Key Engineering Materials Vol. 875

Abstract: In this study optimization of maximum travel speed that can be achieved for Friction Stir Welding of Aluminum Alloy 2014-T6 without compromising the mechanical properties was carried out. Joints were made at different travel speeds of 200, 300, 400, 500 and 600 mm/min with constant tool rotational speed of 800 rpm and tool tilt angle of 2.The samples were characterized by stereo microscopy, optical microscopy, scanning electron microscopy, Vickers microhardness testing and tensile testing. Microstructural features of as-welded samples revealed refined equiaxed grains in nugget zone and grain growth in the heat effected zone. Tensile test results showed that the tensile strength was maximum at travel speed of 500 mm/min but then decreased after further increasing the travel speed. Hardness in the nugget zones of all welds was lower than that of base material. Fractographic analysis exhibited significant variations in fracture surfaces of tensile samples. A relationship between the welding parameters and resultant heat inputs was also discussed.

Abstract: Aluminum 2xxx alloys have been one of the primary materials for the structural applications of aerospace and transportation industry because of their performance, manufacturing and reliable inspection techniques. Welding is very important in the manufacturing process of structural parts and is now known as the most vital process in the manufacturing route. A relatively new process of joining of materials is friction stir welding process, which was invented by The Welding Institute (TWI) in the UK in 1991. The friction stir welding is mainly employed in aerospace, marine and transportation fields that have high safety requirements. The failure by fatigue is the dominant failure mode for structural weld joints. Since fatigue failure of parts accounts for 50 to 90% of all failures, it is of great significance to understand the fatigue properties of friction stir welded joints. The aim of this overview is to summarize the current research on fatigue crack growth behavior of friction stir welded AA2xxx alloys and critical attention is payed to the damage tolerance performance of friction stir welded aluminum joints that can be affected by welding process parameters, residual stress, stress ratio, environment and post weld treatments.

Abstract: Crack repairing of aluminum alloys is done using conventional welding techniques or mechanical methods, which results in the redundancy of mechanical properties due to defects formation. Friction Stir Welding/Processing (FSW/FSP) is a solid-state joining technique which is used to join various different similar and dissimilar metals, along with the fabrication of surface composites to cater the mentioned problem. The objective of this study is to repair the crack produced in 6061 aluminum alloy by the reinforcement of ceramic particles, Al₂O₃ and B₄C, to further increase the efficiency of the joint along the crack line. Weld parameters, equipment used and the processing conditions are emphasized. The mechanical testing and the characterization of the weld as well as base metal was done and compared using tensile testing, micro hardness test and microstructural analysis. X-Ray Diffraction (XRD) was performed for crystallinity and intermetallic study. The dispersion of the particles was investigated using Field Emission Scanning Electron Microscope (FESEM). The crack in the Al-6061 was effectively repaired using FSP. The reinforced samples showed improved mechanical properties as compared to non-reinforced ones.

Abstract: Manufacturing of thick seamless pipes of age-hardenable aluminum alloys are a specialized technology since their application is limited to specific hi-tech areas. Quality criteria for their inspection are propriety items of the very few production facilities that develop these criteria in-house. Present study relates ultrasonic signals reflecting back from non-continuities in the thickness of seamless pipes with their microstructural features. Detailed study of defects leads to the source of their formation and will ultimately help to systematically control them. Signals from ultrasonic testing trace defects as UT waves reflect back from discontinuities in the material. Defective sections of seamless pipes were cut with precision to reveal the defects. The sectioned surfaces were subjected to metallographic preparation and revealed defects were studied using Optical and Field Emission Scanning Electron Microscopes (FESEM). Defects are grouped based on the shape of UT signals as well as the defect morphology as revealed by microscopic studies. Most of the observed cracks are found to grow in the direction of extrusion. Energy Dispersive Spectroscopy (EDS) analysis was conducted to determine the composition of inclusions in the vicinity of the defects. Data from elemental analysis is used to identify the potential sources. The study recommends measures to control the defects and improve the yield.

Abstract: This study focuses on the effect of deposition time and heat treatment on Ni₃Al coatings with respect to mechanical and microstructural properties of the material. Air plasma spraying technique was employed to deposit Ni₃Al on hot work tool steel samples for different deposition times i.e. 15-45 seconds. The coated samples were then heat treated at 900 °C for 20 to 100 hours at an interval of 20 hours each. The characterization tools such as X-Ray diffraction (XRD), optical and scanning electron microscopy (SEM) were used to study the homogeneity, phases formed and structure of coatings. All the coatings showed lamellar structure with distinctive boundaries along with the presence of some porosity and oxide particles. The XRD analysis of as prepared samples showed characteristic peaks of Ni₃Al whereas of heat treated samples revealed NiO formation that increased with increasing heat treatment time. Micro-hardness and wear resistance measurements of the coated layer showed that they were increasing with the deposition time due to formation of more thick and dense layers. Formation of NiO, due to heat treatment imparted greater hardness and wear resistance to the coating. Moreover, the SEM study of heat-treated samples showed presences of alumina and spinel phases which were confirmed by energy dispersive spectroscopic analysis.

Abstract: The article aims to investigate the feasibility of oxidation protection imparted to C-103 Nb-based alloy by fused slurry silicide coating of R512E (60Si20Fe20Cr) carried out at lower temperature (1200 °C) for higher dwell time i.e. 12 hours. The findings reveal that the coating treated can impart sufficient oxidation resistance the alloy, which may withstand the desired application conditions wherein oxidation protection for smaller time period is needed. Moreover, this treatment is not found to deteriorate other mechanical properties of the alloy in 'As coated' condition.

Abstract: High temperature applications of self-lubricated sliding surfaces have gained industrial importance during the recent years. One popular system is based on sintered Ni-Cr composites with addition of solid lubricants. In the present work these composites were prepared under controlled sintering conditions with different combinations of solid lubricants (MoS₂, Ag and CaF₂) at 1200 °C under flowing argon. The physical properties such as sintered density, relative density and porosity were studied. The microstructures and phase studies of the Ni-Cr based composites were conducted using SEM analysis while the hardness of the composites was measured by Vickers Micro Hardness Tester. The friction tests were conducted with ball on disc configuration following ASTM G-99-95a standard. The MoS₂ solid lubricant provides best lubrication at room temperature which is demonstrated by a low friction coefficient compared to pure Ni-Cr composites. The SEM pictures of worn out tracks show solid debris distribution, and filling of pores with solid lubricant phases. The time taken for stabilization of friction coefficient also varies with the type of solid lubricant. Dual and multiple additions of solid lubricants are also able to reduce the friction of coefficient compared to pure Ni-Cr composite.

Abstract: Nickel aluminide (βNiAl) is a bond coat material used in thermal barrier coating (TBC) system of aeroengines. The performance of TBC is significantly influenced by the thermal response of bond coat (BC) material. Usually, the most failures of the TBCs are attributed to poor performance of a BC. There are several factors that affects the performance, such as; oxidation, mechanical damages, manufacturing oriented problems (thermal residual stresses) etc. In this study, βNiAl was deposited onto CMSX-4 superalloy substrates using in-situ chemical vapour deposition (CVD) method. Zirconium was also incorporated as a dopant into βNiAl coating. Residual stresses were measured using X-ray diffraction (XRD) method. In particular, the comparison was made between the doped and undoped coating samples. Results demonstrated minimum thermal stresses in the zirconium doped coating in comparison to its undoped counterpart. Possible mechanism of stress removal is discussed.

Abstract: Hard and adherent wear-resistant thin surface coatings are synthesized on an aerospace alloy AA2219-T6 alloy using plasma-assisted physical vapor deposition. Submicron size TiN coatings with Ti interlayers are deposited and evaluated as a function of different deposition temperatures varying from 100 ^°C to 200 ^°C. Features of deposited surface layers are evaluated using different techniques; these include field emission scanning electron microscopy equipped with energy dispersive spectroscopy and X-ray diffraction. Mathematical models developed by Yu. A. Bykov and S. Hogmark are considered to estimate the coating thickness using the Composite Hardness measurements. The values predicted by Bykov’s model grossly underestimated the thickness values and thus a correction factor is proposed. The actual coating thickness is measured by sandwiching the thin coating between the substrate and electrolytically deposited nickel layer; the observations are made using field emission scanning electron microscope. The present study successfully used corrected Bykov’s Model along with micro-indentation proved to be an easy, quick and accurate method to estimate the coating thicknesses of thin hard coatings on the soft substrate over a wide range coating thicknesses.

Abstract: High cycle fatigue (HCF) caused by the vibratory stresses is the main cause of failure in many machine components, e.g. aircraft engine and gas turbine components, which has caused loss of many lives and billions of dollars. To avoid these kind of failures, vibratory stresses should be attenuated to an acceptable level, especially at resonant frequencies. A lot of previous studies have shown that thin coatings of different materials significantly reduced these vibratory stresses by adding extra damping to the system. These include viscoelastic materials, plasma graded coatings, piezoelectric materials, and magneto-mechanical damping material coatings, but some of these had applicability and performance issues. Among these thin coatings, magneto-mechanical materials are very effective in reducing these vibratory stresses significantly.In this study, the effect of different beam structure thicknesses under same magneto-mechanical coating of 0.2mm was studied. For this purpose, Ni₃Al was applied as magneto-mechanical coating. The natural frequencies, damping ratios and displacements of beams were calculated before and after applying magneto-mechanical coatings using forced response analysis and hammer tests. The results indicated a sharp change in vibration characteristics i.e. natural frequency, damping ratio and beam deflections, of all the beams used. The results showed that the magneto-mechanical coatings were more effective when applied to thin structures as compared to thick structures, because thin structures have higher strains, which enabled magneto-mechanical coatings to dissipate larger amounts of energy of applied loadings, because performance of these coatings is strain dependent.