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  1. A finite element formulation for deformation twinning induced strain localization in polycrystal magnesium alloys

    Deformation twinning induces shear strain localization in hexagonal close-packed crystals and is critical for the material’s ductility and failure. Cracks often occur at twin-twin or twin-grain boundary intersections and propagate along twin bands. However, most crystal plasticity models for deformation twinning are based on a “pseudo-slip” approach and do not capture the localized deformation associated with the formation of each discrete twin band. The few exceptions are discrete twin models that involve very complex numerical algorithms and are often compromised in accuracy due to the numerical convergence. These factors make the discrete twin models hard to adopt. This paper proposesmore » a modification to the conventional finite element weak form, to fully incorporate a twin-induced heterogeneous deformation that does not depend on the “pseudo-slip” assumption. The model starts by splitting the deformation gradient into elastic-slip-twinning components. The twin-induced deformation gradient component is computed separately by solving a microstructural evolution problem and then implemented into finite element weak form by constructing a global “twin-force” vector. The constitutive update (e.g., in the user-defined material subroutine, or UMAT, for ABAQUS) therefore avoids dealing with the twinning and recovers to the form of a regular slip-based crystal plasticity model. The results presented here indicate that the twin-induced strain localization and the associated stress-reversal phenomena near the twin band were naturally captured in the model, which was validated against an in-situ synchrotron X-ray micro-diffraction experiment.« less
  2. Deformation and failure of PrintCast A356/316 L composites: Digital image correlation and finite element modeling

    A356/316 L interpenetrating phase composites can be fabricated by infiltrating additively-manufactured 316 L stainless-steel lattices with a molten A356 aluminum alloy, a new process termed PrintCasting. This work investigates the mechanical properties of PrintCast composites and their relation to the volume-fraction of 316 L reinforcement. Uniaxial tension experiments were conducted with A356/316 L PrintCast composites that had either 30 vol%, 40 vol% or 50 vol% 316 L. When 316 L reinforcement increased from 30 vol% to 40 vol%, a > 200% increase in ductility and 400% increase in absorbed-energy were observed, while a much lower increase was exhibited when reinforcementmore » increased from 40 vol% to 50 vol%. The failure of the 30 vol% sample occurred by localized deformation and a single failure initiation region, in contrast to the 40 vol% and 50 vol% samples which failed by delocalized damage in the entire gauge section. To understand this transition phenomena, digital image correlation (DIC) was coupled with finite element (FE) analysis to capture the deformation and failure processes. The results revealed that, for all samples, stress concentrated and failure initiated in a 316 L strut near the lattice nodes, where the strut underwent localized bending-dominated deformation. In the high 316 L volume-fraction composites, the increase in 316 L-strut diameter reduced local bending stress and stabilized the deformation, leading to improved damage tolerance. Based on the presented analysis, local modifications to the PrintCast structure are suggested.« less
  3. Molecular dynamics study on interface formation and bond strength of impact-welded Mg-steel joints

    It was recently demonstrated that the vaporizing foil actuator welding (VFAW) method can directly join immiscible magnesium and steel alloys without coating or a third chemical element based intermetallic compound layer. The VFAW Mg/steel joint exhibits a mixed interface layer of up to 200μm thickness consisting of Mg matrix and Fe particles. Computer simulations have suggested the formation of the interlayer is from the high-velocity frictional shearing between the Mg/steel substrates during the oblique impact in the VFAW process. This paper investigates the formation of Mg-Fe interlayer under VFAW condition with different shearing velocities using molecular dynamics (MD) model, andmore » studies the bonding strength under different scenarios. Finally, the results elucidate the critical role of shearing velocity and surface roughness in achieving Mg/Fe joint.« less
  4. Surface engineering to enhance heat generation and joint strength in dissimilar materials AZ31 and DP590 ultrasonic welding

    A multiscale simulation approach was developed and employed to optimize the sheet surface conditions for higher interfacial temperature and joint strength in ultrasonic welding of magnesium alloy AZ31 and dual-phase steel DP590. First, a mesoscale model was used to study the relationship between friction coefficient and surface roughness, which can be modified by various engineering methods. Then a macroscopic process model was employed to study the effects of surface roughness on heat generation, indicating that a temperature increase can be achieved with rougher surfaces on two sides of both DP590 and AZ31 sheets. Samples prepared by sanding and filing, asmore » well as grinding, were first characterized for surface roughness and then welded under ultrasonic vibration. An infrared camera was used to measure temperatures in situ for model validation. Overall, lap shear test results for the welded joint showed that the joint strength can be improved by 10~25% using filing and round grinding methods as a result of the enhanced heat generation and mechanical interlocking on the interface.« less
  5. Multi-scale characterization and simulation of impact welding between immiscible Mg/steel alloys

    Vaporizing foil actuator spot welding method is used in this paper to join magnesium alloy AZ31 and uncoated high-strength steel DP590, which are typically considered as un-weldable due to their high physical property disparities, low mutual solubility, and the lack of any intermetallic phases. Characterization results from scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) of the weld interface indicate that the impact creates an Mg nanocrystalline interlayer with abundant Fe particles. The interlayer exhibits intact bonding with both DP590 and AZ31 substrates. To investigate the fundamental bond formation mechanisms at the interface, a finite element (FE)-based processmore » simulation is first performed to calculate the local temperature and deformation at the interface under the given macroscopic experimental condition. Finally, taking the FE results at the interface as inputs, molecular dynamics (MD) simulations are conducted to study the interlayer formation at the Mg/Fe interface during the impact and cooling. The results found a high velocity shearing-induced mechanical mixing mechanism that mixes Mg/Fe atoms at the interface and creates the interlayer, leading to the metallurgical bond between Mg/steel alloys.« less
  6. Linking constituent phase properties to ductility and edge stretchability of two DP 980 steels

    Two DP 980 steels were characterized with multiple methods to determine its microstructure characteristics, tensile properties and edge stretchability. The first (DP980-B) is a commercial grade DP980 steel produced by AK Steel, while the second (DP980-T) is a modification of the first one by a tempering process. Both contain two individual phases: ferrite and martensite with body centered cubic (BCC) and tetragonal (BCT) crystal structure respectively. ASTM standard tensile tests were performed for both materials, revealing that DP980-B has higher work hardening and larger total elongation than DP980-T. Standard hole piercing and subsequent hole expansion and extrusion tests were alsomore » performed. The critical hole expansion ratio (HER) of hole pierced sheet at different nominal clearances, on the other hand, show an opposite trend: the HER of DP980-T almost doubles that of DP980-B. This indicates the tensile ductility and edge stretchability are determined by different mechanisms due to different deformation modes. The ductility of sheet metal during uniaxial tension is determined by deformation instability, i.e. necking, due to less constraint at the region of deformation and fracture will occur quickly after necking. The contribution of post-necking deformation to the total elongation is insignificant for the high strength grades such as DP980 steels. The ductility during hole-expansion, however, is mainly determined by material fracture behavior that is dependent on phase property disparity and material intrinsic fracture parameters. With the individual phase properties for both DP steels determined by in situ tensile tests under high energy X-ray diffraction, the results show that DP980-T steel has lower property disparities between the two phases than DP980-B steel, due to the tempering process. This explains why the DP980-T steel has higher HER than DP980-B steel: lower phase disparity will lead to less local deformation during loading. The higher work hardening rate for DP980-B steel contributes to its higher uniform elongation compared with DP980-T steel based on the maximum load condition of deformation instability. An integrated finite element simulation framework for studying hole expansion is also presented here, based on the calculated individual phase properties from the combined high energy x-ray diffraction (HEXRD) and elastic plastic self-consistent modeling. The simulation results correlate well with the experimental results on the HER difference between the two materials.« less
  7. Predicting forming limit diagrams for magnesium alloys using crystal plasticity finite elements

    The crystal plasticity finite element (CPFE) method was used to predict forming limit strains of hexagonal close-packed (HCP) polycrystalline magnesium alloy sheets, namely AZ31 and ZE10, under an isothermal temperature condition. The strain rate-dependent uniaxial tensile test data along various loading directions and an initial texture were used to determine the constitutive parameters of the crystal plasticity model. A hybrid representative volume element approach, which combines the CPFE and the Marciniak–Kuczynski model, was developed to obtain the forming limit diagram of the magnesium alloys. The predicted forming limits were in excellent agreement with the Nakazima test results. Moreover, the microscopicmore » responses such as slip/twin activation and deformation texture changes during various loading paths from uniaxial tension to the balanced biaxial tension were comprehensively analyzed and discussed from the CPFE results to understand the underlying micro-mechanism for the macro-mechanical responses.« less
  8. Effect of copper content on the tensile elongation of Al–Cu–Mn–Zr alloys: Experiments and finite element simulations

    Microstructures of cast aluminum alloys used in automotive engine applications often consist of intermetallic particles that can impact the tensile elongation of these alloys. Here in this paper, we investigate the effect of intermetallic grain boundary particles on tensile elongation by fabricating a series of alloys with Cu content varying between 6.0 - 9.0 wt% in cast Al–Cu–Mn–Zr (ACMZ) type compositions. The tensile elongation of as-aged ACMZ alloys decreases monotonically with increase in Cu content. While the microstructure within the grains and yield stress of the alloy remains invariant with Cu content, the decrease in tensile elongation correlates well withmore » increase in the size and volume fraction of grain boundary particles. Microstructural observations are combined with finite element simulations to explain the trend in tensile elongation with changing Cu content. Crack initiation is found to occur by brittle fracture of the grain boundary particles. Increase in particle size promotes crack initiation by reduction in size dependent particle fracture strength. Lower inter-particle spacing at higher particle volume fraction further facilitates crack initiation by increasing stress within the particles caused by the interaction between stress fields of neighboring particles. Increase in particle volume fraction also accelerates crack propagation through the formation of macro shear zones in the microstructure. The increase in Cu content of cast ACMZ alloys, therefore, decreases tensile elongation by promoting both crack initiation and crack propagation.« less
  9. Towards an integrated experimental and computational framework for large-scale metal additive manufacturing

    Using the Metal Big Area Additive Manufacturing (MBAAM) system, a thin steel wall was manufactured from a low carbon steel wire. The wall was then characterized comprehensively by high-throughput high-energy X-ray diffraction (HEXRD), electron backscatter diffraction (EBSD), and in-situ HEXRD tensile tests. With the predicted temperature histories from the finite element-based additive manufacturing process simulations, the correlations between processing, microstructure, and properties were established. The correlation between the final microstructure with the predicted temperature history is well explained with the material’s continuous cooling transformation (CCT) diagram calculated based on the composition of low carbon steel wire. The final microstructure ismore » dependent on the cooling rate during austenite to ferrite/bainite transformation during initial cooling and the subsequent reheating cycles. Fast cooling rate resulted in small ferrite grain size and fine bainite structure at the location closest to the base plate. Slower cooling rate at the side wall location and repeated reheating cycles to the ferrite-pearlite regions resulted in all allotriomorphic (equiaxed) ferrite with medium grain size« less
  10. Heat generation and deformation in ultrasonic welding of magnesium alloy AZ31

    A dual-sonotrode edge welding setup and a finite element analysis (FEA) model were developed for ultrasonic welding (USW) of AZ31 magnesium alloy sheets. Sonotrode vibration was measured quantitatively by a high-speed camera and introduced into the model as the driving force. The transient temperature field on the edge of the sheets was captured by an infrared camera. The heat generation mechanism in USW was investigated by a parametric study on the friction coefficient. Friction at faying interface should be smaller than those at other interfaces to enhance heat generation. The model was then validated by the experimental thermal history atmore » the faying interface and the full temperature field as well as sonotrode indentation. Using the established model, USW under different welding powers (or vibration amplitudes) but at the same energy input was analyzed to characterize USW bond condition. Much higher temperature in the weld was observed with increasing vibration amplitudes, while reasonable indentation was maintained. Furthermore, vibration amplitude other than energy input was a critical factor in bond formation between AZ31 sheets.« less
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"Hu, Xiaohua"

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