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All you need to know about sintering
Thinking about metal Binder Jetting or FFF? Here is (almost) everything you need to know about sintering With the arrival of high-volume metal Binder Jet systems and a growing interest in metal Fused Filament Fabrication, the AM industry is set for a new phase of growth. The ability to use this new generation of systems for the production of ‘green’ parts is, however, only half of the story. The sintering of these parts to create large quantities of finished product to a consistent quality requires both an investment in furnaces that can cost in excess of $1 million each, and a thorough understanding of sintering. In this article, Prof Randall German, the leading authority on the science of sintering, outlines the process and its core challenges.
Sintering is a heating process applied to shaped powders to provide strength by bonding the particles into a solid. Often, but not always, densification occurs during sintering. Surface diffusion forms the first sinter bonds without causing a dimensional change. When heated to a higher temperature, grain boundary diffusion induces densification, grain growth, pore rounding, and significant property changes. The optimal sintering conditions depend on the material and particle size, realising excessive heating leads to a loss of properties and even swelling of the component. Beyond classic textbook materials, sintered materials often rely on intentional grain boundary segregants, including liquids, to retard grain growth or accelerate sintering. Within the confines of Additive Manufacturing, it is important to differentiate between the sintering process that is used to densify parts created by metal Binder Jetting and Fused Filament Fabrication, and the Laser Powder Bed Fusion of metal powders, often erroneously referred to as a variant of ‘laser sintering’, in which a very different melting process occurs.
Vol. 5 No. 3 © 2019 Inovar Communications Ltd
The term ‘sintering’ arose from geology where ‘cinder’ was used to describe hardening of mineral phases around geothermal vents. Modern uses for sintering trace to the early 1800s and the fabrication of platinum crucibles for melting glass. By the early 1900s, sintering was used to fabricate tungsten ingots
from which lamp filaments were drawn. Scientific understanding of how sintering occurred emerged as atomic diffusion became accepted in the late 1940s. Predictive models for the role of time, temperature, particle size, and green density emerged in a body of mathematical expressions by the 1980s [1,2]. Unfortunately, the
Fig. 1 Scanning electron micrograph of spherical bronze particles to illustrate sinter neck formation between contacting particles prior to significant densification
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