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.
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.
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).
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.
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.
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.
A typical spray formed billet will contain 1-2% porosity with a pore size dependent on alloy freezing range and various process parameters.
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.
The next generation melting procedures in spray forming applications were bottom pour induction units, which offer many benefits.
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.
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 Ks−1.
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.
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.
Theoretical analysis of the atomisation process to predict droplet size has yielded models providing only moderate agreement with experimental data.
Typically equipment such as closed circuit cameras and optical pyrometry can be used to monitor billet size/position and top surface temperature.
The top surface should be in a mushy condition in order to promote sticking of incoming droplets and partial re-melting of solid particles.
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.
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.
The above text is substantially taken from 'Spray forming of Si-Al alloys for thermal management applications' By Dr Al Lambourne, D.Phil Thesis, 2007, Queens College.