Advanced Materials Research Vols. 83-86

Abstract: In this paper, the laser welding of the 630 precipitation hardening stainless steel with the 321 austenitic stainless steel was considered. The experiments were carried out with the pulsed Nd: YAG laser under various welding parameters. The effects of power of laser, voltage, duration of pulse, welding speed, beam diameter and frequency, on the weld volume are investigated. The results show that the weld volume increases with voltage and frequency. The weld volume decreases with welding speed and beam diameter. Moreover, the study shows two effects for the duration of pulse. Up to some value of duration of pulse, the weld volume increases. Exceeding, this limit reduces the weld volume. The optical and scanning electron microscopic tests were carried out. The results show that the martensitic structure is produced in the 630 stainless steel side and the austenitic stainless steel is produced in the 321 stainless steel side. The microhardness tests across the samples were carried out. The results show the maximum hardness is for the 630 stainless steel side and the minimum weld hardness is for the 321 stainless steel side. The high voltage may cause hot cracks in the 321 stainless steel side. The effects of gas flow rate on the microstructure were also considered.

Abstract: In this study, the microstructure of Nd:YAG pulsed laser weld of C17200 copper beryllium alloy is investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. The SEM images reveals three distinct solidified structures due to various thermal gradients occurred in the fusion zone. The XRD patterns show that the preferred solidification directions are the same as those of FCC materials. The effects of pulsed laser parameters on the weld pool dimensions were also studied. According to metallographic results, pulsed energy should be kept as low as possible to obtain the weld pool with the highest values of penetration and the least width. In addition, the laser beam diameter should be kept as low as possible. Moreover, polarization was carried out to investigate the effect of passive layer on the reflectivity of surface by incident laser beam. It was found that weld pool increases when the Fresnel absorption phenomenon activated at high pulsed energy.

Abstract: In this paper the superplastic properties of a commercial 5083 aluminum alloy is studied. The aims are to achieve a proper temperature and strain rate to obtain maximum elongation and to investigate the strain rate sensitivity and cavitation in this alloy. The material is thermo-mechanically processed to produce fine recrystallized grain size. Then hot tensile test is carried out at various temperatures and strain rates. Velocity jump tests are performed to determine stress-strain rate relation and strain rate sensitivity coefficient (m) at various temperatures and strains. The microstructures are studied by optical and electron microscopy (SEM). The area fraction of cavities is determined by analyzing the micrographs from the surface of tensile specimens at various strains. It is found that maximum elongation about 300% is obtained at 520 oC and strain rate of 10-3 s-1. Maximum strain rate sensitivity coefficient (m) about 0.6 is achieved at strain 0.5. With increasing strain the peak m value decreases and shifts to lower strain rates. The failure surface is wide and failure occurs by cavitation.

Abstract: High-temperature deformation of an artificially aged 6082-Al alloy was conducted in the present investigation. Tensile tests were carried out at temperatures of 623, 673 and 723 K at various strain rates ranging from 5x10-5 to 2x10-2 s-1. The behavior of the alloy is characterized by high stress exponent, n and high apparent activation energy, Qa that are higher than what is usually observed in Al and Al solid-solution alloys under similar experimental conditions, which implies the presence of threshold stress; this behavior results from dislocation interaction with second phase particles. The threshold stress, σo values were seen to decrease exponentially with temperature. By incorporating the threshold stress in the analysis, the true activation energy, Qt was calculated to be close to that of dislocation pipe diffusion in Al. Analysis of the experimental data of the alloy in terms of the Zener- Hollomon parameter vs. normalized effective stress, revealed a single type of deformation behavior with an n value of ~7. Measurements showed that the values of elongation percent at failure increase with strain rate and temperature.

Abstract: The effect of magnesium addition up to 0.9 wt.% on the microstructure and mechanical properties of Al-9Si-0.35Mn alloy has been investigated in both as-cast and heat treated conditions. Generally, Mg addition increases the heat treatability and strength of the alloys at the expense of the lower ductility. High levels of magnesium addition, causes the formation of large and brittle intermetallics, a slight increase in porosity and hence, a decrease in ultimate tensile strength and ductility of the cast alloys. T6 heat treatment increases the strength of the alloys up to 80 percent compared to as-cast samples. Among the studied compositions, heat treated Al-9Si-0.35Mn-0.25Mg alloy, has the maximum value of quality index and can be regarded as a promising material with the optimum mechanical properties for industrial applications.

Abstract: The artificial aging response of Al-Mg-Si 6082 aluminum alloy is investigated over a wide temperature range. Samples aged to under aged, peak aged and over aged conditions are further subjected to plastic deformation by simple compression, plane strain compression and simple shear. The flow behavior and the corresponding hardening rates are documented. Equivalent stress – strain curves are generated for the three stress states for an aging temperature of 160oC. Strain reversal experiments in simple shear were carried out in order to characterize the Bauschinger effect. Strain path change experiments were also conducted, in which the gage section that was first deformed by simple shear was further deformed by simple compression.

Abstract: The object of the present work is the characterization of the corrosion behavior of Al-5%SiC nanocomposites prepared by mechanical alloying and the hot pressing method. Al/SiC metal matrix composite (MMCp) powder with volume fraction of 5% SiC was synthesized by high-energy milling of the blended micro component powder. The mechanically mixed nanocomposite powder was consolidated into bulk compacts using hot pressing at 1 GPa pressure for thirty seconds at three different temperatures of 440, 480 and 520°C. The microstructure was investigated by Secondary electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The corrosion behavior of the coating was evaluated using electrochemical techniques such as potansiodynamic polarization and EIS. It was found that the corrosion resistance of Al-5%SiC nanocomposite when hot pressed at 480°C, in 3.5% NaCl solution was higher than pure Al and Al-SiC nanocomposite that had been hot pressed at other temperatures.

Abstract: 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.

Abstract: 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.

Abstract: Composites are overtaking their traditional counterpart materials by prooving themselves to be superior alternatives even for their application in harsh environmental conditions. The technological innovations and developments in manufacturing processes, which lead to a wide range of new composite products, have made them attractive candidates for applications in oil & gas industry, with their application ranging from oil production to transportation. Lack of enough data regarding their long-term environmental and mechanical durability has been a major hinderance in their full-fledge application. This paper investigates the effect on the tensile and creep strength of filament wounded E-glass/vinylester composite pipes (GFRV) exposed to a combination of crude oil and outdoor temperature conditions of Dhahran. The variation in the tensile and creep properties is considered for exposure periods of 6, 12 and 24 months. The combined crude oil filled GFRV pipes exposed to outdoor environmental conditions for a periods of 6, 12 and 24 months have shown a constant increase in the tensile and creep strengths respectively, when compared with the as-received GFRV samples. This increase in the tensile and creep strengths may be attributed to the dual curing of the GFRV pipes i.e both the outer side and inner side of the pipes due to outdoor temperature and crude oil respectively. Microscopic analysis of fractured surfaces using optical microscope and SEM is used to characterize the failure mechanisms responsible.