Many manufacturers are now looking at metal powder-based additive manufacturing (AM) as a realistic alternative to more traditional manufacturing processes such as casting, forging, and machining. While AM can be expensive it can deliver huge advantages in sustainability, efficiency, and flexibility and requires less raw material consumption than subtractive processes. Parts produced through AM can also be made lighter and more complex, delivering efficiency during use.
A major difference in traditional metal fabrication processes compared with AM processes is the heating-cooling regimes involved. For AM processes, such as selective laser melting (SLM) and electron-beam melting (EBM), the heating-cooling regimes are very fast and location-specific, which can lead to different microstructures than those obtained with conventional processes, even with the same alloy composition. This is important as many engineers are looking to achieve similar microstructures to those achieved with conventional routes.
Consistent metal powder does not necessarily mean consistent properties
Metals can crystalize in different phases (ie. atomic arrangements) that have very different properties. As an example, the different phases of steel are illustrated in Figure 1. The pathways to phase formation in traditional processing are well-known, but in AM a different heating-cooling regime or different atomizing gases can produce products with a different phase composition and, therefore, different mechanical properties. When this powder is melted and rapidly recrystallized during an EBM or SLM process, there is further potential for phase transformation to occur. Grain microstructure can also be affected by processing conditions. It is more difficult to control grain structure in an AM manufactured part and this often results in large grain sizes compared with other methods.
Most engineers and metallurgists are looking for a fine grain structure since this improves material strength. This is why post-treatment is still commonplace for many metal AM processes. Grain orientation (also known as texture) is also important because a textured grain orientation can substantially change mechanical properties such as chemical reactivity, strength, and deformation response. This may lead to improved component strength or weakness, and premature failure.
Residual stress is another important characteristic of AM parts. Residual stresses are stresses that are retained in a component after manufacture and act in addition to any externally applied stress, increasing the risk of mechanical failure. AM components are more prone to residual stress due to highly localized cooling and rapid phase transformations that give insufficient time for stresses to relax to their equilibrium crystal structure. Residual stresses can occur anywhere in a material, but those located near a crack, pore, or at the surface of a component are of greatest concern since this is where stresses become most concentrated.
Why XRD is an important analytical tool
X-ray diffraction is a non-destructive analytical technique used to identify and quantify phases in a material. Every crystalline phase produces a characteristic diffraction pattern (e.g. fingerprint) as illustrated for steel in Figure 1.
In addition to phase analysis, X-ray diffraction can also be used to analyze microstructural features such as texture, residual stress and grain size. Texture produces systematic deviations of peak intensity from the characteristic diffraction pattern of a phase. The intensity deviation can be used to quantify the fraction of grains in a certain orientation by tilting and rotating the sample in the diffractometer
A tensile or compressive residual stress will change the atomic spacing of a phase, which will produce a shift in the diffraction peak position. This can be measured with high sensitivity by X-ray diffraction. A series of measurements determine how peak position varies with sample orientation relative to the incident X-ray beam, which can then be used to precisely determine the atomic strain. If the elastic constant of the material is known, then the stress can be calculated.
X-ray diffraction can also be used to analyze grain size. Small grain sizes produce a broadening effect in the diffraction peak width that can be used to quantify crystallite sizes <200 nm. This makes X-ray diffraction a powerful technique to quantify the size of nanocrystalline materials. Peak broadening may also be produced by defects, such as dislocations or stacking faults, that are created during processing. Analysis of multiple diffraction peaks can be used to separate and quantify both size and defect concentration. In addition, area (2D) detectors can be used to image the Debye diffraction cone, which can reveal large grain sizes. New image analysis techniques can calibrate and quantify grain sizes larger than 10 µm.
For more information about the use of XRD for additive manufacturing check out our new application note – Additive Manufacturing of Inconel 718: Characterising parts and powders with XRD and XRF, or tune into our webinar on November 19 which will cover this blog topic in a lot more detail.