JOURNAL OF SCHOOL OF ENGINEERING
Year:2008 | Volume:20 | Issue:1 (METALLUGICAL ENGINEERING ISSUE)|Papers Count:11

Alloys based on Ti3Al intermetallic compounds have assumed considerable importance in aerospace and automotive industries, because of their high strength to weight ratio and excellent creep resistance up to 800°C. One such alloy is Ti-23Al-11Nb-0.9Si(at.%) which shows attractive creep properties. Nevertheless, the alloy suffers from relatively poor resistance to oxidation at high temperatures. To improve this, the alloy was coated by immersing in Al-7wt%Si liquid followed by diffusional annealing at 850 ◦C for 5, 10 and 15 hours. Microstructure studies revealed that coatings were consisted of exterior TiAl3 and interior TiAl layers. To study the effect of coating on oxidation behavior of the alloy, isothermal and cyclic oxidation tests were carried out before and after coating. The results showed that the oxidation resistance of the coated samples were much better than those of uncoated ones. This was due to the formation of a thin α-Al2O3 layer on the coating surfaces.

The aim of this paper is to determine the effect of addition of various amounts of reinforced-phase on wear properties of martensitic matrix ferrotic composites. In-situ method was used to synthesize the composites by addition of titanium and graphite to iron melt(weight ratio 4 : 1). In order to produce martensitic matrices, some specimens were austenitized at 980 oC followed by quenching in water. The microstructural investigation revealed: the matrix and the amount, size, and distribution of reinforced phase by using secondary electron and optical microscopes. The hardness and wear tests were used to determine the properties of specimens. As the results the wear resistance improved by increasing volume fraction of TiC .The quenched and as cast specimens exhibited the maximum wear resistance at low and high volume percent of TiC, respectively.

Several methods for achieving higher density in ferrous PM parts are possible. Double press/double sinter allows densities in excess of 7.3 g/cm3 but is limited by cost and geometry considerations. Warm compaction is a new technology resulting in increased density in a single step. By this process it is possible to obtain density levels and mechanical properties comparable to double pressing and sintering. Some characteristic properties of Astaloy CrM, a prealloyed water atomized powder, was investigated and compared with those of conventional compaction. The effect of die temperature on the mechanical properties of parts was also studied. The results clearly show the advantages of using warm compaction process over conventional methods. It was found that the best results are achieved when the die temperature is set to 1500C.

Nowadays Duplex stainless steels are widely used in natural corrosive environment such as sea water. Beside biological complexity of this environment, stainless steels alloy suffer from localized corrosion in sea water due to the presence of chloride and bromide aggressive ions. Pitting corrosion of a Duplex stainless steel at the presence of halide ions specially chloride and bromide has been investigated. Potentiodynamic polarization and potentiostaic polarization at applied anodic potentials (150, 300 and 450 mV) in 0.01, 0.1, 1 M NaCl and also constant concentrations of both Cl-+Br- ions at 25 and 45°C were studied. The results revealed that presence of bromide and chloride ions have a detrimental effects on pitting corrosion. These detrimental effects were appeared in increase on magnitude and lifetime of metastable pitting transients and also lowering pitting potential of the alloy. Morphological observation of metastable and stable pits by Scanning Electron Microscopy (SEM) in bromide containing solution proved the detrimental effect of these ions on pitting corrosion of duplex stainless steels.

Adhesion properties of plasma sprayed hydroxyapatite coatings to the Ti alloy roughened under different conditions, using standard tensile adhesion test as well as interfacial indentation method were evaluated. The measurement of adhesion strength based on the standard test showed scattered values for each group of specimens with identical surface morphology, and different failure modes (adhesive, cohesive or mixed one). Adhesion strength of hydroxyapatite coatings was increased by the increase of fractal dimension of the roughened surfaces. Interfacial indentation method was used to evaluate adhesion strength by determination of critical load as the index of adhesion properties. There was a direct relationship between the measured values of standard tensile strength and critical load in interfacial indentation method for the coating system with different surface morphologies of substrate. Interfacial indentation method presents advantages such as no require to use of a bonder as well as obtaining a single value for critical load and/or interfacial apparent fracture toughness, as compared with other methodologies. It can be considered as an alternative method to evaluate adhesion strength of plasma sprayed hydroxyapatite coatings.

In this work, the simultaneous effect of reducing admixed lubricant from 0.6% to 0% and sintering at RT, 130 oC, 150 oC and 165 oC at compaction pressure of 650MPa were investigated. The results show that eliminating the admixed lubricant and increasing compaction temperature to 165 oC linearly increases density and green strength. As compared to samples containing 0.6% admixed lubricant and compacted at room temperature under 650MPa pressure, warm compacted die wall lubricated samples without admixed lubricant showed 0.35g/cm3 increase in density. Sinter density and mechanical properties increased by increasing compaction temperature up to 150 oC. Both parameters were deteriorated above this temperature for admixed powders, while it kept increasing for die wall lubrication. Decreasing lubricant Improves pore morphology and increase load bearing cross sectional area.

In this paper an experiment procedure was conducted to analyze the microstructure of SiC-particle reinforced Al-MMCs via forming temperatures. In this procedure, first the 6061-alloy powder is mixed by 33% SiC-particle reinforced in order to preparation of AA6061/SiCp composite. The obtained alloy powder is compressed into a cold die cylindrical mould at a pressure of 10.3MPa. The cylindrical samples were heated into an electrical furnace until the liqidus- solidus temperatures range of alloy. Just before the samples were transferred to extrusion press, the split die, which was preheated, was placed in the container of the extrusion press. The samples were then transferred from the furnace and placed on top of the split die and a pressure pad placed on top. A computer data logger then measured parameters such as load and extrusion time whilst the die was being filled. Finally, the samples quenched into cold water and were prepared for metallography test. Then was investigated the effect of forming temperature on the interface of reinforcement particles/Matrix and microstructure of semi-solid forming (SSF) samples. Results showed complete die filling was achieved only when the initial sample temperature was within the semi-solid temperature range. At temperatures close to the liquidus temperature of the alloy, reinforcement clustering and segregated regions of matrix occurred. The microstructures and coherency at interface between reinforcement and matrix of the SSF samples appeared to be dependant on the formation of aluminum carbide and the precipitation of silicon factors. Tests of the XRD, FEGSEM and light microscopy were used to verify the effects of these factors. Therefore based on the obtained results it is possible substitute the SSF instead of the HIP stage in the powder metallurgy process, without any aluminum carbide at the interface.

In this research, Al and Pb powders (25 wt% Pb) were subjected to mechanical alloying, using a planetary ball mill. The ball milled mixture was studied by scanning electron microscope (SEM), equipped by EDS. To study thermal behavior, DSC experiments were performed and variation of melting temperature of Pb by ball milling was investigated. In addition, solid solubility of Pb in Al was studied during ball milling. Despite this fact that based on equilibrium phase diagram there is no solid solubility in Al-Pb system, formation of a non equilibrium solid solution of Pb in Al and also a reduction in melting point of Pb by ball milling could be observed. During heating-up of ball milled samples, very fine particles of solute Pb were precipitated at about 300 oC, which caused in lowering of melting point of Pb. Melting point of Pb decreased from 327.25 oC for un-milled to 316 oC in 48 hours milled sample and also the solved Pb in Al matrix increased from 0.65 wt% for 5 hours milled sample to 0.89 wt%in 48 hours milled sample.