This is not strictly a powder metallurgical process. It involves the atomisation of molten metal, but instead of being allowed to solidify as powder, the spray is collected on a substrate to form billets for subsequent forging.

Spray Deposition - Spray deposition is not a powder metallurgical process within the strict definition of that term since metal in actual powder form is not involved.

Molten metal is gas atomised in the normal way and the spray is caused to impinge while still in the liquid or semi-solid state on a solid former where a layer of dense solid metal of a pre-determined shape is produced.

The solid thus produced has a structure similar to that of powder-based material with all the attendant advantages of fine grain, freedom from macro-segregation, etc.

In common with the PM process, spray deposition facilitates the production of alloy compositions that are difficult if not impossible to produce conventionally, and in certain cases the benefits that rapid solidification offers can be obtained also.

Properties even superior to those of powder-based wrought products have been reported; for example superalloy having a much lower inclusion count than that of its powder-based equivalent.

• The range of materials that are being processed in this way is extremely wide and includes Al alloys, Cu alloys, stainless steels, high Cr alloy steels, and superalloys.
• The range of shapes is extensive also; - round billets, tubes, strip and sheet, and near-net shape pre-forms.

  Clad materials are also being produced, for example low alloy steel rolls clad with high speed steel.

• The sizes that can be produced are, naturally, a function of the available plant and they are continually rising.

  A recent installation will produce tube blanks weighing up to 4.5t.

The commercial viability of the process is markedly influenced by the yield of usable product - i.e. the proportion of the metal atomised that is deposited on the substrate.

This in turn is dependent on the design of the equipment, the spray pattern, and the co-ordinated movements of the substrate.

The amount of 'over-spray' has been progressively reduced and yields as high as 90% are being claimed.

• With conventional products such as, for example, stainless steel tubing, the benefit of spray deposition is mainly cost saving, in other cases there are significant property improvements.

  Rolls for metal rolling mills spray-deposited and HIPped have been found to have 2 or 3 times the life of cast rolls of similar composition.

• Among the materials that cannot be made conventionally, but can be made by spray deposition, are rapidly solidified Al-Li alloys, Al-Sn alloys with high Zn content (11%), highly alloyed Cu-Ni-Sn and Cu-Cr, as well as the composites referred to in a previous section.

  In this last case, the re-inforcing particles are injected into the metal stream during the atomisation process. Spray deposition seems destined to have a very interesting future.

In addition to these breakthrough developments, steady progress is being maintained in the traditional areas of powder metallurgy.

The quality of commercially available powders has been improved, die materials and die designs are better, and presses have become more efficient as well as more powerful.

mproved, and more economical sintering furnaces are now in use and better sintering atmospheres are available.

These developments have resulted both in quality, range of product, and cost competitiveness, and there is little doubt that, in addition to the many exciting developments in products that can be made only by PM, the sintering process will continue to take an increasing share of the market for traditional engineering components.