In Situ x-ray Diffraction Study of the Deformation of an AISI 316L Stainless Steel Produced by Laser Powder Bed Fusion

Abstract

Additive manufacturing (AM) has emerged as an outstanding technique for obtaining complex geometries and custom parts, without the material loss of conventional subtractive manufacturing processes. In this work, AISI 316L stainless steel specimens were fabricated by laser powder bed fusion (L-PBF), and its microstructure was characterized by several techniques. Tensile tests with in situ x-ray diffraction (XRD) measurements were performed using synchrotron radiation. Stress–strain curves and diffractograms were obtained for the as-printed AM 316L, annealed AM 316L and conventional/rolled 316L samples for comparison. The results indicated lower ductility for the AM samples when compared to the sheet. This can be a result of the remaining porosity associated with the AM process. The annealing of the AM samples led to a reduction of the residual stress and an improvement of ductility without significant loss on the ultimate tensile strength. In situ XRD data indicated that AM samples did not undergo phase transformation during straining, maintaining a fully austenitic microstructure and preventing a transformation-induced plasticity (TRIP) effect. On the other hand, in the rolled sample, peaks of α′-martensite were identified. Electron backscattered diffraction (EBSD) measurements indicated that a random texture was achieved by the parameters and scanning strategy used. The results indicate that process parameters must be carefully chosen in order to avoid porosity, and excessive residual stresses, features that directly affect the mechanical behavior of the material.

Appendices

Appendix

Applied Corrections of the Diffraction Profiles, and the Procedure for Calculating Phase Fractions (Direct Comparison Method)

The corrected angle position (Δ(2θ)) and corrected intensity (I(2θ)) must be calculated because the diffraction surface moves in a normal direction to the surface plane during the tensile test. The values of (Δ(2θ)) and (I(2θ)) can be calculated by Eq 1 and 2 (Ref 27,28,29).

where 2θ_obs is the observed angle, t is the sample thickness, ω is the beam incident angle, ε is the sample’s strain, g is the distance between the detector center and the irradiated region of the sample, I(2θ) is the corrected integrated intensity, I_obs(2θ) is the observed integrated intensity, and θ is the corrected angle of the diffraction beam.

The direct comparison method is appropriated for calculating the phases fractions of polyphasic, polycrystalline samples (Ref 30). According to this method, the volumetric phase fraction phase V_i is related to the integrated intensity as follows in Eq 3., K and R_i^hkl are calculated from Eq 4 and 5, respectively.

where I_i^hkl is the integrated intensity from the diffracted beam for the hkl plane related to an i phase, K is the instrumental factor, R_i^hkl is the scattering factor, V_i is the volumetric fraction of phase i, v is the unit cell volume, F is the structure factor for the hkl reflection plane, p is the multiplicity factor, ${\text{e}}^{\text{-2m}}$ is the temperature factor, λ is the wavelength of the incident beam, μ is the linear absorption coefficient, A is the section area transversal to the incident beam, I₀ is the intensity of the incident beam, r is the radius of the diffractometer, and e and m are electron charge and mass, respectively.

However, as the experiments were carried in a synchrotron source beam, the diffraction plane is vertical and there is no intensity lost due to the polarization factor (Ref 26, 27). Thus, the polarization factor can be considered as 1. The polarization factor (P) is given in Eq 6 (Ref 30).

This way, the volumetric fraction of phases austenite and α′ martensite can be calculated by Eq 3. with the simplification of the polarization factor.

$$ \Delta \left( {2\theta } \right) = \tan^{ - 1} \left( {\left( \varepsilon \right)\left( \frac{t}{g} \right)\left( {\frac{{\sin \left( {2\theta obs} \right)}}{\sin (\omega )}} \right)} \right) $$

(1)

$$ I\left( {2\theta } \right) = \left( {I_{{{\text{obs}}}} \left( {2\theta } \right)} \right)e^{{ - \left( {1 + \frac{\sin (\omega )}{{\sin \left( {2\theta - \omega } \right)}}} \right)}} $$

(2)

$$ I_{i}^{hkl} = \frac{{KR_{i}^{hkl} V_{i} }}{2\mu } $$

(3)

$$ K = \left( {\frac{{I_{0} A\lambda^{3} }}{32\pi r}} \right)\left[ {\left( {\frac{{\mu_{0} }}{4\pi }} \right)\frac{{e^{4} }}{{m^{2} }}} \right] $$

(4)

$$ R_{hkl} = \left( {\frac{1}{{\nu^{2} }}} \right)\left[ {\left| F \right|^{2} p\left( {\frac{{1 + \cos^{2} 2\theta }}{{sen^{2} \theta \cos \theta }}} \right)} \right]\left( {e^{ - 2m} } \right) $$

(5)

$$ P = \frac{1}{2} (1 + \cos^{2} \theta ) $$

(6)

$$ \Delta K = \frac{0.9}{D} + \sqrt {\left( {\frac{{\pi M^{2} b^{2} }}{2}} \right)\rho } \left( {K\overline{C}^{\frac{1}{2}} } \right) $$

(7)

$$ {\text{b}}_{{{\text{fcc}}}} = \frac{\sqrt 2 }{2}a $$

(8)

$$ b_{{{\text{bcc}}}} \frac{\sqrt 3 }{2}a $$

(9)

$$ C = \overline{C}_{h00} \left( {1 - qH^{2} } \right) $$

(10)