نشریه ریخته گری
پیاپی 93 (پاییز 1388)

In functionally graded composite materials the volume fraction of reinforcement particles varies continuously along specific directions, resulting in graded properties. These types of functionally graded materials (FGMs) are useful in applications where a combination of high surface wear resistance and high toughness of the interior bulk material is required. In the present study functionally graded Al-Si composites were prepared by holding Al-18wt.%Si hypereutectic samples at various temperatures within the freezing range of the alloy and the effect of holding time (0-15min) and temperature (590-650°C) on the distribution of primary Si particles and hardness of composites was quantified. The results showed that the structure of the samples in the upper part was hypereutectic identified by the presence of primary silicon particles. The number of primary Si particles and consequently the hardness of samples decreased gradually from the upper towards the lower parts of the samples. For the lower parts, a hypoeutectic structure with a relatively low hardness value was identified. These results were rationalized in terms of the variation in the microstructure due to segregation of Si towards the upper parts of the sample.

In this study, the practical ingate velocities of some castings produced in air impulse moulding systems, like the back brake drum of Nissan, the crankshaft of Peugeot 405, and the flywheel of Peykan were measured. For each of these segments, the molten metal was filmed as entering the open mould. Afterwards, the theoretical velocities were calculated and were compared with the practical ones obtained from the recorded film.The results showed that the existence of the fraction and heat transfer of the mould cause a considerable difference between the theoretical and practical velocities. It was also found that the ingate velocities in these castings were less than 0.5ms-1, which is lower than the critical velocity.Therefore, the ingate systems used for the production of these castings were suitable according to the ingate velocity of molten metal. So, the defects caused by exceeding the critical velocity were not observed.

Functionally graded materials (FGMs) are a group of inhomogeneous composite materials in which the size, volume fraction and distribution of reinforcement phase have changed continuously and functionally along some directions and result in gradual change in physical, chemical and mechanical properties in those directions. In this article, cylindrical samples with hypereutectic Al-40Wt.% alloy have been prepared in Al-Cu system by horizontal centrifugal casting method and influence of pouring temperature 650, 700, 800 oC and sample thickness on graded microstructure of primary Al2Cu particles were studied. The microstructural analysis indicates that through increasing the superheating or decreasing the sample thickness, the segregation of the primary Al2Cu particles towards the outer sections of the cylindrical speciments is increased.

In this investigation a study has been done to improved structure and a mechanical property of ferritic-pearlitic ductile iron with continues heat treatment and tin trace element. For these reason standard Y-block that designed with in-mold process. In each experiment was included different chemical analysis. In order to determine the cooling curve used the S-thermocouple (Pt-Rh) in the mold, after casting with control of cooling curve, the Y-block shakeout from molds and then cooled in the air. Then machined standard specimen for mechanical and metallographic exam. The metallographic exam show that with increase the rate of shakeout time, the pearlite in microstrure has been increased, as well as the mechanical exam indicate that with increase the rate of shakeout time, hardness and tensile strength increase, although the percentage of elongation and impact energy has been decreased.

The effects of austempering time and temperature on the wear resistance of carbidic austempered ductile iron (CADI) were studied in the present work using a pin-on-disk wear test machine. Results showed that matrix micro structure contains upper and lower bainites associated with a lower amount of carbide and martensite in terms of austempering time and temperature. Based on XRD analysis, retained austenite volume fraction increased with austempering temperature upto 375ºC, and the high carbon austenite volume fraction also increased with austempering time upto 60 min. The carbide formed using the metallic chillers during casting process were finely distributed within the matrix upon austenitazing process. If was shown that the wear resistance decreased when increasing austempering temperature. If was also found that the wear resistance increased with decreasing austempering time (upto 2min) at the low applied load of 85N, while at the higher applied load the increase wear resistance was resulted from applying a longer austempering time.

Honeycombs are known as structures which present special and unique properties. They are used in various industries and in particular aerospace industry. The effect of casting temperature and inner wall thickness of honeycombs on fillability and structural integrity of the samples were studied in this research. Various honeycomb samples of 13mm height were prepared and pressed along their diameters. Then the fracture sections were studied by a Scanning Electron Microscope. The results showed that the higher the pouring temperature, the lower were the fillability of the mold and pressure strength of the honeycombs. It is also found that as the inner wall thicknesses of honeycombs increased their pressure strength increased too. Furthermore, the fracture mode in all of the fractured surfaces was ductile and inclusions have a significant effect on the formation of such fracture surfaces.

In this study, the effect of the casting conditions such as; pre-heating temperature and rotation velocity on the mechanical properties and micro structure of the as-centrifugal cast sample were investigated and then compared with gravity cast samples.Low weight small Si blades with small dimensions move to the inside of the sample because of segregation during casting. On the other hand, a few number of bigger Si blades with larger weight come to the surface of the sample that lead to the increasing of hardness from the outside to the inside of the sample. The segregation decreases with increasing of the rotation velocity and leads to the homogeneity of microstructure. The higher pre-heating temperature and lower rotation speed lead to the increasing of Si amount in the matrix of the samples. The aluminum concentration in the matrix of all samples is approximately equal, but it decreases in the Si blades and precipitated particles from outside to the inside of them. Fe concentration in the precipitation grains is more than other areas. Owing to segregation, precipitations with higher Fe concentration move to the outside of sample. Fe amount of the ascentrifuged sample in the near- surface is larger than that of the as-gravity cast samples.

In this research, a study was carried out to detect segregation of alloying elements in ductile iron by testing on sample made by the melt produced in induction furnace. Point analysis was made by SEM attached with spot EDS. The results showed that Ni is segregated next to graphite nodules because its distribution coefficient is 6/75. On the other hand, Mo is segregated between graphite nodules and its distribution coefficient is 0/16.As common methods were not efficient, partial melting method had been used to homogenize alloying elements in ductile iron matrix. Homogenization was carried out at 1130°C for cast specimens. Molybdenum segregation, between graphite nodules, decreased melting point in these region. On the contrary, adjacent to graphite nodules, nickel segregation increased melting point. Homogenizing at eutectic temperature resulted in forming of melted regions. Amount of formed molten pool during PMH increase gradually at first and after reaching a maximum point, start to reduce and ultimately vanish completely because molten phase had been solidified. Spot analyze of specimens after heat treatment and studies of concentration profiles for alloying elements showed that alloying elements distribution, in ductile iron matrix had been homogenized. So effective segregation coefficients were decreased from 6.75 to 1.37 for nickel and increased from 0.16 to 0.7 for molybdenum.