Spray forming
Encyclopedia
Spray forming, also known as spray casting, spray deposition and in-situ compaction, is a method of casting near net shape
metal components with homogeneous microstructure
s via the deposition
of semi-solid sprayed droplets onto a shaped substrate. In spray forming an alloy
is melted, normally in an induction furnace
, then the molten metal is slowly poured through a conical tundish
into a small-bore ceramic
nozzle
. The molten metal exits the furnace as a thin free-falling stream and is broken up into droplets by an annular array of gas jets, and these droplets then proceed downwards, accelerated by the gas jets to impact onto a substrate. The process is arranged such that the droplets strike the substrate whilst in the semi-solid condition, this provides sufficient liquid fraction to 'stick' the solid fraction together. Deposition continues, gradually building up a spray formed billet
of metal on the substrate.
Professor Singer at the Swansea University
first developed the idea of gas atomised spray forming in the 1970s in which a high pressure gas jet impinges on a stable melt stream to cause atomisation. The resulting droplets are then collected on a target, which can be manipulated within the spray and used to form a near-dense billet of near-net shape. Spray forming has found applications in specialist industries such as: stainless steel
cladding
of incinerator tubes; nickel
superalloy
discs and rings for aerospace
-engine
s; aluminium
-titanium
, aluminium-neodymium
and aluminium-silver
sputter targets; aluminium-silicon
alloys for cylinder liners; and high speed steel
s. The history of spray forming
of how spray forming then developed is an example of how the creative contributions of many researchers were necessary over a number of years to produce the innovation of a now widely-used industrial process.
The gas atomised spray forming (GASF) process typically has a molten alloy flow rate of 1–20 kg/min-1, although twin atomizer systems can achieve metal flow rates of up to 80 kg/min-1. Special steel billets of 1 tonne or more have been produced by spray forming on a commercial basis, together with Ni superalloy ring blanks of up to 500 kg and Al alloy extrusion billets of up to 400 kg.
metallurgy
and more specialised techniques such as powder metallurgy
. Firstly, it is a flexible process and can be used to manufacture a wide range of materials, some of which are difficult to produce by other methods, e.g. Al-5wt% Li alloys or Al-SiC, Al-Al2O3 metal matrix composites (MMCs). The atomisation of the melt stream into droplets of 10-500 µm diameter, some of which, depending on diameter, cool quickly to the solid and semi-solid state provide a large number of nucleants for the residual liquid fraction of the spray formed material on the billet top surface. The combination of rapid cooling in the spray and the generation of a large population of solid nucleants in the impacting spray leads to a fine equiaxed microstructure, typically in the range 10–100 µm, with low levels and short length scales of internal solute partitioning. These microstructural aspects offer advantages in material strength because of fine grain size, refined distribution of dispersoid and/or secondary precipitate
phases, as well as tolerance to impurity ‘tramp’ elements. This fine structure in the ‘as sprayed’ condition means homogenising heat treatment
s can often be avoided. Because of the complex solidification path (i.e. the rapid transition from superheated melt to solid, liquid or semi-solid droplet to temperature equilibration at semi-solid billet top and final slow cooling to fully solid) of the spray formed material, extended solubility of alloying elements and the formation of metastable and quasi-crystalline phases has also been reported.
One of the major attractions of spray forming is the potential economic benefit to be gained from reducing the number of process steps between melt
and finished product. Spray forming can be used to produce strip, tube, ring, clad bar / roll and cylindrical extrusion feed stock products, in each case with a relatively fine-scale microstructure even in large cross-sections. The benefits of GASF over powder metallurgy accrue from the reduced number of process steps where powder sieving, pressing, de-gassing and handling steps and their attendant safety and contamination issues may be removed.
or deposition rate for a given alloy. Much of the control is based on operator experience and empirical relationships. It is partly the process complexity and lack of robust process control that has prevented the widespread commercialisation of this process. Some developments using feed-back control have proved successful in improving the variations in billet diameter and improving yield in specific systems but these have yet to find widespread implementation.
Porosity resulting from gas entrapment and solidification shrinkage is a significant problem in spray formed materials. A typical spray formed billet will contain 1-2% porosity with a pore size dependent on alloy freezing range and various process parameters. Hot isostatic pressing
(HIPing) or thermo-mechanical processing can heal these pores if they are small (less than 30 µm). Despite these disadvantages, spray forming remains an economic process for the production of difficult to manufacture, niche alloys. Large-scale porosity is more difficult to heal effectively and must be minimised by careful process control. In some cases, porosity is controlled by alloy additions which react with dissolved and entrapped gas to form a solid phase e.g. titanium added to copper billets to form titanium nitride
with dissolved and entrapped nitrogen gas. Porosity, even after consolidation, can limit the applications of spray formed material, for example rotating gas turbine components must have zero porosity because of the detrimental effect on high-cycle fatigue (HCF).
this plug melts allowing the contents of the furnace to drain through the nozzle. Another problem associated with bottom pour furnaces is the change in flow rate associated with the reducing metalo-static head in the crucible. In some cases, introducing an inert gas
overpressure during pouring can compensate for this effect.
An alternative approach is the tilt-pour furnace whereby an induction furnace is tilted to pour the melt into a conical tundish that in turn delivers the molten metal to the melt delivery nozzle. The tilt pour system provides the advantage that melting is decoupled from the spraying procedure so that melting problems and remedial solutions do not effect or disturb the critical set-up of the melt delivery nozzle.
In the most complex melting arrangement, used only for production of nickel superalloy turbine forging
blanks by spray forming, vacuum induction melting
, electroslag re-melting and cold hearth crucibles have been combined by GE
to control alloy impurity levels and the presence of refractory inclusions in the molten metal supply. Clean metal spray forming (CMSF) combines the electroslag refining process, cold walled induction guide and gas atomised spray forming. This approach has led to a reduction in the number of melt related defects (pores, inclusions, etc.), a finer average grain size, the ability to produce larger ingots and the ability to process a wider range of alloys.
and viscous
forces so the melt is fragmented into droplets. Droplet diameters produced by centrifugal atomisation are dependent primarily on the rotation speed, (up to 20,000 rpm) and are typically in the range 20–1000 µm with cooling rates of the order 104 Ks−1. Centrifugal atomisation is generally conducted under an inert atmosphere of Ar or N2 to prevent oxidation of the fine droplets or can be operated under vacuum
.
The atomising gas mass flow rate to molten metal mass flow rate ratio is a key parameter in controlling the droplet diameter and hence the cooling rate, billet temperature and resulting solid particle nucleant density. The gas-metal ratio (GMR) is typically in the range 1.5 to 5.5, with yield decreasing and cooling rates in the spray increasing with increasing GMR. Typically at low (1.5) GMR, yield is 75%, if the GMR is increased to 5.0 with all other parameters remaining constant, the process yield is reduced to 60%.
Scanning atomisers have been developed which allow the production of billets of up to 600 mm diameter, approximately twice the diameter possible with a static atomiser. The atomiser head is oscillated mechanically through 5 to 10° at a typical frequency of 25 Hz, to deflect the melt stream creating a spray path that is synchronised with the rotation speed of the collector plate in order to deposit a parallel-sided billet. By using programmable oscillating atomiser drives it was possible to improve the shape and shape reproducibility of spray formed deposits. It has been demonstrated that parallel sided, flat topped billets could be sprayed in a reproducible manner if the substrate rotation and atomiser oscillation frequency were synchronised and optimised for specific alloys and melt flow rates. Twin atomiser systems combine a static and scanning atomiser, making it possible to spray billets of up to 450 mm diameter with economic benefits.
Atomising gas used in spray forming is generally either N2 and can be either protective or reactive depending on the alloy system, or Ar which is generally entirely inert but more expensive than N2. Reactive gasses can be introduced in small quantities to the atomising gas to create dispersion strengthened alloys e.g. 0.5–10% O2 in N2 used to generate oxide dispersion strengthened (ODS) Al alloys. Comparisons of N2 and Ar based spray forming showed that with all other factors remaining constant, the billet top temperature was lower with N2 than with Ar, because of the differences in thermal diffusivity
of the two atomising gases: Ar has a thermal conductivity of 0.0179 W/mK which is approximately a third less than N2 with a thermal conductivity of 0.026 W/mK.
The mechanisms of melt break up and atomisation have been extensively researched, showing that atomisation typically consists of 3 steps: (1) primary break up of the melt stream; (2) molten droplets and ligaments undergo secondary disintegration; (3) particles cool and solidify. Theoretical analysis of the atomisation process to predict droplet size has yielded models providing only moderate agreement with experimental data.
Investigations show that in all cases gas atomisation of molten metal yields a broad range of droplet diameters, typically in the range 10-600 µm diameter, with a mean diameter of ~100 µm. Droplet diameter governs the dynamic behaviour of the droplet in flight which in turn determines the time available for in-flight cooling which is critical in controlling the resulting billet microstructure. At a flight distance of 300–400 mm, predictions show droplet velocities of 40-90 ms−1 for droplet diameters in the range 20-150 µm respectively, compared to measured velocities of ~100 ms−1, and at distances of up to 180 mm from the atomiser, droplets were still being accelerated by the gas. Droplets cool in-flight predominantly by convection and radiation, and can experience undercooling of up to 300 °C (572 °F) prior to nucleation. Models and experimental measurements show that small droplets (<50 µm) very rapidly become fully solid prior to deposition, 50-200 µm droplets will be typically semi-solid and droplets of diameters >200 µm will be liquid at deposition. The range of droplet dynamic and thermal histories result in a billet top surface of 0.3 to 0.6 solid fraction. Not all material that impacts the surface is incorporated into the billet: some solid droplets will bounce or splash-off the billet top surface or be directed out of the deposition region by turbulent gas movement in the chamber. The proportion of droplets that impact the surface compared to the proportion that are incorporated into the billet has been termed the sticking efficiency: dependent on the geometric sticking which is a function of the spray angle relative to substrate and the thermal sticking efficiency dependent on spray and billet solid/liquid fraction.
The final phase of solidification occurs once droplets have impacted the mushy billet surface and thermal equilibration has taken place between the droplets and the billet. At this stage residual liquid is present as continuous network delineating polygonal grain boundaries, with a typical liquid fraction of 0.3 – 0.5. The cooling rates during solidification of the billet is several orders of magnitude slower than the cooling rate in the spray, at 1-20 Ks−1.
Although one of the benefits of spray forming is purportedly the ability to produce bulk material with fine scale microsegregation and little or no macrosegregation work on Al-Mg-Li-Cu alloys showed that as a consequence of the interconnected liquid in the billet there was significant macrosegregation in large spray formed wrought Al billets. The distribution of Cu, Mg and Li in, for example, Al alloy 8091 showed surprisingly pronounced macrosegregation with the variation of Cu(wt%) in a spray formed 8091 billet, ranging from approximately 1.4 at the billet centre to 1.92 seconds the billet periphery. These macrosegregation patterns were explained in terms of inverse segregation in which solute rich liquid from the billet centre is sucked back through the primary Al-rich network to feed solidification shrinkage at the billet periphery. This effect was suggested to be exacerbated by centrifugal effects from the billet rotation.
As sprayed the billet porosity is typically 1-2% with a region of higher porosity in the splat-quenched region adjacent to the substrate. The very top of the billet often shows increased porosity because the top is rapidly chilled by the atomising gas which continues to chill the billet for 10–60 seconds after spraying. There has also been little progress in understanding and quantifying the underlying physics that controls as-sprayed porosity.
In most cases, the higher porosity at the billet base and top are scalped and recycled. Ultrasonic inspection is sometimes used to determine the depth of the chill zone regions to prevent unnecessary wastage. Depending on the alloy system and the final application, the remaining bulk material is usually processed to close porosity and subjected to a range of thermo-mechanical treatments. Spray formed materials are rarely used in the as-sprayed condition and are often treated by HIPing to remove porosity. In some cases, the residual atomising gas in pores may react with alloying elements to form allegedly beneficial phases e.g. N2 reacting with titanium in nickel superalloy Rene 80 to form a dispersion of TiN.
Near net shape
Near net shape is an industrial manufacturing technique. The name implies that the initial production of the item is very close to the final shape, reducing the need for surface finishing...
metal components with homogeneous microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
s via the deposition
Deposition (Aerosol physics)
In aerosol physics, Deposition is the process by which aerosol particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: dry and wet deposition. The rate of deposition, or the deposition velocity,...
of semi-solid sprayed droplets onto a shaped substrate. In spray forming an alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
is melted, normally in an induction furnace
Induction furnace
An induction furnace is an electrical furnace in which the heat is applied by induction heating of metal. The advantage of the induction furnace is a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting...
, then the molten metal is slowly poured through a conical tundish
Tundish
The word tundish originates from a shallow wooden dish with an outlet channel, fitting into the bunghole of a tun or cask and forming a kind of funnel for filling it. These were originally used in brewing.- Plumbing :...
into a small-bore ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
. The molten metal exits the furnace as a thin free-falling stream and is broken up into droplets by an annular array of gas jets, and these droplets then proceed downwards, accelerated by the gas jets to impact onto a substrate. The process is arranged such that the droplets strike the substrate whilst in the semi-solid condition, this provides sufficient liquid fraction to 'stick' the solid fraction together. Deposition continues, gradually building up a spray formed billet
Billet
A billet is a term for living quarters to which a soldier is assigned to sleep. Historically, it referred to a private dwelling that was required to accept the soldier....
of metal on the substrate.
Professor Singer at the Swansea University
Swansea University
Swansea University is a university located in Swansea, Wales, United Kingdom. Swansea University was chartered as University College of Swansea in 1920, as the fourth college of the University of Wales. In 1996, it changed its name to the University of Wales Swansea following structural changes...
first developed the idea of gas atomised spray forming in the 1970s in which a high pressure gas jet impinges on a stable melt stream to cause atomisation. The resulting droplets are then collected on a target, which can be manipulated within the spray and used to form a near-dense billet of near-net shape. Spray forming has found applications in specialist industries such as: stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
cladding
Cladding (metalworking)
Cladding is the bonding together of dissimilar metals. It is distinct from welding or gluing as a method to fasten the metals together. Cladding is often achieved by extruding two metals through a die as well as pressing or rolling sheets together under high pressure.The United States Mint uses...
of incinerator tubes; nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
superalloy
Superalloy
A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
discs and rings for aerospace
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
-engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
s; aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
-titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, aluminium-neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
and aluminium-silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
sputter targets; aluminium-silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
alloys for cylinder liners; and high speed steel
High speed steel
High speed steelMost copyeditors today would tend to choose to style the unit adjective high-speed with a hyphen, rendering the full term as high-speed steel, and this styling is not uncommon . However, it is true that in the metalworking industries the styling high speed steel is long-established...
s. The history of spray forming
History of spray forming
Spray Forming – The Pioneering Years Spray forming has now become very attractive to the metal industry because of its versatility and its capability of producing products with properties not easily obtainable by other means...
of how spray forming then developed is an example of how the creative contributions of many researchers were necessary over a number of years to produce the innovation of a now widely-used industrial process.
The gas atomised spray forming (GASF) process typically has a molten alloy flow rate of 1–20 kg/min-1, although twin atomizer systems can achieve metal flow rates of up to 80 kg/min-1. Special steel billets of 1 tonne or more have been produced by spray forming on a commercial basis, together with Ni superalloy ring blanks of up to 500 kg and Al alloy extrusion billets of up to 400 kg.
Advantages
Spray forming offers certain advantages over both conventional ingotIngot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
and more specialised techniques such as powder metallurgy
Powder metallurgy
Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape , and then heating the compressed material in a controlled atmosphere to bond the material . The powder metallurgy process generally consists of four basic steps: powder manufacture, powder...
. Firstly, it is a flexible process and can be used to manufacture a wide range of materials, some of which are difficult to produce by other methods, e.g. Al-5wt% Li alloys or Al-SiC, Al-Al2O3 metal matrix composites (MMCs). The atomisation of the melt stream into droplets of 10-500 µm diameter, some of which, depending on diameter, cool quickly to the solid and semi-solid state provide a large number of nucleants for the residual liquid fraction of the spray formed material on the billet top surface. The combination of rapid cooling in the spray and the generation of a large population of solid nucleants in the impacting spray leads to a fine equiaxed microstructure, typically in the range 10–100 µm, with low levels and short length scales of internal solute partitioning. These microstructural aspects offer advantages in material strength because of fine grain size, refined distribution of dispersoid and/or secondary precipitate
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
phases, as well as tolerance to impurity ‘tramp’ elements. This fine structure in the ‘as sprayed’ condition means homogenising heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
s can often be avoided. Because of the complex solidification path (i.e. the rapid transition from superheated melt to solid, liquid or semi-solid droplet to temperature equilibration at semi-solid billet top and final slow cooling to fully solid) of the spray formed material, extended solubility of alloying elements and the formation of metastable and quasi-crystalline phases has also been reported.
One of the major attractions of spray forming is the potential economic benefit to be gained from reducing the number of process steps between melt
Melt
Melt can refer to:* Melting, in physics, the process of heating a solid substance to a liquid*Melt , the semi-liquid material used in steelmaking and glassblowing*Melt inclusions, a feature of igneous rock...
and finished product. Spray forming can be used to produce strip, tube, ring, clad bar / roll and cylindrical extrusion feed stock products, in each case with a relatively fine-scale microstructure even in large cross-sections. The benefits of GASF over powder metallurgy accrue from the reduced number of process steps where powder sieving, pressing, de-gassing and handling steps and their attendant safety and contamination issues may be removed.
Disadvantages
There are two major disadvantages to the gas atomisation spray forming process. The most significant disadvantage is the relatively low process yield with typical losses of ~30%. Losses occur because of overspray (droplets missing the emerging billet), splashing of material from the billet surface, and material ‘bouncing’ off the semi-solid top surface. Many operators of the spray forming process now use a particle injector system to re-inject the overspray powder, and thus recycle material that would otherwise be lost, or sell the overspray powder as a product in its own right. The second major disadvantage is one of process control. As it is essentially a free-forming process with many interdependent variables, it has proved difficult to predict the shape, porosityPorosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
or deposition rate for a given alloy. Much of the control is based on operator experience and empirical relationships. It is partly the process complexity and lack of robust process control that has prevented the widespread commercialisation of this process. Some developments using feed-back control have proved successful in improving the variations in billet diameter and improving yield in specific systems but these have yet to find widespread implementation.
Porosity resulting from gas entrapment and solidification shrinkage is a significant problem in spray formed materials. A typical spray formed billet will contain 1-2% porosity with a pore size dependent on alloy freezing range and various process parameters. Hot isostatic pressing
Hot isostatic pressing
Hot isostatic pressing is a manufacturing process used to reduce the porosity of metals and influence the density of many ceramic materials. This improves the material's mechanical properties and workability....
(HIPing) or thermo-mechanical processing can heal these pores if they are small (less than 30 µm). Despite these disadvantages, spray forming remains an economic process for the production of difficult to manufacture, niche alloys. Large-scale porosity is more difficult to heal effectively and must be minimised by careful process control. In some cases, porosity is controlled by alloy additions which react with dissolved and entrapped gas to form a solid phase e.g. titanium added to copper billets to form titanium nitride
Titanium nitride
Titanium nitride is an extremely hard ceramic material, often used as a coating on titanium alloys, steel, carbide, and aluminium components to improve the substrate's surface properties....
with dissolved and entrapped nitrogen gas. Porosity, even after consolidation, can limit the applications of spray formed material, for example rotating gas turbine components must have zero porosity because of the detrimental effect on high-cycle fatigue (HCF).
Commercialisation
In spite of the problems associated with the spray forming process there has been sustained industrial interest in spray forming over the last 35 years. Sandvik-Osprey (former Osprey Metals Ltd) of Neath, South Wales holds the patents on the process and have licensed the technology to a range of industries. There are currently approximately 25 licensees operating around the world, ranging from small research and development plants to full-scale commercial operations. Main applications are prematerial for low temperature Nb3Sn super conductors (CuSn), oil drilling equipment (high strength material CuMnNi) and for forming tools (CuAlFe with high Al-content). In all of these applications, research concerns the reconciliation of the cost disadvantages and complexity of spray forming with the demand for high performance alloys in niche applications.Melting
The earliest spray forming work was based on a resistively heated electric holding furnace. The melt then passed through a 3 mm diameter Al2O3 nozzle. However the low flow rate made a high superheat necessary to prevent solidification in the nozzle. The next generation melting procedures in spray forming applications were bottom pour induction units, which offer many benefits. In this system, the melting crucible is directly above the atomiser head with a ceramic nozzle feeding directly from the furnace to the atomiser. A stopper rod runs through the melt to the top of the pouring nozzle, the rod is withdrawn when the melt reaches the designated temperature for spraying, typically 50 to 150 °C (122 to 302 F) above the alloy's liquidus. Alternatively a pre-prepared plug of alloy to block the nozzle is used, and at a specified superheatSuperheat
Superheat is a live album by Dutch alternative rock band The Gathering, released on 25 January 2000 by Century Media. The album was recorded at Paradiso, Amsterdam, Netherlands on 16 April 1999, with the exception of "Rescue Me" & "Strange Machines", which were recorded at 013, Tilburg, Netherlands...
this plug melts allowing the contents of the furnace to drain through the nozzle. Another problem associated with bottom pour furnaces is the change in flow rate associated with the reducing metalo-static head in the crucible. In some cases, introducing an inert gas
Inert gas
An inert gas is a non-reactive gas used during chemical synthesis, chemical analysis, or preservation of reactive materials. Inert gases are selected for specific settings for which they are functionally inert since the cost of the gas and the cost of purifying the gas are usually a consideration...
overpressure during pouring can compensate for this effect.
An alternative approach is the tilt-pour furnace whereby an induction furnace is tilted to pour the melt into a conical tundish that in turn delivers the molten metal to the melt delivery nozzle. The tilt pour system provides the advantage that melting is decoupled from the spraying procedure so that melting problems and remedial solutions do not effect or disturb the critical set-up of the melt delivery nozzle.
In the most complex melting arrangement, used only for production of nickel superalloy turbine forging
Forging
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...
blanks by spray forming, vacuum induction melting
Vacuum Induction Melting
Vacuum induction melting is a process for melting metal under vacuum conditions using electromagnetic induction. It works by creating electrical eddy currents in the metal which heats the "charge" to melt it...
, electroslag re-melting and cold hearth crucibles have been combined by GE
Gê
Gê are the people who spoke Ge languages of the northern South American Caribbean coast and Brazil. In Brazil the Gê were found in Rio de Janeiro, Minas Gerais, Bahia, Piaui, Mato Grosso, Goias, Tocantins, Maranhão, and as far south as Paraguay....
to control alloy impurity levels and the presence of refractory inclusions in the molten metal supply. Clean metal spray forming (CMSF) combines the electroslag refining process, cold walled induction guide and gas atomised spray forming. This approach has led to a reduction in the number of melt related defects (pores, inclusions, etc.), a finer average grain size, the ability to produce larger ingots and the ability to process a wider range of alloys.
Atomisation
There are many different techniques for atomisation of molten metals, many of which are derived from the powder metallurgy industry and have been extensively reviewed elsewhere. There are two major atomisation techniques used in spray forming: centrifugal atomisation for the manufacture of near net shape rings and gas atomisation for the manufacture of billets, tube and strip.Centrifugal atomisation
Centrifugal atomisation involves pouring molten metal at relatively low flow rates (0.1– 2 kg/min) onto a spinning plate, dish or disc, whereby the rotation speed is sufficient to create high centrifugal forces at the periphery and overcome surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
and viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
forces so the melt is fragmented into droplets. Droplet diameters produced by centrifugal atomisation are dependent primarily on the rotation speed, (up to 20,000 rpm) and are typically in the range 20–1000 µm with cooling rates of the order 104 Ks−1. Centrifugal atomisation is generally conducted under an inert atmosphere of Ar or N2 to prevent oxidation of the fine droplets or can be operated under vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
.
Gas atomisation
The melt stream exits the melt delivery nozzle into the spray chamber. The melt stream is protected from being destabilised by the turbulent gas environment in the spray chamber by primary gas jets operating at intermediate inert gas pressure of 2 to 4 bar, the resulting gas flow is parallel to the melt stream to stabilise the melt stream. The secondary atomiser uses high velocity (250 to 350 ms−1), high-pressure (6 to 10 bar) gas jets to impinge on the melt stream to achieve atomisation. The atomiser jets are usually arranged as an annulus or as discrete jets positioned symmetrically about the melt delivery nozzle, or less commonly, arranged as a linear nozzle for the production of strip products. Typical droplet diameters follow a log-normal distribution with powder diameters up to ~600 µm with a mass median diameter of ~150 µm.The atomising gas mass flow rate to molten metal mass flow rate ratio is a key parameter in controlling the droplet diameter and hence the cooling rate, billet temperature and resulting solid particle nucleant density. The gas-metal ratio (GMR) is typically in the range 1.5 to 5.5, with yield decreasing and cooling rates in the spray increasing with increasing GMR. Typically at low (1.5) GMR, yield is 75%, if the GMR is increased to 5.0 with all other parameters remaining constant, the process yield is reduced to 60%.
Scanning atomisers have been developed which allow the production of billets of up to 600 mm diameter, approximately twice the diameter possible with a static atomiser. The atomiser head is oscillated mechanically through 5 to 10° at a typical frequency of 25 Hz, to deflect the melt stream creating a spray path that is synchronised with the rotation speed of the collector plate in order to deposit a parallel-sided billet. By using programmable oscillating atomiser drives it was possible to improve the shape and shape reproducibility of spray formed deposits. It has been demonstrated that parallel sided, flat topped billets could be sprayed in a reproducible manner if the substrate rotation and atomiser oscillation frequency were synchronised and optimised for specific alloys and melt flow rates. Twin atomiser systems combine a static and scanning atomiser, making it possible to spray billets of up to 450 mm diameter with economic benefits.
Atomising gas used in spray forming is generally either N2 and can be either protective or reactive depending on the alloy system, or Ar which is generally entirely inert but more expensive than N2. Reactive gasses can be introduced in small quantities to the atomising gas to create dispersion strengthened alloys e.g. 0.5–10% O2 in N2 used to generate oxide dispersion strengthened (ODS) Al alloys. Comparisons of N2 and Ar based spray forming showed that with all other factors remaining constant, the billet top temperature was lower with N2 than with Ar, because of the differences in thermal diffusivity
Thermal diffusivity
In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It has the SI unit of m²/s...
of the two atomising gases: Ar has a thermal conductivity of 0.0179 W/mK which is approximately a third less than N2 with a thermal conductivity of 0.026 W/mK.
The mechanisms of melt break up and atomisation have been extensively researched, showing that atomisation typically consists of 3 steps: (1) primary break up of the melt stream; (2) molten droplets and ligaments undergo secondary disintegration; (3) particles cool and solidify. Theoretical analysis of the atomisation process to predict droplet size has yielded models providing only moderate agreement with experimental data.
Investigations show that in all cases gas atomisation of molten metal yields a broad range of droplet diameters, typically in the range 10-600 µm diameter, with a mean diameter of ~100 µm. Droplet diameter governs the dynamic behaviour of the droplet in flight which in turn determines the time available for in-flight cooling which is critical in controlling the resulting billet microstructure. At a flight distance of 300–400 mm, predictions show droplet velocities of 40-90 ms−1 for droplet diameters in the range 20-150 µm respectively, compared to measured velocities of ~100 ms−1, and at distances of up to 180 mm from the atomiser, droplets were still being accelerated by the gas. Droplets cool in-flight predominantly by convection and radiation, and can experience undercooling of up to 300 °C (572 °F) prior to nucleation. Models and experimental measurements show that small droplets (<50 µm) very rapidly become fully solid prior to deposition, 50-200 µm droplets will be typically semi-solid and droplets of diameters >200 µm will be liquid at deposition. The range of droplet dynamic and thermal histories result in a billet top surface of 0.3 to 0.6 solid fraction. Not all material that impacts the surface is incorporated into the billet: some solid droplets will bounce or splash-off the billet top surface or be directed out of the deposition region by turbulent gas movement in the chamber. The proportion of droplets that impact the surface compared to the proportion that are incorporated into the billet has been termed the sticking efficiency: dependent on the geometric sticking which is a function of the spray angle relative to substrate and the thermal sticking efficiency dependent on spray and billet solid/liquid fraction.
Spray formed microstructure
During spraying it is essential to maintain a constant top surface temperature and hence maintain steady-state conditions if a billet with consistent microstructure is to be produced. At the billet surface, during spraying an enthalpy balance must be maintained where the rate of enthalpy lost (Hout) from the billet by conduction to the atomising gas and through the substrate, convection and radiation must be balanced with the rate of enthalpy input (Hin) from the droplets in the spray. There are a variety of factors that can be adjusted in order to maintain these conditions: spray height, atomiser gas pressure, melt flow rate, melt superheat and atomiser configuration, being those parameters most readily adjusted. Typically equipment such as closed circuit cameras and optical pyrometry can be used to monitor billet size/position and top surface temperature. If Hout is much greater Hin then a steady temperature is maintained at the billet top surface. The top surface should be in a mushy condition in order to promote sticking of incoming droplets and partial re-melting of solid particles. The necessary partial re-melting of solid droplets explains the absence of dendritic remnants from pre-solidified droplets in the final microstructure. If Hin is insufficient to cause significant re-melting, a ‘splat’ microstructure of layered droplets will form, typical of thermal spray processes such as vacuum plasma spraying (VPS), arc spraying and high velocity oxy-fuel. Processing maps have been produced for plasma spraying and spray forming using a steady-state heat balance in terms of the interlayer time (time between deposition events) against average deposition rate per unit area. These maps show the boundaries between banded un-fused microstructure and an equiaxed homogeneous structure.The final phase of solidification occurs once droplets have impacted the mushy billet surface and thermal equilibration has taken place between the droplets and the billet. At this stage residual liquid is present as continuous network delineating polygonal grain boundaries, with a typical liquid fraction of 0.3 – 0.5. The cooling rates during solidification of the billet is several orders of magnitude slower than the cooling rate in the spray, at 1-20 Ks−1.
Although one of the benefits of spray forming is purportedly the ability to produce bulk material with fine scale microsegregation and little or no macrosegregation work on Al-Mg-Li-Cu alloys showed that as a consequence of the interconnected liquid in the billet there was significant macrosegregation in large spray formed wrought Al billets. The distribution of Cu, Mg and Li in, for example, Al alloy 8091 showed surprisingly pronounced macrosegregation with the variation of Cu(wt%) in a spray formed 8091 billet, ranging from approximately 1.4 at the billet centre to 1.92 seconds the billet periphery. These macrosegregation patterns were explained in terms of inverse segregation in which solute rich liquid from the billet centre is sucked back through the primary Al-rich network to feed solidification shrinkage at the billet periphery. This effect was suggested to be exacerbated by centrifugal effects from the billet rotation.
As sprayed the billet porosity is typically 1-2% with a region of higher porosity in the splat-quenched region adjacent to the substrate. The very top of the billet often shows increased porosity because the top is rapidly chilled by the atomising gas which continues to chill the billet for 10–60 seconds after spraying. There has also been little progress in understanding and quantifying the underlying physics that controls as-sprayed porosity.
In most cases, the higher porosity at the billet base and top are scalped and recycled. Ultrasonic inspection is sometimes used to determine the depth of the chill zone regions to prevent unnecessary wastage. Depending on the alloy system and the final application, the remaining bulk material is usually processed to close porosity and subjected to a range of thermo-mechanical treatments. Spray formed materials are rarely used in the as-sprayed condition and are often treated by HIPing to remove porosity. In some cases, the residual atomising gas in pores may react with alloying elements to form allegedly beneficial phases e.g. N2 reacting with titanium in nickel superalloy Rene 80 to form a dispersion of TiN.