Papers by Author: Farshad Akhlaghi

Abstract: Al-Mg₂Si composites have gained considerable attention because of their attractive properties such as low density, improved wear resistance and good castability. However, when in-situ routes are used for processing of these composites, the formation of coarse Mg₂Si particles with sharp cornersis inevitable and is detrimental for the composite properties. This problem is intensified when a hypereutectic composition such as Al-25wt.% Mg₂Si alloy is processed. In the present study, an innovative semisolid technique termed as the Vibrating Cooling Slope (VCS) has been applied to produce a sample of in-situ Al-25wt.% Mg₂Si composite.This technique combines the conventional cooling slope and vibration casting methods into an integrated one for producing fine and globular structures in the as-cast condition. An inclined plate was prepared from a 10mm thickness copper plate and was coated by boron nitride. This platecould vibrate mechanically in the vertical direction at a predetermined frequency by the aid of four springs and an electric motor. The molten Al-16.5wt.%Mg-9.4%Sialloy with 100^o C superheat was poured on the surface of this cooling slope (set at 45° inclined angle)while it was vibrating at the frequency of 40 Hzandamplitude of 400 μm.The semisolid alloy travelled the length of 40 cm on the slope before being poured into a steel mold (60 mm internal diameter and 35 mm in height). Also for the purpose of comparison, gravity casting (GC) and conventionally still cooling slope casting (CS)were carried outboth using the same mold and with the same superheat.The samples were sectioned, polished and subjected to metallographicstudies,porosity and hardness measurements. It was concluded that CS and VCS techniques resulted in a decrease in the size of Mg₂Si particles by about 50%and 70% respectively when compared with the gravity casting. However, the increased shape factor of Mg₂Si particles in the VCS and CS processed samples was insignificant as compared with GC. Although in comparison with GC, the VCS processed sample showed a higher porosity level, it exhibited a higher hardness value. These results were attributed to the finer and modified microstructure obtained via this newly developed technique.

Abstract: In the present study high energy ball milling was utilized to produce aluminum (Al-6061) matrix nanocomposite powders reinforced with nanosilicon carbide (SiC) particles. The starting materials containing different percentages (1,3 or 5 wt.%) of nanoSiC particles (25-50 nm) and Al (38-63 μm) were co-milled for different times (16, 20, 24 h) to achieve nanocomposite powders. The crystal size of powders were evaluated by quantitative XRD analysis. Laser particle size analysis was used to evaluate the size of powders during milling. The microstructure of powders and their microhardness values were evaluated by Scanning Electron Microscopy (SEM) and a microhardness tester respectively. The results indicated that the crystal size of the matrix alloy decreased by milling time. The increased SiC content up to 3% resulted in increased microhardness of the powders. However, further increase of SiC to 5% resulted in decreased microhardness due to agglomeration. It was concluded that the maximum microhardness together with a uniform distribution of SiC particles within the matrix alloy was obtained after 20 h milling of powder mixture containing 3% of SiC nanoparticles.

Abstract: In the present investigation, the in-situ powder metallurgy (IPM) method was utilized to synthesis aluminum alloy matrix composite powders containing SiC nanoparticles. Specified amounts of SiC particles (with a size in the range of 250-600 µm) together with SiC nanoparticles (average size of 60 nm) were preheated and added to aluminum melt. This mixture was stirred via an impeller at a certain temperature for a predetermined time. The kinetic energy of the impeller was transferred to the melt via the non-wetting SiC particles and resulted in melt disintegration. The liquid droplets created by this process were then solidified upon cooling the blend resulting in a mixture of Al powders and SiC particles. This blend was passed through a 250 µm sized sieve and a mixture of Al powders and SiC nanoparticles was produced which could be subsequently used as a feedstock for preparation of Al-SiC nano-composites via the standard powder metallurgy methods. The results confirmed that the surface condition (oxidized vs. as-received), amount and different proportions of the added nano-sized and micron sized SiC particles as well as the chemistry of the metallic charge (CP aluminum or Al-1wt.%Mg) affect the size distribution and yield of the resultant sub-250 µm sized powders. The scanning electron microscopy (SEM) studies revealed that Al/SiC composite powders containing nano-sized SiC particles could be produced by using Al-1 wt.% Mg as the matrix alloy.

Abstract: The Al/Gr/SiC hybrid composites have attracted a considerable attention owing to their high wear resistance combined with a low friction coefficient. In these composites graphite acts as a solid lubricating agent and lowers the friction coefficient. However, it reduces the mechanical properties of the composite. The presence of hard SiC particles in these hybrid composites increases the hardness and strength and compensates for the weakening effects of graphite. Powder metallurgy (P/M) is an important processing technique for processing of these MMCs but requires a relatively long mixing time for obtaining a uniform distribution of graphite and SiC particles in the matrix alloy. In the present study for the first time a new method, namely "in situ powder metallurgy (IPM)" is applied for preparation of Al/SiC/Gr hybrid composites. In this work, the effects of different contents (0-40 vol.%) of SiC particles on the morphology and size distribution of the IPM produced Al/Gr/SiC powder mixtures containing 9 vol.% of graphite particles was investigated. Then Al/Gr/SiC compacts were prepared by cold pressing of different powder mixtures and after sintering, the effects of SiC content on the density, microstructure, hardness and wear properties of the resultant hybrid composites was investigated.

Abstract: Semi-solid processing (SSP) technology, due to its ability to provide near-net-shape components with properties far exceeding those of other casting technologies is considered as an alternative for forgings and investment castings. Conventional semi-solid forming, involving the use of heated billets, melt stirring or using cooling slopes require many processing steps and supplementary equipments. This article describes Narrow Melt Stream (NMS), as an alternative process for semi-solid processing of aluminum alloys that eliminates capital cost expenditures, reduces the number of steps required, and hence reduces the costs of making components with a globular structure. However, the applicability of this technique in producing globular structures in the large molds has not been explored. In the present study the results of a systematic investigation on the effects of mold size on the size and morphology of the globular structures formed by NMS processing of Al 356 alloy is reported. For this purpose, five different series of molds were employed. Each series consisted of three molds with identical volumes but different casting moduluses. By using these molds, the effects of casting modulus at constant casting size as well as the effect of casting volume on the size and shape factor of the globular structures in aluminum A356 samples prepared by NMS technique is reported.

Abstract: The production of silicon carbide whiskers (SiCw) by using rice husks has attracted a considerable attention due to a lower production cost as compared to the other processing routs. In the present investigation, the effect of pyrolysis furnace type (vertical tube, horizontal tube, and graphite chamber) on the yield of the resultant SiCw was investigated. It was concluded that the maximum yield was achieved by using a horizontal tube furnace whereas the minimum yield was obtained in a vertical tube furnace. These results were rationalized in terms of the different conditions for the evacuation of the produced gases from the different pyrolysis furnaces.

Abstract: In the present study the effect of Mg addition on the characteristics of Al/SiC nanocomposite powder particles produced via a relatively new method called in situ powder metallurgy (IPM) is investigated. Commercially pure Al and Al-Mg alloy melts containing different amounts of Mg were used as the matrix alloy. Nano-sized SiC particles with the average size of 60 nm were used as the reinforcing material. The effect of Mg content on the fluidity of the melt as an influencing factor affecting both the process yield and wettability of SiC with molten metal was investigated. The size distribution of produced powders was characterized using a laser particle size analyzer. Scanning electron microscopy was utilized to investigate the possibility of embedding of SiC nanoparticles within the metallic matrix. Results of microhardness measurements together with SEM micrographs and EDS analysis showed that nano-sized SiC particles could be embedded in the relatively coarse Al-Mg powders containing at least 1 wt.% Mg.

Abstract: In the present study, Al-Si samples with a graded structure were produced via horizontal centrifugal casting. The distribution of primary and eutectic silicon particles within the produced specimens was investigated by optical microscopy. The microstructure of one end of the sample (closed to rotating axis) consisted of primary Si particles distributed within a eutectic matrix. The volume fraction of these particles decreased towards the other end of the sample and exhibited mostly primary Al-α and eutectic structure. Microhardness and macrohardness measurements conducted along the sample length showed a gradual decreasing value from one end (near the rotation axis) towards the other end of sample demonstrating functionally graded properties. These results were rationalized in terms of the lower density of Si compared to Al alloy which provided the movement of Si particles under the action of centrifugal forces during rotation.