Hamed Khosravi

Modeling and optimization of cooling slope process parameters for semi-solid casting of A356 Al alloy

Abstract Semi-solid processing (SSP) of A356 aluminum alloy was discussed via cooling slope (CS) method. The D-optimal design of experiment (DODE) was employed for experimental design and analysis of results. 38 random experiments obtained by software were carried out. In experimental stage, the molten aluminum alloy was poured on an inclined plate with different lengths of 100, 300 and 500 mm set at 30°, 45° and 60° of slope angles respectively. Three different pouring temperatures of 660, 680 and 700 °C were also used. After the casting process, the partial re-melting treatment was carried out at 590 °C for different isothermal time of 5, 8 or 12 min. The combined effect of these factors on globularity of the primary α(Al) crystals was investigated and optimized using DODE. The results indicated that the primary dendritic phase in the conventionally cast A356 alloy was transformed into a non-dendritic one in ingots cast over a cooling plate. The CS processed samples exhibited a globular structure only after re-heating to semi-solid region. The optimum values of pouring temperature, cooling length, slope angle and isothermal holding time were found to be 660 °C, 360 mm, 48°, and 9 min, respectively. In this case, the globularity of primary crystals was obtained, about 0.91. The obtained model is highly significant with a correlation coefficient of 0.9860.

Comparison of microstructure and wear resistance of A356–SiCp composites processed via compocasting and vibrating cooling slope

The influences of SiC content on the microstructure, porosity, hardness and wear resistance of A356–SiCp composites processed via two different methods of compocasting and vibrating cooling slope (VCS) were compared with each other. In the as-cast condition, the matrix of VCS and compocast processed composites exhibited globular and dendritric structures, respectively. While a more uniform distribution of SiC particulates in the matrix alloy as well as higher hardness values were obtained for the VCS processed samples, the composites produced via compocasting exhibited less porosity. The increased SiC content (up to 20% in volume fraction) resulted in a more uniform distribution of SiC particles within the matrix alloy and improved wear resistance for both the composite series. However, for the VCS processed composites, the increased SiC content, resulted in the decreased size and shape factor of globules as well as better tribological properties when compared with compocast composites. It was concluded that the improved properties of the VCS processed composites when compared with their compocast counterparts was a consequence of a more uniform distribution of SiC particulates in the matrix alloy as well as the globular microstructure generated during the VCS process.

On the mechanical characterizations of unidirectional basalt fiber/epoxy laminated composites with 3-glycidoxypropyltrimethoxysilane functionalized multi-walled carbon nanotubes-enhanced matrix

This study represents the effects of multi-walled carbon nanotubes at various contents with respect to the matrix (0–0.5 wt% at a step of 0.1 wt%) on the mechanical responses of unidirectional basalt fiber/epoxy composites. Toward this end, multi-walled carbon nanotubes were firstly functionalized with 3-glycidoxypropyltrimethoxysilane to improve their dispersion state and interfacial compatibility with the epoxy. Subsequently, unidirectional basalt fiber/epoxy and multiscale 3-glycidoxypropyltrimethoxysilane–multi-walled carbon nanotubes/unidirectional basalt fiber/epoxy composites were prepared. The mechanical properties of the composites were determined by tensile, flexural, and quasi-static compression tests. The compressive strength of the composites was obtained through performing the compression test on the off-axis specimens and extracting their longitudinal compressive strength. Results demonstrated that the highest values in tensile strength, flexural strength, and compressive strength were attained at 0.4 wt% multi-walled carbon nanotubes with 12%, 38%, 41% increase, respectively, compared to the basalt fiber/epoxy composite. Potential mechanisms behind these were implied. Furthermore, compressive modulus and strength of the specimens were obtained theoretically using an Euler-Bernoulli beam-based approach. Knowing the flexural strength as well as the tensile and flexural moduli from the experimental testing, compressive modulus and strength were obtained and the results were compared to those obtained experimentally. The theoretical and experimental results confirmed that there was a good agreement between them.

Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting route

This article reports the effects of stirring speed and T6 heat treatment on the microstructure and mechanical properties of Al-2024 alloy synthesized by a rheocasting process. There was a decrease in grain size of α-Al particles corresponding to an increase in stirring speed. By increasing the stirring speed, however, the globularity of matrix particles first increased and then declined. It was also found that the hardness, compressive strength, and compressive strain increased with the increase of stirring speed. Microstructural studies revealed the presence of nonsoluble Al15(CuFeMn)3Si2 phase in the vicinity of CuAl2 in the rheocast samples. The required time for the solution treatment stage was also influenced by stirring speed; the solution treatment time decreased with the increase in stirring speed. Furthermore, the rheocast samples required a longer homogenization period compared to conventionally wrought alloys. Improvements in hardness and compressive properties were observed after T6 heat treatment.

Effects of compocasting process parameters on microstructural characteristics and tensile properties of A356−SiCp composites

Abstract The effects of compocasting process parameters on some structural and tensile characteristics of the A356–10% SiC p (volume fraction) in;, 600 ognized by weak composites were studied. Semisolid stirring was carried out at temperatures of 590, 600 and 610 °C with stirring speeds of 200, 400 and 600 r/min for 10, 20 and 30 min. The distribution of the SiC particles within the matrix, porosity content and tensile properties of the obtained samples were examined. The structural evaluations show that by increasing the stirring time and decreasing the stirring temperature, the uniformity in the particle distribution is improved; however, by increasing the stirring speed the homogeneity firstly increases and then declines. It is also found that by increasing all of the processing parameters, the porosity content is enhanced. From the tensile characteristics viewpoint, the optimum values of the speed, temperature and time are found to be 400 r/min, 590 °C and 30 min, respectively. The contribution of the reinforcement distribution uniformity prevails over that of the porosity level to the tensile properties.

Fabrication method and microstructural characteristics of coal-tar-pitch-based 2D carbon/carbon composites

The lignin-cellulosic texture of wood was used to produce two-dimensional (2D) carbon/carbon (C/C) composites using coal tar pitch. Ash content tests were conducted to select two samples among the different kinds of woods present in Iran, including walnut, white poplar, cherry, willow, buttonwood, apricots, berry, and blue wood. Walnut and white poplar with ash contents of 1.994wt% and 0.351wt%, respectively, were selected. The behavior of these woods during pyrolysis was investigated by differential thermal analysis (DTA) and thermo gravimetric (TG) analysis. The bulk density and open porosity were measured after carbonization and densification. The microstructural characteristics of samples were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared (FT-IR) spectroscopy. The results indicate that the density of both the walnut and white poplar is increased, and the open porosity is decreased with the increasing number of carbonization cycles. The XRD patterns of the wood charcoal change gradually with increasing pyrolysis temperature, possibly as a result of the ultra-structural changes in the charcoal or the presence of carbonized coal tar pitch in the composite’s body.

Evaluating the Mechanical Behavior of Basalt Fibers/Epoxy Composites Containing Surface-modified CaCO3 Nanoparticles

Polymer matrix composites (PMCs) owing to their outstanding properties such as high strength, low weight, high thermal stability and chemical resistance are broadly utilized in various industries. In the present work, the influence of silanized CaCO3 (S-CaCO3) with 3-aminopropyltrimethoxysilane (3-APTMS) coupling agent at different values (0, 1, 3 and 5 wt.% with respect to the matrix) on the mechanical behavior of basalt fibers (BF)/epoxy composites was examined. BF-reinforced composites were fabricated via hand lay-up technique. Experimental results from three-point bending and tensile tests showed that with the dispersion of 3 wt.% S-CaCO3, flexural strength, flexural modulus, tensile strength and tensile modulus enhanced by 28 %, 35 %, 20 % and 30 %, respectively. Microscopic examinations revealed that the development of the mechanical properties of fibrous composites with the incorporation of modified CaCO3 was related to enhancement in the load transfer between the nanocomposite matrix and BF as well as enhanced mechanical properties of the matrix part.