Joining of steel to aluminium and stainless steel to titanium for engineering applications

Dissimilar welding has been subject of several investigations due to its potential importance in various industrial fields such as transportation, energy generation and management. Dissimilar welding can increase the design efficiency, by the use of complementary alloys with different properties, cost cutting and light weighting structures. The use of different materials within a component or structure to best suit a particular task, requirement or increase its life and performance has always been an ambition of several designers and engineers. This project investigated the joining steel and aluminium for the automotive industry and also stainless steel and titanium to be applied in the civil nuclear energy generation industry. These dissimilar metallic combinations are metallurgically incompatible and the formation of brittle intermetallic phases (IMC) need to be controlled or eliminated. To join steel to Al, laser spot welding process was selected, to avoid the bulk melting of steel and Al at the joint interface that enhance the formation of brittle IMC. This part of the work was focused in controlling the joining process to control the IMC formation of galvanized and uncoated steel to Al and verify if it was possible to have a sound and reliable joint in the presence of an IMC layer. In the second part of this study, stainless steel to titanium joining, a different approach was taken with the application of weld metal engineering to modify or eliminate the IMC formation. Several metals were evaluated as potential interlayers to use and laser welding with a Ni interlayer was evaluated with moderate success, due to the modified IMC with improved mechanical properties and the good compatibility between Ni and the stainless steel. A further improvement was achieved when Cu was brazed between stainless steel and Ti using CMT (Cold Metal Transfer) a low heat input MIG process. The final attempt was to use a different interlayer that was 3D printed and deposited in several layers. This interlayer was composed Cu and Nb that were selected as candidates to avoid the IMC formation between the stainless steel and Ti. With this approach it was possible to build an IMC free component and possibly improve and avoid IMC formation in several other dissimilar metallic combinations

Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components

Laser welding-brazing technique, using a continuous wave (CW) fibre laser with 8000 W of maximum power, was applied in conduction mode to join 2 mm thick steel (XF350) to 6 mm thick aluminium (AA5083-H22), in a lap joint configuration with steel on the top. The steel surface was irradiated by the laser and the heat was conducted through the steel plate to the steel-aluminium interface, where the aluminium melts and wets the steel surface. The welded samples were defect free and the weld micrographs revealed presence of a brittle intermetallic compounds (IMC) layer resulting from reaction of Fe and Al atoms. Energy Dispersive Spectroscopy (EDS) analysis indicated the stoichiometry of the IMC as Fe2Al5 and FeAl3, the former with maximum microhardness measured of 1145 HV 0.025/10. The IMC layer thickness varied between 4 to 21 μm depending upon the laser processing parameters. The IMC layer showed an exponential growth pattern with the applied specific point energy (Esp) at a constant power density (PD). Higher PD values accelerate the IMC layer growth. The mechanical shear strength showed a narrow band of variation in all the samples (with the maximum value registered at 31.3 kN), with a marginal increase in the applied Esp. This could be explained by the fact that increasing the Esp results into an increase in the wetting and thereby the bonded area in the steel-aluminium interface

Fabrication of functionalised surfaces on gum metal (Ti-30Nb) using micromachining

Structured surfaces are attracting deep interest, as they allow tailoring the functionality via changes in the surface topography. Applications for these surfaces range greatly, including, optical surfaces for antireflective surfaces, thermal structures to assist in heat dispersion and anti-fouling surfaces to reduce micro-organisms from adhering to components. Gum metal is a relatively newer kind of beta titanium alloy that has earmarked its place as the next generation Ortheopedic implant material. In a timely effort, this work investigated the generation of micron level structured surfaces on Gum metal (Ti-30Nb – a beta titanium alloy) to explore micromilling as the robust scalable process to achieve low dimensional surfaces in titanium alloy. During micromilling, the feedrate, spindle speed, axial depth of cut and tool step over were varied to optimise these parameters for achieving superior quality of machining

High temperature performance of wire-arc additive manufactured Inconel 718

In developing a wire-arc directed energy deposition process for superalloys used in high-speed flight environments, Inconel 718 was deposited using a plasma arc process and tested for its high temperature performance. The deposited material was tested in both the as deposited condition and after an age-hardening industry standard heat-treatment for this alloy. Results showed a reduced performance in both deposited conditions, with heat-treated material significantly outperforming as deposited material up to 538 °C. The difference in performance was less significant from 760 to 1000 °C, owing to an in-test aging process which increased the performance of the as deposited material. The microstructure of deposited material showed significant cracking throughout the alloy and formation of secondary phases throughout the matrix, with significantly more precipitation after heat-treating.DST

High‑temperature failure and microstructural investigation of wire‑arc additive manufactured Rene 41

In developing a wire-arc plasma direct energy deposition process for creep-resistant alloys used in high-speed flight applications, structures were built from nickel-based superalloy Rene 41. Samples of additive manufacturing (AM) material were analysed for their microstructural and mechanical properties, in both as-deposited (AD) and heat-treated (HT) conditions. Tensile specimens were tested at room temperature, 538, 760, and 1000 °C. Macroscopically, large columnar grains made up of a typical dendritic structure were observed. Microscopically, significant segregation of heavier elements, grain boundary precipitates, and secondary phases were observed, with key differences observed in HT material. There was a clear distinction between failure modes at different testing temperatures and between AD and HT variants. A fractographic investigation found a progressive move from brittle to ductile fracture with increasing testing temperature in both AD and HT conditions, as well as microstructural features which support this observation.DST

Performance Evaluation Between HarperDB, Mongo DB and PostgreSQL

Several modern-day problems, like information overload and big data, need to deal with large amounts of data. As such, to meet the application requirements, for instance, performance and consistency, more and more systems are adapting to the specificities. The existing Relational Database Management System (RDBMS)’s the processing of massive data has become an issue because these databases do not deal with a massive amount of data. NoSQL is a database management system that makes processing massive and/or unstructured data easier because it uses key-value to store the data, collections or document stores instead of tables. Many companies today tend to start a project using NoSQL. However, HarperDB aims to produce a relational and nonrelational DBMS, allowing developers to choose between different solutions. This paper aims to show the most relevant differences between HarperDB, MongoDB and PostgreSQL and compare their performances. Preliminary results show that PostgreSQL performs better with structured data, but HarperDB can integrate NoSQL and SQL, which can be a significant advantage to HarperDB compared to the other solutions.info:eu-repo/semantics/publishedVersio

Strong piezoelectricity in single-layer graphene deposited on SiO2 grating substrates

Electromechanical response of materials is a key property for various applications ranging from actuators to sophisticated nanoelectromechanical systems. Here electromechanical properties of the single-layer graphene transferred onto SiO2 calibration grating substrates is studied via piezoresponse force microscopy and confocal Raman spectroscopy. The correlation of mechanical strains in graphene layer with the substrate morphology is established via Raman mapping. Apparent vertical piezoresponse from the single-layer graphene supported by underlying SiO2 structure is observed by piezoresponse force microscopy. The calculated vertical piezocoefficient is about 1.4 nm V-1, that is, much higher than that of the conventional piezoelectric materials such as lead zirconate titanate and comparable to that of relaxor single crystals. The observed piezoresponse and achieved strain in graphene are associated with the chemical interaction of graphene's carbon atoms with the oxygen from underlying SiO2. The results provide a basis for future applications of graphene layers for sensing, actuating and energy harvesting

Selection of processing parameters in laser microwelding. Part 1: Continuous wave (CW) mode

A phenomenological model which specifies the penetration depth and width of the fusion zone in laser microjoining can be a very useful tool in achieving the required welding parameters for a desired application. In this study the power factor model, previously established and validated in macrowelding, has been tested in fibre laser microwelding, enabling achievement of a particular weld independently of a laser system. Differ-ent weld profiles in aluminium and stainless steel were correlated with various combinations of parameters for a wide range of beam diameters. It has been shown that the same penetration depth can be achieved with different weld profiles. A similar trend, as previously found in macrow-elding, has been confirmed in microwelding. It was demonstrated that the depth of penetration can be kept constant independently of the laser sys-tem until certain limit of beam size

Efficient reduced-order thermal modelling of scanning laser melting for additive manufacturing

Additive manufacturing (AM) with a scanning laser (SL) to independently control melt pool shape has the potential to achieve part building with high geometric accuracy and complexity. An innovative dynamic convection boundary (DCB) method is proposed to develop a reduced-order finite element (FE) model to accelerate the thermal analysis of a SL process for AM. The DCB method approximates the thermal conduction of the adjacent material around the bead region by using a convection boundary condition that can be dynamically adjusted during the numerical solution. Thereby, a smaller problem domain and fewer elements are involved in the reduced-order FE modelling. A non-oscillating equivalent bar-shaped heat source was also introduced as a simplified substitution for a high oscillation frequency SL heat source. The DCB-based reduced-order thermal model achieved over 99% accuracy compared to the full-scale model but reduced the element amount by 73% and the computational time by 58%. The use of the bar-shaped equivalent heat source can further enhance computational efficiency without compromising the prediction accuracy of a high oscillation frequency SL process. The DCB-based reduced-order thermal modelling method and equivalent heat source could be adopted to boost extensive parametric analysis and optimisation for novel AM processes. Study on large structures AM could also be facilitated by simplifying the computation at critical regions. This study can also enable efficient thermal analyses of different manufacturing processes, such as welding, cladding, and marking.Engineering and Physical Sciences Research Council (EPSRC): EP/R027218/

A novel cold wire gas metal arc (CW-GMA) process for high productivity additive manufacturing

Wire-arc directed energy deposition (DED) is suitable for depositing large-scale metallic components at high deposition rates. In order to further increase productivity and efficiency by reducing overall manufacturing time, higher deposition rates are desired. However, the conventional gas metal arc (GMA) based wire-arc DED, characterised by high energy input, normally results in high remelting and reheating at relatively high deposition rates, reducing the process efficiency and deteriorating the mechanical performance. In this study, a novel wire-arc DED process with the combination of a GMA and an external cold wire, namely cold wire-gas metal arc (CW-GMA), was proposed for achieving high deposition rate and low material remelting. The maximum deposition rates at different levels of energy input were investigated, with the highest deposition rate of 14 kg/h being achieved. An industrial-scale component weighing 280 kg was built with this process at a high deposition rate of around 10 kg/h, which demonstrated the capability of the process for high productivity application. It was also found that, due to the addition of the cold wire, the remelting was reduced significantly. The working envelope and geometric process model for the CW-GMA process was developed, which can be used to avoid defects in parameter selection and predict the geometry of single-pass wall structures. Moreover, the addition of the cold wire in the CW-GMA process reduced the specific energy density, leading to a reduction in both grain size and anisotropy, which improved the mechanical properties with increased strength and reduced anisotropy.Innovate UK: 53610 Engineering and Physical Sciences Research Council (EPSRC): EP/R027218/