Compression and stress-strain characterization of powder particles for additive manufacturing by instrumented indentation

Additive manufacturing (AM) is a rapidly growing domain where metallic powder is often used as a precursor for creating of components with complex shapes. Mechanical properties of the new components and parts depend on the build process but also on the properties of the original powder material. Since the powder particles usually have dimensions in tens of micrometers, nanoindentation is one of the few suitable techniques for characterization of their mechanical properties. This application report shows the use of the nanoindentation technique for measuring of powder particles by compression and also for spherical indentation to obtain their stress-strain properties.

Additive manufacturing (AM) is a fast growing domain for production of mainly small series of complex components or prototypes in the medical, automotive and aerospace sectors. The AM (also called 3D printing) is often based on solidification of melted powder particles in precise areas in order to build the desired part. Many types of powders for AM are already available but more materials are constantly being developed. Rapid solidification is one of the methods for production of such powders. The rapid cooling (cooling rate of 104 K/s and more) can cause undercooling of up to ~100 °C prior to solidification, which results in formation of metastable microstructures with usually improved mechanical properties. This can include improvement of wear resistance due to refined structure, increased hardness or elastic modulus. Among suitable materials for enhancement of mechanical properties using rapid solidification belong D2 tool steel, Cobased alloys such as Stellite and Al33Cu alloys. The D2 steel is expected to increase its wear resistance whereas rapid  solidification of the Cobased alloys should increase their ductility, which is necessary for successful cladding process. Although rapid solidification of metals or metallic alloys is beneficial for improvement of mechanical properties, it is often resulting in creation of powders with diameter typically between ~30 μm and ~350 μm. Thus it is quite challenging to characterize the mechanical properties of these newly created compounds. The nanoindentation technique is a suitable tool for testing of such small particles. This technique allows for testing of the particles in two main modes:
▪ compression (standalone particles) by a flat punch
▪ indentation (particles embedded in resin and polished)
Instrumented indentation is also one of the few techniques that are able to extract the stress-strain properties of such small particles. For this, a spherical indenter has to be used and the evaluation is generally done using the Tabor’s formulae.

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