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

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
Spray formed aluminium alloys

An 80kg Al and Mg 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 various laser based flow measurement techniques.

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

The spray chamber

Spray formed Si-Al alloys for electronic packaging applications applications

 

Alloys containing up to 70wt%Si are being manufactured by spray forming. These alloys offer a unique combination of low thermal expansion, high thermal conductivity and low density, and are under investigation for electronic package thermal management applications in the avionics, satellite and other industries. The research concerns the characterisation of the key mechanical and microstructural properties of Al-70Si; the optimisation of spray forming; and the enhancement of alloy properties by ternary alloy additions and novel variants of the spray forming process. (Funded by EPSRC and Osprey Metals Ltd). A Si-Al alloy billet

Spray formed Al-Li-Mg alloys for airframe applications

Research focuses on production and evaluation of low density Al-Mg-Li alloys by spraycasting; 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. (Funded by EPSRC and Joint Infrastructure Fund, and in collaboration with Southampton University, Imperial College, BAE Systems and QinetiQ).

Recently, 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.