3D modelling of Ti–6Al–4V linear friction welds

Linear friction welding (LFW) is a solid-state joining process that significantly reduces manufacturing costs when fabricating Ti–6Al–4V aircraft components. This article describes the development of a novel 3D LFW process model for joining Ti–6Al–4V. Displacement histories were taken from experiments and used as modelling inputs; herein is the novelty of the approach, which resulted in decreased computational time and memory storage requirements. In general, the models captured the experimental weld phenomena and showed that the thermo-mechanically affected zone and interface temperature are reduced when the workpieces are oscillated along the shorter of the two interface contact dimensions. Moreover, the models showed that unbonded regions occur at the corners of the weld interface, which are eliminated by increasing the burn-off

Modelling of Friction Stir Welding

This thesis investigates the modelling of friction stir welding (FSW). FSW is a relatively new welding process where a rotating non-consumable tool is used to join two materials through high temperature deformation. The aim of the thesis is the development of a numerical model to improve process understanding and to assist in the design of new tools. The early part of the thesis describes the process, defines the modelling problem and describes why a computational fluid dynamics package (FLUENT) was selected for the subsequent work. A systematic series of friction stir welding experiments in 7075 aluminium alloy, used to provide validation data for a numerical model of the process, are described in chapter 2. The trials examined how the welding conditions and tool type affected the weld temperature and heat input. From this data a thermal model of the welds was developed that included the convective heat flow due to material mixing. Chapters 3 to 6 describe the model development, from a preliminary model of a standard tool, to a detailed analysis of 2 dimensional profiles incorporating a novel slip boundary condition, and finally to a full 3 dimensional model of a new tool design, including material slip. The preliminary model with a standard tool assumed that the material stuck to the tool surface and included features such as the tool tilt, heat generation and heat flow. The model captured many of the real process characteristics, but gave poor predictions of the welding forces and heat generation. This identified the need for a more complex treatment of the tool-material interface that allowed material slip. The slip model was first implemented in a 2 dimensional study of flow around profiled tooling (chapter 4). This enabled a first order visualisation of the flow and the quantitative comparison of different 2 dimensional pin profiles. In chapter 5 an optimised 2 dimensional pin profile was determined by selecting the shape that minimised the traversing force. Two prototype tools based on this profile were manufactured: the plain 'Trivex™' and the threaded 'MX-Trivex™'. These were tested against a conventional 'MX-Triflute™' tool with the results showing that the traversing force was reduced by 18-25%. Chapter 6 describes 3 dimensional models of the 'Trivex™' and 'Triflute™' tools, which extended the slip model to 3 dimensions. The model correctly predicted that the Trivex™ tool had lower traversing and down forces than its Triflute™ counterpart, as observed experimentally. The thesis successfully demonstrates the application of fluid dynamics modelling to friction stir welding, enhancing visualisation of the flow, and guiding the development of new tooling

Microstructure of interpass rolled wire + arc additive manufacturing Ti-6Al-4V components

Mechanical property anisotropy is one of the issues that are limiting the industrial adoption of additive manufacturing (AM) Ti-6Al-4V components. To improve the deposits’ microstructure, the effect of high-pressure interpass rolling was evaluated, and a flat and a profiled roller were compared. The microstructure was changed from large columnar prior beta grains that traversed the component to equiaxed grains that were between 56 and 139 μm in size. The repetitive variation in Widmanstätten alpha lamellae size was retained; however, with rolling, the overall size was reduced. A “fundamental study” was used to gain insight into the microstructural changes that occurred due to the combination of deformation and deposition. High-pressure interpass rolling can overcome many of the shortcomings of AM, potentially aiding industrial implementation of the process.EPSRC, AirBu

Investigation of low current gas tungsten arc welding using split anode calorimetry

Most previous split anode calorimetry research has applied high weld currents which exhibit pseudo Gaussian distributions of arc current and power density. In this paper we investigate low current arcs and show that both the current and power distributions have minima in the centre – varying significantly from the expected Gaussian profile. This was postulated due to the formation of the arc with the copper anode and the tungsten cathode. Furthermore, a number of parameters were varied including the step size between measurements, anode thickness and anode surface condition as well as cathode type and tip geometry. The step size between measurements significantly influenced the distribution profile and the anode thickness needed to be above 7 mm to obtain consistent results

2D linear friction weld modelling of a Ti-6Al-4V T-joint

Most examples of linear friction weld process models have focused on joining two identically shaped workpieces. This article reports on the development of a 2D model, using the DEFORM finite element package, to investigate the joining of a rectangular Ti-6Al-4V workpiece to a plate of the same material. The work focuses on how this geometry affects the material flow, thermal fields and interface contaminant removal. The results showed that the material flow and thermal fields were not even across the two workpieces. This resulted in more material expulsion being required to remove the interface contaminants from the weld line when compared to joining two identically shaped workpieces. The model also showed that the flash curves away from the weld due to the rectangular upstand "burrowing" into the base plate.Understanding these critical relationships between the geometry and process outputs is crucial for further industrial implementation of the LFW process.EPSRC, The Welding Institut

Measuring the process efficiency of controlled gas metal arc welding processes

The thermal or process efficiency in gas metal arc welding (GMAW) is a crucial input to numerical models of the process and requires the use of an accurate welding calorimeter. In this paper, the authors compare a liquid nitrogen calorimeter with an insulated box calorimeter for measuring the process efficiency of Fronius cold metal transfer, Lincoln surface tension transfer and RapidArc, Kemppi FastRoot and standard pulsed GMAW. All of the controlled dip transfer processes had a process efficiency of ∼85% when measured with the liquid nitrogen calorimeter. This value was slightly higher when welding in a groove and slightly lower for the RapidArc and pulsed GMAW. The efficiency measured with the insulated box calorimeter was slightly lower, but it had the advantage of a much smaller random err

A computationally efficient thermal modelling approach of the linear friction welding process

Numerical models used to simulate LFW rely on the modelling of the oscillations to generate heat. As a consequence, simulations are time consuming, making analysis of 3D geometries difficult. To address this, a model was developed of a Ti-6Al–4 V LFW that applied the weld heat at the interface and ignored the material deformation and expulsion which was captured by sequentially removing row of elements. The model captured the experimental trends and showed that the maximum interface temperature was achieved when a burn-off rate of between 2 and 3 mm/s occurred. Moreover, the models showed that the interface temperature is reduced when a weld is produced with a higher pressure and when the workpieces are oscillated along the shorter of the two interface dimensions. This modelling approach provides a computationally efficient foundation for subsequent residual stress modelling, which is of interest to end users of the process

Defining next-generation additive manufacturing applications for the Ministry of Defence (MoD)

“Additive Manufacturing” (AM) is an emerging, highly promising and disruptive technology which is catching the attention of the Defence sector due to the versatility it is offering. Through the combination of design freedom, technology compactness and high deposition rates, technology stakeholders can potentially exploit rapid, delocalized and flexible production. Having the capability to produce highly tailored, fully dense, potentially optimized products, on demand and next to the point of use makes this emerging and immature technology a game changer in the “Defence Support Service” (DS2) sector. Furthermore, if the technology is exploited for the Royal Navy, featured with extended and disrupted supply chains, the benefits are very promising. While most of the AM research and efforts are focusing on the manufacturing/process and design opportunities/topology optimization, this paper aims to provide a creative but educated and validated forecast on what AM can do for the Royal Navy in the future. This paper aims to define the most promising next generation Additive Manufacturing applications for the Royal Navy in the 2025 – 2035 decade. A multidisciplinary methodology has been developed to structure this exploratory applied research study. Moreover, different experts of the UK Defence Value Chain have been involved for primary research and for verification/validation purposes. While major concerns have been raised on process/product qualification and current AM capabilities, the results show that there is a strong confidence on the disruptive potential of AM to be applied in front-end of DS2 systems to support “Complex Engineering Systems” in the future. While this paper provides only next-generation AM applications for RN, substantial conceptual development work has to be carried out to define an AM based system which is able to, firstly satisfy the “spares demands” of a platform and secondly is able to perform in critical environments such as at sea

Alternative friction stir welding technology for titanium–6Al–4V propellant tanks within the space industry

Friction stir welding (FSW) offers an appealing solid state joining alternative to traditional fusion welding techniques for titanium alloys because it reduces problems associated with high temperature processing. Propellant tanks are a critical component of every spacecraft and contain several weld seams and a prime candidate for this innovative technology. This paper reviews the current technological maturity of FSW relative to titanium alloys and considers the application with respect to a pressure vessel. FSW is currently in a period of significant investment by large engineering companies and international research institutions. The technology is advancing and evolving to cater for high temperature alloys. Stationary shoulder FSW and hybrid techniques show promising potential with respect to Ti–6Al–4V. The tool material and limited process window for this material are restrictive factors at present but can be overcome with future development