Isothermal hot stamping process, which is composed of stamping and subsequent stress-relaxation steps, is an important technology to form complex thin-walled titanium components in the aerospace industry. It is a key issue to enable the accurate simulations of these two steps simultaneously for the process design and optimization. In this study, a unified constitutive model connecting both the plastic flow behaviour in stamping and the stress-relaxation behaviour in subsequent step is developed by considering the continuous evolution of key microstructures, i.e., dislocation density, in the whole process. A series of basic mechanical tests, including tensile and stress-relaxation tests, of a typical titanium alloy Ti-6Al-4V at 750°C was performed to calibrate the developed model. The unified model was then implemented into the commercial software ABAQUS via the VUMAT subroutine, and simulations of the complete hot stamping process were done, including stamping, stress-relaxation and final springback. In addition, a typical curve-shape component was hot-stamped at 750°C and stress-relaxation for 5 minutes was performed. The predicted result from the developed constitutive model and FE model shows a good agreement of the springback with the corresponding experimental result, verifying the effectiveness of the developed model for the further applications in hot stamping process design and optimization in the industry.

A volume of an additively manufactured 316L stainless steel sample has been tracked during its recrystallization using near- and far-field High Energy Diffraction Microscopy (HEDM) and absorption tomography at Advanced Photon Source beamline 1-ID. A near-field compatiblein situfurnace allows monitoring of Bragg diffraction signals as they evolve out of a weak and diffuse background while the sample temperature is ≈ 1250° C. The sample is rapidly cooled to room temperature after observation of significant signal evolution and ∼ 0.035 mm³ is mapped by the near-field method. Four cycles of heat treatment follow the structure from a state of small, isolated grains through impingement of domains to near completion of recyrstallization. Here, the experiment and reconstructions are described, and recrystallized fractions, twin domains, and distributions of grain boundary types are discussed.

In this work, it is investigated how grain boundaries influence the local strain determination by the microstructural feature tracking (MFT) algorithm. In this method, tetrahedra are used as the strain calculation unit. We apply the MFT processing procedure on data obtained by a crystal plasticity finite element modeling (CPFEM) simulation to explore the uncertainties in the calculated strains caused by grain boundaries. Effects of tetrahedron types and radius ratios are discussed.

Shielding gas flow is essential to the additive manufacturing (AM) process, and the effects of argon shielding gas flow variation on the macroscopic homogeneity of additive manufactured stainless steel parts have been studied using an open-architecture AM system. Such a variation manifests itself layer-by-layer within one part and part-by-part across the build plate. Within one build, a combination of balling behavior, conduction melting and keyhole melting is observed across the build plate using the same laser parameters. Quantitative characterization of the melt pool shapes show that the melt pool width and the penetration depth exhibit the largest variations. Possible relations between the gas flow condition and macroscopic structure variations are discussed and guidelines for improved design of a gas flow system as well as future research directions are suggested for achieving macroscopically uniform metal AM.

A wire of a β-Titanium alloy known as Gum Metal was cold rolled to a reduction of 80% and heat treated for times in the range 3-120 minutes at a fixed temperature of 740 °C to yield a set of partially recrystallized microstructures. The recrystallization course was evaluated by light optical microscopy and Electron Backscatter Diffraction, based on which the recrystallization kinetics was analysed using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. This analysis showed that the kinetics of this material does not follow the traditional JMAK behaviour but has two distinct stages with very different Avrami exponents.

The BaZrO₃/YBa₂Cu₃O₇ (BZO/YBCO) interface has been found to affect the vortex pinning efficiency of one-dimensional artificial pinning centers (1D-APC) of BZO. A defective BZO/YBCO interface due to a lattice mismatch of ∼7.7% has been blamed for the reduced pinning efficiency. Recently, we have shown incorporating Ca_0.3Y_0.7Ba₂Cu₃O_7-x spacer layers in BZO/YBCO nanocomposite film in multilayer (ML) format can lead to a reduced lattice mismatch ∼1.4% through the enlargement of lattice constant of YBCO via Ca diffusion and partial Ca/Cu replacement on Cu-O planes. In this work, the effect of this interface engineering on the BZO 1D-APC pinning efficiency is investigated at temperatures of 65-81 K through a comparison between 2 and 6 vol.% BZO/YBCO ML samples with their single-layer (SL) counterparts. An overall higher pinning force ( F_p ) density has been observed on the ML samples as compared to their SL counterparts. Specifically, the peak value ofF_p( F_p,max ) for the 6% BZO/YBCO ML film is about ∼ 4 times of that of its SL counterpart at 65 K. In addition, the location of theF_p,max( B _max) in the ML samples shifts to higher values as a consequence of enhanced pinning. For the 6% BZO/YBCO ML sample, a much smaller "plateau-like" decrease of theB_maxwith increasing temperature was observed, which is in contrast to approximately linear decrease ofB_maxwith increasing temperature in the 6% SL film. This result indicates the importance of restoring the BZO/YBCO interface quality for better pinning efficiency of BZO 1D-APCs especially at higher BZO doping concentration.