Metal Sprays and Spray Deposition

Near net shape processing or net shape processing has been and continues to be a pursuit of the Materials Science and engineering community. Net shape processing is a type of manufacturing that produces a product that does not require any further treatment. Near net shape processing is similar except that minor treatment of the product is considered necessary. There are many motivations for developing such routes. Processing metallic and metallic based composite products are capital intensive operations; thus any process that generates a product closer to its final form using less processing steps will require less capital equipment and result in reduced capital investments. Concomitant with the reduction in process steps is the requirement that superior product performance and properties be achieved while reducing the waste generated in processing the part. It is desired to process complex shaped parts with significant throughput and the ability to apply automation in processing. This increases the reliability of products while achieving high volume production. An additional advantage of these processing routes is that they are considered to be green processes.

Atomization is simply defined as the breakup of a liquid into droplets. It can be achieved in many ways including spraying through nozzle, pouring on to a rotating disc, etc. Atomization practice and research usually involve materials processing in their liquid state either at or near room temperature (oil-based liquids, paint spraying, aerosol sprays, etc.) or at high temperature (metal melts). Most of the literature describing atomization mechanisms pertains to two fluid atomization in which a second fluid is applied to break up a melt stream into droplets. Two fluid atomization techniques for molten metals are described in Chap. 3.

In this chapter we review the major types of atomization process configurations: free-fall gas atomization with an unconfined melt stream and close-coupled gas atomization with controlled melt introduction to an energetic gas flow. Studies will be reported of several types of devices, termed atomization nozzles, which are used to perform two-fluid atomization processes that involve the disintegration of a molten metal by interaction with a high velocity atomization gas. The resulting atomization process is a complex physical phenomena consisting of stages that start with melt stream pre-filming and distribution to the primary atomization zone, where melt sheets or ligaments form and initial droplet breakup (primary atomization) occurs by the interaction of a high density, hot melt with a high velocity (high kinetic energy, but low temperature) atomization gas, typically. Primary atomization is followed in the near-field region by secondary breakup, if a high enough gas velocity and sufficient mismatch velocity with the melt fragments are maintained to cause significant production of further droplets. Thus, the atomization processes described in this chapter essentially involve momentum and heat exchange between gas and melt, while other chapters will discuss the subsequent processes of droplet solidification, droplet-droplet or particle-droplet collisions and other spray phenomena that are important to spray deposition. Primarily, this chapter will deal with our state of understanding of melt breakup physics and the various types of gas atomization nozzles that can be used to generate an atomized molten metal spray.

This chapter will present insights into the spray evolution and spray transport process during melt atomization and sprays based on multiphase flow analysis with momentum and energy transfer. Spray processes typically involve the liquid atomization stage and the multiphase phase flow within the spray. In the present example the spray consists of a two-phase flow with melt droplets and gas (metal melt atomization for powder production or spray forming) or even a three-phase flow of solid particles, melt droplets and gas (spray processing of metal-matrix-composites). The evolution of the spray depends on a series of physical phenomena involved initiated by bulk liquid disintegration (i.e. primary atomization), breakup of primary fragments like ligaments and droplets (i.e. secondary atomization), momentum and heat exchange between gas and melt, droplet solidification, droplet-droplet or particle-droplet collisions. The gas flow dynamics, especially in a twin-fluid atomization process, is an important topic here. The physics of atomization of liquid metal into dispersed phases and subsequent spray of those dispersed phases is mainly governed by very high and rapid momentum and heat transfer between the high speed atomization gas phase and the molten metal stream. A detailed introduction is given to the fundamentals of liquid atomization, in which the up-to-date understandings in liquid jet/sheet disintegration mechanism and droplet breakup mechanism, as well as the recent progress in melt atomization and spray process modelling, are presented. The research progress on the kinetic dynamics and thermal dynamics of dispersed phases in spray process, as well as the conclusions from the investigations of droplet-droplet or particle-droplet collision process will be given. At last, a multiscale description of particle-droplet interactions in spray processing of metal-matrix-composite (MMC) particles is described, and thereby the optimized operation condition and spray configuration for the maximum production efficiency of MMC particles in spray processes can be derived.

Chapter 5 will review the dynamics of both single droplets and sprays of molten metals landing on solid surfaces. Droplets landing on a solid substrate flatten out and solidify; the splats may either be disc shaped or fragmented, depending on impact conditions. Multiple droplets impacting on a surface fuse with each other to form a solid layer. The dynamics of single droplet impact and solidification are discussed. The thermal contact resistance between the droplet and substrate on the solidification rate is important in determining the solidification rate and the splat shape. An overview is given of numerical models to simulate droplet impact and solidification. The impact and coalescence of multiple droplets in a spray to form a solid layer is described. Monte Carlo and smoothed particle hydrodynamics methods can be used to simulate droplet impact and coalescence in a spray and predict properties such as coating thickness and porosity.

The structure and material properties of spray formed products depend directly on the thermal state of the semi-solid droplets before their impact, of the substrate and of the already deposited layer. Monitoring specific droplet properties as i.e. droplet temperature, velocity and size as well as mass and enthalpy fluxes provide a unique tool for optimizing the material properties as well as controlling spraying conditions during deposition (as sketched in Fig. 6.1).

Spray forming is a casting process in which the molten metal is directly converted to a solid bulk with unique characteristics. When processed under optimum conditions, spray formed materials typically present microstructures composed of refined polygonal (non-dendritic) grains, uniformly distributed with low levels of micro- and macro-segregation. This set of characteristics is achieved regardless of the alloy system, making spray forming an attractive process to produce alloys where processing by conventional casting techniques is complicated. This chapter is dedicated to presenting the mechanisms that take place when the atomized droplets arrive at the deposit surface, and how the spray-formed microstructures evolve during deposition. It will be seen that spray forming is a self grain-refining casting process and cannot be considered a rapid solidification technique. Section 7.6 will address the main differences between the microstructural evolution in spray forming and other spray deposition or “thermal spray” processes. These processes include plasma spraying, high velocity oxy-fuel, wire arc spraying, detonation gun spraying, etc. In this way, it will show why spray forming is such a unique process. This chapter is also dedicated to presenting how the porosity—an intrinsic feature of spray-formed microstructures—is generated and how the processing parameters affect its type, size and distribution. Furthermore, the generation of other defects related to the solidification and/or to the cooling of the spray formed product after deposition—such as residual stresses and hot cracks—and their influence on the product quality and material properties will be presented. Finally, this chapter will also discuss the effect of the atomization gas (Ar, N₂ or He) on the final product quality in terms of porosity and chemical composition of steels, superalloys, and copper alloys.

Several atomization techniques have been used to spray form a bulk material. Detailed information about those techniques is described in Chapter 2. This chapter focuses on differences in process conditions in spray forming using single fluid and gas atomization as well. Specially, the conditions (e.g. mass flux and enthalpy distribution) in the spray cone of a free fall atomizer are reported in detail. Furthermore, the deposition process is described including topics like overspray, yield, sticking efficiency and temperature history of the deposit. These are important issues with relevance to processes similar to spray forming, such as current thrusts in Additive Manufaturing (AM).

Spray-formed materials are characterized by fine and homogeneous microstructures and typically show much better hot workability than conventional materials. On the other hand, porosity is commonly present in spray-formed materials, and in the worst case cracks and significant segregation may appear. Over the past two decades, a considerable amount of research has been devoted towards characterization of spray-formed materials. A variety of techniques have been used to investigate them with regard to material homogeneity, porosity, grain structure, phases, solid solubility, inclusions and hot workability. For example, segregation has been investigated by means of SOES/GDOES on the macro scale and by means of electron microprobe analysis on the micro scale. Porosity has been measured and studied by means of the Archimedes’ method, optical microscopy and image analysis techniques. Microstructures have been examined and evaluated by means of optical microscopy, XRD, SEM + EDX, etc. Size, shape, and distribution of precipitates/carbides have been quantitatively evaluated by means of image analysis. Deformation behavior of spray-formed materials has been studied by means of compression tests. In addition, soundness has been analyzed by color penetration tests. This chapter will focus on the characterization of spray-formed materials in the last two decades. The most effective and widely used techniques for the material characterization will be described, and the representative results of the investigations will be presented.

As mentioned in Chap. 1, Prof. Singer claimed in 1990 four mean targets to make spray forming a success story, which are valid for aluminium alloys, too [90]. These four targets are:

Spray forming as a process innovation in the copper industry has opened the door to several cutting-edge technologies for copper alloys. Indeed, spray-formed copper alloys have become “mature” materials within the last two decades and have seen industrial applications in several major key production technologies of the twenty first century. In several fields they have become competitors to classical engineering materials such as steel due to their homogeneous and tailored microstructure allowing production of complex alloy systems with a good combination of strength, ductility, workability and physical properties.

This contribution presents an overview on production, microstructure, properties, applications and quality-control of industrially relevant spray-formed copper alloys. In particular, spray-formed tin bronzes as pre-materials for low-temperature superconductors, copper-manganese-nickel for the oil drilling industry, high strength aluminium bronzes as cold working tools and finally copper-nickel-silicon as a replacement for copper-beryllium are discussed in detail. Furthermore, the potential of modified spray forming (e.g. reactive spraying, injection of a second component) as a shaping method for copper-containing composite materials is outlined briefly. Finally, the effect of process parameters during spray forming is discussed and typical quality issues such as cracks, porosity, and segregation are taken into account and assessed critically.

Spray forming of a wide range of steels and iron based alloys have been investigated since the 1970s. These range from low-alloy carbon steels to high-carbon, high-alloy tool steels. The preform types include round billets, flat deposits, tubular preforms, clad structures, gradient deposits, and molds/dies. While the size of the deposits produced in pilot-scale plants is typically less than 100 kg, the industrial plants are in some cases capable of producing preforms with weight up to several tons. Microstructure and properties of the spray formed steels are usually far superior to those of cast material, typically resembling those of the equivalent powder metallurgy steels. The main advantage of spray forming over powder metallurgy route is the possibility to eliminate powder handling steps. This not only minimizes the risk of contamination but also results in cost savings.

Superalloy parts are conventionally manufactured using either cast, cast & wrought, or powder metallurgy processing. When cast, superalloys are usually precision-cast to final shape using the lost wax investment casting process. In the cast & wrought process, an ingot is cast and remelted, then converted into a forging billet via thermomechanical processing. In the powder metallurgy process, a billet is prepared by gas atomizing liquid metal to form powder that is consolidated into a fully dense solid. Spray forming offers a high-production-rate alternative that bypasses many of the steps associated with both cast & wrought and powder metallurgy processes.

Spray forming (SF) can be classified as a three-stage manufacturing process where liquid is disintegrated into a spray of small droplets, droplets solidify in the spray under a relatively rapid solidification condition during their flight and finally ends as the spray deposit builds up on a substrate, with the remaining liquid/semi-solid droplets solidifying at considerably slower rates. Due to a high cooling rate experienced during the atomization and the special conditions of deposit build up, with incoming droplets dynamically refining the solidifying material, as-sprayed deposits typically display a fine-scale microstructure, which may also exhibit some extended solid solubility and metastable phases. In the last few decades, a number of new materials’ classes and process routes have been developed. The world inclination towards the development of newer materials to cater to the presently stringent requirements has led to these innovations. Among them, very promising materials are the amorphous alloys and metallic glasses that show high strength as well as stiffness far above the conventional material classes of similar compositions. Despite such incremental developments, a paradigm shift has been observed in the design of new alloys with low cost alloying elements such as iron, aluminium and magnesium, instead of the costly Pd-, Zr- and La-alloy systems, and in the development of viable processing routes. However, the lower GFA of many of the alloys poses challenge on the process selection and modification. A few research works have demonstrated the development of bulk amorphous, nanocrystalline or a combination of amorphous-nanocrystalline-crystalline materials by spray forming. This chapter describes the results reported so far on spray forming of aluminum- and iron-based alloys, whose compositions are derived from rapid solidification studies aimed at obtaining amorphous structures. Due to the unique effect of the combinations of various process parameters, the processed aluminium-based glass-forming alloys show the formation of amorphous phase throughout the deposit of the Al-based alloy at high Gas to Metal ratio. This is generally not observed in the Fe-based alloys. However, some iron-based compositions displaying the highest glass-forming ability showed a high volume fraction of amorphous phase up to 4 mm thickness of the deposit. A similar value is obtained for this class of material when processed by copper mold casting. The present review, therefore, is an attempt to look into the alloy systems, their glass formability and the efficacy of spray forming in particular, to produce bulk metallic amorphous materials. The chapter also attempts to bring out the prevailing mechanisms during the development of amorphous phase in the bulk deposits, and in light of the process characteristics points out directions for future developments.