Papers by Keyword: Interface

Abstract: Optimizing the performance and reliability of welding techniques for dissimilar aluminum (Al) to titanium (Ti) is a promising way to establish new applications in aerospace industry. Due to structural weight reduction, lightweight materials can help to minimize fuel consumption and save emissions. Solid-state welding technologies allow short joining cycles and metallurgical changes, residual stresses and severe intermetallic compound formation can be reduced by limited thermal exposure. Besides temperature and plastic deformation, intimate contact plays an important role for diffusion. In this work, AlMgSi alloys with systematic variations of Mg and Si alloying elements, were welded to Ti6Al4V (Ti64) by refill Friction Stir Spot Welding. The focus lays on the effect of Ti64 sheet surface roughness, varied by different surface preparations. Additionally, the influence of the plunge depth, the distance between the tool and the Ti64 sheet surface is analyzed. It was found that a reduced tool to interface spacing has a beneficial influence on joint integrity. Grinding trenches allowed better bonding compared to the pit-like surface structure generated by sandblasting, which led to an increase in mechanical lap-shear properties. Knurling the grinded surfaces resulted in high standard deviation, as most likely not the whole interface area was bonded. However, the partially outstanding properties showed that a beneficial effect can be expected due to mechanical interlocking mechanisms, when sufficient diffusion is ensured.

Abstract: Optimizing the mechanical properties of aluminum to titanium welds is crucial to establish applications for dissimilar lightweight structures in the aerospace industry. In this context, solid-state welding technologies have proven effective in terms of short joining cycles, allowing the combination of cost-effective production and structural weight optimization. However, metallurgical effects between aluminum and titanium in the joint interface are still not completely understood due to differences in physical as well as chemical characteristics. In this study, aluminum alloy 6013 was welded to Ti6Al4V by refill Friction Stir Spot Wel ding, including systematic variations of Mg and Si alloying element content in the used AA6013 sheets. In total five different Al alloys were welded to the titanium to investigate the influence of Mg and Si during processing. Apart from the material selection, the weld strength is mainly influenced by the intermetallic compound thickness at the interface, which in turn primarily depends on the exposed temperature cycle. Consequently, major interest during this study was given on the temperature evolution, interfacial features and the global mechanical properties.

Abstract: The early failure of single-lap adhesive joints in carbon fiber reinforced polymer (CFRP) composites is typically induced by stress concentration at the edges of the overlap region. To address this issue, this study proposes a novel local pre-curing process system based on gradient thermal curing regulation. Through multi-physical field modeling of the temperature-curing coupling effect and a gradient curing control strategy, active optimization of the adhesive layer stress distribution is achieved. By optimizing the interface stress distribution, the proposed technique demonstrates the potential to enhance the overall joint performance by 3-6%. This research combines Abaqus finite element simulation and experimental verification. A CFRP single-lap joint model considering the temperature-curing coupling effect was established to analyze the influence of local pre-curing on the stress distribution of the adhesive layer. The results show that: 1. Local pre-curing can reduce the peel stress in the critical edge danger zone by about 3 - 5% while improving the shear strength in the middle region. 2. The preferential curing at the center of the adhesive layer can induce stress redistribution, relieve the stress concentration at the edges, and thus improve the overall load-bearing capacity of the joint. This study provides a low-cost and easily implementable solution for optimizing the performance of CFRP joints, showing potential applications in the field of lightweight aerospace structures. Through the synergistic effect of precise thermal management and regulation of the adhesive's rheological properties, it offers new insights for advanced composite joining technologies.

Abstract: The electronic structure of ultrathin Na/GaN interfaces was studied using photoelectron spectroscopy with synchrotron radiation in the photon energy range of 75–770 eV. The experiments were carried out in situ in ultrahigh vacuum of 5·10^–10 Torr with submonolayer sodium coverages on the gallium nitride surface. The photoemission spectra of the Ga 3d and N 1s core levels were studied at different excitation energies. It was found that Na adsorption causes a decrease in the intensity and a shift in the spectra of the Ga 3d and N 1s core levels towards higher binding energies. It was found that the sodium adsorption leads to some changes in the spectra due to charge transfer between the Na adlayer and the surface Ga or N atoms.

Abstract: In this study, the bonding of WC-Co cermet to AISI 304L stainless steel was achieved through the flash spark plasma sintering (FSPS) process under a steady pressure of 5 MPa and ultra-short holding durations. The investigation focused on the impact of holding time on interfacial characteristics, diffusion behavior, and mechanical performance. The results demonstrated that prolonged holding times, particularly up to 12 seconds, led to pronounced interfacial deformation and significant diffusion of Co, Ni, and Fe elements across the joint interface. Toughness assessment of the WC-Co cermet near the bonded region was carried out using the Vickers indentation fracture (VIF) technique. The analysis revealed a decline in mechanical integrity with extended holding times, increasing the brittleness of the joint despite the enhanced elemental diffusion between the cermet and the stainless steel.

Abstract: Amongst various strategies to mitigate the environmental impact of non-degradable polymers, the integration of Cork with fossil fuel-derived Nylon is considered an attractive option to develop a lightweight, strong composite. To optimize the integration of these materials for processing as 3D printed structures requires the exploration of functional compatibilizers to enhance the homogeneity and 3D printability of the Nylon-Cork composite. In this paper, Nylon-12 (PA-12) was mixed with cork in varying melted compositions using one coupling agent/stabilizer/compatibilizer, namely: 3-aminopropyl triethoxysilane (APTS), to improve the interfacial bond between the components and amenability for 3D printed composite structures. This paper examines the characteristics of this composite using scanning electron microscope (SEM), rheology experiments and differential scanning calorimetry (DSC). These findings are used to understand and explain the ensuing 3D printed characteristics using the APTS compatibilizer and Nylon-Cork ratios. This work is expected to be critical for developing low-density engineering products using PA-12-Cork composites and for sustainable processing, using 3D printing technologies.

Abstract: A structural composite material is obtained by incorporating continuous and strong fibres in a polymer matrix. Such a design leads to materials with exceptional mechanical properties over a very small density. This family of composite materials can be extended further by combining special designs of composite sub-parts, like in honeycomb structures. Thanks to their performances, these composites are increasingly used in a range of applications mainly in the energy, construction, automotive and aerospace sectors. However, it is very difficult to dismantle composite materials in multi-material structures for recycling purposes; currently, they are mainly incinerated to produce energy. The present paper proposes adding “smart chemical additives” during composite manufacturing and assembly, which will facilitate both the separation of multi-material structures into single blocks, and the separation of composite sub-parts into raw materials. This innovative “debonding on-demand” function provides a significant incentive to using composite materials in a circular economy, i.e. promoting the repair, reuse and recycling of these materials.

Abstract: A process when different materials are combined to produce a product with multiple layers is called co-extrusion. During this process, polymers are melted in separate machines and then extrudate from different die channels. Once these channels converge, the polymers meet and flow through a single channel. The surface where the two fluids face is called “interface”. It is crucial to maintain the interface's uniformity and stability in order to achieve the desired multi-layered structure. Most of the issues in co-extrusion are related to issues that can be classified into two categories such as polymer encapsulation/interfacial distortion and die swell. To solve these problems, designers focus on improving the interface's stability. This paper examines effects of cross-section modification of the two-channel feedblock on the interface location and velocity and pressure distributions of the flow. The ANSYS software was used to simulate the co-extrusion of polymers, LLDPE and HDPE, in two-channel feedblock with rectangular, circular, and straight slot cross-sections. The results show that sharp corners increase the thickness of dead zones, while rounding them decreases the thickness. Additionally, stadium-shaped (or straight-slot) cross-section channels can move the flow with a higher maximum velocity and thinner boundary layer combining the results of rectangular and circular feedblocks.

Abstract: In the ultrasonic bonding process, oxides existing on the metal surfaces are removed, and bonding is achieved by bringing clean surfaces to be in contact with each other. However, the bonding process with microstructure variation is not well understood due to experimental difficulties. In this study, using a newly developed sample holder, which enables ultrasonic bonding in a TEM, we directly observed the bonding process at the nanoscale. The bonding process of Au foils with a clean surface was investigated and compared to that of Al foils with a stable oxide film, a bonding inhibitor, on the surface. During the Al ultrasonic bonding process, the nanoparticles generated dispersed over the entire bonding interface and finally formed a fine grain region at the interface. In contrast, in Au bonding, the nanoparticles generated tended to accumulate at the local area of the Au surface and form bridge-like connections between Au foils. It was considered that these differences in bonding behavior were caused by the surface conditions of the materials to be bonded.

Abstract: Silicon carbide (SiC) metal-oxide semiconductor (MOS) power devices such as metal-oxide semiconductor field-effect transistors (MOSFETs) require a stable and low defect-density interface, and a high-quality dielectric, for good device performance and reliability. Notably, the interface and dielectric properties determine the threshold voltage stability, the field-effect channel mobility, and the device lifetime as limited by dielectric breakdown in both the forward on-state and reverse blocking conditions. Here we discuss the present state of SiC MOS processing and properties and point to directions for future development. Important items to address are: 1) interface passivation approaches; 2) dielectrics; 3) device design; and 4) in-depth measurements of the interface quality and reliability.