Shipin Qin

Bio: Shipin Qin is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Plasticity & Stress (mechanics). The author has an hindex of 5, co-authored 12 publications receiving 100 citations.

Characterization of the strength of support structures used in powder bed fusion additive manufacturing of Ti-6Al-4V

Abstract: Support structures are required in powder bed fusion (PBF) additive manufacturing of metallic components with overhanging structures in order to reinforce and anchor the part, preventing warping during fabrication. In this study, we tested the tensile structural strength of support structures with four different 2-dimensional lattice geometries by fabricating samples composed of solid material on the bottom, followed by support material in the middle, followed by solid material on the top. The support structure regions were fabricated with a lower linear heat input than the solid material, providing deliberate geometrical stress concentrations to enable the removal of support material after processing. These samples were subjected to tension in the vertical direction to measure the strengths of the support structure-solid material interfaces. Two strengths were computed: an effective structural strength defined as the total force that the structure withstood normalized by the full cross-sectional area, and a ligament structural strength, defined as the effective structural strength normalized by the density of the solid material, thereby ignoring the volume of the surrounding powder and voids that do not contribute to the strength of the lattice. The effective structural strength was 14–32% of the strength of fully dense Ti-6Al-4V made by PBF and the ligament structural strength was 34–49% of the strength of fully dense material. These interface strengths are lower than that of fully-dense material due to the stress concentrations at the support structure-solid material interfaces, not any intrinsic difference in the intrinsic strength of support structure versus solid material. These results can be used to tailor the support structure geometry to balance sufficient anchoring strength during fabrication and ease of part removal and subsequent machining during post-processing.

Anisotropic multiaxial plasticity model for laser powder bed fusion additively manufactured Ti-6Al-4V

Abstract: The multiaxial yield and plastic flow behavior of Ti-6Al-4V manufactured in two orientations via laser powder bed fusion (L-PBF) additive manufacturing was investigated. The mechanical properties of L-PBF Ti-6Al-4V were evaluated under uniaxial tension, plane strain tension, pure shear, and combined tension/shear loading. The mechanical behavior was found to be stress state dependent and slightly anisotropic. A plasticity model, consisting of a Hill 1948 anisotropic yield criterion, associated flow rule, and an isotropic hardening law was calibrated and used to describe the yield and plasticity behavior of this material. Validation of the plasticity model under multiaxial stress states demonstrated that the model was able to predict the stress state dependent anisotropic plasticity behavior of this material.

Plasticity and fracture behavior of Inconel 625 manufactured by laser powder bed fusion: Comparison between as-built and stress relieved conditions

Abstract: In this study, the influence of stress relief on the plasticity and fracture behavior of Inconel 625 fabricated through laser powder bed fusion additive manufacturing (AM) was investigated. The as-built versus stress relieved microstructures were compared, showing similar grain structures but the presence of ~10 vol % δ phase in the stress relieved condition, and no δ phase in the as-built condition. Mechanical tests under plane strain tension were performed on the stress relieved samples, and an anisotropic plasticity model was calibrated and validated using finite element simulations. Uniaxial and notched tension tests were performed on both as-built and stress relieved samples to probe the effect of stress relief on stress state- and direction-dependent fracture behavior. It was found that on average, the fracture strain of the stress relieved samples along the build direction was 30% higher than that along the perpendicular build direction in the stress state range studied, and the stress relief heat treatment resulted in a 45% decrease in fracture strain. The fracture strain in stress relieved samples was more strongly dependent on stress state than in as-built samples.

Unveiling dislocation characteristics in N i 3 Al from stacking fault energy and ideal strength: A first-principles study via pure alias shear deformation

Abstract: Nickel aluminide $(\mathrm{N}{\mathrm{i}}_{3}\mathrm{Al})$ is an important material for a number of applications, especially when used as a strengthening constituent in high-temperature Ni-based superalloys. Despite this, there is minimal information on its mechanical properties such as strength, plasticity, creep, fatigue, and fracture. In the present work, a first-principles based pure alias shear deformation has been applied to shed light on dislocation characteristics in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Al}$ using the predicted stacking fault energy (i.e., the \ensuremath{\gamma} surface) and ideal shear strength $({\ensuremath{\tau}}_{\mathrm{IS}})$. Results include direct evidence for the splitting of a $1/2[\overline{1}10]$ dislocation into two Shockley partials on the ${111}$ plane, which is further supported by the equivalence of the complex stacking fault (CSF) energy ${\ensuremath{\gamma}}_{\mathrm{CSF}}$ and the antiphase boundary (APB) energy ${\ensuremath{\gamma}}_{\mathrm{APB}111}$. Estimates of the Peierls stresses using ${\ensuremath{\tau}}_{\mathrm{IS}}$ and elastic properties suggest the prevalence of edge dislocations in Ni and screw dislocations in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Al}$, agreeing with experimental observations regarding the dominance of edge dislocations in the first stage of crystal deformation in fcc metals and the yield-strength anomaly related to screw dislocations in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Al}$. The present calculations further point out that the CSF and APB111 are easily formed by shear due to the low-energy barriers, although the lowest planar energies are for the superlattice intrinsic stacking fault and the APB001. Through the case of $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Al}$, the present work demonstrates that the pure alias shear methodology is not only computationally efficient but also provides valuable insight into the nature of shear-related properties.

Laser and Electron Beam Additive Manufacturing Methods of Fabricating Titanium Bone Implants

Abstract: Additive Manufacturing (AM) methods are generally used to produce an early sample or near net-shape elements based on three-dimensional geometrical modules. To date, publications on AM of metal implants have mainly focused on knee and hip replacements or bone scaffolds for tissue engineering. The direct fabrication of metallic implants can be achieved by methods, such as Selective Laser Melting (SLM) or Electron Beam Melting (EBM). This work compares the SLM and EBM methods used in the fabrication of titanium bone implants by analyzing the microstructure, mechanical properties and cytotoxicity. The SLM process was conducted in an environmental chamber using 0.4–0.6 vol % of oxygen to enhance the mechanical properties of a Ti-6Al-4V alloy. SLM processed material had high anisotropy of mechanical properties and superior UTS (1246–1421 MPa) when compared to the EBM (972–976 MPa) and the wrought material (933–942 MPa). The microstructure and phase composition depended on the used fabrication method. The AM methods caused the formation of long epitaxial grains of the prior β phase. The equilibrium phases (α + β) and non-equilibrium α’ martensite was obtained after EBM and SLM, respectively. Although it was found that the heat transfer that occurs during the layer by layer generation of the component caused aluminum content deviations, neither methods generated any cytotoxic effects. Furthermore, in contrast to SLM, the EBM fabricated material met the ASTMF136 standard for surgical implant applications.