Efficacy of Vickers indentation method to measure residual stress in additively manufactured IN718 specimens

Abstract

Selective laser melting of Inconel 718 powder (a nickel-based superalloy) is regularly used to manufacture gas turbine parts for the aerospace industry. Residual stresses that remain in components after manufacturing are known to be detrimental, often resulting in geometric distortion and inadequate mechanical properties. It is therefore essential to be able to measure these residual stresses. Many non-destructive residual stress measuring techniques are used in the industry, and the Vickers indentation method has been proposed as a less expensive, rapid evaluation technique of residual stresses. The efficacy of the Vickers indentation method as a residual stress measurement technique was evaluated in the study. It was observed that, for the conditions present in the study, the Vickers micro-indentation method provided inaccurate data. Residual stresses measured using this technique varied considerably across the surfaces of the specimens, with only slight tendencies observable. Compared to X-ray diffraction measurement on cubes manufactured under the same conditions, the residual stress values calculated from the Vickers micro-indentations were very low in magnitude. Differences in hatch distance overlap percentage did not show substantial differences in calculated surface residual stress values; however, slight increases in compressive stresses were observed with increasing hatch distance overlap. Sand-blasting of specimen surfaces preceded dry electrochemical polishing and resulted in increased compressive stresses calculated by the Vickers micro-indentation method. The ability to ensure even and smooth surfaces for Vickers micro-indentations would provide more accurate data on surface residual stress values. Ultimately, it was concluded that the Vickers micro-indentation method is not suitable for surface residual stress measurements of selective laser melted components because of the inability to provide adequately smooth surfaces for indentations to provide accurate data without removing substantial amounts of component surfaces. Finally, the inherent strain method was employed to simulate residual surface stresses on the selective laser melted components. It was found that these values did not correlate with the values obtained from Vickers micro-indentation tests and X-ray diffraction tests. These discrepancies were attributed to the inability of the simulation software to account for certain processes used during the manufacturing and preparation of the specimens.