An emerging technology in industry is additive manufacturing. Over the past many centuries, parts such as a component in an automotive assembly, or a finished good such as a toy, are created by forming a solid material in a die or by removing portions of the material using a cutting tool. Additive manufacturing uses a layering technology, typically a 3-dimensional (3D) printer. Following a proscribed template such as a CAD drawing, the printer deposits as series of layers of material to form a solid structure.
While a good deal of 3D printing uses polymer materials, additive manufacturing can also use metallic powders to create metal parts and pieces. Metals have several properties that polymers cannot replace. 3D printing processes that involve metals use a laser to scan and selectively fuse the metal powder particles, building a part layer-by-layer. This process uses a single melting temperature that fully melts the particles, or it uses a powder which is composed of materials with variable melting points that fuse on a molecular level at elevated temperatures. Understanding both processes is critical to the success of the method, whether in product development or in fabrication.
Crystallography and 3D Printing
Crystals are homogeneous, anisotropic solid-state 3-dimensional bodies whose constituent atoms, ion or molecules are periodically ordered. Most metals are crystalline in their solid state, and the crystalline structure of the manufactured part has a direct effect on its quality and function. Whether a researcher is experimenting with new formulation or in an inspection or a finished part, the ability to understand the printed part’s crystallography is an essential phase of the process. (Pardon the pun!)
X-ray diffraction (XRD) is an ideal analytical technique to examine both metallic and polymeric 3D crystalline structures. XRD is non-destructive and relatively fast. This is helpful in planning temperature treatment steps, analyzing the anisotropy of the printing process, or characterize the mechanical properties of the part. The Thermo Scientific™ ARL™ EQUINOX 100 X-Ray Diffractometer is a benchtop instrument that can be situated in a plant near the manufacturing equipment, or on a chemistry bench in a laboratory.
Understanding Anisotropy in Structured Materials
Because material parameters may change during the additive process of 3D printing—if for example a new batch of feedstock is used, or output of the laser or heating source drifts—understanding the material’s anisotropy is important. The root suffix of the word, “-tropic,” relates to direction (such as towards the direction of the sun). “iso” means equal (as in isometric), and “an” means without (as in anesthesia which means “without feeling”). In crystallographic terms, anisotropy indicates unequal physical properties along different axes. And since 3D printing involves depositing a material along an axis, understanding its anisotropy is critical knowledge.
Case Study – Examining 3D Printed Stainless Steel
A recent study of a stainless-steel part created with additive manufacturing demonstrates XRD’s utility in examining the anisotropy of such a material. An austenitic stainless-steel alloy has exceptional corrosion resistance and useful mechanical properties. The primary crystal structure of this alloy is a face-centered cubic structure that possesses “austenite,” a metallic and non-magnetic allotrope of iron or a solid solution of iron with an alloying element.
The ARL EQUINOX 100 X-Ray Diffractometer was equipped with sample holder that facilitated height adjustment and sample rotation of the 3D object. The XRD data acquisition included a qualitative and quantitative analysis. Diffraction measurements longitudinal and transversal to the printing direction revealed an anisotropy in the microstructure of the steel due to variations in the austenite content of the steel. Longitudinal measurements to the printing direction did not detect austenite, whereas transversal to the printing direction reveals 1.3 weight percent of austenite.
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