Spray Forming - Disadvantages

Disadvantages

There are two major disadvantages to the gas atomisation spray forming process. The most significant disadvantage is the relatively low process yield with typical losses of ~30%. Losses occur because of overspray (droplets missing the emerging billet), splashing of material from the billet surface, and material ‘bouncing’ off the semi-solid top surface. Many operators of the spray forming process now use a particle injector system to re-inject the overspray powder, and thus recycle material that would otherwise be lost, or sell the overspray powder as a product in its own right. The second major disadvantage is one of process control. 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. Much of the control is based on operator experience and empirical relationships. It is partly the process complexity and lack of robust process control that has prevented the widespread commercialisation of this process. Some developments using feed-back control have proved successful in improving the variations in billet diameter and improving yield in specific systems but these have yet to find widespread implementation.

Porosity resulting from gas entrapment and solidification shrinkage is a significant problem in spray formed materials. A typical spray formed billet will contain 1-2% porosity with a pore size dependent on alloy freezing range and various process parameters. Hot isostatic pressing (HIPing) or thermo-mechanical processing can heal these pores if they are small (less than 30 µm). Despite these disadvantages, spray forming remains an economic process for the production of difficult to manufacture, niche alloys. Large-scale porosity is more difficult to heal effectively and must be minimised by careful process control. In some cases, porosity is controlled by alloy additions which react with dissolved and entrapped gas to form a solid phase e.g. titanium added to copper billets to form titanium nitride with dissolved and entrapped nitrogen gas. Porosity, even after consolidation, can limit the applications of spray formed material, for example rotating gas turbine components must have zero porosity because of the detrimental effect on high-cycle fatigue (HCF).