Papers by Author: Mahmoud Nili-Ahmadabadi

Abstract: Thixoforming or semi-solid metal forming offers many advantages in comparison with casting and conventional forging. However, due to the high-melting temperature and related difficulties, there is relatively a few experimental data on the semi-solid processing of steels. Therefore, this study is subjected to study the microstructure evolution during partial remelting of 304 stainless steel which is priorely deformed in solid state by equal channel angular pressing (ECAP) at room temperature. ECAP is a promising technique to induce a great homogeneous strain in bulk metals known as a new method for strain induced melt activation (SIMA) process for preparing thixoforming ingot. Simultaneously, the effect of number of passes during ECAP was also discussed in terms of the microstructure change. The results showed that the microstructure was well refined with formation of fine twins and partial martensite along with many fine equiaxed grains which in turn lead to fine and spheroidal semi-solid slurry. It can be confirmed that ECAP followed by reheating was an effective method to produce semi-solid ingot for thixoforming.

Abstract: Fe-10Ni-7Mn (wt. %) steel is a member of ultrahigh strength steels which shows good ductility in the solution annealed condition and excellent age hardenability. In the current research, this alloy was subjected to heavy cold rolling in which the reverse transformation of martensitic to austenite was brought about. From the XRD, DSC and dilatometric analyses, it is resulted that after 60 % cold rolling the austenite phase may be formed by displacive mechanism. Stability of austenite at room temperature is referred to the ultrafine/nanograin size of austenite after deformation which prevents the austenite to martensite transformation. The presence of ultrafine/nanoaustenite formed by displacive mechanism leads to the observation of new mechanical properties during cyclic tensile test. This behavior is known as pseudoelastic phenomenon. In this behavior, during loading-unloading tensile cycle, the shape of the specimen return to its original configuration with a hysteresis loop in its path to the zero strain point.

Abstract: There is a growing interest for investigation of free-volume type defects in SPD-processed ultrafine-grained materials. In the present work, excess free volume in pure Fe prepared by shaped rolling is studied by high-resolution differential dilatometer as well as Archimedes method. The results show the formation of excess volume with increasing deformation strain. The volume fraction of free volume detected by dilatometer is below the evaluation of Archimedes method maybe because of the existence of void in the deformed metal. Microstructural study by field emission type scanning electron microscope confirms the presence of voids.

Abstract: Thixoforming is a new forming technology which has been studied by many researchers during the last years. Mold filling is one of the most important steps for casting engineers which should be controlled to have a sound part. In the semisolid alloy forming die design, viscosity variations during forming and temperature decreasing, solid fraction plus globularity are the main parameters which affect final product. In this study, a stepped die was designed and some billets in different weights were injected into the die. This process helps to characterize the flow pattern during mold filling. The effect of two phase flow was also studied by microstructural investigating. Numerical simulation is the second method which is used in this study. The ProCast software was used to indicate the flow pattern of the fluid in the mold. Some comparisons were also done to show that this software is a suitable simulating software to predict the flow behavior of semisolid alloys. Also, the step casting method shows the exact pattern of flow in different parts of mold and is a reliable method for researchers to investigate the fluid pattern.

Abstract: The inclined Cooling Plate process can be considered as a reliable method for the production of semi-solid ductile cast iron. By utilizing aforesaid method, the main limitation of common semi-solid methods, i.e. Mg fading, will be eliminated. The present research is intended that the importance of alloying design through the inclined cooling plate method and its profound effect on obtaining the optimum solidification range for thixo-forming process have been illustrated. The optimization of solidification range for thixo-forming process has been gained by the simulation in both equilibrium and non-equilibrium conditions. Afterwards the alloy selection has been performed amongst several alloying composition with a view to their effect on the thixo-forming process and austempering.

Abstract: Elements such as Mn and Mo may be added to ductile iron to improve its hardenability and mechanical properties, but segregation of these elements could result in the reduction of mechanical properties and must therefore be considered during the manufacturing of thick section ductile iron. In this research the microstructures of cast specimens containing 2% Mn were studied by optical microscopy and scanning electron microscopy. Results showed that manganese segregates in the intercellular areas while silicon segregates in the regions around graphite nodules. Partial Melting Homogenization (PMH) was carried out on the samples to reduce segregation. Analysis of the results showed that homogenization has been done successfully and segregation in the ductile iron matrix was significantly reduced. The successful effect of homogenization was also studied on austempered samples. PMH prior to austempering resulted in a very uniform bainitic microstructure. Tensile tests were carried out on the samples which showed improved mechanical properties due to combined PMH process and austempering process.

Abstract: In this investigation the role of alloying elements on austemperability of heavy section ductile irons was studied. Four different chemical compositions were chosen in a way to specify an optimal chemical composition with suitable austemperability. Austempering was carried out at 315 and 350oC for 1 hour on specimens prepared from cast Y-blocks with 75 mm thickness. Metallographic examinations and hardness tests indicated that thickness of bainitic layer was positively dependant on the amount of the alloying elements which deferred pearlitic transformation in TTT diagram to the longer time. In addition, by increasing the amount of alloying elements, the morphologies of ferrite in bainitic structure changed from featherlike to acicular. According to the results of this study, in order to achieve a full bainitic structure (in the specimens whit 75mm thickness), the optimal amounts of alloying elements were: 0.35 % Mn, 0.22 % Mo, 1.1-1.4% Ni and 0.6% Cu. In addition, regarding to the thick sections of Y-blocks, graphite degeneration defect was also studied, and it was noticed that this defect could be completely eliminated by adding 50ppm of antimony.

Abstract: Microstructure and mechanical properties of high Si bainitic steel, before and after two passes of equal channel angular pressing (ECAP) at room temperature were investigated. SEM and TEM microscopy were used for microstructural study. Shear punch test and Vickers hardness test of as received and ECAPed samples were carried out to measure the influence of ECAP process on the mechanical properties of the samples. The results showed that tensile strength and shear strain were increased as a consequence of ECAP processing.

Abstract: Fe-Ni-Mn martensitic steels show excellent age hardenability but suffer from embrittlement after aging. Discontinuous coarsening of grain boundary precipitates was found as the main source of embrittlement. Effect of cold rolling and equal channel angular pressing on the mechanical properties of an Fe-10Ni-7Mn steel was investigated. Cold rolling for 20%, 40%, 60%, 80% and 90% and equal channel angular pressing for four passes through the Bc route were carried out on a solution annealed material with subsequent aging at 753 K. Hardness measurement, tensile test and scanning electron microscopy were used to study mechanical properties and microstructural features of the as-deformed and aged alloys. Improvement in tensile properties of the as-deformed and aged alloys was found. A tensile strength of about 1840 MPa along with 3% elongation were determined for cold rolled by 90% thickness reduction and aged alloy, while conventional steel shows a premature fracture stress of 820 MPa with zero ductility. It was also indicated that after heavy cold rolling ductility increases in comparison to the equal channel angular pressed and aged alloy.

Abstract: NiTi shape memory alloys are a group of materials which have a lot of applications especially in aerospace industries and medical equipments because of their excellent properties. Shape memory effect (SME), pseudo-elasticity (PE), high corrosion resistance and biocompatibility is special properties of these alloys which lead to their extensive applications. The superior behavior of NiTi alloy is due to thermoelastic martensitic phase transformation. In the present paper, two NiTi shape memory alloys were prepared by non-consumable vacuum arc melting technique in copper water cooled crucible. One of them had commercial elements and the other had high purity elements. Metallographic investigation, chemical analysis, XRD and DSC were carried out on two alloys. Metallographic observation and XRD shows that structure at ambient temperature consists of austenite phase besides Ti2Ni, Ni3Ti intermetallic compounds and martensite phase. Transformation investigation determines that the impurity such as iron in commercial alloy causes two stage phase transformation B2→R→B19′.