1,721,059 research outputs found
Metal Matrix Nanocomposites; potentials, challenges and feasible solutions
No full textThis thesis focuses on the processing and properties of three different metal matrix nanocomposites, Al and copper matrix composites produced by classical powder metallurgy techniques and magnesium (Elektron21) matrix composites fabricated by an ultrasound assisted casting method in the frame of EXOMET project. In fact, aluminum and copper matrix composites are reinforced by graphene nanoplatelets (GNPs) whereas magnesium matrix composites are reinforced by some nano-ceramic particles such as aluminum nitride and aluminum oxide. Regarding the Al and copper matrix composites, the targets were to obtain a uniform dispersion of graphene within the metallic matrix, to avoid the undesirable reaction between GNPs and the metal matrix, to obtain a strong interfacial bonding, to improve the mechanical properties, decrease the coefficient of thermal expansion and improve the thermal and electrical conductivities. The experimental results showed that fabrication of MMNCs reinforced by graphene is not an easy task and it is vital to take into account all the possible issues in order to exploit all the potentials of graphene in the composites. In General, the aim was to fabricate composite materials and determine the relationship between the production, microstructure and final properties of the bulk composites. Regards to the dispersion of reinforcement within the matrix, it is found out in the case of aluminum and copper matrix nanocomposites the wet mixing method has a great potential to disperse the graphene nanoplatelets not only homogeneous but also without damaging the structure of graphene. However, the dispersion of graphene nanoplatelets at higher contents is still puzzling. On the other hand, although AlN and Al2O3-AlOOH nanoparticles were dispersed uniformly within the molten electron 21 alloy by means of ultrasonication, but during the solidification, particles were pushed and agglomerated after solidification. On the whole, it can be concluded that despite all the efforts that have been undertaken during this work, and although some promising results were obtained at low graphene content, the fabrication of MMNCs reinforced by high graphene content is still challenging. In any case, it was proved that the wet mixing method which was developed during this research can be easily and successfully applied to the fabrication of MMNCs reinforced with low graphene content and allowed to achieve improved thermal and mechanical properties. Moreover, in El21/AlN and El21/Al2O3-AlOOH several complex phenomena such as particle pushing, particle agglomeration, reaction between the particle and the matrix and in-situ formation of some particles have been identified. Microstructural analysis showed that the grain size slightly reduced through the addition of AlN particles. All mechanical properties of El21-AlN at room temperature such as hardness, compression and tensile strength, did not show any improvement with respect to the unreinforced alloy. However, a significant improvement was observed in the creep resistance of El21-AlN so that at low stresses the minimum creep rate of composite is one order of magnitude lower than base alloy. Nevertheless, this minimum creep rate is almost equal to the minimum creep rate of El 21 after solution treated followed by ageing up to the maximum hardness. In the case of El21-AlN composites some aluminum was transferred to the melt, while zirconium reacts with the nanoparticles, originating some complex interfacial reaction that could form strong interfacial bondings between the matrix and the reinforcement. On the other hand, in El21-Al2O3 composites a reaction between alumina and magnesium leads to the in-situ formation of MgO particles and to the presence of Al in the alloy. The presence of Al in the alloy can bring to reactions with rare earth, with the formation of highly stable phases (i.e. Al2Nd), whose dissolution seems to be impossible even at high solution treatment temperature. The overall trend of precipitation hardening of composites was completely different from the base alloy that shows the effect of nanoparticles on the dissolution of precipitates and kinetics of precipitation during the ageing treatment. Finally, it can be concluded that the fabrication of metal matrix composites either by powder metallurgy or by casting faces with several challenges. . In particular, some of these difficulties can be attributed to the nature of the involved materials, while some of them are related to the preparation technique. It seems that these latter can be often solved by changing the production process or by using post-processing techniques. More challenging, instead are the problems related to the composition of matrix and reinforcement, their reactivity and to the dispersion of particles, in particular if nano-sized. These topics still bring a significant challenge to the materials scientists, and it would be worth to say that fabrication of MMNCs with uniform dispersion of reinforcement, strong interfacial bonding, without detrimental reactions and improved isotropic properties is still a puzzling issue
EFFECT OF DIFFERENT PREPARATION ROUTES ON COLOR METALLOGRAPHY OF ZIRCONIUM ALLOYS BY HEAT TINTING
No full textSo as to investigate the color metallography of Zirconium alloys via heat tinting method, different grades of this alloy such as Zircaloy-4, Zr-1% Nb, Zr-2.5% Nb has been applied as a base metal. For this purpose, each sample was prepared by different preparation routes such as polishing, polishing and etching, electropolishing. On the other hand, two important parameters (temperature and time) which play a significant role in the surface quality of specimens are considered in such a way that the surface of each sample is thermally etched at temperatures of 300 and 400ºC for 15 - 30 minutes. Results show that the best route for sample preparation is polishing and etching by using an electro-polishing apparatus. Outcomes show that the best temperature of heat tinting for different samples is about 400ºC and the best time is between 15 - 20 minutes. By comparison the optical microscopic images, it can be found that the heat tinting method is not suitable for samples with superplastic deformation microstructure while for annealed samples with equiaxed microstructure shows distinct image
On the processability of copper components via powder-based additive manufacturing processes: Potentials, challenges and feasible solutions
No full textThe high electrical and thermal conductivities make copper the most suitable material for producing components where a high heat transfer capability is required. The material efficiency can be enhanced by designing shell and tube exchangers that allow high heat-transfer coefficient and high turbulence. However, high-performance design often required many manufacturing operations, including welding, that compromise the heat exchanger theoretical efficiency. Additive Manufacturing (AM) techniques can solve this problem definitely, but copper manufacturing is still particularly challenging for AM. Over the last decade, many studies have been carried out to face this topic and show possible solutions. This paper offers an overview of research advances on copper manufacturability via powder-based AM processes. The solutions are grouped into two categories: technological modifications and material modifications. This review highlights the best practices that may be considered for future works to accelerate the development of copper processed by AM. Overall, this work points out the importance, challenges and opportunities when the potentialities of AM processes are integrated with the unique characteristics of copper
Comparing the Cold, Warm, and Hot Deformation Flow Behavior of Selective Laser‐Melted and Electron‐Beam‐Melted Ti–6Al–2Sn–4Zr–2Mo Alloy
No full textThe cold, warm, and hot deformation flow behavior and mechanical properties of the Ti-6242 alloy produced by selective laser melting (SLM) and electron beam melting (EBM) are compared. Hot compression tests are conducted over a wide temperature range (100–1000 °C) at a constant strain rate of 0.001 s−1. The yield and overall strength levels of the SLMed specimens are higher than those of the EBMed specimens at all temperatures. A thermally stable region is observed for SLMed specimens in the temperature range of 100–700 °C, but the strength level drops significantly (approximately 300–500 MPa) at above 700 °C. The stability region shifts to the lower temperature range of 100–600°Cfor the EBMed specimens, and strength starts to decrease at 600 °C. Both specimens experience fracture in a cold-temperature regime but exhibit a high strain tolerance of approximately 0.4. The SLMed samples display a completely brittle behavior at a warm temperature regime, whereas, the EBMed specimens demonstrate better formability, tolerating higher compressive strains. The softening fraction substantially increases at high temperatures, indicating safe domains for thermomechanical processing of additively manufactured Ti6242 alloy
Effect of Graphene Nanoplatelets (GNPs) on Properties of Pure Copper
No full textGraphene nanoplatelets (GNPs)/copper composites has been prepared by conventional powder metallurgy. Although GNPs/Al composites are widely studied, homogeneous dispersion of GNPs is still a big challenge for the researchers, which limits its use in practical applications. Our novel nano-processing route is free of ball milling which can damage the structure of GNPs during mixing. Therefore, our method can be an alternative of ball milling and it has a great potential for the synthesis of Cu based matrix nano-composite which is considered good for engineering applications. Nevertheless, the interface of GNPs/Cu plays a critical role in electrical resistivity in such a way that by increasing the GNPs/Cu interface the electrical resistivity increased.Nonetheless the hardness of the composite was increase by the introduction of GNPs
Graphene Reinforced Aluminum Matrix Composites Produced by Powder Metallurgy; Microstructure, Thermophysical And Mechanical Properties
No full textAluminum matrix nanocomposites reinforced by different graphene nanoplatelets content were produced by a novel wet mixing followed by conventional powder metallurgy. The microstructure, thermal conductivity, thermal expansion coefficient and compressive strength of the nanocomposites were investigated. Microstructural evaluations show that at lower graphene contents a uniform dispersion of GNPs can be achieved while at the higher GNPs content they start to form some agglomerates. Outcomes of the mechanical properties evaluation indicate that the addition of low GNPs content improve the hardness and compressive strength of the composite, while the effect of high GNPs content is opposite. Furthermore, the thermal conductivity of composites decreases as a function of GNPs content. It is also found that the coefficient of thermal expansion of composites is reduced by the introduction of GNPs. The agglomeration of GNPs is exacerbated as a function of graphene content, which is the main reason for the deterioration in most of the properties
Innovative Approach to Evaluate the Mechanical Performance of Ti–6Al–4V Lattice Structures Produced by Electron Beam Melting Process
Full text linkAdditive manufacturing processes allow producing complex geometries which include structures with enhanced mechanical performance and biomimetic properties. Among these structures, the interests on the use of lattice are increasing for
both medical and mechanical applications. The mechanical behaviour of the structure is closely correlated to its shape and
dimension. However, up to now, far too little attention has been paid to this aspect. Hence, this work aims to explore the
efect of geometry, dimension and relative density of the cell structure on the compressive strength of specimens with lattice
structures. For this purpose, various Lattice structures are designed with diferent geometries and dimensions. This approach
leads to having structures with diferent relativity densities. Replicas of the designed structure are produced using Ti–6Al–4V
powder processed by electron beam melting process. The samples are tested under compression. A new approach to calculate the absorbed energy up to failure by the lattice structure is presented. The results show a close relationship between the
mechanical performance of the structure and the investigated parameters. In contrast with the current literature, the presented
experimental data and a collection of the literature data highlight that the lattice structures with similar relative density do
not exhibit the same Young’s modulus values
An investigation on the effect of deposition pattern on the microstructure, mechanical properties and residual stress of 316L produced by Directed Energy Deposition
Full text linkIn this work, 316L cubes were produced by Directed Energy Deposition (DED) process. To evaluate the effect of deposition patterns on the microstructure, mechanical performance and residual stress of 316L samples, two different deposition strategies are selected (67° and 90°). The general microstructure is revealed, and then the effect of deposition pattern on the microstructure of 316L alloy is evaluated through the Primary Cellular Arm Spacing (PCAS) analysis. The cooling rate in each sample is estimated according to the PCAS values. Interestingly, it is found that by increasing the rotation angle per layer, the PCAS value decreases as a consequence of increment in the cooling rate. On the other hand, in both cases, by increasing the distance from the substrate, due to the changes in cooling mechanisms, the cooling rate at first decreases and then at the last layers increases again. The phase composition analysis of 316L samples confirms the predictions that suggested the presence of residual δ-ferrite in the final microstructure. In fact, the final microstructure of samples is characterized by austenitic dendrites together with some residual δ-ferrite in the interdendritic regions. Moreover, the microstructural evaluations exhibit that during the DED process, some metallic inclusions are formed within the 316L samples that consequently deteriorates their mechanical properties. Tensile results show that the samples with 90° rotation per layer have a better mechanical performance such as slightly higher ultimate tensile strength and almost 35% higher elongation to fracture, mainly owing to their finer microstructure and slightly less oxide content. However, in both cases, the elongation of the 316L samples is lower than the typical elongation of this material produced via DED. This discrepancy is found to be as a result of higher inclusions contents in the samples produced in this work with respect to those of literature. Lastly, it is found that the residual stresses on the top surfaces are similar for both deposition patterns, although higher stress values are observed on the lateral surfaces of the cubes produce using the 90° rotation per layer
Laser powder bed fusion in situ alloying of AISI 316L-2.5%Cu alloy: microstructure and mechanical properties evolution
Full text linkThis work investigates the effects of copper addition on the microstructure and mechanical properties of AISI 316L austenitic stainless steel fabricated by the laser powder bed fusion (L-PBF) method. The outcomes reveal that the copper atom dissolves into iron and forms a complete austenitic structure under the condition of the L-PBF process. Microstructural observations demonstrate that the microstructure of the new alloy is characterised by columnar grains consisting of finer cellular structures, as compared to the as-built AISI 316L. The appearance of such a finer sub-structure could be originated from the effect of copper on the cooling rate during the L-PBF process. The energy-dispersive X-ray spectroscopy maps indicate that the distribution of copper in the AISI 316L matrix is homogeneous, and no significant segregation of elements in the matrix is revealed. The results of the tensile tests show that the ultimate tensile strength of AISI 316L-Cu alloy is 558 MPa, whereas the yield strength value and the tensile elongation are 510 MPa and 30.4%, respectively. Two mechanisms of solid solution strengthening, and refinement of cell sizes improve the mechanical properties of AISI316L-Cu alloy compared with AISI 316L one. The microscale fractography of the fracture surface shows ductile fracture with massive dimple networks and brittle fracture with a quasi-cleavage plane, which may indicate the melt pool boundary. All these results confirm that the development of new alloys following the in situ alloying approach is economical and reliable
The electrochemical behaviour of Ti-48Al-2Cr-2Nb produced by electron beam powder bed fusion process
Full text linkTitanium aluminides (TiAl) are distinguished by their exceptional strength-to-weight ratio, making them ideal for aerospace and medical applications. Notably, TiAl alloys offer a unique combination of high-temperature resistance and corrosion resilience, contributing to their growing prominence in advanced engineering and biomedical fields. Although initially developed for aerospace applications, TiAl alloys have demonstrated promising potential as implant materials over time. Hence, this research focuses on producing γ-TiAl alloy through electron beam powder bed fusion (EB-PBF) technology, utilising a powder with a composition of Ti-48Al-2Cr-2Nb. For comparative purposes, the corrosion characteristics of Ti6Al4V produced via EB-PBF were also evaluated under identical conditions. The findings indicate that the EB-PBF γ-TiAl exhibits exceptional resistance to corrosion. This is supported by the significantly high polarisation resistance and corrosion potential values, as well as the notably low corrosion current value. However, based on the analysis of the polarisation and impedance curves, it can be observed that the γ-TiAl sample displayed a less protective passive film formation. This occurrence can be attributed to the presence of aluminium ions within the passive layer, resulting in the formation of unstable oxides. As a consequence, it can be inferred that γ-TiAl exhibits inferior resistance to pitting corrosion when compared to Ti6Al4V alloy. The point defect model and Mott-Schottky test further revealed that the γ-TiAl alloy exhibited increased oxygen vacancies. Additionally, the presence of aluminium ions as impurities or dopants led to their substitution for titanium ions, creating cationic vacancies within the passive film. The accumulation of excessive cation vacancies ultimately led to the initiation of pitting corrosion
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