Spray forming

Spray forming, also called spray casting or spray deposition, is the inert gas atomization of a liquid metal stream into variously sized droplets (10-500 microns) that are then propelled away from the region of atomization by the fast flowing atomizing gas. The droplet trajectories are interrupted by a substrate which collects and solidifies the droplets into a coherent, near fully dense preform. By continuous movement of the substrate relative to the atomizer as deposition proceeds, large preforms can be produced in a variety of geometries including billets, tubes and strip at flow rates of 5-30kg/min.

Schematic of the spray forming process

Our thoughts on microstructural evolution in spray forming can be found at: doi:10.1007/s11661-006-9015-3

A review from 1995 of the spray forming process as a whole can be found at: doi:10.1016/0079-6425(95)00004-6 

Please see our selected journal papers for some of our other papers relating to spray forming.

Spray forming of aluminium alloys

An 80kg Al and Mg Sandvik-Osprey spray forming unit is fully operational and has been specially designed by Sandvik-Osprey, Neath, UK to allow the installation of a range of process monitoring devices such as infrared thermal imaging and laser based flow measurement techniques. The equipment is run remotely from a control room using a combination of manual control guided by CCTV and automatic computer control. Because each run generates overspray aluminium alloy powder, care is taking to control the atomosphere during spraying to prevent any oxidation.

Like the unit for Ni superalloys and speciality steels, the equipment uses the Sandvik-Osprey two stage scanning atomising technology. These atomisers are very robust and can operate for long times without melt build up on the tip or clogging - essential for steady-state atomisation, deposition and control of microstructure.

The light alloy spray forming unit is configured to produce billet geometries (below)

Billets are usually hot isostatically pressed (HIPed) or vacuum hot pressed to remove any as sprayed porosity. Further downstream processing may then be employed, such as forging, rolling or extrusion.

A general view of the 80kg Al alloy spray forming plant at Oxford.
The spray chamber

Al-Li alloys

Research has focused on production and evaluation of low density Al-Mg-Li alloys by spray forming; characterisation of microstructure; investigation of secondary processing on the development of the microstructure and the resulting mechanical properties; definition of new compositions and processing conditions for optimised alloys; and scale-up to billet sizes suitable for forging and component trials. Much of this work was funded intially by a consortiun comprising EPSRC through the Joint Infrastructure Fund, and then by EPSRC in collaboration with Southampton University, Imperial College, BAE Systems and QinetiQ.

Subsequently, a new range of Al-Li-Mg based alloys have been produced by spray forming followed by forging. These alloys have shown room temperature mechanical properties better than those achieved by a similar AA5091 alloy variant made by mechanical alloying, and approaching those of high strength aluminium alloys such as AA7010. These alloys also show superplastic behaviour that is now being investigated for aerospace and other applications.

Most recently (summer 2008) research has continued with an industrial consortium to exploit these alloys for a specific industrial application that requires high strength and low density without the requirement for complex heat treatments that are problematical in complex cross-sections.

Bulk nanostructured alloys

Current research funded by EPSRC and the Defence Science and Technology Laboratory (DSTL) is investigating the potential of spray forming (alongside conventional rapid solidification processes) to produce bulk nanostructured materials. This work builds on an earlier successful feasibility study at Oxford and is currently at an early stage. Come back later for latest news!

Ni superalloys and speciality steels

Spray forming plant featuring a 50kg vacuum induction melt furnace for Ni and Fe alloys is being used to produce a variety of ring and billet preforms. The ability to spray form a near net shape billet on a substrate or a ring directly onto a large diameter mandrel offers a reduction in the total number of process steps in comparison with conventional casting and downstream forging operations. Furthermore, in contrast to the dendritic microstructure produced by conventional casting, spray forming produces a refined, homogeneous equiaxed microstructure (left) that amongst other benefits exhibits superior hot working capabilities. It is this combination of near net shape processing and the ability to process conventionally hard to forge alloys which makes spray forming a candidate technology for use in the aerospace industry.

The Ni superalloy/speciality steel spray forming equipment
As spray formed IN718 Ni superalloy ring
As spray formed IN718 Ni superalloy billet
Typical polygonal as sprayed microstructure

Processing of Advanced Materials