نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 50 (پاییز 1398)

Nowadays, the production of industrial components by semi-solid methods is considered as a new method. These methods are time consumable due to slurry preparation process, leading to the development of methods that allow the creation of non-dendritic structures in the shortest possible time. For example, the method of enthalpy exchange Material (EEM) is one of the most common processes that results from the proximity of two materials with high and low enthalpy. In this research, the effect of adding swarf on the structure of die casting parts has been studied on aluminum alloy A380 and the possibility of structural changes in different temperature conditions has been investigated. The microstructure of the parts has changed from dendrites to globular and the microstructure consists mainly of alpha phase, eutectic and intermetallic compounds. SEM results showed that the distribution of copper and iron elements is in grain boundaries of aluminum alpha particles. Also, the solid weight fraction was calculated using the thermal analyze results and confirmed by microscopic images.

In this paper, the effect of adding a low amount of platinum to slurry Si-modified aluminide coatings containing silicon less than the amount required to provide complete protection in these coatings is investigated. In addition to the coatings characterization, the role of platinum on the type II hot corrosion behavior of coatings has been discussed. To create silicon and silicon-platinum aluminide coatings, a slurry silicoaluminzing method was applied to samples of Ni-base superalloy GTD-111 specimens, which was previously Pt-electroplated on some of them with a thickness of 2 μm. The amount of silicon in the slurry was 10% by weight relative to total solid of the slurry, so that the amount of silicon in the coating is less than the critical amount required for complete protection. The type II hot corrosion test was carried out using a furnace method with Na2SO4-60mol% V2O5 at 700 °C. After 20 hours of hot corrosion, Al2O3 protective oxide and non-protective oxide NiAl2O4 were detected in silicon-platinum- and silicon-aluminide coatings, respectively. In the absence of platinum, the Ni3V2O8 phase was also found to be a product of NiO and NiAl2O4 dissolution in type-II hot corrosion condition. At the end of the 80-hour hot corrosion, unlike the silicon-aluminide coating, silicon remained in the silicon-platinum-aluminide coating structure.

In this research, the microstructure of the pulsed laser surface melted AISI H13 tool steel was studied. Then, by laser surface alloying with TiC powder, the effect of particle size and powder concentration on superficial composite microstructure was investigated. For this purpose, TiC powders with particle sizes of 1 micrometer and 45 micrometers in layers of different thicknesses were pre-placed on the surface of H13 steel and then subjected to pulsed laser operation. The results showed that in the surface melting, an intermittent cell/dendritic structure developed from the depth to the surface of the molten pool with a higher concentration of alloying elements in the boundary network. With the selected laser parameters, the cooling rate was estimated at one million K/s. In the surface alloying process, the preplaced TiC particles were completely (fine powders) or a partially (coarse powders) dissolved in the melt pool. During subsequent cooling, TiC-type MC carbides precipitated from the melt. Increasing the thickness of the preplaced layer caused the morphology of carbides to be more diverse. The size of precipitated MC carbides was reduced by decreasing the concentration of TiC powder in the melt pool and increasing the particle size of preplaced TiC powder. As the number of MC carbides increased, the cellular/dendritic structures of the steel matrix replaced by coaxial grains.

It is necessary to use a physical model for the relationship between welding parameters and hot cracks. These models are available in micro, meso, and macro-scale. In this research, a sheet of 6061 aluminum alloy was welded by a Nd:YAG laser machine. For the first time, the diameter of the dendritic arm spacing in the aluminum laser weld was measured and the results were compared with the solidification models. Contrary to the prediction of hot crack models, increasing the dendritic arm spacing, decreasing the solidification rate, and the reduction of the strain rate did not reduce hot cracking. However, based on the pre-existing models, preheating should reduce hot cracks, but inversely increases the amount of cracks. The images of high speed cameras and the assessment of crack surface by a field emission scanning electron microscopy showed that in pulsed laser welding, hot cracks will be created in three steps: 1) initiation 2) the first step of propagation 3) the second step of propagation. Propagation in the second step will occur in the newly solidified weak grain boundary of the weld metal. What is finally seen as a crack in the weld seam is the solidification and high temperature cracks and therefore, the models that are considered for continuous fusion welding are required to be modified based on the conditions of the pulsed solidification and melting and the fracture of weak grain boundaries after solidification should also be taken into account.

In this study, the effect of nickel particle size and green density on the formed phases, ignition time, and porosity distribution in NiAl-TiB2-TiC composite produced by combustion synthesis method was investigated. For this purpose, reactive powders were weighed and mixed to form a mixture of Ni + Al + x (3Ti + B4C). The green mixture was compressed in the form of a cylinder. The samples were synthesized using an induction furnace which the sample with a constant heating rate. The reaction was ignited in a quartz tube under a flow of argon gas. The microstructure of the products was examined using Scanning Electron Microscopy (SEM), and the phases were determined using X-ray diffraction (XRD). The amount of open, closed and total porosity was measured using Archimedes method, and the distribution of porosity was analyzed by using image analysis software. The results showed that by decreasing the average particle size of nickel from 87µm to 3µm, due to increase in contact surfaces, there was a decrease in ignition time of the samples, and more intensive reactions were encountered. Using finer nickel resulted in a ca. 10% decrease in the porosity of products. The minimum ignition time was obtained at green density of 85%. Increase in the relative density of green sample reduced the amount of secondary open pores; the porosity of the samples with a relative density of 65% and 95% was 58.8% and 26.9% after synthesis, respectively.

Biometals such as stainless steels, tantalum, titanium, cobalt and their alloys are widely used for medical implants for bone and dental healing. Researches indicates that metals such as cobalt, chromium, nickel, aluminum and vanadium, release ion in vivo, which make the human body sensitive to the allergy and so implant lessening. The biocompatibility and non-toxic ion release of titanium is suitable for human body implantation, but it is needed to improve its strength. Nano-structured pure titanium is a newly developed method to increase the strength of implants, and increase their efficiency. In this research some parameters of Equal Chanel Angular Pressing (ECAP) like number of passes and temperature were investigated. After ECAP processing on pure titanium, the mechanical, metallurgical and biological properties of the products were investigated using standard tests. Results indicated that using ECAP process and reducing the titanium grain size from 14 micron to 440 nanometer, its mechanical and biological properties improved. Results of biocompatibility measurement indicated that the processing temperature of 240 ◦C make a very good biocompatibility with Adipose-derived stem cells (ASCs). Also the biocompatibility of titanium increased by increasing the number of the ECAP passes. The processed titanium at 240 ◦C for 4 passes, could be introduced as the choice during the other candidates.