Introduction

SPRAY FORMING is a near-net-shape solidification process that takes place by capturing a spray of gas atomized metal or alloy droplets on a moving substrate. The metal flow rates are high, typically 0.2 to 2.0 kg/s, and multiple sprays produce even larger rates. On the basis of experimental studies and process modeling (Ref 1), it is known that at the moment of impact the heat content of the spray is equivalent to a solid fraction in the range 0.5 to 0.8. This is material, in the form of small droplets, that has been rapidly solidified by high velocity gas while in flight to the substrate in a few milliseconds. If the process is performed correctly, a large fraction of the sprayed material, typically 0.8 or more, is captured on the substrate where it forms a deposit with the geometry of a billet, tube, sheet/plate, or other shape with dimensions ranging from a few to hundreds of millimeters. The remaining solidification then occurs within the deposit at a much slower rate over times that correlate with the size of the deposit, ™10 to 100 s or more (Ref 1, 2). The resulting as-sprayed deposits exhibit low porosity and a homogeneous, fine-grained structure. Given appropriate control of the spray and substrate movement, deposits with well-controlled shapes can be produced (Ref 3). Thus, the technology is capable of producing shaped castings with microstructures comparable to or superior to those characteristic of wrought ingot alloys.

Grant (Ref 4) provides an overall review of all aspects of the process and Lavernia and Wu (Ref 5) give an extensive overview of both atomization and deposition, the two major processes integral to spray forming. Their monograph provides a complete coverage of publications in the area up until 1995. The reports of three international conferences on spray forming (Ref 6, 7, 8) give an account of the range of application of the process and show how the subject has evolved in recent years with increasing sophistication of process modeling.

In this article, a description is given of the spray forming process and the attendant metallurgical characteristics of the product. This description is followed by a review of the commercialization and application of spray forming illustrated in terms of several product forms. The article concludes with a detailed analysis of modeling and evolution of microstructure in spray forming.

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