Fe(iron) is beneficial in die-cast aluminum alloys, but its Fe-rich phases are highly detrimental to ductility, which limits the tolerable Fe content. In this study, a strategy of partially substituting Mn with Fe combined with Al-SrRE composite modification is proposed to improve the mechanical properties. Under a constant total (Mn + Fe) content, the effects of Fe content on the three-dimensional (3D) morphological evolution of Fe-rich phases and the mechanical properties of Al-7Si-0.2 Mg-xFe-(0.8-x)Mn alloy were investigated using OM, SEM, EDS, DSC, and synchrotron radiation X-ray computed tomography (SRXCT). The results show that with increasing Fe content and decreasing Mn content, the morphology of the Fe-rich phases evolves: needle-like + Chinese-script -* Chinese-script -* needle-like + Chinese-script -* coarse needle-like. When both Fe and Mn contents reach 0.40%, corresponding to an Mn/Fe ratio of approximately 1, the Fe-rich phases are dominated by fine and compact Chinese-script structures with a multi-branched polyhedral 3D morphology. At this composition, the alloy achieves the optimal mechanical properties, with a tensile strength of 176 MPa and an elongation of 9.6%, respectively. DSC results and thermodynamic calculations indicate that when the contents of Fe and Mn are close, the precipitation temperature range of the Fe-rich phases overlaps with the alpha-Al eutectic reaction. The competitive growth between them promotes the formation of fine and compact Fe-rich phases, which is the main reason for achieving a good combination of strength and ductility. This study provides new insight for the highvalue utilization of recycled aluminum with high Fe content.
High-strength AA7075 remains difficult to process by laser powder bed fusion because its broad solidification interval, high thermal conductivity, and evaporation of volatile Zn and Mg promote lack-of-fusion defects, keyhole porosity, and solidification cracking. In this study, TiB 2 nanoparticles were incorporated into gas-atomized AA7075 powder to improve melt-pool stability, refine the solidification structure, and enhance mechanical performance. A Taguchi L9 design was employed to investigate the combined effects of laser power, scan speed, hatch spacing, and TiB 2 content over ranges of 200 to 250 W, 500 to 800 mm s −1 , 70 to 100 μm, and 0.5 to 1.5 wt%, respectively. Relative density, porosity, lack-of-fusion fraction, keyhole-pore fraction, crack density, maximum crack length, melt-pool geometry, and Vickers microhardness were evaluated to establish the processing window. Three distinct processing regimes were identified. Insufficient energy input produced severe lack-of-fusion defects, as observed in Run 3, which exhibited a relative density of 93.12%, a total porosity of 6.88%, and a LoF fraction of 6.55%. Excessive thermal input promoted unstable keyhole formation and cracking, with Run 8 showing a keyhole-pore fraction of 2.53%, a crack density of 0.68 mm mm −2 , and a maximum crack length of 195.2 μm. Run 9 provided the most favorable overall consolidation response, achieving a relative density of 99.52%, total porosity of 0.48%, crack density of 0.14 mm mm −2 , maximum crack length of 52 μm, and keyhole porosity of 0.26%. Tensile specimens were fabricated in a second build using the Run 9 laser-processing parameters, with TiB 2 content varied from 0.5 to 1.0 wt% to assess the influence of the reinforcement level under the selected processing conditions. The optimized tensile condition exhibited an ultimate tensile strength of 518 ± 9 MPa, a yield strength of 432 ± 8 MPa, and an elongation of 16.2 ± 0.6%. The maximum measured microhardness within the Taguchi matrix was 158.7 HV. The observed microstructures were consistent with TiB 2 -assisted heterogeneous nucleation, cellular refinement, interruption of continuous epitaxial growth, and reduced crack continuity; these mechanisms are interpreted from the available microscopy and require matched-control quantitative EBSD and TEM confirmation. X-ray diffraction showed retention of the α-Al matrix and TiB 2 phase with no additional crystalline reaction products detected within the XRD detection limit. Overall, the results demonstrate that the processability and mechanical performance of LPBF-fabricated AA7075 cannot be optimized by energy input or reinforcement content alone. Simultaneous control of laser parameters, melt-pool stability, defect evolution, and TiB 2 dispersion is required to produce dense, crack-resistant components with a favorable strength-ductility balance.
Double-layer Gd 2 Zr 2 O 7 (GZ)/yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were deposited using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS) to investigate the effect of microstructure design on mechanical properties and wear behavior. Scanning electron microscopy (SEM) of the APS-deposited TBC showed a characteristic lamellar microstructure with relatively low porosity but pronounced microstructural heterogeneity associated with the presence of un-melted zones, microcracks and interlamellar defects. In contrast, the SPS coatings exhibited a fine-grained columnar microstructure with homogeneously distributed porosity. These differences strongly influenced the deformation response and tribological behavior. Nanoindentations performed on polished cross-sections showed that the APS coating exhibited nearly identical hardness and Young's modulus for both GZ (14 ± 3 GPa; 177 ± 34 GPa) and YSZ (14 ± 3 GPa; 182 ± 36 GPa) layers, characterized by relatively high standard deviations. In contrast, the SPS-deposited layers followed an inverse trend, with lower scatter of mechanical properties values and with a higher hardness and stiffness for GZ (11 ± 2 GPa; 155 ± 18 GPa) compared to YSZ (9 ± 2 GPa; 128 ± 17 GPa). Wear behavior was evaluated using a ball-on-disk configuration under reciprocating sliding conditions with a ZrO 2 counterbody. Under an applied load of 2 N, the APS coating demonstrated a lower wear rate ((4.5 ± 0.3) × 10 −4 mm 3 /Nm), whereas the wear rate of the SPS coating was extremely high ((19.8 ± 1.6) × 10 −4 mm 3 /Nm). At a higher load (5 N), the specific wear rates were (6.6 ± 0.6) × 10 −4 mm 3 /Nm for APS and (8.5 ± 1.1) × 10 −4 mm 3 /Nm for the SPS-deposited TBC. The dominant wear mechanisms in the APS coatings involved brittle fracture, interlamellar cracking, and splat pull-out, leading to progressive material loss with increasing load. The SPS coatings exhibited a transition from unstable contact conditions due to surface micro-fragmentation and debris accumulation at low load to a more stable wear regime at higher load. This transition was associated with progressive debris compaction, pore collapse and the formation of mechanically consolidated layers.
Carbon-fibre-reinforced polyether ether ketone (CF/PEEK) offers a lightweight alternative to metallic fastening components, but its vibration response and mechanical performance can be further tailored through particulate reinforcement. This study investigates TiB2-reinforced CF/PEEK hybrid composites and their application in lightweight polymer bolts, with particular emphasis on the relationship between TiB2 content, microstructural evolution, mechanical behaviour, thermal stability, and dynamic response. TiB2 was incorporated at 1–4 wt.% through melt compounding followed by low-speed planetary milling and compression moulding. The resulting composites were evaluated using microstructural, mechanical, thermal, and vibration characterization, while hybrid polymer bolts were assessed through frequency-response and impact-hammer testing. TiB2 incorporation improved hardness and compressive strength, with maximum values of 106 HRM and 359.97 MPa, respectively, at 4 wt% TiB2 for the 450CA30 configuration. For the dynamically investigated 3 wt% TiB2 configuration, the damping ratio increased from 0.88 to 0.94 for 450CA30 and from 0.86 to 0.91 for 150CA30, accompanied by substantial attenuation of resonance response. The 3 wt% TiB2 hybrid bolts also exhibited tensile-strength improvements of approximately 4.5% and 3.5% for 450CA30 and 150CA30, respectively. The principal novelty of this work lies in demonstrating the potential of TiB2-reinforced CF/PEEK as a lightweight polymeric fastening material with experimentally verified vibration-attenuation capability at the bolted-joint level. The results establish a composition–microstructure–property–dynamic-response relationship and identify 3 wt% TiB2 as a promising configuration among those subjected to dynamic testing, while acknowledging that definitive dynamic optimization requires characterization of higher TiB2 loadings.
The response of Ni-Cr alloys to exposure to molten chloride and fluoride salts is typically characterized by Cr dealloying with the formation of a Cr-depleted porous sub-surface layer. The exact mechanism behind Cr loss over distances unattainable by lattice diffusion alone is still debated. Here, two different surface finishes, namely electropolished and sanded, of a Ni-30Cr alloy were exposed to LiCl–KCl-2 wt% EuCl3 eutectic salt at 500 °C for 96 h. In the absence of fast diffusion pathways, dissolution occurred layer-by-layer and was consistent with near congruent Ni and Cr removal being rate-limited by the slower-dissolving Ni, as observed over the grain interiors of the electropolished sample. Grain boundaries were subject to diffusion-induced grain boundary migration (DIGM), leading to the formation of Ni islands above grain boundaries. This overall behavior contrasted with the sanded surface response characterized by several micrometer deep interconnected porosity and complete Cr depletion. DIGM of the dense grain boundaries created by recrystallization of the sanded surface was responsible for the observed sub-surface microstructure. While the salt redox chemistry set by EuCl3 fixes the driving force and selectivity for dissolution, the evolving grain boundary microstructure controls the kinetics, depth, and morphology of the sub-surface layer.
