Spray forming combines the metallurgical benefits of fast cooling during solidification to produce low segregation, refined microstructures with the cost advantages associated with near net shape processing. It can offer particular advantages in manufacturing certain components, such as rings and casings for aero-engines. As aero-engine diameters increase to deliver more power, manufacturing the large diameter rings and casings that are required becomes more difficult. Spray forming offers a route to manufacture these large components more easily, and it also allows the possibility of using alloys which cannot currently be used in other manufacturing methods. It provides a route for producing components with enhanced properties and ultimately could lead to larger, more powerful aeroplanes.
Spray forming was commercialised by Osprey Metals of Neath in the 1970s and consequently is commonly referred to as the Osprey process. Holders of Osprey licenses include major Al, Cu, Fe and Ni alloy manufacturers. Production facilities at PEAK, Germany produce high Si containing Al alloys (3,500 tonnes per year) to be used as cylinder liners for Mercedes, and Sprayform Technologies International (STI), USA manufactures large diameter Ni rings (500 tonnes per year per shift) for the aerospace industry.
The spray forming process is shown schematically in figure 1. The first step is Ar or N2gas atomisation of a melt stream to produce a spray of 10-500μm droplets. These droplets are deposited at a growing sprayed preform surface translated under the spray cone, where any residual liquid in the sprayed preform cools relatively slowly and solidifies.
Figure 1. Schematic of the spray forming process.(The copyright belongs to AZO Materials)
Characteristics of Spray Formed Materials
Billet, ring and plate geometries can be manufactured, as well as particulate reinforced metal matrix composites (MMCs) by the co-deposition of a solid ceramic phase, typically 5-15μm SiC particulate. Sprayed preforms have characteristically equiaxed grains of 15-50μm diameter at all preform locations, and low- levels of micro-segregation and macro-segregation. The microstructure of the sprayed preform is crucially dependent upon the average spray solid fraction at deposition, which usually lies critically in the range 0.3 to 0.8. Sprayed preforms always contain a residual porosity of 1-3 Vol%, and require downstream processing by extrusion, hot isostatic pressing (HIPing), rolling etc to full density.
Advantages of Spray Forming
Consolidated spray formed materials have shown consistently equivalent or superior tensile, toughness, fatigue and creep behaviour in comparison with conventionally processed equivalents. The economic benefit of near net shape processing by spray forming is component sensitive but, for example, the removal of processing steps in the manufacture of large diameter Ni superalloy aero-engine rings is expected to realise savings of up to 30% compared with cast and wrought methods or powder processing.
Other Aluminium Alloys
Similar alloy development programmes have been carried out for high strength 7XXX and high temperature 2XXX series alloys. In 7XXX alloys, commercial DC cast alloys can contain up to 8 wt% Zn (eg 7449 and 7055). However, spray formed alloys can be produced with Zn contents up to 11-12%, and with higher than normal levels of Zr, such as Al-11.5Zn-2.5Mg-1.0Cu-0.2Zr. Forgings of these alloys have shown improved combinations of strength and toughness compared with conventional 7075 and 7050. 2XXX series alloys have also been developed with higher levels of Cu, Mn and other dispersoid forming elements such as Zr, V and Ti to obtain improved elevated properties compared with conventional high temperature alloy 2618. UL40 and a high Zn 7XXX series alloy, as well as other aerospace alloys are available from Osprey Metals.
Whereas a number of new Al alloys have been developed specifically for spray forming, the different market and engineering requirements of Ni alloys for aero-engine applications have focused spray forming developments on reductions in manufacturing costs for existing alloys. However, as spray forming is established progressively in niche markets such as that being developed by STI, alloy developments will be pursued for Ni superalloys.