ZEK100 Magnesium Alloy Research Articles

Dissimilar ultrasonic spot welding of rare-earth containing ZEK100 magnesium-to-aluminum alloy with a zinc interlayer: Microstructural evolution and mechanical properties

The purpose of the present study was to identify microstructural changes at both Mg/Zn and Al/Zn interfaces of ultrasonic spot welded joints between a rare-earth containing ZEK100 magnesium alloy and an AA6022 aluminum alloy with a zinc interlayer, focusing on their effect on the tensile lap shear strength. During ultrasonic spot welding (USW), the diffusion or reaction was observed to occur at both interfaces, with the reaction being much faster at the Mg/Zn interface than at the Al/Zn interface. This was attributed to the higher diffusion rate of Zn in Mg and lower eutectic temperature in the Mg-Zn system along with the higher specific heat capacity and lower thermal conductivity of magnesium. With increasing welding energy, the diffusion layer at both interfaces increased. However, the diffusion layer at the Al/Zn interface was appreciably thinner than that at the Mg/Zn interface. While the eutectic (or eutectoid) structure containing Mg7Zn3, MgZn2 and MgZn intermetallic phases appeared at the Mg/Zn interface, the diffusion layer at the Al/Zn interface consisted of only Al and Zn based solid solution. The joining mechanisms were mainly mechanical interlocking and metallurgical bonding which were enhanced by grain refinement at both interfaces stemming from dynamic crystallization during USW. The peak tensile lap shear strength of the present Mg-Al dissimilar joints was attained at a welding energy of 1000 J, and the failure occurred in the diffusion layer at the Mg/Zn interface.

Warm Forming Simulation of a ZEK100 Magnesium Door Panel

The warm forming of a door panel made of commercial grade ZEK100 magnesium alloy sheet was modelled using finite element techniques (Autoform). The operation consisted of a heating step, a forming step and a cooling/springback step. The material properties of the ZEK100 blank were modelled using a set of temperature- and strain rate-dependent stress-strain curves, which were derived based on a Zerilli-Armstrong constitutive model. In order to simplify the material model to enable its complex response to be represented within a commercial finite element code, the anisotropy of the magnesium sheet was approximated using a Banabic-2005 yield surface and yield asymmetry was neglected, a reasonable approach for warm forming. Necking was predicted using a set of forming limit curves obtained at different isothermal temperatures. The entire forming model was run at a punch speed of 160 mm/s, and two initial blank temperatures: 215 and 230 °C. The tooling was initially at room temperature. The model predicted that the blank cracked when its initial temperature was 215 °C due to excessive cooling. The best formability (lack of wrinkling and necking) was predicted when the blank was heated to 230 °C. These predictions agree well with the forming trial outcomes.

Improving the corrosion resistance of ZEK100 magnesium alloy by combining high-pressure torsion technology with hydroxyapatite coating

Magnesium alloys have better biocompatibility and biodegradability than conventional biomedical metal materials, but the corrosion resistance is so poor that the implant loses its mechanical integrity before the damaged tissue completely recovers. In this study, the ZEK100 magnesium alloy was pre-deformed with a high-pressure torsion (HPT) process and then fabricated hydroxyapatite (HA) coatings with different contents of Mg(OH)2 nanopowder via hydrothermal method. The specimens were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). Meanwhile, the corrosion behavior of the specimens was evaluated by electrochemical impedance spectroscopy (EIS) and hydrogen evolution tests. Results showed that HPT processing refined the grain size and introduced a great number of twins, resulting in the enhancement of microhardness and Young's modulus of ZEK100. The abundant fine grains, twins and grain boundaries in HPT-ZEK100 can provide more nucleation sites for the HA crystal, resulting in a denser, thicker HA coating with smaller crystal sizes. In terms of the amount of Mg(OH)2 nanopowder, a 0.3 mg/mL content was most appropriate for the deposition of HA. In addition, HPT technique and surface modification by HA coating simultaneously reduced the corrosion rate of ZEK100 magnesium alloy, which virtually expands the biological application of magnesium alloys.

Effect of hot rolling on microstructure and properties of the ZEK100 alloy

Abstract Attempts to expand manufacturing capabilities of magnesium-based sheet materials are described. The as-cast ZEK100 (Mg 1.2Zn 0.35Zr 0.17Nd, in wt%) magnesium alloy was subjected to hot rolling at temperatures from 350 oC to 450 oC using the laboratory 50 ton reversible mill with preheated rolls to manufacture the 1.5–1.7 mm thick sheet. The rolling temperature affected the sheet properties and an increase in the rolling temperature from 250 to 450 oC caused reduction of tensile strength from 257 to 228 MPa accompanied by a reduction in tensile yield stress from 237 to 185 MPa. At the same time, the alloy elongation increased from 17 to 21%. For the same rolling temperature range, the compressive strength reduced from 418 to 351 MPa. The post-rolling annealing at 450 oC led to reduction of both tensile and compressive properties with the largest changes corresponding to the rolling temperature of 350 °C. The correlation between the alloy grain size after rolling and the sheet properties was established where a reduction in grain size was accompanied by an increase of both the tensile/compressive strength and yield stress. Based on Hall–Petch relationship and an average grain size the correlation developed in this study may be used as the prediction model for properties of the hot rolled magnesium sheet. The results are discussed in terms of recent developments in magnesium sheet alloys and the vital role played in this process by rare earth elements.

Mechanism-based constitutive modeling of ZEK100 magnesium alloy with crystal plasticity and in-situ HEXRD experiment

The constitutive behavior of a hexagonal close-packed (HCP) polycrystalline ZEK100 magnesium alloy was investigated using combined high energy X-ray diffraction (HEXRD) from a synchrotron source and crystal plasticity modeling approach. The in-situ tensile test data coupled with the HEXRD enabled the tracking of the lattice strain evolution during deformation. The microscopic behavior represented by lattice strain and the macroscopic behavior represented by stress-strain curves were then used together as objective function to estimate the critical resolved shear stress (CRSS) and hardening parameters of available slip and deformation twin systems in the ZEK100 alloy. An enhanced predominant twinning reorientation (ePTR) scheme was proposed in the current work, and the ePTR parameters were determined for the first time by the use of basal plane peak intensity along loading direction measured from HEXRD. Two crystal plasticity models, the computationally efficient elastic-plastic self-consistent (EPSC) and crystal plasticity finite element (CPFE) models, were developed incorporating the deformation twinning for the HCP-structured metals. The determined constitutive parameters were further validated by comparing the predicted deformation texture with the measured one. The work provides a useful and computationally-efficient modeling scheme to understand the slip/twin induced deformation behaviors of the ZEK100 alloy in micro- and macro-scales.

Fracture characterization of ZEK100 magnesium alloy sheet under monotonic and non-proportional loading histories

In the present paper, a comprehensive investigation was conducted into the fracture response of a rare-earth magnesium alloy sheet, ZEK100-O, under quasi-static conditions. Various types of specimen geometries were fabricated in different orientations with respect to the rolling direction of the sheet to reveal the anisotropic fracture response of the alloy under proportional loading conditions. To visualize directional dependency of the fracture response, experimental fracture loci for different orientations were constructed. Furthermore, non-proportional tests were performed by abrupt changes of stress states to study the role of the loading history on fracture behaviour of the alloy. The non-proportional tests entailed pre-straining the material in uniaxial and equi-biaxial tension up to a prescribed deformation level, followed by extreme changes of stress states to plane-strain tension or shear. Based on the results of the proportional and non-proportional tests, the validity of employing “damage indicator” approaches commonly utilized in phenomenological modelling of fracture is assessed.

Crystal plasticity based constitutive modeling of ZEK100 magnesium alloy combined with in-situ HEXRD experiments

In the current works, both micro- and macro-mechanical properties of a hexagonal close-packed (HCP) polycrystalline ZEK100 magnesium alloy were investigated. In the experimental perspective, in-situ high energy X-ray diffraction (HEXRD) from a synchrotron source was conducted during the uniaxial tension along the rolling direction (RD) and the transverse direction (TD) to measure the lattice strain evolutions and stress-strain behaviors. In the modeling perspective, crystal plasticity finite element (CPFE) model was developed incorporating the deformation twinning for the HCP-structured metals. The HEXRD experiments and crystal plasticity models were then coupled to characterize the constitutive behaviors of the ZEK100 alloy. The lattice strain data representing the microscopic behavior of the material and the macroscopic stress-strain behavior were then tied together as objective values to estimate the critical resolved shear stress (CRSS) and hardening parameters of available slip and twin systems of the ZEK100 alloy using the developed CPFE model. The stress-strain behavior as well as the lattice strain variation during the uniaxial tension tests are presented using the CPFE model and compared with the actual HEXRD data.

Computer aided design of the laser dieless drawing process of tubes from magnesium alloy with take into account ductility of the material

Paper is devoted to the laser dieless drawing of the tubes made of magnesium alloy ZEK100. Important limitation of this process is a low ductility of magnesium alloys. The purpose of this work was the development and implementation into the FEM code, the ductility model of ZEK100 magnesium alloy and experimental validation of proposed methodology of process design. Ductility and flow stress models were developed based on tension and compression tests in temperature interval 20-350 °C. As result of this work, the technological diagrams for determination of diameter of tube after laser dieless drawing process were obtained.

Ratcheting behavior of ZEK100 magnesium alloy with various loading conditions and different immersing time

Abstract

In vitro and in vivo degradation and mechanical properties of ZEK100 magnesium alloy coated with alginate, chitosan and mechano-growth factor

The biocompatibility, ultimate loading capacity and biodegradability of magnesium alloy make it an ideal candidate in biomedical fields. Fabrications of multilayered coatings carrying sodium alginate (ALG), chitosan (CHI) and mechano-growth factor (MGF) on fluoride-pretreated ZEK100 magnesium alloy have been obtained via layer by layer (LBL) to reduce the degradation rate of magnesium alloy in this study. The modified surfaces of ZEK100 substrates were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) and CARE EUT-1020 tester. Results reveal that multilayer-coated magnesium alloy can be successfully obtained with smooth surface morphology, and the mechanical properties of coated samples are almost the same as those of uncoated samples. However, the fatigue life of coated ZEK100 is slightly larger than that of uncoated samples after 1day of immersion. By comparing the degradation of uncoated and multilayer-coated ZEK100 samples in vitro and in vivo, respectively, it is found that the degradation rate of ZEK100 samples can be inhibited by LBL modification on the surface of the sample; and the corrosion rate in vivo is lower than that in vitro, which help solve the rapid degradation problem of magnesium alloy. In terms of the visible symptom of tissues in the left femur medullary cavity and material responses on the surface, multilayer-coated ZEK100 magnesium alloy has a good biocompatibility. These results indicate that multilayer-coated ZEK100 may be a promising material for bone tissue repair.

Cyclic deformation and anelastic behavior of ZEK100 magnesium alloy: Effect of strain ratio

Wrought magnesium alloys with low rare-earth (RE) contents are being considered for lightweight automotive applications. The purpose of this study was to identify the effect of strain ratio on cyclic deformation behavior of a rolled ZEK100-O Mg alloy with 0.2wt% neodymium. The microstructure in its annealing condition consisted of equiaxed grains which were oriented with most c-axes perpendicular to the rolling direction. This alloy exhibited a superior combination of tensile strength and ductility due to its weak basal texture and decreased stacking fault energy. Significant plastic deformation occurred in the tensile phase of the first cycle at strain ratios of Rε=0 and 0.5. The asymmetry of initial hysteresis loops tended to diminish towards the mid-life cycles. With increasing strain ratio, fatigue life first increased, reached its maximum at a strain ratio of Rε=−1, and then decreased. This was attributed to a combined effect of mid-life stress amplitude, plastic strain amplitude and mean stress, where the stress amplitude increased and plastic strain amplitude decreased with increasing strain ratio. The closer the strain ratio to Rε=−1 was, the lower the absolute value of mean stress was. The mean stress relaxation occurred mainly in the initial stage and for the strain ratios more remotely from Rε=−1. The anelastic behavior of this alloy largely remained arising from the twinning and detwinning, with the strain ratio identified as an influential parameter via sensitivity analyses. The anelastic strain amplitude, along with three newly-defined parameters (eccentricity, angle deviation, and relative slope change) all decreased with increasing strain ratio, reflecting more symmetric hysteresis loops.

An elasto-plastic constitutive model for evolving asymmetric/anisotropic hardening behavior of AZ31B and ZEK100 magnesium alloy sheets considering monotonic and reverse loading paths

Deformation twinning and texture evolution in hexagonal close-packed (HCP) metals lead to evolving flow stress asymmetry/anisotropy and evolving plastic anisotropy. These phenomena cause a significant change in the shape of the yield surface with accumulated plastic deformation which cannot be modeled accurately with traditional hardening laws. In this paper, an anisotropic continuum-based plasticity model is proposed to capture the large strain cyclic hardening behavior of magnesium alloys. Key in the current formulation is the incorporation of distortional hardening to model the evolving asymmetric/anisotropic hardening response of magnesium alloys for both monotonic and reverse loading paths. The hardening behavior is classified into three deformation modes: Monotonic Loading [ML], Reverse Compression [RC], and Reverse Tension [RT]. The deformation modes correspond to the different loading regimes of the cyclic hardening curve. Specifically, the ML mode corresponds to the initial in-plane tension and the initial in-plane compression from the annealed state, the RC mode corresponds to the in-plane compression following previous tension and the RT mode corresponds to the in-plane tension following previous compression. Three separate hardening laws are used to define the hardening response within each deformation mode. Moreover, a multi-yield surface modeling approach is used where a CPB06 type anisotropic yield surface is assigned to each deformation mode. The evolution of the anisotropy coefficients involved in the expression of the yield function, is considered to model distortional hardening within each deformation mode. The evolving anisotropy parameters are found by minimizing the difference between the model predictions and the experiments, together with the interpolation technique proposed by Plunkett et al. (2006). The proposed model is calibrated using monotonic and reverse loading experimental data for AZ31B and ZEK100 magnesium alloys. A strain rate independent elasto-plastic formulation is used to implement the proposed constitutive model as a user material subroutine (UMAT) in the commercial finite element software LS-DYNA®. The predictions of the model are compared against the experimental monotonic and cyclic (CTC and TCT) flow stresses and r-values of AZ31B and ZEK100 sheets along different test directions. An excellent agreement is found between the simulated and experimental results.

Biodegradable behaviour and fatigue life of ZEK100 magnesium alloy in simulated physiological environment

ABSTRACTThe fatigue life of ZEK100 magnesium alloy in the phosphate buffered solution for various immersion intervals was investigated by experiments and theoretical predictions. The biodegradable behaviours of ZEK100 magnesium alloy were also studied. Microstructure observation showed that the corrosion behaviours were characterized by pitting corrosion. The corrosion rate decreased a lot in the initial 7 d and then almost stayed unchanged. After 28 d immersion, the elastic modulus almost kept stable, while the yield strength and the ultimate strength decreased a lot, which indicated that corrosion had important effects on the tensile mechanical properties. It showed that the fatigue life of the samples under the same stress conditions decreased with increasing immersion time under the asymmetric stress‐controlled cyclic loading. Considering the effect of corrosion on the material failure, a modified fatigue life model was proposed for magnesium alloy under corrosion.

Strain-controlled low cycle fatigue properties of a rare-earth containing ZEK100 magnesium alloy

Low rare-earth (RE) containing magnesium alloys are being considered for the lightweight automotive applications to reduce fuel consumption and emissions. Design of magnesium components requires strain-controlled low-cycle fatigue (LCF) behavior. This study was aimed to evaluate the cyclic deformation characteristics and LCF life of a low (0.2 wt.%) Nd-containing ZEK100-O alloy. The alloy contained equiaxed grains along with some Mg12Nd particles, and exhibited a relatively weaker basal texture. While slight cyclic softening occurred at high strain amplitudes, cyclic stabilization remained at lower strain amplitudes. Fatigue life of ZEK100 alloy was longer than that of the extruded RE-free AZ31 and AM30 alloys, due to a fairly good combination of strength with ductility. The asymmetry and skewness of hysteresis loops, which were characterized by eccentricity, angle deviation, and relative slope change, respectively, were effectively improved relative to the extruded RE-free alloys, arising from less extensive twinning caused by texture weakening and grain refinement. While the pseudo-elastic behavior tended to decrease with RE addition, it largely remained. An additional term was thus introduced to calculate the total strain range, i.e., Δεt=Δεe+Δεp+Δεan, where the total anelastic strain range (Δεan) consisted of both tensile and compressive components. Fatigue crack initiated from the near-surface imperfections, and crack propagation was characterized by fatigue striation-like features.

Microstructure, texture, and mechanical properties of friction stir spot welded rare-earth containing ZEK100 magnesium alloy sheets

The effect of friction stir spot welding process parameters on the microstructure, texture, and mechanical properties of ZEK100 (Mg–1.0 wt% Zn–0.5 wt% RE–0.5 wt% Zr) Mg alloy was investigated. Lap-shear joints were prepared using two different tool rotational speeds (1500 and 2250 rpm) and three different shoulder plunge depths (0.0, 0.2, 0.6 mm). Microstructure analysis revealed significant grain refinement in the stir zone, when compared to the base material. Electron backscatter diffraction analysis revealed a strong texture development in the keyhole periphery and adjacent regions despite the presence of RE-elements, however, no significant texture variation was observed within the process parameters. These results suggest that the ultimate failure of the weld is more attributed to macroscopic features such as the bond width and upper sheet thickness rather than texture development.

Constitutive Behavior of Commercial Grade ZEK100 Magnesium Alloy Sheet over a Wide Range of Strain Rates

The constitutive behavior of a rare-earth magnesium alloy ZEK100 rolled sheet is studied at room temperature over a wide range of strain rates. This alloy displays a weakened basal texture compared to conventional AZ31B sheet which leads to increased ductility; however, a strong orientation dependency persists. An interesting feature of the ZEK100 behavior is twinning at first yield under transverse direction (TD) tensile loading that is not seen in AZ31B. The subsequent work hardening behavior is shown to be stronger in the TD when compared to the rolling and 45 deg directions. One particularly striking feature of this alloy is a significant dependency of the strain rate sensitivity on orientation. The yield strength under compressive loading in all directions and under tensile loading in the TD direction is controlled by twinning and is rate insensitive. In contrast, the yield strength under rolling direction tensile loading is controlled by non-basal slip and is strongly rate sensitive. The cause of the in-plane anisotropy in terms of both strength and strain rate sensitivity is attributed to the initial crystallographic texture and operative deformation mechanisms as confirmed by measurements of deformed texture. Rate-sensitive constitutive fits are provided of the tensile stress–strain curves to the Zerilli–Armstrong[1] hcp material model and of the compressive response to a new constitutive equation due to Kurukuri et al.[2]

Friction stir spot welding of rare-earth containing ZEK100 magnesium alloy sheets

In this study, rare-earth containing ZEK100 magnesium alloy sheets were joined using friction stir spot welding. In order to determine the influence of welding parameters on lap-shear strength, overlap joints were created under various tool rotational rate and shoulder plunge depths. Relationships between the maximum lap-joint strength and key features of the weld including effective sheet thickness, interfacial hook width, and shoulder plunge depth were established. Based on the results of this study, welds with larger effective upper sheet thickness and small interfacial hook height generally exhibited the highest lap-shear strength. In addition, experiments indicated that overlap joints welded at 1500 rpm and 0.2 mm tool shoulder plunge depth exhibited the highest lap-shear strength.

Sheet texture modification in magnesium-based alloys by selective rare earth alloying

The current study examines the influence of select rare earth elements; Gd, Nd, Ce, La and mischmetal (MM) on the sheet texture modification during warm rolling and annealing of a ZEK100 magnesium alloy, and the resulting formability and anisotropy during subsequent tensile testing at room temperature. It was found that all the investigated RE elements led to weak sheet textures and hence promoted enhanced ductility and reduced anisotropy over conventional Mg sheet. Gd was of a particular interest because it gave rise to a desired Mg sheet texture despite its coarsest grain size resulting in promising mechanical properties. It is suggested that solute related effects on the grain boundary migration and the relative strengths of different deformation mechanisms are responsible for altering the common concepts of recrystallization and grain growth during annealing, and the activation scenarios of slip and twinning during deformation.