14. Spray Forming of Novel Materials

Spray forming (SF) can be classified as a three-stage manufacturing process where liquid is disintegrated into a spray of small droplets, droplets solidify in the spray under a relatively rapid solidification condition during their flight and finally ends as the spray deposit builds up on a substrate, with the remaining liquid/semi-solid droplets solidifying at considerably slower rates. Due to a high cooling rate experienced during the atomization and the special conditions of deposit build up, with incoming droplets dynamically refining the solidifying material, as-sprayed deposits typically display a fine-scale microstructure, which may also exhibit some extended solid solubility and metastable phases. In the last few decades, a number of new materials’ classes and process routes have been developed. The world inclination towards the development of newer materials to cater to the presently stringent requirements has led to these innovations. Among them, very promising materials are the amorphous alloys and metallic glasses that show high strength as well as stiffness far above the conventional material classes of similar compositions. Despite such incremental developments, a paradigm shift has been observed in the design of new alloys with low cost alloying elements such as iron, aluminium and magnesium, instead of the costly Pd-, Zr- and La-alloy systems, and in the development of viable processing routes. However, the lower GFA of many of the alloys poses challenge on the process selection and modification. A few research works have demonstrated the development of bulk amorphous, nanocrystalline or a combination of amorphous-nanocrystalline-crystalline materials by spray forming. This chapter describes the results reported so far on spray forming of aluminum- and iron-based alloys, whose compositions are derived from rapid solidification studies aimed at obtaining amorphous structures. Due to the unique effect of the combinations of various process parameters, the processed aluminium-based glass-forming alloys show the formation of amorphous phase throughout the deposit of the Al-based alloy at high Gas to Metal ratio. This is generally not observed in the Fe-based alloys. However, some iron-based compositions displaying the highest glass-forming ability showed a high volume fraction of amorphous phase up to 4 mm thickness of the deposit. A similar value is obtained for this class of material when processed by copper mold casting. The present review, therefore, is an attempt to look into the alloy systems, their glass formability and the efficacy of spray forming in particular, to produce bulk metallic amorphous materials. The chapter also attempts to bring out the prevailing mechanisms during the development of amorphous phase in the bulk deposits, and in light of the process characteristics points out directions for future developments.