Hassan Farhangi

Structural characterization of AA 2024-MoSi2 nanocomposite powders produced by mechanical milling

Nano-sized MoSi2 powder was produced successfully from commercially available MoSi2 by a mechanical milling process carried out for 100 h, and mechanical alloying was employed to synthesize AA 2024-MoSi2 nanocomposites. The effects of MoSi2 reinforcement and mechanical milling on the structure, morphology, and iron contamination of the produced materials were investigated using X-ray diffraction, scanning electron microscopy, and atomic absorption spectrometry. It is revealed that the morphology of the aluminum alloy changes continuously during milling from spherical to plate-like, irregular, and finally equiaxed. The presence of MoSi2 reinforcement accelerates the milling process and results in a smaller average particle size. The Williamson-Hall method determined that the crystallite size of the aluminum alloy in the composite powder is smaller than that of the unreinforced alloy at the same milling time and this size reaches 45 nm after 16 h milling time. The Fe contamination content is higher for the nanocomposite in comparison with the unreinforced alloy because of the wearing role of MoSi2 hard particles.

Effects of Cooling Condition on Hot Tearing during Investment Casting

In the present study, the effect of cooling condition on hot tearing tendency and hot tearing fracture surface morphology were investigated. Results show that, the hot tear fracture surface characteristics are nearly similar under different cooling conditions. The hot tear surface exhibits two main features; the brittle region and the ductile region. The results also indicate that cooling condition has multifaceted effects on hot tearing phenomenon. Increasing cooling rate increases the thermal gradient, which raises the hot tearing tendency; concomitantly it decreases the grain size and dendrite arm spacing which increases the strength of the material. The occurrence of hot tearing phenomenon under different cooling conditions is discussed and evaluated based on the competition between these opposing factors during the solidification process.

Microstructure and Mechanical Properties of Dissimilar Ferritic and Austenitic Steel Joints with an Intermediate Inconel-182 Buttering Layer

In this study, microstructure and mechanical properties of dissimilar weld joints between 2.25Cr-1Mo ferritic steel and 316L austenitic stainless steel, with and without an Inconel-182 buttering layer, have been investigated. The buttering layer widths produced on the machined edges of the ferritic steel plate were 3 and 5 mm. The dissimilar weld joints were butt-welded using a SMAW process with Inconel-182 electrodes. The results indicate that the ferritic base metal dilution effects are minimized due to buttering and a more uniform distribution of Fe, Ni, Cr and Nb contents is established over a broad region within the fusion zone. Moreover, a microstructure consisting of combined columnar and equiaxed dendrite with interdendritic Nb-rich particles is developed within the fusion zone as a result of buttering. Mechanical tests show that the average hardness, tensile ductility and impact energy of the weld metal were enhanced with increasing width of the buttering, while tensile strength properties were unaffected. It is observed that fracture surfaces of tensile specimens exhibit ductile features composed of ductile tear ridges with numerous interspersed dimples. However, the dominant fracture mode is noted to change from interdendritic to transdendritic with the use of a buttering layer.

The Impact of Ceramic Shell Strength on Hot Tearing during Investment Casting

The effect of ceramic shell strength on hot tearing susceptibility during solidification was inspected practicing investment casting of the cobalt‐base superalloy samples with the same casting conditions, but different ceramic shell systems. Results showed that the lower the ceramic shell strength upon using polymer additives, the lower the hindered contraction rate, and the lower the hindered contraction rate, the smaller the hot tearing tendency. Optical microscopy and electron microscopy scanning revealed that the hot tear propagated along the last solidified interdendritic phase, and that the hot tear surface had two major modes: 1) the ductile region in the outer layer; and 2) the inner region of liquid embrittlement.

Nanocomposites of aluminum alloy-MoSi2: Synthesis and characterization

Nanocomposites of AA 2024 aluminum alloy matrix reinforced with nano-sized MoSi2 particles were produced using mechanical alloying method followed by cold and hot pressing. Phase stability analysis, hardness, and room temperature compression tests of the composites were performed. In addition, absorb energy calculation and fracture behavior evaluation of the compression test specimens were carried out. The results indicated that the composite constituents exhibit good stability during the high temperature processing. The hardness and compressive strength of the composites show a steady increase for nano-sized MoSi2 particle contents up to 3 vol.% and exceed values of 150 HB and 800 MPa, respectively followed by a small drop at higher contents, most probably due to the particle agglomeration. Fractographic investigations of compression test specimens revealed that the failure mode of the composites with lower amount of reinforcement is mainly dimple ductile fracture while, with increasing the reinforcement amount, multiple cracking dominates.

STRUCTURAL AND MORPHOLOGICAL EVALUATION OF NANO-SIZED MoSi2 POWDER PRODUCED BY MECHANICAL MILLING

Nano-sized intermetallic powders have received great attention owing to their property advantages over conventional micro-sized counterparts. In the present study nano-sized MoSi2 powder has been produced successfully from commercially available MoSi2 (3 μm) by a mechanical milling process carried out for a period of 100 hours. The effects of milling time on size and morphology of the powders were studied by SEM and TEM and image analyzing system. The results indicate that the as-received micrometric powder with a wide size distribution of irregular shaped morphology changes to a narrow size distribution of nearly equiaxed particles with the progress of attrition milling up to 100 h, reaching an average particle size of 71 nm. Structural evolution of milled samples was characterized by XRD to determine the crystallite size and lattice microstrain using Williamson-Hall method. According to the results, the crystallite size of the powders decreases continuously down to 23 nm with increasing milling time up to 100 h and this size refinement is more rapid at the early stages of the milling process. On the other hand, the lattice strain increases considerably with milling up to 65 h and further milling causes no significant changes of lattice strain.

The Influence of Welding Parameters on Tensile Behavior of Friction Stir Welded Al 2024-T4 Joints

In the present study, the relationships between friction stir welding parameters and the tensile behavior of Al 2024-T4 joints was investigated. The aluminum alloy plates were butt-welded using a hardened steel tool with a threaded and fluted cylindrical pin at various tool rotation speed to advancing speed ratios. Metallographic observations, EDS analysis and microhardness measurements show that the band spacing in the periodic microstructure of the stir zone and the average microhardness of this region decrease with increasing speed ratio. Tensile ductility is strongly affected by welding parameters and final elongation increases significantly with speed ratio at the constant rotating speed of 900 rpm. This behavior is found to be associated with a change in tensile fracture location. Formation of microscopic voids at low speed ratios leads to premature fracture in the nugget zone, while in the defect-free joints produced at higher speed ratios the fracture location shifts into the HAZ on the retreating side, which exhibits the lowest microhardness value within the weld joint. At the optimum rotation speed of 900 rpm and speed ratio of 11.2 rev/mm the tensile strength and final elongation of the joints are equivalent to 97% and 77% that of base metal, respectively.

INFLUENCE OF GEOMETRY AND DESIGN PARAMETERS ON FLEXURAL BEHAVIOR OF DYNAMIC COMPRESSION PLATES (DCP): EXPERIMENT AND FINITE ELEMENT ANALYSIS

This work aimed to study the effect of various geometric parameters on bending behavior in orthopedic dynamic compression plates (DCPs) in order to achieve suitable criteria in an optimum design of these plates. Modeling, simulation, and analysis were performed through the finite element software of ABAQUS. In order to verify the model, four-point bending tests on several actual plates were conducted. In addition, the classical beam theory was applied for the theoretical estimation of the maximum tensile stress in the outer fiber and the longitudinal stresses of plates. Finite element analysis (FEA) results indicated relatively good conformity with the empirical results and those of beam theory. Based on the results, the distance of the holes from the plate edge was observed to be the most effective parameters on flexural behavior. It was also found that the flexural properties are maximized at a unique distance between the outside edge of the hole and the edge of the plate.

Fracture Analysis of Generator Fan Blades

Critical gas turbine rotating component, such as turbine blades, compressor disks, spacers and cooling fan blades are subjected to cyclic stresses during engine start-up, operation and shut-down. The lifetime of these components are usually established on the basis of probabilistic crack initiation criterion for a known fracture-critical location (Koul & Dainty, 1992). Therefore, periodic inspections are carried out to detect the probable cracks and prevent suddenly fractures.

Influence of Hot Extrusion Process on the Mechanical Behavior of AA6061/SIC Composites

The effect of hot extrusion process on the microstructure, mechanical properties and fracture behavior of metal-matrix composites (MMCs) of AA6061 alloy reinforced with 10 volume percent particulate SiC with the average size of 46 µm was studied. The MMC ingots were fabricated by the stir casting method and were extruded at 450°C at a ram speed of 1mm/s and at the extrusion ratios of 6:1, 12:1 and 18:1. Various techniques including metallography, density measurement, tensile testing, and SEM fractography were utilized to characterize the mechanical behavior of the MMCs. Results demonstrated that extruded composites possessed considerably lower porosity contents, higher strength, and enhanced ductility in comparison with the as-cast samples. In addition, further improvement in the mechanical properties of the extruded composites was noticed by increasing the extrusion ratio. Fractographic observations revealed that the brittle fracture behavior of the as-cast specimens was promoted by cracking of the large SiC particle clusters. Whereas, the fracture surfaces of extruded composites showed extensive tear ridge formation by initiation and growth of shallow dimples, around the cracked particles, which is characteristic of a ductile fracture process. This change in the fracture behavior and improvement in mechanical properties is attributed to the break up of particle clusters and diminishment of pores during the extrusion process.