Spray Simulation - Modelling and Numerical Simulation of Sprayforming Metals

Cambridge University Press
0521820987 - Spray Simulation - Modelling and Numerical Simulation of Sprayforming Metals - by Udo Fritsching
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1 Introduction

Modelling of technical production facilities, plants and processes is an integral part of engineering and process technology development, planning and construction. The successful implementation of modelling tools is strongly related to one’s understanding of the physical processes involved. Most important in the context of chemical and process technologies are momentum, heat and mass transfer during production. Projection, or scaling, of the unit operations of a complex production plant or process, from laboratory-scale or pilot-plant-scale to production-scale, based on operational models (in connection with well-known scaling-up problems) as well as abstract planning models, is a traditional but important development tool in process technology and chemical engineering. In a proper modelling approach, important features and the complex coupled behaviour of engineering processes and plants may be simulated from process and safety aspects viewpoints, as well as from economic and ecologic aspects. Model applications, in addition, allow subdivision of complex processes into single steps and enable definition of their interfaces, as well as sequential investigation of the interaction between these processes in a complex plant. From here, realization conditions and optimization potentials of a complex process or facility may be evaluated and tested. These days, in addition to classical modelling methods, increased input from mathematical models and numerical simulations based on computer tools and programs is to be found in engineering practice. The increasing importance of these techniques is reflected by their incorporation into educational programmes at universities within mechanical and chemical engineering courses.

The importance of numerical models and simulation tools is increasing dramatically. The underlying physical models are based on several input sources, ranging from empirical models to conservation equations for momentum, heat and mass transport in the form of partial differential or integro differential equations. Substantial development of modelling and simulation methods has been observed recently in academic research and development, as well as within industrial construction and optimization of processes and techniques. For the process or chemical industries, some recent examples of the successful inclusion of modelling and simulation practice in research and development may be found, for example, in Birtigh et al. (2000). This increasing importance of numerical simulation tools is directly related to three different developments, which are individually important, as is the interaction between them:

First, the potential of numerical calculation tools has increased due to the exploding power of the computer hardware currently available. Not only have individual single processor computers increased their power by orders of magnitude in short time scales, but also interaction between multiple processors in parallel machines, computer clusters or vector computers has recently raised the hardware potential dramatically.
Next, and equally important, the development of suitable sophisticated mathematical models for complex physical problems has grown tremendously. In the context of the processes to be described within this book, a variety of new complex mathematical models for the description of exchange and transport processes in single- and multiphase flows (based on experimental investigations or detailed simulations) has recently been derived.
Last, but not least, developments in efficient numerical analysis tools and numeric mathematical methods for handling and solving the huge resulting system of equations have contributed to the increasing efficiency of simulated calculations.

Based on state-of-the-art modelling and simulation tools, a successful and realistic description of relevant technical and physical processes is possible. This story of success has increased the acceptance of numerical simulation tools in almost all technical disciplines. Closely connected to traditional and modern theoretical and experimental methods, numerical simulation has become a fundamental tool for the analysis and optimization of technical processes.

The process of spray forming, which will be discussed here in terms of modelling and simulation, is basically a metallurgical process, but will be mainly described from a fundamental process technology point of view. Metal spray forming and the production of metal powders by atomization, i.e. the technical processes evaluated in this book, are fundamentally related to the disintegration of a continuous molten metal stream into a dispersed system of droplets and particles. Atomization of melts and liquids is a classical process or chemical engineering operation, whereby a liquid continuum is transformed into a spray of dispersed droplets by intrinsic (e.g. potential) or extrinsic (e.g. kinetic) energy. The main purpose of technical atomization processes is the production of an increased liquid surface and phase boundary or interfacial area between liquid and gas. All transfer processes across phase boundaries directly depend on the exchange potential, which drives the process, and the size of the exchange surface. In a dispersed system, this gas/liquid contact area is equal to the total sum of surfaces of all individual drops, i.e. of all droplets within the spray. By increasing the relative size of the phase boundary in a dispersed system, the momentum, heat and mass transfer processes are intensified between the gas and the liquid. The total exchange flux within spray systems may thereby be increased by some orders of magnitude.

Atomization techniques in process technology or chemical engineering processes/plants can be applied to:

impact-related processes, and
spray-structure-related processes.

Some examples of spray process applications in engineering following this subdivision are listed below.

Impact-related spray applications requiring a continuous fine spatial distribution of a liquid continuum, e.g. in the field of coating applications:
– for protection of metallic surfaces from corrosion in mechanical engineering through paint application;

– for coating of technical specimens/parts for use in private or industrial applications, including surface protection and colour (paint) application;

– for thermal plasma- or flame-spraying of particles to provide protective ceramic or metallic coatings in the metal industry;

– for spray granulation or coating of particles (for example in pelletizers or in fluidized beds), e.g. for pharmaceutical or food industry applications;

– for spray cooling in steel manufacturing or in spray heat treatment of metallic specimens.
Spray-structure-related applications whereby the structure or properties of liquids or particulate solids are altered by gas/dispersed phase exchange processes, within:
– thermal exchange processes, e.g. for rapid cooling and solidification of fluid metal melts in metal powder generation;

– coupled mass and thermal transfer processes, e.g. spray drying (or spray crystallization) in food or dairy industries or in chemical mass products production;

– particle separation from exhaust gases, e.g. from conventional power plants (wet scrubbing);

– reaction processes within fuel applications in energy conversion, automotive, or aeroplane or aerospace engine or fuel jet applications.
Combination of impact- and spray-structure-related spray process applications:
– within droplet-based manufacturing technologies, e.g. for rapid prototyping;

– for the generation of specimens and preforms by spray forming of metals.

In spray forming, a combination of nearly all the features, subprocesses and examples of atomization processes listed, may be found. Spray forming is, in its unique composition, an ideal and typical example of a complex technical atomization process. The numerical modelling and simulation techniques derived for analysis and description of the spray forming process may be easily transferred to other atomization and spray process applications.

In a first analysis approach, the complex coupled technical process is subdivided into single steps for further study. In the context of spray forming, subdivision of the technical atomization process into modular subprocesses can be done. This is illustrated in Figure 1.1, where the three main subprocesses discussed below are shown:

Fig. 1.1 Subdivision of an atomization process into subprocesses

atomization: the process of fluid disintegration or fragmentation, starting with the continuous delivery of the fluid or melt, and necessary supporting materials (such as gases or additives), to the resulting spray structure and droplet spectrum from the atomization process;
spray: the establishing and spreading of the spray, to be described by a dispersed multiphase flow process with momentum, heat and mass transfer in all phases, and the exchange between the phases, as well as a possible secondary disintegration process of fluid ligaments or coalescence of droplets;
impact: the impact of spray droplets onto a solid or liquid surface and the compaction and growth of the impacting fluid or melt mass, as well as the building of the remaining layer or preform.

Central, integrative and common to all spray-related subprocesses is the fluids engineering and process or chemical engineering discipline of fluid dynamics in multiphase flows involving integral heat and mass transfer. The fundamental properties and applications of this discipline are central to the theme of this book.

Based on this method of analysis, modelling and numerical simulation are introduced as scientific tools for engineering process development, as applied to metal spray forming. Then, the individual physical processes that affect spray forming are introduced and implemented into an integral numerical model for spray forming as a whole. Recent modelling and simulation results for each subprocess involved during metal spray forming are discussed and summarized. Where possible, simulated results are compared to experimental results during spray forming, to promote physical understanding of the relevant subprocesses, and are discussed under the heading ‘spray simulation’.

Numerical modelling and simulation of the individual steps involved in spray forming are presently of interest to several research groups in universities and industry worldwide. In this book, the current status of this rapidly expanding research area will be documented. But despite the emphasis given to metal spray forming processes, it is a major concern of this book to describe common analysis tools and to explain general principles that the reader may then apply to other spray modelling strategies and to other complex atomization and spray processes.

To enhance the general integral spray forming model further, additional physical submodels need to be developed and boundary conditions determined. It is hoped that the combination of experimental, theoretical and numerical analyses presented here will contribute to the derivation and formulation of such additional subprocess models. Integration of these models into a general operational model of the spray forming process will then be possible.

In conclusion, the main aims of this book are:

to introduce a general strategy for modelling and simulation of complex atomization and spray processes,
to review relevant contributions on spray form modelling and simulation, and
to analyse and discuss the physical behaviour of the spray forming process.

2 Spray forming of metals

In this chapter, fundamental features of the metal spray forming process are introduced in terms of their science and applications. Chapter 1 saw the division of the process into three main steps:

(1) disintegration (or atomization),
(2) spray establishment, and
(3) compaction.

Now, a more detailed introduction to those subprocesses that are especially important for application within the spray forming process, will be given.

2.1 The spray forming process

Spray forming is a metallurgical process that combines the main advantages of the two classical approaches to base manufacturing of sophisticated materials and preforms, i.e.:

metal casting: involving high-volume production and near-net shape forming,
powder metallurgy: involving near-net shape forming (at small volumes) to yield a homogeneous, fine-grained microstructure.

The spray forming process essentially combines atomization and spraying of a metal melt with the consolidation and compaction of the sprayed mass on a substrate. A typical technical plant sketch and systematic scheme of the spray forming process (as realized within several technical facilities and within the pilot-plant-scale facilities at the University of Bremen, which will be mainly referenced here) is illustrated in Figure 2.1. In the context of spray forming, a metallurgically prepared and premixed metal melt is distributed from the melting crucible via a tundish into the atomization area. Here, in most applications, inert gas jets with high kinetic energy impinge onto the metal stream and cause melt disintegration (twin-fluid atomization). In the resulting spray, the droplets are accelerated towards the substrate and thereby cool down and partly solidify due to intensive heat transfer to the cold atomization gas. The droplets and particles in the spray impinge onto the substrate thereby consolidating and depositing the desired product.

Fig. 2.1 Principle sketch and plant design of a spray forming process facility

The basic concept of metal spray forming was established in the late 1960s in Swansea, Wales, by Singer (1970; 1972a,b) and coworkers. In the 1970s, the spray forming process was further developed as an alternative route for the production of thin preforms directly from the melt. The spray forming process developed as a substitute for the conventional production processes of casting and subsequent hot and cold rolling of slabs (often in combination with an additional thermal energy source). The spray forming process was first used commercially by a number of Singer’s young researchers (Leatham et al., 1991; Leatham and Lawley, 1993; Leatham, 1999), who founded the company Osprey Metals in Neath, Wales. For this reason, the spray forming process is sometimes referred to as the Osprey process. Since then, worldwide interest in the physical basics and application potential of the spray forming process has spawned several research and development programmes at universities and within industries. An overview of the resulting industrial applications and the aims of industrial spray forming are given, for example, in Reichelt (1996) or Leatham (1999) and Leatham et al. (1991). Also, the actual position and application potential of spray forming are reviewed, for example, in Lawley (2000). In almost all metallurgical areas, spray forming has been, or is aimed to be, applied because of its unique features and potentials. It is referred to as one of the key technologies for future industrial applications. For example, in the aluminium industry, spray forming was recently identified as the ‘highest priority’ research area (The Aluminium Association, 1997).

Fig. 2.2 Spray forming of different preform shapes (Fritsching et al., 1994a)

In technical applications, nowadays, different preform shapes of several materials and alloys are produced via spray forming, such as:

conventional metallic materials and alloys;
materials and alloys that tend to segregate within conventional casting processes and are therefore complicated to handle, such as some alloys cast on an aluminium, copper or iron basis;
super alloys (e.g. on a nickel base) for applications in aeroplane and aerospace industries;
intermetallic composite materials (IMCs);
metal-matrix-composite materials (MMCs), e.g. ceramic particle inclusions in a metal matrix.

The main geometries of spray formed preforms and materials produced at present are summarized in Figure 2.2. These include the following:

Flat products that are formed by distributing the spray over a specific area of a flat substrate that is moved linearly. The spray is distributed either by oscillating (or scanning) the atomizer or is based on atomization of the melt within so-called linear nozzles, where the melt flow exits from the tundish via an elongated slit. Flat products are of interest for several technical applications, but, until now, problems associated with process reproducibility and control in flat product spray forming have meant that the industrial application of this type of spray forming is not always practicable.
Tubes or rings are spray formed by applying the metal spray in a stationary or scanning movement onto a cylindrical or tube-shaped substrate. The inner material (base tube) may either be drilled off afterwards or, for realization of a twin-layered clad product or material (combining different properties), remain in the spray formed preform. Spray formed tubular products are used in chemical industrial applications, while spray formed rings of larger dimension may be used, for example, in aeroengine or powerplant turbines.
Cylindrical billets are at present the most successful industrially realized spray formed geometries. Such billets are used, for example, as near-net shaped preforms for subsequent extrusion processes, where the advanced material properties of spray formed preforms are needed to produce high-value products. For spray forming of a billet (as shown in the figure), the spray is applied at an inclination or spray angle, mostly not directed onto the centre of the preform, but off-axis onto a rotating cylindrical substrate. The substrate is moved linearly (in most applications the substrate is moved vertically downwards, but attempts have also been made to spray billets horizontally), thereby maintaining a constant distance between the atomization and spray compaction area. To increase the mass flow rate and also to control the heat distribution in the billet properly during spray forming, multiple atomizers are used simultaneously in some applications (Bauckhage, 1997). In industrial applications, spray forming of billets in large dimensions of up to half a metre in diameter and above two metres in height, is typical.

The main advantage of spray formed materials and preforms compared to conventionally produced materials is their outstanding material properties. These can be summarized as follows:

absence of macrosegregations;
homogeneous, globular microstructure;
increased yield strength;
decreased oxygen contamination;
good hot workability and deformability.

Literature on spray forming fundamentals and applications, and spray formed material properties, can be found in a number of specialist publications, as well as in general review journals and special conference proceedings. Material-related specific, expected or realized advantages, and properties of spray formed materials and preforms, from a research and industrial point of view, are frequently reported at the International Conferences on Spray Forming (ICSF; Wood, 1993, 1997, 1999; Leatham et al., 1991). Also, the collaborative research group on spray forming at the University of Bremen edits a periodical publication, Koll. SFB, on research and application results of spray forming (Bauckhage and Uhlenwinkel, 1996a, 1997, 1998, 1991, 2001). In the latter, the main advantages of the different material groups produced by spray forming have been discussed:

for copper-based alloys see Müller (1996), Hansmann and Müller (1999), as well as Jordan and Harig (1998) and Lee et al. (1999);
for aluminium materials see Hummert (1996, 1999), Rückert and Stöcker (1999) and Kozarek et al. (1996);
for iron-based alloys and steels see Wünnenberg (1996), Spiegelhauer and coworkers (1996, 1998) and Tsao and Grant (1999), as well as Tinscher et al. (1999);
for titanium and ceramic-free super alloys see Müller et al. (1996) and Gerling et al. (1999, 2002);
for composite materials and metal-matrix-composites (MMCs) see Lavernia (1996); and
for lightweight materials (e.g. magnesium) see Ebert et al. (1997, 1998).

Presentation and exchange of scientific ideas and results of the spray forming process and the related process of thermal spraying was combined for the first time at the International Conferences on Spray Deposition and Melt atomization (SDMA), which took place in Bremen in 2000 and 2003 (Bauckhage et al., 2000, 2003). Besides review papers, several papers looking specifically at microstructure and material properties, melt atomization, technical synergies from thermal spraying, process diagnostics and process analysis, new process developments, and modelling and simulation of spray forming, were presented. Select papers from these conferences have also been published in special volumes of the International Journal of Materials Science and Engineering A (Fritsching et al., 2002).

2.2 Division of spray forming into subprocesses

From a chemical engineering and process technology viewpoint spray forming needs to be divided into a number of subprocesses. The first step is the division into the three main categories described in Chapter 1, each of which can, in turn, be further disseminated to derive requisite individual processes and process steps. A possible subdivision of the whole spray forming process is illustrated in Figure 2.3.

Fig. 2.3. Subdivision of the spray forming process into subprocesses

These subprocesses and their tasks and descriptions, as well as the aims of their modelling and numerical simulation in the frame of the integral spray forming process are described below.

(1) Melt delivery in a conventional spray forming process plant is realized either directly from the melting crucible (by a bottom pouring device, e.g. by control with a stopper rod) or more frequently by pouring the melt from the melting crucible into a tundish and from this through the melt nozzle towards the atomization area. Process instabilities caused by possible freezing of the melt in the narrow passage through the melt nozzle tip, especially in the transient starting phase of the process, are well known and feared by all industrial users of spray forming. Several attempts to improve the stability of the melt exiting from the nozzle have been made. Description and analysis of the transient melt, in terms of its temperature distribution within the tundish and its velocity profile within the nozzle help to develop suitable process operation strategies and thereby to prevent nozzle clogging.