جستجوی مقالات مرتبط با کلیدواژه « Ultrafine » در نشریات گروه « مکانیک »

Magnesium and its alloys have received much attention not only in the aerospace and electronics industry, but also in medical applications due to its low density, excellent physical properties, and biocompatibility. However, magnesium and its alloys have low ductility and poor strain hardening ability because of the hexagonal crystal structure with the limited number of slip systems at room temperature. Therefore, it seems necessary to improve their ductility and other mechanical properties via novel technologies. In this research, hydrostatic cyclic expansion extrusion has been used to produce ultrafine-grained magnesium rod. Properties of produced rods have been investigated morphologically and mechanically. The numerical investigation has also been performed to show the effects of hydrostatic pressure on strain distribution. Due to the brittleness of magnesium, the process has been conducted at elevated temperatures. Also, due to the fluid limitation at high temperatures, melted polyethylene has been used as the fluid in the process. The results showed that the yield and ultimate strength increased by 54% and 43% after only one pass of the hydrostatic cyclic expansion extrusion process, respectively. Also, elongation increased by 46%. Furthermore, microhardness has also increased with an average of 57 Hv to 70 Hv. The microstructure result showed that the grains become ultrafine-grained after only one pass of the process. Finite element investigation revealed that high hydrostatic pressure has a good effect on improving the strain distribution and the microstructure. This process seems very appropriate for industrial applications due to its ability to produce long ultrafine-grained rods.

One of the most important factors affecting the mechanical, physical and chemical properties of metals, are crystal structure and grain size. Ultrafine metals with an average grain size of 1000-100nm and nanostructured metals have an average grain size of less than 100nm. Ultrafine and nanostructured materials are known as a new generation of metal products, which have remarkably mechanical and physical properties in comparison to coarse-grained metals. In the last two decades, due to good properties such as high strength, high ductility and toughness, good corrosion resistance and high superplasticity properties, these materials have been considered by many researchers. In recent years, many methods have been proposed for severe plastic deformation and are now being developed and expanded. In these processes, in spite of the high hydrostatic pressure and the unaltered dimensions of the sample during the process, it is possible to apply very high strains, which results in the desired mechanical properties and ultrafine grained and nanostructured materials. The accumulative roll bonding process is one of the methods of SPD. ARB process is simple, extensive use, low-cost, industrially capability method that can produce ultrafine and nanostructured metals. In this research, light metals such as aluminum, magnesium and titanium and also extremely used metal such as copper and steel are discussed. Also, mechanical properties, fractography and microstructural properties of ultrafine and nanostructured metals produced by ARB are compared with initial samples and the mechanisms governing of ARB process that cause changes in mechanical and microstructural properties are analyzed.

In this article, it is tried to be mentioned the functional structure of production methods of ultrafine-grained (UFG) and nanograined tubes. As well as metallurgical and mechanical effects of these methods on the matter are fully investigated. Ultrafine grained materials contain grains with an average size of 100-1000 nm and if the grain size is less than 100 nm, the material is classified as nanograined material which have a lot of applications in different industries such as aerospace, automobile, military and medical. Generally, the methods presented in this paper has been done on common materials like aluminum and pure copper and magnesium alloy AZ91. Extremely large plastic deformations lead to ultrafine-grain or nearly nanomaterial in the severe plastic deformation (SPD) methods. Most severe plastic deformation methods for producing ultra-fine grain bulk, whereas in the past decade due to the increasing need tube components with high strength and good ductility, The research was conducted to produce UFG tubes. Advances in this field presented formally so that the advantages and disadvantages of each process are clearly comparable. The most important advantage of ultrafine-grain materials is an enhanced mechanical strength in comparison with their coarse grain counterparts. The microstructural reasons are discussed. Furthermore, this article reviews the refinement and deformation mechanisms, e.g. dislocation deformation mechanism, twin deformation mechanism, grain boundary sliding etc. of SPD methods.

In this research, a novel severe plastic deformation (SPD) method entitled cyclic flaring and sinking (CFS) is presented for producing of the ultrafine-grained (UFG) thin-walled cylindrical tubes. Finite element (FE) results showed that CFS process has a good strain homogeneity and requiring a low load. CFS process includes two different flaring and sinking half-cycles. At flaring half cycle, the flaring punch with two stepped regions is pressed into the tube. Shear and normal tensile strains are applied as a result of the existence of shear zones and increase in the tube diameter. In the second half cycle, the tube is then pressed to sinking die that applies same shear strains and normal compression strain so that the initial diameter of the tube is achieved and high plastic strain is applied. This process can be run periodically on the tube to exert more strain and consequently finer grain size and ultimately achieve better mechanical properties. The results indicated that the yield and ultimate strengths of the CFS processed Al (1050) tube were significantly increased to 165 MPa, and 173 MPa, respectively from the initial values of 50 MPa, and 115 MPa. The elongation to failure was decreased to about 14% after three cycles from the initial value of 42%. In addition, the hardness increases to ~38 Hv after ten cycles of CFS from ~23 Hv.