Hot Pressing Research Articles

Exploring the potential of agricultural waste enset fibers reinforced poly lactic acid biocomposites

This study explores the environmental sustainability of composite materials by reinforcing agricultural residue enset fiber as a potential reinforcement. Initially, enset fibers underwent a 5 wt% sodium hydroxide treatment to eliminate impurities and enhance fiber-matrix adhesion in the subsequent biocomposite fabrication process. Various enset fiber-to-polylactic acid ratios were blended and opened to enset/PLA fibers mat using a fiber carding machine, and the resulting biocomposite material was prepared through the hot press. A comprehensive assessment covering mechanical, thermal, dynamic mechanical, water absorption, and morphological properties was conducted on the enset/PLA biocomposites. The results showed increased mechanical (tensile strength of 24.12 MPa, bending stress of 33.02 MPa, flexural modulus of 2.01 GPa, the impact resistance of 12 kJ/m2) and thermomechanical properties with increased enset fiber loading, with optimum value at a 30 % weight percentage. The Differential scanning calorimetry results showed the addition of enset fiber increases the glass transition temperatures and degree of crystallinity. SEM images confirmed robust adhesion between enset fibers and PLA. This study highlights the potential applications of enset/PLA composites in automotive, furniture, packaging, and diverse industries, emphasizing their promising role in sustainable material development.

Improving the fatigue life of laser powder bed fusion Scalmalloy® by friction stir processing

A major concern about parts produced by laser powder bed fusion (L-PBF) are intrinsic defects or porosities that are difficult to overcome by simply optimizing the process parameters. As these defects and porosities play a crucial role in the mechanical behaviour, especially in fatigue, additive manufactured parts are often subjected to thermo-mechanical post-treatments. To this end, this work proves Friction Stir Processing (FSP) to be an effective post-treatment to drastically reduce the porosity level. FSP leads to an improvement of 60 % of the technical fatigue strength and by two orders of magnitude of the fatigue life of L-PBF Scalmalloy® specimens. The fatigue performances obtained on FSPed and heat-treated specimens are equivalent or even better than the best fatigue life reported in the literature, whatever their L-PBF conditions and post-treatments, while avoiding Hot Isostatic Pressing. However, FSP reduces the beneficial effect of the conventional strengthening heat-treatment applied to L-PBF Scalmalloy®, lowering the high tensile strength for which the alloy is normally reputed. Advanced characterisation by X-ray microtomography and Transmission Electron Microscopy allows us to reach a better understanding of the involved phenomena: drastic reduction of the biggest defects and heterogeneous nucleation of Sc- and Zr-rich precipitates on grain boundaries and dislocations.

Development and manufacturing of beryllium-armoring ITER enhanced heat flux FW toward series production in China

ITER’s enhanced heat flux (EHF) first wall (FW) provides thermal shielding to the other components behind it and limits its plasma boundary, consequently bearing high surface heat loads up to 4.7 MW m−2. China developed the EHF FW technologies in steady stages by making small mock-ups, semi-prototypes and a full-scale prototype (FSP), working toward series production at the Southwestern Institute of Physics. All the key technologies for bimetallic diffusion bonding, welding and assembly have been fully qualified. The Be armor tile size effects on thermal-fatigue performance have been evaluated using a pulsed high heat flux test with an EMS-400 electron beam facility. It was found that both the Be/CuCrZr bonding and its thermal-fatigue life are highly dependent on the tile size, and 12 × 12 mm2 Be tiles are acceptable for the required performance, either as individual tiles or in castellation by cutting larger ones. Small-scale mock-ups with such Be tiles survived 16 000 thermal cycles at 4.7 MW m−2, while EHF FW fingers with 16 × 16 mm2 Be tiles demonstrated unstable performance. In contrast, only BE tiles larger than 24 × 12 mm2 showed reliable diffusion bonding with the CuCrZr heat sink by hot isostatic pressing and consequently should be castellated into smaller ones. The manufacturing of the FSP finger goes through complex thermal cycles, using CuCrZr alloys in age-strengthening plus cold-rolling states, enabling it to maintain its tensile strength to the level of 285 MPa and its mean grain size less than 100 µm. The pipe welding for finger pairing and its assembly with the central beam are demonstrated, including enabling welding to be carried out twice for repair in a narrow space. The FSP showed good finger alignment, dimensional control and reliable vacuum tightness without any blockage of cooling channels.

Cr-Diamond/Cu Composites with High Thermal Conductivity Fabricated by Vacuum Hot Pressing.

Chromium-plated diamond/copper composite materials, with Cr layer thicknesses of 150 nm and 200 nm, were synthesized using a vacuum hot-press sintering process. Comparative analysis revealed that the thermal conductivity of the composite material with a Cr layer thickness of 150 nm increased by 266%, while that with a Cr layer thickness of 200 nm increased by 242%, relative to the diamond/copper composite materials without Cr plating. This indicates that the introduction of the Cr layer significantly enhanced the thermal conductivity of the composite material. The thermal properties of the composite material initially increased and subsequently decreased with rising sintering temperature. At a sintering temperature of 1050 °C and a diamond particle size of 210 μm, the thermal conductivity of the chromium-plated diamond/copper composite material reached a maximum value of 593.67 W∙m-1∙K-1. This high thermal conductivity is attributed to the formation of chromium carbide at the interface. Additionally, the surface of the diamond particles in contact with the carbide layer exhibited a continuous serrated morphology due to the interface reaction. This "pinning effect" at the interface strengthened the bonding between the diamond particles and the copper matrix, thereby enhancing the overall thermal conductivity of the composite material.

Honeycomb network structure constructed by silver nanoparticles achieving negative permittivity at low percolation threshold

Continuous conductive network is associated with the percolation effect, yet the method of fabricating network structure at low content is still a challenge. Herein, this work proposed a “honeycomb” structure via hot pressing to realize weak negative permittivity at low percolation threshold. By polydopamine (PDA) self-polymerization and silver mirror reaction plating, Ag nanoparticles coated polystyrene (PS) microsphere (PS@PDA@Ag) was prepared. Through hot pressing, the microspheres were compressed into “honeycomb”. The percolation threshold was reduced because the thin silver layer oscillated at low frequency plasma. Besides, by controlling the slivering time, weak negative permittivity in the range of −139 to −95 was observed and the percolation threshold was merely 2.23 vol%. The ac conductivity increased by four orders of magnitude and the thermal conductivity enhanced 79.08 %. The electric field and power loss density were simulated by finite element method. More than 100 V/m of electric field mode and 3 × 1011 W/m3 of power loss density were presented, explaining the generation of negative permittivity and the enhancement of dielectric loss. This work presented a method of achieving weak negative dielectric via honeycomb structure, and provided a novel perspective for the development of metal-based metacomposites.

Formation mechanism of TiC twin during densification of SiCf/Ti2AlNb composites

SiCf/Ti2AlNb composites were fabricated using the magnetron sputtering precursor wires method in conjunction with the hot isostatic pressing (HIP) technique. The morphology, elemental distribution, and structural characteristics of the interfacial reaction layer (RL) were examined through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and wavelength dispersive spectroscopy (WDS). The results showed that the interface between SiC fibers and Ti2AlNb reacted to form TiC and α2. Due to the interdiffusion of elements at the interface, a small amount of Al atoms existed in the TiC layer. These Al atoms reduced the stacking fault energy (SFE) of TiC, which is beneficial for the formation and stability of Σ3{111} twin boundaries.

Solution temperature influence on CRSS of the secondary γ′ phase in nickel-based superalloy FGH4096

Nickel-based superalloy components are capable of being manufactured by direct hot isostatic pressing (HIP) process with near-net shape and nearly 100 % raw materials utilization. However, they cannot be applied directly, due to the poor mechanical properties of HIPed components. Fortunately, heat treatment can improve the properties via regulating the microstructure. Therefore, the effects of varied solution temperatures on the tensile properties of FGH4096 nickel-based superalloy disk at room and high temperatures were studied. The results revealed that the disk after sub-solution treatment (1130 °C) has higher tensile properties (room temperature yield strength: 1194 MPa, elongation: 15.5 %; high temperature yield strength: 1075 MPa, elongation: 10.5 %). The antiphase boundary (APB) shear stress, Orowan bypass stress, stacking faults (SF) shear stress, and micro-twins (MT) shear stress affected by the size and distribution of the secondary γ′ phase were calculated. The results show that the main strengthening mechanism at room temperature is APB shear, and the APB shear stress of solution treatment at 1190 °C is the highest. The high temperature strengthening mechanism is due to SF shear and MT strengthening effects. The difference in SF shear stress between various solution temperatures is minimal. 1190 °C has a higher MT strengthening effect than 1090 °C. This is consistent with the actual result that the tensile properties at 1190 °C are superior to those at 1090 °C. The conclusion is that the combination of high strength reflected by several critical resolved shear stresses and considerable plasticity reflected by the MT strengthening effect can lead to a comprehensive improvement in alloy tensile properties.

Microstructure and properties of hot isostatic pressed/laser remelted Fe3Al/Cr3C2 composites

The effects of Cr3C2 content on the microstructure, hardness and wear resistance of Fe3Al/Cr3C2 composites by hot isostatic pressing and laser remelting were studied. The results showed that, the carbides in the hot isostatic pressed (HIPed) Fe3Al/Cr3C2 composites are in large block or irregular shape, while the in situ formed carbides in the laser remelted (LRed) Fe3Al/Cr3C2 composites are small and dispersed. With the increase of Cr3C2 content, the microhardness of HIPed and LRed Fe3Al/Cr3C2 composites gradually increased, and the microhardness of LRed Fe3Al/Cr3C2 composites is higher than that of HIPed ones. With the increase of Cr3C2 content, the wear resistance of HIPed Fe3Al/Cr3C2 composites increased gradually, while the wear resistance of LRed Fe3Al/Cr3C2 composites first increases and then decreases, and the best wear resistance is obtained when the volume fraction of Cr3C2 is 18%.

Effect of hot isostatic pressing on microstructure and properties of cooled hot dip aluminum coating in magnetic field

The effect of hot isostatic pressing (HIP) on the microstructure and properties of hot dip aluminum coating cooled in a magnetic field was investigated in this study. In order to improve the microstructure and properties of magnetic dip aluminum coating, hot isostatic pressing technology was used for post-treatment. Initially, a traditional aluminum-impregnated coating was prepared on the surface of titanium alloy TA15, an alternating electromagnetic field was applied during the forming and solidification process of the coating. Finally, the coating was treated with hot isostatic pressing technology. Analyzed three different coatings of the microstructure and element distribution, and tested the microhardness of the coatings at various positions. The test results revealed that the TA15 titanium alloy hot-dip aluminum coatings obtained through the three different processes exhibited a gradient structure. Compared with the traditional hot-dipped aluminum air-cooled coating, when an appropriate intensity of alternating electromagnetic field was applied during the coating solidification process, the outer coating structure was enhanced, the number of holes was reduced, the microstructure density increased, and the number of cracks significantly decreased. The defects of the 800 °C hot isostatic magnetic cold and hot dip aluminum coating were repaired under high temperature and pressure, resulting in a uniform and fine microstructure. The comprehensive properties of the magnetic cold and hot dip aluminum coating on the surface of the titanium alloy were effectively enhanced through hot isostatic pressing.

Polyphenylene oxide/SiO2 composites: Towards ultra-low dielectric loss and high thermal conductivity of microwave substrates

With the rapid development of electronic information industry has raised the requirements for the performance of electronic components. Therefore, it is essential to research dielectric substrate materials with excellent dielectric properties and good thermal conductivity at high frequencies. This study introduced vinyl trimethoxysilane (VTMS) surface-functionalized silicon dioxide (SiO2) into the polyphenylene oxide/styrene–butadiene–styrene/triallyl isocyanurate (PST) composite resin to prepare PST/V-SiO2 composite. The microwave composite substrates for high-speed signal propagation were prepared through a process involving scraping coating and hot pressing. The study investigated the impact of varying amounts of V-SiO2 on the dielectric property, thermal stability, and thermal conductivity of the composites. The microwave composite substrates exhibit excellent dielectric properties and good thermal conductivity. When the V-SiO2 content is 80 wt%, the dielectric loss is 1.8 × 10-3 with a low dielectric constant (≤3.23@10 GHz). Furthermore, the PST/V-SiO2 composite displays higher thermal conductivity (0.735 W/(m·K)) and low moisture absorption. This research provides a novel strategy to develop microwave composite substrates with low dielectric loss and high thermal conductivity.

Study on synergistic friction between surface micro-texture and hot-pressing filling coal-to-oil soot solid lubricant

To study the combination of triangle micro-texture (TM) and coal-to-oil soot (CS) solid lubricants to improve the tribological properties of steel balls and disc surface, three filling processes (hot pressing, mechanical pressing, and alcohol coating) were used to prepare CS composite lubricating film on TM surface (TM-CS). The tribological behavior of CS was examined across various experimental parameters using the WTM-2E controlled atmosphere micro-friction and wear tester. The structure and morphology of CS were scrutinized with high-resolution transmission electron microscopy. The tribological mechanisms were involved by X-ray photoelectron spectroscopy and scanning electron microscopy. The results reveal that the hot press filling is the most effective method, which can achieve a 97 % filling rate. The optimal tribological performance is observed at a speed of 900 rpm and a load of 2.5 N. Under this condition, the TM-CS surface reduces by 75 % in the average friction coefficient (AFC) and 88 % in the average wear scar diameter (AWSD) compared to the TM surface. The analysis of the friction mechanisms indicates a substantial synergistic effect between the TM surface and the CS solid lubricant. This synergy is significantly potentiated by the electrostatic adsorption phenomenon and the impact of airflow at high rotational speeds, which play a pivotal supportive role in enhancing tribological performance.

Superior uniform deformation ability of intragranular nano TiB reinforced Ti-6.5Al–2Zr–Mo–V fabricated by hot isostatic pressing

In order to improve the uniform deformation ability of titanium matrix composites, the intragranular nano-TiB whiskers (TiBnw) reinforced Ti-6.5Al–2Zr–1Mo–1V (TA15) composite was designed and fabricated by hot isostatic pressing (HIP) at the temperature of 1000 °C. The TiBnw was successfully planted into the pre-alloyed powders before HIP. The composite sintered for 6h (HIP-6h) realized a complete densification process benefiting from a high sintering pressure of 150 MPa. While the 2h sintering process (HIP-2h) was insufficient to densify. The HIP-6h exhibited an unparalleled uniform elongation of 9.5 %, which was superior to the survey TA15 matrix titanium composites. The excellent uniform deformation ability was demonstrated to be associated with the equiaxed structure and the blocking and multiplication effect on dislocations from the intragranular TiBnw.

Enhancing mechanical properties and tribological performance through boron carbide hybridization in Novolac matrix graphite composites under dry sliding condition

This study extensively investigates the effects of 10 wt% graphite reinforcement and varying B4C content (1 wt%, 2 wt%, 4 wt%) on the properties of Novolac matrix polymer composites prepared by mechanical milling-assisted hot pressing. The microstructures, wear surfaces, wear debris, and fracture surfaces of the produced polymer composites were examined using SEM and EDS. The density measurements were conducted according to the Archimed’s principle. Their tribological behavior was evaluated with a reciprocating ball-on-flat sliding wear test at 50 N, 75 N, and 100 N loads. After the wear tests, surface topography was analyzed in 3D with an optical profilometer. The results indicated that composites with 2 wt% B4C had the lowest specific wear rate and the highest wear resistance.

High-performance glued-bamboo through activation of chemical bonding interface

Bamboo glue products are widely used in decoration, construction and transportation. Bamboo is fast growing, strong and inexpensive, making it one of the ideal processing materials to replace steel and wood. Here we present a process for the production of high performance bamboo glue products. In this process, the conversion of natural bamboo surface (NBS) to oxidized bamboo surface (OBS) is a critical step prior to hot pressing, as it provides the bamboo interface with active sites (aldehyde groups) for chemical cross-linking with the adhesive. Subsequent covalent crosslinking with branched chain amine (PAXN) compounds is based on Schiff base chemistry. The improved properties of the oxidized bamboo specimens were attributed to chemical cross-linking within the oxidized bamboo-binder interface (OBI-PAXN) and further densification of the bamboo adhesive interface. The dry/hot/boiling water bond strength of OBI-PA6 N was increased by 130 % (7.5 MPa)/148 % (5.05 MPa)/130 % (3.26 MPa) compared to NBI-PA6 N (3.26 MPa, 2.04 MPa, 0.88 MPa). Its bonding strength and bending strength are superior to commercial PF resins. The combination of excellent mechanical properties with the rich, renewable and sustainable nature of bamboo makes it very attractive for engineering applications.

High strength and fatigue performance achieved for L-PBF processed hybrid particle reinforced Al-Cu-Mg composite

This study highlights the successful manufacturing of a crack-free, dense, hybrid ex-situ/in-situ particle reinforced (Ti + B4C)/Al-Cu-Mg composite, fabricated by laser powder bed fusion and exhibiting exceptional mechanical performance. In its as-built (AB) state, the composite displays a unique microstructure characterized by equiaxed grains with an average grain size of 1.0 ± 0.3 μm, notable interdendritic microsegregation of Cu, Mg, Mn, and Fe, randomly distributed ex-situ added Ti and B4C particles featuring a surface interaction layer with the metal matrix, and in-situ formed reinforcing particles, such as TiB2 and TiC. After subjecting the material to hot isostatic pressing (HIP) and subsequent aging treatment, dissolution of interdendritically segregated elements occurs, and precipitation of Al2Cu, Al12Mg17, and Al-Fe-Cu-Mn phases is observed. Significantly enhanced fatigue performance is recorded, reaching to 107 cycles at 250 MPa in AB and 330 MPa in HIP state, marking a 32 % improvement. The current study highlights the intricate relationship between the different microstructural features in AB and HIPed state, leading to fracture during tensile and fatigue loading conditions.

Multifunctional performances of structural battery composite full-cells based on carbon fiber anode and LiFePO4 loaded carbon fiber cathode

In this work, the structural battery composite (SBC) full-cells based on carbon fiber (CFs) were fabricated using a three-step hot pressing method. LiFePO4 (LFP) was loaded onto CF fabrics considering the influences of hot pressing parameters including the pressure and the temperature. The SBC full-cells were subsequently fabricated using the LFP loaded CF cathode, the CF anode, the glass fiber (GF) separator via a structural electrolyte (SE) filming process, followed by the second hot pressing. The multifunctional efficiencies were assessed for SBCs with SE containing different components. To reduce the capacity loss, the SBC was eventually encapsulated with the GF/Vinyl Ester prepreg and thermally cured in the third hot pressing. The capacity retention of the SBC was significantly improved after encapsulation. This work could be seen as a further step forward the engineering fabrication of the SBC full-cells based on both CF anodes and cathodes.

Structure, microstructure, mechanical and luminescent properties of (Y0.96Еu0.01Sm0.01Tb0.01Er0.01)NbO4 ceramics obtained by uniaxial hot pressing

Fine powders of yttrium orthoniobate activated by Eu, Sm, Tb, Er have been synthesized by the sol-gel. The synthesized powders have been calcined at different temperatures (1000, 1100 and 1180 °C). Luminescent ceramics (Y0.96Еu0.01Sm0.01Tb0.01Er0.01)NbO4 were prepared from fine powders by uniaxial hot pressing (UHP) via various technological modes. For the first time, the crystal structure, microstructure, spectral composition, photoluminescence properties of (Y0.96Еu0.01Sm0.01Tb0.01Er0.01)NbO4 ceramics have been studied depending on the temperature conditions of the synthesis of powders and UHP conditions during sintering of ceramics. The mechanical properties of these ceramics (Young's modulus, microhardness and critical stress intensity factor for the mode I KIC) have been evaluated.

Thermal Conductivity of AlSi12/Al2O3-Graded Composites Consolidated by Hot Pressing and Spark Plasma Sintering: Experimental Evaluation and Numerical Modeling

Functionally graded metal matrix composites have attracted the attention of various industries as materials with tailorable properties due to spatially varying composition of constituents. This research work was inspired by an application, such as automotive brake disks, which requires advanced materials with improved wear resistance on the outer surface as combined with effective heat flux dissipation of the graded system. To this end, graded AlSi12/Al2O3 composites (FGMs) with a stepwise gradient in the volume fraction of alumina reinforcement were produced by hot pressing and spark plasma sintering techniques. The thermal conductivities of the individual composite layers and the FGMs were evaluated experimentally and simulated numerically using 3D finite element (FE) models based on micro-computed X-ray tomography (micro-XCT) images of actual AlSi12/Al2O3 microstructures. The numerical models incorporated the effects of porosity of the fabricated AlSi12/Al2O3 composites, thermal resistance, and imperfect interfaces between the AlSi12 matrix and the alumina particles. The obtained experimental data and the results of the numerical models are in good agreement, the relative error being in the range of 4 to 6 pct for different compositions and FGM structure. The predictive capability of the proposed micro-XCT-based FE model suggests that this model can be applied to similar types of composites and different composition gradients.

High Power Factor Flexible Ag2Te Film on Nylon by a Wet Chemical Method for Power Generator.

Herein, we develop a facile wet chemical method for the synthesis of Ag2Te powders at room temperature and flexible Ag2Te/nylon thermoelectric (TE) films are prepared by vacuum-assisted filtration of the synthesized Ag2Te powders and then hot pressing. Because of the good crystallinity of Ag2Te grains and continuous grain boundaries, an optimized film exhibits a power factor of 513 μW m-1 K-2 at 300 K, which stands among the highest values reported for Ag2Te-based films to date. In addition, the film also has good flexibility. A four-leg flexible TE device assembled with the film generates a power density of 5.46 W m-2 at a temperature gradient of 31.8 K. This work provides a facile and environmentally friendly method for preparing flexible Ag2Te films.

New processing routes for Zr-based ODS ferritic steels

In this work, Zr addition is proposed to refine the nanoparticle dispersion in an ODS RAF steel of composition Fe-14Cr-2W-0.3Zr-0.24Y (wt.%). Three batches of material are obtained using pre-alloyed atomized powder, where yttrium is directly introduced in the melt, and manufactured through three different processing routes. First route is based on the newly developed STARS route that aims to avoid subsequent mechanical alloying. The second route explores the impact of mechanical alloying in pre-oxidized powders. The third route uses mechanically alloyed powders without the pre-oxidation process. The ODS-powders were individually consolidated by hot isostatic pressing and later hot rolled. The obtained materials were characterized by small-angle neutron scattering (SANS) and X-ray absorption spectroscopy (XAS) techniques. SANS and XAS analysis point out the absence of oxide nanoparticles in the material based on the STAR route. SANS analysis confirms that the mechanically alloyed materials do exhibit the presence of nanoparticles. These are identified as Zr-O-rich nanoprecipitates by XAS and the calculated A-ratio by SANS is linked with the phase Y2Zr2O7. Their radii are in the range of 3–3.6 nm. XAS results show that mechanical alloying minimizes the initial differences regarding the oxidation state between the ODS powders with and without pre-oxidation.