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 microstructures via the deposition of semi-solid sprayed droplets onto a shaped substrate. In spray forming, an alloy is melted, normally in an induction furnace, and 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.
The gas atomised spray forming (GASF) process typically has a molten alloy flow rate of 1–20 kg/min, although twin-atomizer systems can achieve metal flow rates of up to 80 kg/min. Special steel billets of 1 tonne or more have been produced by spray forming on a commercial basis, together with nickel super-alloy ring blanks of up to 500 kg and aluminium alloy extrusion billets of up to 400 kg.
A Double Spray Forming process, developed recently, can deposite two types of alloys simultaneously under a protective high-purity argon atmosphere which allows production of in-situ layered composites. This method allows direct layer by layer deposition of different metallic materials (layer 1 as hard material and layer 2 as a soft one) from two opposite jets with good interlayer adhesion without formation of internal oxides .
History 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-engines; aluminium-titanium, aluminium-neodymium, and aluminium-silver sputter targets; aluminium-silicon alloys for cylinder liners; and high-speed steels. The history of spray forming 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.
Advantages Spray forming offers certain advantages over both conventional ingot metallurgy and more specialized 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, such as Al-5wt% lithium alloys or Al-SiC and 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, provides a large number of nucleants for the residual liquid fraction of the spray-formed material on the billet's 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 or secondary precipitate phases, and tolerance to impurity "tramp" elements. This fine structure in the "as-sprayed" condition means that homogenising heat treatments can often be avoided. Because of the complex solidification path (that is, 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.
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, porosity, 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 (HIP) 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 niche, difficult-to-manufacture 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 that react with dissolved and entrapped gas to form a solid phase, such as 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).
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, 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 superconductors, 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-millimetre-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 above the alloy's liquidus. Alternatively, a pre-prepared plug of alloy to block the nozzle is used, and at a specified superheat 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 affect 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.
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 tension 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 K/s. Centrifugal atomisation is generally conducted under an inert atmosphere of argon or N2 to prevent oxidation of the fine droplets or can be operated under vacuum.
Gas atomisation