Precision spray forming gets much faster

It is now possible to spray form forging and mould die inserts to an accuracy of plusminus 0.05mm at 20 to 30 kg/minute. The process makes rapid manufacturing a tools in rapid prototyping time scales. According to Dr Yunfeng Yang, a senior researcher at VTT Technical Research Centre of Finland, Precision Sprayforming (PSF) achieves much higher deposition rates and produces more solid tools with less thermal stress than the Novarc arc spraying process described in Eureka January 2003. PSF pours molten tool steel into a tundish with an atomising nozzle in its base. The molten metal is driven out through the nozzle and atomised under gas pressure, usually nitrogen. The basic idea has been around for some time, known as the Osprey process. The breakthrough is in developing a new series of hot work alloy steels and a new ceramic moulding process to make best use of the process and achieve very high precision in the final result. The process begins by converting a CAD file to a pattern that can either be machined or made by stereolithography. Machining is quicker. The pattern is used to make a ceramic mould, onto which the metal is spray formed. Mirror finishing work and/or surface polishing may be undertaken if necessary. The rapid solidification leads to refined microstructures and nearly no segregation. The process can easily be used to make tools out of materials difficult to process using conventional casting methods, such as those containing large amounts of vanadium. (Wear life of AISI A11 tool steel containing 9.75% vanadium for example, is 3 to 5 times longer than AISI D2 containing 0.9%). As spray formed hardnesses range up to Rockwell C 57. Tensile strength is typically 25% higher and ductility 50% to 100% improved compared to traditional processes. There is no need for high temperature hardening, minimising die distortion. Die costs are reduced by 30% to 50% compared to traditional processes. As with the Novarc process, PSF offers the possibility of laying shaped cooling channels into the die inserts during manufacture, improving die function at elevated temperatures. Our photographs show the process applied to the manufacture of axes for Fiskars, which for those who don't know, is a world leading, Finnish headquartered, maker of scissors and garden tools originally established in 1649. Fiskars is one of seven partners currently engaged in developing the process. It should come as no surprise that one of the other companies interested is Nokia, which came out with 50 new models last year alone, and is looking for ways to get lead times down to no more than a week. VTT Dr Yunfeng Yang Pointers * PSF achieves similar results to the Novarc arc spraying process but achieves higher densities and improved mechanical properties * Lead times for die making can be reduced to a few days * Hardnesses of dies are up to Rockwell C 57, tensile strengths are increased by 25%, and ductility by 50 to 100% owing to rapid solidification and consequent formation of finer final grain structures