Journal of Metallurgical and Materials Engineering
Year:2011 | Volume:23 | Issue:1|Papers Count:7

Due to the great variety of applications of magnesium alloys with superior mechanical properties, recent studies have been focused on the hot deformation behavior and microstructure of magnesium alloy castings. Although there have been several recent investigations on the microstructure of wrought magnesium alloys after hot deformation, a limited number of studies have been conducted with regard to the evolution of grain structure during hot working processes. The present work examines the microstructural evolution during hot deformation of magnesium alloy AZ91. The specimens were hot compressed to the peak strain values of 0.3 and 0.5 at the temperature range of 350 to 425 oC with the strain rate of 0.1 s-1. Structural changes were studied using the optical and scanning electron microscopes. The results showed that the recrystallized grains nucleate along the pre-existing grain boundaries during the compression process. The fraction of dynamically recrystallized grains increased with an increase in the strain value in a sigmoidal way expressed by Avrami equation. The dynamically recrystallized grain size also increased at the beginning with strain, and then decreased after its maximum value being reached.

In this study, the formation of different parts of a cup during deep drawing process has been investigated analytically using the geometrical relationships. For a given punch travel, the effects of the bending angle, and those of the die and shoulder radii on the flange and die shoulder strains were examined. In addition, different components of the deformation force were calculated and the punch force–punch travel diagrams for different die and punch shoulder radii were studied. The results showed that the die and punch shoulder radii have similar effects on the punch travel at the maximum force. There was a very good agreement between the calculated results in this research and those obtained by other investigators.

In this paper, a 3D stochastic model has been presented to predict the coating thickness and porosity in a thermal spray coating process. Different values of processing parameters were obtained. The model was developed on the basis of prescribed rules in calculating the splat size during the impact of individual droplets on the surface of the substrate. Due to the thermal stresses produced during droplet solidification, splats were curled up in the edge regions. This mechanism was assumed to be the sole reason for the formation of porosity. The simulation was performed for a small section of the substrate. The calculated thickness and porosity were found to be in good agreement with the experimental results in this investigation, and with those already reported in the literature. The effects of different processing parameters including the gun distance from the substrate, the gun speed as well as the amount of powder feed rate on the coating properties were also investigated.

In this research, effect of the number of nodular graphite particles, as an important characteristic of ductile cast irons, on the distribution of marten site in dual-phase ductile iron has been studied. Ductile iron samples were produced by step casting and the samples were then annealed in order to achieve a fully ferritic structure. A special heat treatment was conducted to produce dual matrix structures (DMSs) in the ductile iron samples. This heat treatment consisted of preheating the samples at 600 oC for 10 minutes, partial austenitizing at 900 oC in molten aluminum bath for 10, 15, 20, and 25 seconds, and quenching in oil. The volume fractions of different phases were determined according to ASTM-E582 standard. The microstructures of the samples were examined using the optical microscope as well as the scanning electron microscope equipped with the energy dispersive spectrometer (EDS) to determine the composition of the regions around the graphite nodules and the intercellular areas. The results showed that for all the austenitizing times, the marten site volume fraction in the samples increases with an increase in the thickness of the steps. It was also shown that for the thick steps, the graphite nodules are surrounded by the marten site phase whereas for the case of thin steps, the marten site phase has been formed mostly at the intercellular regions.

The chemical composition and uniform distribution of alloying elements in the matrix of austempered ductile irons (ADIs) have significant influences on their properties, e.g. segregation of the alloying elements have detrimental effects on the mechanical properties of these materials. Unlike previous studies, the homogenization treatment via partial melting was used in this study in order to reduce the segregation of alloying elements within the matrix. The optical and electron microscopic studies showed the efficiency of homogenization treatment using partial melting in lowering the amount of segregation in the matrix of ADI. The XRD phase analysis was used for estimating the fraction of phases. The results of Charpy impact and tensile tests as well as the fractographic examinations of the fractured samples before and after being homogenized indicated that the homogenization process with partial melting is an efficient method for improving the mechanical properties of ADIs.

Although the solubility of oxygen in molten aluminum alloys is said to be negligible, the fresh surface of the melt oxidizes quickly in the air and an oxide film is formed on the melt surface. Oxide-metal-oxide sandwich technique is a method for the study of short-time surface oxide films. In this research, air was blown at 0.5 atmosphere into the melts of Al-1Mg and Al-6Mg alloys by means of a compressor in order to form air bubbles within the melts. The sandwiches of oxide-metal-oxide were formed at locations where these bubbles reach each other. Some of the characteristics of these films inside the bubbles were evaluated using the scanning electron microscope. In Al-1Mg alloy, the formation of crystalline aluminum oxide as well as the granules of magnesium oxide at the same time was confirmed whereas the magnesium oxide in the form of layer and granule was detected for the case of Al-6Mg alloy.

In this paper, the properties and microstructure of foam glasses obtained from the wasted cathode ray tube (CRT) display panels were investigated. The foam glasses were prepared at the temperatures of 850, 950 and 1050°C using 2, 4, 6 and 8 wt.% silicon carbide (SiC) as the foaming agent. The physical and mechanical properties of the samples including the density, porosity, four-point bending strength were evaluated in this research. The microstructures of samples were examined by means of a scanning electron microscope. The results showed that SiC as the foaming agent is able to control the pore size and also to cause a narrow distribution of pores in the microstructures. The decomposition of SiC in the presence of oxygen at the temperature of 950oC led to the formation of gas bubbles and therefore, the foam glass was produced. The sample with 6 wt.% SiC exhibited the highest amount of porosity (about to 55%) and relatively uniform microstructure after being sintered at 950oC for 1 hour. The four-point bending strength of the latter sample was measured to be 12 MPa. The properties and microstructure of this sample were found to be comparable with those of the commercial samples.