Advanced Materials Research Vol. 445

Abstract: Tube-tubesheet joints are critical in some applications, where contact between shell and tube side fluids is not tolerable. To ensure joint tightness, standards (ASME and TEMA) recommend performing a combination of rolling-or expansion of tube-tubesheet and seal welding. Available techniques for seal welding are based on fusion welding that sometimes results in a number of defects such as cracking and porosity formation, and such defects may take a newly fabricated heat exchanger out of service. In this work, friction stir welding (FSW) was used for tube-tubesheet seal joint and simulated using a 3D thermo-mechanical finite element model (FEM). The model was analyzed using a commercial finite element (FE) package. The model included the thermal effect of the tool workpiece interaction along with axial load, ignoring the metal flow around the tool. The material model took into account temperature dependency of thermal and mechanical properties. The model objectives were to evaluate the temperature distribution and residual stress in the workpiece resulting from the thermal cycle and axial load during welding for various process parameters, and to study how residual stresses in adjacent roller expanded tubes are affected during welding. The FE results show that the maximum temperature at the welding zone does not exceed the solidus temperature (except at high tool rotational speeds); the process can thus be classified as cold working. Moreover, adjacent tubes temperature does not exceed the annealing temperature. An experimental setup was designed and manufactured to show the feasibility of the process in this constrained size joints and to validate the numerical results. A test cell and a special FSW tool were designed and manufactured for this purpose. Many tests were performed with welding quality depending on process parameters.

Abstract: In the current context of fossil energy scarcity, car manufacturers have to optimize vehicles energy efficiency. This and continuous improvement includes a change of the exhaust manifold design. Usually in cast iron, exhaust manifolds tend to be mechanically welded in order to fit new constraints such as lightness, durability, efficiency and small size. To achieve such requirements, ferritic stainless steels with high chromium content (19%) and molybdenum (2%) are developed. For the welding, the use of existing filler wire does not satisfy fully the application requirements. This leads to oxidation problems and / or thermal fatigue strength that drastically reduces assembly lifetime. New flux cored wires are developed in the context of this study in order to provide molten zone characteristics close to those of the base metal. Different chemical compositions are tested in order to highlight the influence of stabilizing element on microstructure. Welding tests revealed the major influence of titanium on the grain refinement in the molten zone. A minimum Ti content of 0.45 weight % in the filler wire is required to be efficient as grain refiner.

Abstract: Stresses in brazing joints of different differed in properties were appraised as a result of technological experiments and FEM analysis. Evaluation of microstructure and mechanical properties of large dimensional vacuum brazed joints of Ferro Titanit Nicro 128 sinters and precipitation hardened stainless steel of X5CrNiMoCuNb14-5 using copper as the brazing filler metal. Structure of the joint was described. Shear strength R_t and tensile strength R_m of the joints have been defined. It have been state, that the basic factors decreasing quality of the joint, which can occur during vacuum brazing of the Ferro Titanit Nicro 128 sinter Cu brazing filler metal steel joints are diffusive processes leading to exchange of the cermets and brazing filler metal elements and creation of intermetallic in the joint. It can have an unfavourable influence on ductility and quality of the joint. The effect of joint geometry structure on stresses and deformations as well as on the process of plate cracking has been determined. Results of numerical calculations of three-dimensional models of brazed joints for different sizes of surfaces brazed at a constant width of solder gap are presented. Results of the investigate proved that joints microstructure and mechanical properties depend on filler and parent materials, diffusion process during brazing, leading to exchange of the cermets components and filler metal as well as joint geometry. The thickness of the joints has an essential influence on the values of the local stress and the significant influence on the joint rigidity. In a case of the considered joints the values of the local stress differences have been considerable in dependence of a fixed load manner.

Abstract: Friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. This process uses a non-consumable tool to generate frictional heat in the abutting surfaces. The welding parameters such as tool rotational speed, welding speed, axial force, etc., and tool pin profile play a major role in deciding the weld quality. Friction stir tool plays a major role in friction stir welding process. In this investigation, it is tried to evaluate the effect of tool pin thread and tool shoulder curvature on FSW zone formation in AA6061 aluminium alloy. In this regard, six different tool pin geometries (threadless triangular pin with/without conical shoulder, threaded triangular pin with conical shoulder, threadless square pin with/without conical shoulder, threaded square pin with conical shoulder) are used to fabricate the joints. The formation of FSP zones are analyzed macroscopically. Tensile properties of the joints are evaluated and correlated with the FSP zone formation. Consequently, it is obtained that welding creates a higher quality compared to other tool pin profiles using the square tool with curved shoulder and having threaded pin.

Abstract: This document aims at presenting and explaining the mechanism of a simple green process, called Graftfast^©, recently developed in order to graft polymer films onto any type of materials. This process is of great interest as it works in a short one step reaction at room temperature, atmospheric pressure in water. Particularly since this method is a redoxinduced process consisting in the reduction of diazonium salts into aryl radicals in presence of vinylic monomer, the involvement of such radicals was investigated. Moreover, this work demonstrates the efficiency of such process for the preparation of functionalized TiO₂ nanoparticles. The composition and the grafted polymer quantities were investigated showing the successful grafting of the polymer onto the nanoparticles while conserving their morphology.

Abstract: nanostructured coating materials become demanding, since it improves the mechanical properties, such as hardness, through grain refinement. One of the methods of producing nanostructured coatings is to use an arc spray coating process using nanostructured wires. Although the arc spraying process is well developed, the influence of nanoparticles on the fracture toughness of the coating surface has not been examined in details. Consequently, in the present study, arc spraying of nanostructured wires on carbon steel surface is carried out. The influence of coating thickness on the microstructure and fracture toughness of the coating is investigated. It is found that the self-annealing due to large coating thicknesses has a noticeable effect on the microstructure and fracture toughness.

Abstract: Main objective of this work is to manufacture the graphene platelet (GPL)-epoxy nanocomposite and to characterize the nanocomposite using nanoindentation technique. Thermal reduction of graphite oxide is the method used to obtain bulk quantities of graphene platelets (GPL) which comprise multiple graphene sheets. Dispersion of GPL in epoxy matrix is done with sonication and high speed shear mixing is used for mixing curing agent and resin. Following the manufacturing of graphene platelet-epoxy nanocomposites, characterization of the material was performed by nanoindentation. Nanoindentation experiments are performed under load or displacement control at different load/displacement rates to investigate rate dependent behavior of the nanocomposite. The primary mechanical properties obtained from the nanoindentation tests which are the hardness and the elasticity modulus are determined.

Abstract: Owing to its many exceptional properties, aluminium finds many applications in theaerospace, automotive, building and packaging industries. Enhancing its properties through alloyingor thermal treatments has been the focus of researchers’ interests for a long time. In this work, purealuminium powders were mechanically milled for up to 12 hrs and then were cold compacted andextruded to produce bulk nanostructured material. Both tensile and compressive tests wereconducted and the results compared. Post extrusion annealing treatments for up to 3 hrs wereconducted on additional samples.It was found that increasing the process control agent (PCA) content as well as the milling durationresulted in a finer microstructure and hence enhanced mechanical strength. This was accompaniedby a reduction in the ductility of the material. Moreover, compression tests revealed that thesamples are significantly more ductile in compression than in tension and that the decrease inductility with increase in milling time is less significant than in the case of tension. The differencein mechanical response is attributed to plastic instabilities. Annealing was found to enhance thetensile ductility of the samples without sacrificing strength.

Abstract: Thermal and acid wash treatment of rice husk has been conducted 550 to 600 °C in air atmosphere and chemical treatments consisted of acid, H₂SO₄. Purity, pore size distribution, FT-IR and SEM micrographs of treated and non-treated samples are presented. In this study the acid treated rice husk ash was used as SiO₂ source in the preparation of zeolite Y, a medium siliceous zeolite used as an ion exchanger in water treatment process. Obtained zeolite Y, BET Surface area 621.18 square meters per gram, pore volume is 0.33 cubic centimeters per gram and its average pore diameter is 21.222 Å. Ion-exchange capacities were tested through a column packed in zeolite Y by the artificially polluted water filtration, filtrate was analysed by ICP-MS and elements (As, Ba, Ca, Cd, Co, Cr, Bi, Be, Pb Ni etc.) in filtered waters was found in negative amounts. Zeolite Y is found a highly efficient ion-exchange material. So, mini water filtering system is a good for water treatment, even water is highly polluted by toxic elements.

Abstract: Complex metallic alloys (CMA) are new crystalline intermetallic phases representing an upcoming field in materials science. The β-Al₃Mg₂ intermetallic compound belongs to this new class of materials. Different amounts of pre-alloyed β-Al₃Mg₂ nanoparticles (from 0 to 20 wt.%) were mixed with aluminum matrix powder and then co-milled in attrition ball mill for 10 hours. Consolidated samples were prepared by hot pressing of blended composite powders. Microstructural characterization, applying an optical microscope (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyzer confirmed the formation of uniformly distributed β-Al₃Mg₂ nanoparticles in the matrix. Furthermore the results indicated that, increasing the amounts of β-Al₃Mg₂ nanoparticles leads the matrix grain size to be reduced. Effects of reinforcement contents on mechanical properties of nanocomposite samples were also investigated via hardness, compressive and wear tests. The results revealed that with increasing reinforcement content in the matrix, mentioned attributes of the composites are significantly improved.