Neutron diffraction provides deeper analysis of residual stresses

Neutron diffraction provides deeper analysis of residual stresses

A pre-proof study from the journal Materials science and technology: A used a neutron diffraction method and FEM simulation to show that substrate properties can alter the residual stress created in parts by direct energy deposition.

Study: Effect of substrate yield strength and grain size on the residual stress of direct energy deposition fabrication measured by neutron diffraction. Image credit: Zyabich/Shutterstock.com

The Direct Energy Deposition (DED) process can produce nearly net-like objects and repair damaged ones. Residual stress is generated inside the parts machined using this method.

In an upcoming studyresearchers have used a softer substrate with larger grain sizes and low yield strength to lower residual stresses in parts of direct energy deposition.

Residual stress and its importance in metal additive manufacturing

Direct Energy Deposition (DED) is a new metal additive manufacturing process that uses metal powder as raw material. It melts the deposited metal powder by exposing it to a concentrated laser source.

Compared to other metal additive manufacturing processes, direct energy deposition machines are simpler, allowing direct application of the DED process to existing parts. That makes them an excellent choice for parts maintenance and add-on procedures.

However, rapid and repeated temperature variation during the direct energy deposition process causes the surface temperature to change rapidly.

Rapid cooling induces compression in the center of the material.

The outer surface is a constraint on the material in the core as it tries to cool. As a result, residual tensile stress is generated in the inner region, while the outer part will have residual compressive stress.

Even in the absence of temperature gradients or external loads, these stresses remain in a solid material.

Residual stress significantly affects the corrosion resistance, dimensional stability, crack development, and mechanical properties of metal additive manufacturing specimens. It can cause unwanted internal cracks or distortions in the geometry of the parts.

The material is permanently deformed when the residual stress exceeds the yield strength. This leads to deflection as the part separates from the base plate.

Residual stress can be seen as a decisive element in cost management and improving the efficiency and quality of products. However, it is still difficult to evaluate residual stresses in machined parts for metal fabrication.

Limitations of traditional residual stress measurement techniques

X-ray diffraction, contour, and hole drilling methods were previously used to determine residual stress. However, X-ray diffraction is only applicable to subsurface analysis, while contour and borehole techniques can give inaccurate results due to their destructive measurement methods.

Measurement of residual stresses using the neutron diffraction method

The pre-proof study published in Materials science and technology: A used a non-destructive neutron diffraction method to measure the residual stresses induced during the direct energy deposition process. The neutron’s lack of electrical charge allows it to penetrate deeper into the material.

The substrates were 10 mm thick stainless steel sheets. The direct energy deposition samples were heat treated to create two distinct substrates with varying grain sizes and yield strengths to evaluate the influence of the substrate properties on the residual stresses created during direct energy deposition.

Finite element method (FEM) simulations helped to study the evolution of residual stresses and the influence of the yield strength of the substrate on the residual stresses of direct energy deposition parts.

The FEM analysis was compared with the experimentally obtained residual stress values.

The electron backscatter diffraction (EBSD) method investigated the microstructures of the deposited metal and the stainless steel substrate. Both non-heat treated (NH) and heat treated (HT) stainless steel substrates were subjected to tensile tests to validate their mechanical properties.

Important results of the study

The majority of the direct energy deposit developed compressive residual stresses as the height of the deposit increased, while tensile residual stresses were generated closer to the top surface.

Neutron diffraction and FEM simulation showed that substrate yield strength and grain size affect the magnitude of residual stress generation in the deposited metal and stainless substrate.

Comparison of the residual stresses for the NH and HT stainless steel substrates showed that the HT substrate has significantly lower residual stresses than the NH substrate. The substrate’s low yield strength allows more thermal shrinkage compensation during the direct energy deposition process, lowering residual stress.

The substrate properties affect the grain size and texture of the direct energy deposited metal due to epitaxial growth at the deposit-substrate interface.

The grain size affects the yield strength, but the texture affects the resistance to plastic deformation, which affects the residual stress.

However, these microstructural variations had no significant effect on the tensile properties of the direct energy deposits.

This work showed that the change in the properties of the substrate could be used to lower the residual stresses from the direct energy deposits and improve the quality of the part.

This method can be used when typical stress reduction techniques, such as preheating and post-annealing, are not feasible.

Reference

SG Jeong, SY Ahn, ES Kim, GM Karthik, Y. Baik, D. Seong, YS Kim, W. Woo, HS Kim. (2022) The Effect of Substrate Yield Strength and Grain Size on the Residual Stress of Direct Energy Deposition Fabrication as Measured by Neutron Diffraction. Materials science and technology: A. https://www.sciencedirect.com/science/article/pii/S0921509322010164

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