مجله مهندسی متالورژی
پیاپی 68 (زمستان 1396)

In this study, change in microstructures and corresponding mechanical properties during friction stir welding are investigated. Microstructural analysis was carried out by optical microscope and electron microscope. Tensile test is done by uniaxial tensile test machine. The results proved that Abrasion of welding tool happened when the transverse speed is relatively low and as a result some WC particles originated from the welding tool is observed in the weldment. In contrast, in relatively high transverse speed causes formation of tunneling defect in weldment. The results from uniaxial tensile test illustrated that the specimen welded by transverse speed of 100 mm/min was fractured from base metal, however in case of transverse speed of 50 mm/min fractured was appeared in weldment area. Significant grain refinement in welded specimens resulted in increment of yield strength. Furthermore, harness measurements demonstrated that in weldment area the hardness value increased from 155 HV to 210 HV.

The most important drawback of hyper-eutectic Al-Si alloys is the presence of coarse, primary, irregular and brittle silicon phases, which strongly weakens the material. Therefore, modifying the morphology, decreasing the size and uniforming the distribution of primary silicon phases are essential. The aim of this paper is to use friction stir processing (FSP) for modification of surface microstructure of hyper-eutectic Al-Si alloy. Also, the effect of processing parameters on microstructure and tribological properties was investigated by the light microscopy and microhardness measurements. For this reason, several plate-shaped samples were prepared by casting in both metallic and sand molds. It was observed that microstructure is uniformed, porosities are removed and silicon phases are finely distributed by friction stir processing, leading to improvement of microstructure and mechanical properties of the alloy. The results also showed when Mg added and cooling rate increased, particles’ size changes from 33.09 to 21.05 µm, micro hardness increased from 53 to 69 HV, and microstructure morphology is modified.

The degradation mechanisms are occurred in gas turbine nozzle imposed in the critical high stress and temperature. These mechanisms could lead to reduce the service life and failure. Rejuvenation heat treatment could recover some microstructural changes to the initial state and increase the service life. In this paper, the heat treatment influence on the microstructure of the first-stage gas turbine nozzle made of IN738LC was studied after 33,000 hr of operation in a 160 MW gas turbine. The microstructure was investigated by optical and scanning electron microscopes to show the changes of the precipitated phases size, distribution morphology. Precipitated phases are γ' and carbides in grain boundaries or inside grains. The results show the proper size and distribution of the γ' and carbides similar to the initial structure obtained by the combination of the full solution heat treatment and the standard heat treatment compared. Furthermore, the tensile test was performed to evaluate the performance of heat treated sample. The tensile results of full Solution heat treatment cycle confirm the microstructural observation and show the higher tensile properties (8% in 0.2%YS, 12% in UTS & 100% in El) Related to the Standard heat treated samples.

In this study, the solid state recycling of aluminum waste (3000 and 5000 series) through mechanical milling, leads to production of nano-crystalline aluminum powders. For this goal, aluminium used beverage cans, which usually consist of body (75%), lid (22%) and opening part (3%), are used as raw material. The body, lid and opening of cans usually are alloyed 3004, 5182 and series of 5017, 5042 or 5082, respectively. After two-steps hammer milling of cans, fine chips with 10 mm in size was achieved. The de-coating and chemical cleaning operations were performed to remove the paint layer and other contaminations (such as oil and oxides), respectively. The ball milling was accomplished on clean aluminum fine chips in a planetary ball mill under argon atmosphere for various times up to 48h. The results show that the short time would not be enough for milling. On the other hand too long times are unwelcome either, apparently as a consequence of microstructure leads to agglomeration. In the case of process controlling agent, however it leads to finer powder production, it contaminates the final product due to decomposition during milling. Since mechanical milling without PCA would be suitable. Totally, at least for the condition used in this investigation, 24h mechanical milling without PCA for production clean nano crystalline Al powders finer than 50nm in crystalline size with uniform size (smaller than 40 µm) will be recommended.

In this study, the effect of the heat input on the microstructure and mechanical behavior of the 5456 series aluminum thin sheets in two processes of gas tungsten arc welding (GTAW) and gas metal arc welding of (GMAW) was investigated. For this purpose, the aluminum base metal with a thickness of 0.9 mm, ER5356 as metal filler with a diameter of 1.2 mm and argon gas were used. Microstructure and fracture surface were studied by optical microscopy and scanning electron microscopy respectively. Also, to study the mechanical properties of the joint, the tensile and microhardness tests were used. The results shows that the microstructure of TIG and MIG welding process is coaxially dendrite and dendritic columnar respectively. The microstructure of TIG welded sample was finer than MIG welded sample as a result of lower heat input and lower temperature gradient. This event cause increase in strength and elongation in TIG welded sample compared to MIG welded sample. Also, the fracture surface of TIG sample is consists of the fine dumplings and funnel-shaped cavities, and so the ductile fracture has occurred, but the semi brittle failure was done in the MIG sample.

Ductile irons are a type of engineering materials that used in many industrial applications, extensively. Thus, improvement of mechanical properties of ductile irons is very important. In this study, AISI 1045 continuous steel chips applied into the ductile iron matrix as reinforcement and ductile iron composites were manufactured through sand mold casting process. Therefore, the casting ductile iron included 0, 5, and 10 volume fraction of steel chips. Light microscopy and scanning electron microscopy (SEM) equipped energy dispersive X-ray spectroscopy (EDS) used to investigation of the microstructure and interface region. In addition, the mechanical properties of the ductile iron composites were studied by tensile and hardness tests. As a result, graphite morphology and microstructure of ductile iron matrix affected on the reinforcement fraction. In addition, a compatible interface was observed between the reinforcement and ductile iron matrix. Formation of this interface can be probably related on the melting the chips surfaces and solidification of this region. Thus, improvement of the tensile properties by increasing the reinforcement fraction can be related on the formation of the compatible interface between the matrix and reinforcement.

Ferrites exhibits unique structural, electrical, magnetic and mechanical properties in extreme conditions that are of great research interest. In this work, the effects of sintering atmosphere and temperature on elastic moduli properties of Ni0.3Cu0.2Zn0.5Fe2O4 nanoparticles have been investigated. The elastic constants of the samples have been calculated by using the values of the structural parameters and also the vibration frequencies obtained from infrared spectra. The values of shear (VS) and longitudinal (Vl) wave velocities obtained from force constants have been used to determine the values of Young’s modulus (E), rigidity modulus (G), bulk modulus (B), Debye temperature (θD). By comparison of the elastic results of the studied samples, we can observe that the elastic moduli of the samples sintered under reducing atmosphere are higher than the other samples, whereas they have been improved with increase of sintering temperature under reducing atmosphere. In addition, using the values of the compliance sij obtained from elastic stiffness constants, the values of Young’s modulus E(hkl) and Poisson’s ratio σ(hkl,θ) along the oriented direction [hkl] have been calculated for the samples.

Challenge of change of alloy properties during service life of extra safety parts was always considered by industrialists and consequently researchers. Fuselage and wings of airplane repeatedly in ascent and descent is affected respectively by cooling to -55 0C and heating to environment temperature. Hence, the effect of above temperature changes on tensile and fatigue properties is unknown. In this study in laboratory, the situation of airplane body working is simulated and then the changes of microstructure and consequently the changes of tensile properties and hardness after holding for 10 and 4 hours were studied respectively in temperature -60 and -196 0C. Results showed that by using of sub-zero treatment, hardness is without change but tensile properties are increased to control sample and when the cooling rate was low (less than 1 degree per minute), more improvement will be achieved in tensile properties. While results showed that by using of sub-zero treatment, fatigue strength is decreased at least 20% to control sample