Laser sintering (LS) is a fabrication process in the powder bed fusion family of additive manufacturing. LS is commonly used to fabricate polymer, metal, and composite components. The most commonly used polymer, Polyamide 12 (PA 12, Nylon 12), is a semicrystalline thermoplastic with two main crystalline phases, α and γ. The thermophysical properties of Nylon 12 make it ideal for LS.Laser sintering offers a great deal of design flexibility beyond what is possible with traditional manufacturing operations. The design flexibility of LS, however, comes at the cost of inferior material properties. Although laser sintered PA 12 has similar strength to injection molded PA 12, its ductility is substantially reduced. Post-processing may be a way to improve ductility in PA 12 parts made by laser sintering. This paper considers the application of hot isostatic pressure and heat treatments individually or in combination for altering the final properties of the parts.&nbsp;Hot isostatic pressing (HIP) is a post-processing method commonly used to remove porosity in cast metal and powder metallurgy parts. HIP involves heating the material and applying pressure to remove pores, producing parts with nearly 100% density, as well as improved tensile and fatigue properties. A series of modified HIP treatments and annealing and cooling cycles were designed to enhance mechanical properties in PA 12 parts through changes in the density, crystalline phase, and crystallite size.HIP parameters (temperature, pressure, exposure time) were determined experimentally to maximize part density or minimize percent α phase crystallinity. Parts were heated near their melting temperature before high pressure was applied by nitrogen gas. Tensile bars were HIP treated, annealed, and quenched in a custom HIP vessel. Samples were cooled in the furnace or quenched in either an ice water bath or oil. Samples were measured for density, porosity, crystalline phase, and tensile properties before and after treatment. Density was measured according to Archimedes principle. Porosity was observed and calculated in 10-micron thick microtome sections under a microscope. Crystalline phase was determined by X-ray diffraction of virgin PA 12 powder and processed LS samples. Finally, each sample was subjected to destructive tensile testing to determine ultimate tensile strength and strain at break.With all treatments applied, microtome images show that HIP drastically reduces porosity, with a corresponding increase in density. However, pores reopen, and density decreases when samples are reheated. Any heat treatment reduces the amount of the brittle α phase in the crystalline matrix, but this does not correlate directly with an increase in ductility. HIP treated samples increase in tensile strength but decrease in elongation at break. Annealing near the crystal melting temperature causes HIP treated samples to revert to initial mechanical properties. The more quickly a sample cools after HIP or annealing, the more ductile it tends to be. Post-processing can produce PA 12 parts with a wide range of properties, which could be tailored according to end-use application.

Post-Processing and Material Properties of Nylon 12 Prepared by Laser-Powder Bed Fusion

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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