagate in bending indicating adequate levels of toughness. Fatigue performance was poor in comparison with results from similar welds however, it was found to be acceptable in terms of class recommendations for fusion welding. This lower performance was predominantly attributed to a weld root flaw. Strain gauge measurements indicated that the local stress at the weld root was up to 25% lower than the nominal stress determined prior to testing, thus artificially improving fatigue performance. Welds of good quality and refined microstructure were found, however process related flaws on the top and bottom surface emphasise the need for optimisation of the tool material and welding parameters

Numerical optimisation of laser assisted friction stir welding of structural steel

Significant progress has been made on the implementation of friction stir welding (FSW) in the industry for aluminium alloys. However, steel FSW and other high-temperature alloys is still the subject of considerable research, mainly because of the short life and high cost of the FSW tool. Different auxiliary energies have been considered as a means of optimising the FSW process and reducing the forces on the tool during the plunge and traverse stages, but numerical studies on steel are particularly limited. Building on the state-of-art, laser-assisted steel FSW has been numerically developed and analysed as a viable process amendment. Laser-assisted FSW increased the traverse speed up to 1500 mm min −1, significantly higher than conventional steel FSW. The application of laser assistance with a distance of 20 mm from the rotating tool reduced the reaction force on the tool probe tip up to 55% when compared to standard FSW

Recent developments in steel friction stir welding : Project HILDA

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. Preliminary studies have identified many positive effects on the properties of welded steel components. However, the fundamental knowledge of the process in relation to structural steel remains relatively limited, hence industrial uptake has been essentially non-existent to this date. The European-funded project HILDA, the first of its kind in terms of breadth and depth, is concerned with enhancing the understanding of the process on low alloy steel, establishing its limits in terms of the two more significant parameters which can be directly controlled, tool traverse and rotational speed, thus improving its techno-economic competitiveness to fusion welding. A detailed study investigated the effect of process parameters on the evolved microstructure. In parallel, a full programme of mechanical testing was undertaken to generate data on hardness, impact toughness and fatigue. From this, it has been established that friction stir welding of steel produces high integrity joints that exhibit excellent fatigue properties. From a simulation perspective, a local microstructural numerical model has been developed to predict the microstructural evolution within the weld zone during friction stir welding of low alloy steel. This model concentrates on predicting grain size evolution due to dynamic recrystallization with respect to tool traverse and rotational speed. Furthermore, a computational efficient local-global numerical model capable of predicting the thermal transients, stir and heat affected zone, residual stresses and distortion produced by friction stir welding of DH36 plates is presented

Evaluation of the synergistic erosion-corrosion behaviour of HVOF thermal spray coatings

The present study examines three High Velocity Oxy Fuel deposited coatings, Tungsten Carbide, Chromium Carbide and Aluminium Oxide, under slurry erosion-corrosion conditions. Coatings produced in this manner typically exhibit superior density and hardness over alternative thermal spray technologies, therefore are suitable for use in corrosive and highly erosive environments. The scope of the study concentrates on isolation of the contributing factors of erosion, corrosion and synergy through applied electrochemistry, as well as metallographic analysis to evaluate the mechanisms causing coating degradation. The aim of which is to provide comprehensive data on the performance of the mentioned coatings under erosion-corrosion in conditions representing a flowing environment. Results demonstrate the breakdown of Chromium Carbide and Aluminium Oxide coatings result in enhanced mass loss over the uncoated S355 steel. Despite this, results have shown Tungsten Carbide with a Cobalt binder to be an effective protective coating, resulting in a significant reduction in total material loss over uncoated S355 steel

Thermomechanical deformation behaviour of DH36 steel during friction stir welding by experimental validation and modelling

Friction stir welding is a solid state thermo-mechanical deformation process from which the plasticisation behaviour of the stirred material can be evaluated throuh the study of flow stress evolution. Flow stress data also supporting the development of a local microstructural numerical model have been generated. Hot compression testing of DH36 steel has been performed at a temperature range of 700ºC-1100ºC and strain rates from 10-³ s-¹ to 10² s-¹ to study the alloy's thermo-mechanical deformation behaviour in conditions which simulate the actual friction stir welding process. It has been found that the evolution of flow stress is significantly affected by the test temperature and deformation rate. The material's constitutive equation and constants have been calculated after analysis of these data. Preliminary numerical analysis results are in good agreement with experimental observations

A techno-economic evaluation of friction stir welding of DH36 steel

Friction stir welding of steel presents an array of advantages across many industrial sectors such as shipbuilding when compared to conventional fusion welding techniques. However, there seems to be very limited techno-economic assessment studies on its potential introduction in industry, and particularly in shipbuilding. A microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in ship and marine applications has been undertaken. In this comprehensive study, steel plates were butt welded together at increasing traverse speeds in order to improve the technical competitiveness of the process. Samples were examined microscopically and by traverse tensile testing, Charpy impact testing and micro-hardness testing in various regions of the weld. The study has examined a wide range of traverse speeds; from this, initial process parameter data have been established that are able to produce commercially attractive excellent quality welds through a substantial increase in the conventionally recognised welding traverse speed. In parallel, a comparative economic evaluation between friction stir welding and submerged arc welding has revealed a number of areas where the former is superior. However, the cost of the friction stir welding tool for steel has been exposed as the dominant obstacle for the wider commerical acceptance of the process on steel

Advances in friction stir welding of steel : Project HILDA

A microstructure and property evaluation of friction stir welded DH36 6mm plate has been undertaken. The study examined a wide range of process parameters and, from this, a process parameter envelope has been developed and an initial process parameter set established that gives good welding properties. Thermo-mechanical deformation studies were developed to generate flow stress regimes over a range of stain rates and temperatures and these data will support the on-going local numerical modelling development. A preliminary thermo-fluid model has been developed to predict temperature and material flow during the FSW of steel grade DH36. In this model, materials are considered as highly viscous incompressible fluid. The welded material is flowing around the rotating tool thanks to the modelling of the friction at tool/workpiece interface. In parallel, a global numerical model is being developed to predict the inherent residual stresses and distortion of FSW butt welded assemblies often in excess of 6m long plate

Cold gas dynamic spraying of metal matrix composite coatings with subsequent friction stir processing

The present study forms an initial investigation in to the development of an innovative process to apply wear resistant surface layers to a chosen substrate material. Tungsten carbide – cobalt chromium, chromium carbide – nickel chromium and aluminium oxide coatings were cold spray deposited on to AA5083 grade aluminium and subsequently friction stir processed. In order to improve the deposition efficiency of the cold spray process, coatings were co-deposited with powdered AA5083. Friction stir processing (FSP) has been used in combination with the cold spray deposited coating to produce an engineered surface layer containing evenly dispersed reinforcing particles that reflects the constituent phases of the feedstock powder. Microstructural characterisation was performed on the test specimens making use of micro-hardness testing, light optical and scanning electron microscopy with electron dispersive spectroscopy to establish the elemental composition of the processed layer. The resulting data was contrasted with as-deposited coatings (no FSP) to highlight the variation in microstructure between the two conditions. The results demonstrate that FSP has improved the dispersal of reinforcing particles within the metal matrix composite layer with the average interparticle spacing decreasing by up to 68%. The micro-hardness of friction stir processed material shows an increase of approximately 540% over the unaltered substrate and 118% increase over the as-deposited MMC layer, in the case of the tungsten carbide reinforced coating