Showing papers in "Journal of Materials Engineering and Performance in 2014"

Metal Additive Manufacturing: A Review

Abstract: This paper reviews the state-of-the-art of an important, rapidly emerging, manufacturing technology that is alternatively called additive manufacturing (AM), direct digital manufacturing, free form fabrication, or 3D printing, etc. A broad contextual overview of metallic AM is provided. AM has the potential to revolutionize the global parts manufacturing and logistics landscape. It enables distributed manufacturing and the productions of parts-on-demand while offering the potential to reduce cost, energy consumption, and carbon footprint. This paper explores the material science, processes, and business consideration associated with achieving these performance gains. It is concluded that a paradigm shift is required in order to fully exploit AM potential.

Recent Developments in Friction Stir Welding of Al-alloys

Abstract: The diversity and never-ending desire for a better life standard result in a continuous development of the existing manufacturing technologies. In line with these developments in the existing production technologies the demand for more complex products increases, which also stimulates new approaches in production routes of such products, e.g., novel welding procedures. For instance, the friction stir welding (FSW) technology, developed for joining difficult-to-weld Al-alloys, has been implemented by industry in manufacturing of several products. There are also numerous attempts to apply this method to other materials beyond Al-alloys. However, the process has not yet been implemented by industry for joining these materials with the exception of some limited applications. The microstructures and mechanical properties of friction stir welded Al-alloys existing in the open literature will be discussed in detail in this review. The correlations between weld parameters used during FSW and the microstructures evolved in the weld region and thus mechanical properties of the joints produced will be highlighted. However, the modeling studies, material flow, texture formation and developments in tool design are out of the scope of this work as well as the other variants of this technology, such as friction stir spot welding (FSSW).

Innovative and Highly Productive Joining Technologies for Multi-Material Lightweight Car Body Structures

Abstract: Driven by increasing costs for energy and raw material and especially by the European CO2-emission laws, automotive industry faces the challenge to develop more lightweight and at the same time still rigid and crash-stable car bodies, that are affordable for large-scale production. The implementation of weight-reduced constructions depends not only on the availability of lightweight materials and related forming technologies, but also on cost-efficient and reliable joining technologies suitable for multi-material design. This article discusses the challenges and requirements for these technologies, based on the example of joining aluminium with press-hardened boron steels, what is considered as a very important material combination for affordable future lightweight mobility. Besides a presentation of recent developments for extending the process limits of conventional mechanical joining methods, new promising technologies such as resistance element welding are introduced. In addition, the performance, advantages, and disadvantages of the presented technologies are compared and discussed.

Microstructure and Mechanical Behavior of 17-4 Precipitation Hardenable Steel Processed by Selective Laser Melting

Abstract: The mechanical behavior and the microstructural evolution of 17-4 precipitation hardenable (PH) stainless steel processed using selective laser melting have been studied. Test coupons were produced from 17-4 PH stainless steel powder in argon and nitrogen atmospheres. Characterization studies were carried out using mechanical testing, optical microscopy, scanning electron microscopy, and x-ray diffraction. The results show that post-process heat treatment is required to obtain typically desired tensile properties. Columnar grains of smaller diameters (<2 µm) emerged within the melt pool with a mixture of martensite and retained austenite phases. It was found that the phase content of the samples is greatly influenced by the powder chemistry, processing environment, and grain diameter.

Sliding Wear Characteristics and Corrosion Behaviour of Selective Laser Melted 316L Stainless Steel

Abstract: Stainless steel is one of the most popular materials used for selective laser melting (SLM) processing to produce nearly fully dense components from 3D CAD models. The tribological and corrosion properties of stainless steel components are important in many engineering applications. In this work, the wear behaviour of SLM 316L stainless steel was investigated under dry sliding conditions, and the corrosion properties were measured electrochemically in a chloride containing solution. The results show that as compared to the standard bulk 316L steel, the SLM 316L steel exhibits deteriorated dry sliding wear resistance. The wear rate of SLM steel is dependent on the vol.% porosity in the steel and by obtaining full density it is possible achieve wear resistance similar to that of the standard bulk 316L steel. In the tested chloride containing solution, the general corrosion behaviour of the SLM steel is similar to that of the standard bulk 316L steel, but the SLM steel suffers from a reduced breakdown potential and is more susceptible to pitting corrosion. Efforts have been made to correlate the obtained results with porosity in the SLM steel.

Young’s Modulus of Austenite and Martensite Phases in Superelastic NiTi Wires

Abstract: Young’s moduli of superelastic NiTi wires in austenite and stress-induced martensite states were evaluated by three different experimental methods (tensile tests, in situ synchrotron x-ray diffraction, and dynamic mechanical analysis) and estimated via theoretical calculation from elastic constants. The unusually low value of the Young’s modulus of the martensite phase appearing in material property tables (<40 GPa) is generally ascribed in the literature to the fact that stress-driven martensitic transformation and/or twinning processes continue even beyond the transformation range and effectively decrease the value of the tangent modulus evaluated from macroscopic stress-strain curve. In this work, we claim that this low value is real in the sense that it corresponds to the appropriate combination of elastic constants of the B19′ martensite phase forming the polycrystalline wire. However, the Young’s modulus of the martensite phase is low only for wire loaded in tension, not for compression or other deformation modes. It is shown that the low value of the martensite Young’s modulus in tension is due to the combination of the unique coincidence of elastic anisotropy of the B19′ martensite characterized by the low elastic constant C55, austenite drawing texture, and strong martensite texture due to the martensite variant selection under tensile stress.

On Optimization of Surface Roughness of Selective Laser Melted Stainless Steel Parts: A Statistical Study

Abstract: In this work, the effects of re-melting parameters for postprocessing the surface texture of Additively Manufactured parts using a statistical approach are investigated. This paper focuses on improving the final surface texture of stainless steel (316L) parts, built using a Renishaw SLM 125 machine. This machine employs a fiber laser to fuse fine powder on a layer-by-layer basis to generate three-dimensional parts. The samples were produced using varying angles of inclination in order to generate range of surface roughness between 8 and 20 µm. Laser re-melting (LR) as post-processing was performed in order to investigate surface roughness through optimization of parameters. The re-melting process was carried out using a custom-made hybrid laser re-cladding machine, which uses a 200 W fiber laser. Optimized processing parameters were based on statistical analysis within a Design of Experiment framework, from which a model was then constructed. The results indicate that the best obtainable final surface roughness is about 1.4 µm ± 10%. This figure was obtained when laser power of about 180 W was used, to give energy density between 2200 and 2700 J/cm2 for the re-melting process. Overall, the obtained results indicate LR as a post-build process has the capacity to improve surface finishing of SLM components up to 80%, compared with the initial manufactured surface.

The Role of Shear-Thickening Fluids (STFs) in Ballistic and Stab-Resistance Improvement of Flexible Armor

Abstract: This paper provides a detailed review on relevant literature related to rheological properties of STFs with particular emphasis on efforts to improve their impact resistance. The review has concisely demonstrated that there are many factors affecting shear-thickening behavior of colloidal suspensions. Nanoparticle characteristics such as shape, size, distribution, solid volume fraction, and interaction with other particles as well as properties related to suspending phase and also flow field could affect the rheological properties of STFs. Recent studies on application of STFs to textile fabrics, preparation techniques, and the factors determining the performance of STF/fabric composites are summarized. Particular emphasis is laid on researches that explore the ballistic, stab, and puncture protective properties of STF-based materials and body armors.

Microhardness and Surface Integrity in Turning Process of Duplex Stainless Steel (DSS) for Different Cutting Conditions

Abstract: The objective of the investigation was to identify microhardness of surface integrity (SI) after turning with wedges of coated sintered carbide. SI is important in determining corrosion resistance, and also in fatigue crack initiation. The investigation included microhardness analyses in dry and wet machining of duplex stainless steel. The microhardness of SI for various cutting speeds was compared. It has been shown that wet cutting leads to the decrease of SI hardening depth, while increasing the rounded cutting edge radius of the wedge increases the maximum microhardness values and the hardening depth. An infinite focus measurement machine has been used for the rounded cutting edge radius analysis. The study has been performed within a production facility during the production of electric motor parts and deep-well pumps as well as explosively cladded sheets.

Evaluation of Microstructure and Mechanical Properties in Dissimilar Austenitic/Super Duplex Stainless Steel Joint

Abstract: To study the effect of chemical composition on microstructural features and mechanical properties of dissimilar joints between super duplex and austenitic stainless steels, welding was attempted by gas tungsten arc welding process with a super duplex (ER2594) and an austenitic (ER309LMo) stainless steel filler metal. While the austenitic weld metal had vermicular delta ferrite within austenitic matrix, super duplex stainless steel was mainly comprised of allotriomorphic grain boundary and Widmanstatten side plate austenite morphologies in the ferrite matrix. Also the heat-affected zone of austenitic base metal comprised of large austenite grains with little amounts of ferrite, whereas a coarse-grained ferritic region was observed in the heat-affected zone of super duplex base metal. Although both welded joints showed acceptable mechanical properties, the hardness and impact strength of the weld metal produced using super duplex filler metal were found to be better than that obtained by austenitic filler metal.

A Comparative Study of High-Power Diode Laser and CO 2 Laser Surface Hardening of AISI 1045 Steel

Abstract: The study investigates laser surface hardening in the AISI 1045 steel using two different types of industrial laser: a high-power diode laser (HPDL) and a CO2 laser, respectively. The effect of process parameters such as beam power, travel speed on structure, case depth, and microhardness was examined. In most cases, a heat-affected zone (HAZ) formed below the surface; a substantial increase in surface hardness was achieved. In addition, big differences were found between the hardened specimens after HPDL surface hardening and CO2 laser surface hardening. For HPDL, depths of the HAZ were almost equal in total HAZ o, without surface melting. For CO2 laser, the depths changed a lot in the HAZ, with surface melting in the center. To better understand the difference of laser hardening results when use these two types of laser, numerical (ANSYS) analysis of the heat conduction involved in the process was also studied. For HPDL method, a rectangular beam spot and uniform energy distribution across the spot were assumed, while for CO2 laser, a circular beam spot and Gaussian energy distribution were assumed. The results showed that the energy distribution variety altered the thermal cycles of the HAZ dramatically. The rectangular HPDL laser beam spot with uniform energy distribution is much more feasible for laser surface hardening.

Improved Thermal Performance of Diamond-Copper Composites with Boron Carbide Coating

Abstract: B4C-coated diamond (diamond@B4C) particles are used to improve the interfacial bonding and thermal properties of diamond/Cu composites. Scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were applied to characterize the formed B4C coating on diamond particles. It is found that the B4C coating strongly improves the interfacial bonding between the Cu matrix and diamond particles. The resulting diamond@B4C/Cu composites show high thermal conductivity of 665 W/mK and low coefficient of thermal expansion of 7.5 × 10−6/K at 60% diamond volume fraction, which are significantly superior to those of the composites with uncoated diamond particles. The experimental thermal conductivity is also theoretically analyzed to account for the thermal resistance at the diamond@B4C-Cu interface boundary.

Non-linear Correlation Between Uniaxial Tensile Properties and Shear-Edge Hole Expansion Ratio

Abstract: Stretch flanging of steel sheets is an important formability issue for automobile industry. Finite element simulation study confirms that the edge of the hole deforms in a uniaxial tensile manner during the hole expansion process. To understand the effect of various tensile properties on hole expansion ratio, current experimental data and collected data from published work have been used. Yield stress, ultimate tensile stress, coefficient of normal anisotropy, total elongation, and post uniform elongation are closely related to hole expansion ratio. A non-linear relationship between hole expansion ratio and tensile properties (ultimate tensile stress, coefficient of normal anisotropy, and total elongation) is developed in the present investigation.

Interface Phenomena and Bonding Mechanism in Magnetic Pulse Welding

Abstract: Magnetic pulse welding (MPW) is a solid-state impact welding technology that provides metallurgical joints while exhibiting a negligible heat-affected zone. The MPW process is a high speed single shot welding technique used mainly for joining tubular components in a lap configuration and characteristic length scales of few millimeters to centimeters. It is similar in operation to explosive welding and shares the same physical principles. The nature of bonding in MPW is not sufficiently understood yet and some controversial explanations are reported in the literature. The two major ideas are based on either solid state bonding or local melting and solidification. The present work summarizes our current understanding of the bonding mechanism and the structure in various similar and dissimilar metal pairs joined by MPW.

Experimental Study of the Forces Acting on the Tool in the Friction-Stir Welding of AA 2024 T3 Sheets

Abstract: In this paper, AA 2024 T3-rolled sheets were joined in butt joint configuration through the friction stir welding process. Different joints were carried out varying the principal process parameters (i.e., tool welding speed and tool rotational speed). The aim of this work was the study and the experimental characterization of the influence of the process parameters on the forces acting on the tool during the FSW process. Furthermore, it was studied the correlation between the forces and the grain size, in particular with the extension of the heat-affected zone. Forces acting along the axis parallel to the tool are actually greater than those acting along welding direction. All the recorded forces are strictly dependant on the process parameters adopted. No correlation has been found between the grain dimension within the weld bead and the recorded forces, while the greater the forces, the narrower the extension of the heat-affected zone.

Effect of Thermomechanical Processing on the Microstructure and Mechanical Properties of Nb-Ti-V Microalloyed Steel

Abstract: The paper presents the results of thermomechanical treatment via forging on the microstructure and mechanical properties of newly obtained microalloyed steel containing 0.28% C, 1.41% Mn, 0.027% Nb, 0.028% Ti, and 0.019% V. The investigated steel is assigned to the production of forged elements for the automotive industry. Conditions of forging using the thermomechanical processing method were developed based on plastometric tests. Continuous and double-hit compression tests were conducted using the Gleeble 3800 thermomechanical simulator. The samples were investigated in a temperature range from 900 to 1100 °C and a strain rate of 1 and 10 s−1. To determine the recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of samples using the applied deformation were conducted in a temperature range from 900 to 1100 °C with isothermal holding of the specimens between successive deformations for 2-100 s. Observations of the microstructures of thin foils were conducted using a TITAN80-300 FEI transmission electron microscope. The applied thermomechanical treatment allows to obtain a fine-grained microstructure of the austenite during hot-working and production of forged parts. These acquire advantageous mechanical properties and guaranteed crack resistance after controlled cooling from the end plastic deformation temperature and successive tempering. Forgings produced using the thermomechanical treatment method, consecutively subjected to tempering in a temperature range from 550 to 650 °C, reveal values of YS0.2 which equal from 994 to 892 MPa, UTS from 1084 to 958 MPa, KV from 69 to 109 J, KV−40 from 55 to 83 J, and a hardness ranging from 360 to 300 HBW.

Modeling Precipitation Kinetics During Heat Treatment with Calphad-Based Tools

Abstract: Sophisticated precipitation reaction models combined with well-developed CALPHAD databases provide an efficient way to tailor precipitate microstructures that maximize strengthening via the optimization of alloy chemistries and heat treatment schedules. The success of the CALPHAD approach relies on the capability to provide fundamental phase equilibrium and phase transformation information in materials of industrial relevance taking into consideration composition and temperature variation. The newly developed TC-PRISMA program is described. The effect of growth modes, alloy chemistries, and cooling profiles on the formation of multimodal microstructures has been examined in order to understand the underlying thermodynamics and kinetics. Practical issues that are critical to the accuracy and applicability of the current simulations, such as modifications that overcome mean field approximations, compatibility between CALPHAD databases, and selections of key parameters (particularly interfacial energy and nucleation site densities), are also addressed.

Effect of Ultrasonic Shot Peening on Microstructure and Mechanical Properties of High-Nitrogen Austenitic Stainless Steel

Abstract: The effect of ultrasonic shot peening (USSP) was studied on microstructural modification and mechanical properties such as microhardness, yield strength, tensile strength, and low cycle fatigue (LCF) life of a nitrogen stabilized austenitic stainless steel, at room temperature. There was grain refinement up to nano scale in surface region of the shot peened specimens and the microhardness was increased markedly up to the depth of approximately 100 µm. There was insignificant increase in yield and tensile strength, but drastic reduction in LCF life, particularly at low strain amplitude, from USSP. The nominal increase in yield and tensile strength was due to grain refinement in the surface region and drastic fall in LCF life was due to surface cracking resulting from USSP.

Effects of Mn additions on the structure, mechanical properties, and corrosion behavior of Cu-Al-Ni shape memory alloys

Abstract: The influences of different amount (0.4, 0.7, and 1 wt.%) of Mn addition on the structure, mechanical properties, and corrosion behavior of Cu-Al-Ni shape memory alloys have been studied using differential scanning calorimetry, field emission scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile test, shape memory effect test, hardness test, and electrochemical test. It was observed that the transformation temperatures, microstructural characteristics, and mechanical properties are highly sensitive to the composition variations. The obtained results show that the transformation temperatures and mechanical properties of Cu-Al-Ni SMAs exhibited the best results with 0.7 wt.% of Mn addition. These kinds of enhancements are mainly due to the type, amount, and morphology of the martensite phase, including the grain refinement. The result of electrochemical test showed that an increment in Mn content up to 0.7 wt.% improved the corrosion resistance of Cu-Al-Ni SMA. However, further increase of Mn content decreases the corrosion resistance of the alloy.

The Effect of Two Amine-Based Corrosion Inhibitors in Improving the Corrosion Resistance of Carbon Steel in Sea Water

Abstract: This study investigates the effect of two amine-based corrosion inhibitors in reducing the corrosion rate (CR) of 1018 carbon steel in formulated sea water. For discussion purposes, the two inhibitors are named Inhibitor A (belongs to the alkyl pyridine benzyl chloride quaternary family of inhibitors) and Inhibitor B (belongs to the alkyl amines family of inhibitors). The two inhibitors are routinely considered for applications by Saudi Aramco, the world’s largest oil producing and processing company, for reducing its corrosion problems in carbon steel pipelines carrying oil and gas. The experimental apparatus was a circulating flow loop system inside an autoclave. The experimental work was performed at pH 8.2, 55 °C, and 1000 rpm. The inhibitors were evaluated at three different concentrations of 5, 10, and 15 ppm. The CR was determined using the weight loss method and electrochemical methods such potentiodynamic sweep and linear polarization resistance. The experimental results indicate that Inhibitor B reduced the CR more than that of Inhibitor A.

Fracture Behavior and Characterization of Lead-Free Brass Alloys for Machining Applications

Abstract: The stricter environmental, health, and safety regulations address the harmful effects of lead and provide the driving force for the development of lead-free brass alloys. Conventional leaded brass rods are widely used in several manufacturing sectors (i.e., fabrication of hydraulic components, fittings, valves, etc.) due to their superior workability in extrusion and drawing as well as their superior machinability. As machinability performance involves shear and dynamic fracture processes evolved under high strain-rate conditions, the understanding of the mechanical behavior/microstructure interaction is critical in order to successfully tailor candidate lead-free alloys for improved machinability without compromising the reliability of manufactured components. In this work, the mechanical behavior under static and dynamic loading of three lead-free brass alloys (CW510L-CW511L-C27450) in comparison to a conventional leaded brass alloy (CW614N) was studied. The fractographic evaluation of the texture of conjugate fracture surfaces was performed to identify the involved fracture mechanisms and their relation to the alloy microstructure. It was shown that the CW510L lead-free brass alloy is a potential candidate in replacing conventional CW614N leaded brass, combining high tensile strength and fracture toughness, due to the prevalence of the β-intermetallic phase in the alloy microstructure.

Mechanical and Tribological Characterization of Al-Mg 2 Si Composites After Yttrium Addition and Heat Treatment

Abstract: In this study, the effect of heat treatment and yttrium additions on the microstructure, mechanical properties, and tribological behavior of Al-15% Mg2Si cast composites was investigated. The microstructural study revealed the presence of both primary and secondary Mg2Si phases in all composite specimens and also Y-containing intermetallics (Al2Y phases) at higher concentrations. It was also found that Y addition does not change the size and morphology of primary Mg2Si particles considerably, but the pseudo-eutectic Mg2Si changed from a flake-like morphology to fine fibrous or rod-like one. The results show that proper content of Y additions can reduce the amount of Mg2Si phase through dissolving it into the matrix, lead to the precipitation of Al2Y phase and improve the mechanical properties. Modified composites with 0.5% Y exhibited an ultimate tensile strength (UTS) of 290 MPa with an elongation of 4.3%. After exposing the composite to solution treatment at 520 °C for 4 h, the tensile strength of the composite continuously increased with the increase of Y content, and reached the maximum at 1% Y. The maximum UTS and elongation at room temperature for the heat-treated composites are 294 MPa and 7.4%, respectively. In the cast specimen, fracture surfaces are covered by packets with coarse steps, suggesting a brittle mode of failure. Modified composites with 0.5 wt.% Y contain several cracked particles together with a few decohered primary Mg2Si particles. In solution heat-treated state, dimples present at the fracture surface are rather coarse but homogenous, showing a semi-ductile mode of fracture. Wear test results showed that the wear resistance of all specimens increases with the addition of Y up to 0.3 wt.%. Scanning electron microscopic observations of the worn surfaces revealed that the dominant wear mechanism was abrasive wear accompanied by some delamination wear mode.

Microstructure Characterization of Laser-Welded Nb-Microalloyed Silicon-Aluminum TRIP Steel

Abstract: This work presents the results of a microstructural characterization of welds in Nb-microalloyed TRIP steel with silicon partially replaced by aluminum. Tests of laser welding of thermomechanically processed sheet samples were carried out using keyhole welding and a solid-state laser. Welding penetration tests were conducted for heat input values between 0.037 and 0.048 kJ/mm. Identification of different microstructural constituents was carried out using light microscopy and scanning electron microscopy in the fusion zone (FZ), heat-affected zone (HAZ), and base metal. Special focus was put on the effect of cooling conditions on the stabilization of retained austenite in different zones. The intercritical, fine-grained, and coarse-grained regions of the HAZ were identified. It was determined that enriching austenite with carbon in the intercritical HAZ stabilizes this phase at a level close to the base metal, i.e., a 15% volume fraction. Despite a high cooling rate in the FZ and HAZ, interlath retained austenite is also present in these zones. The research involved microhardness measurements and characterizing non-metallic inclusions formed in the fusion zone. A good correlation between microstructures formed in different weld regions and microhardness results was obtained.

Effect of Tool Rotation Rate on Microstructure and Mechanical Behavior of Friction Stir Spot-Welded Al/Cu Composite

Abstract: In this study, the friction stir spot welding of Al/Cu composite produced by accumulative roll-bonding process was performed using a triangular pin with no features. The influence of tool rotation rate on the microstructure, surface hardness, and tensile shear strength was examined. The results indicated that the weld made at lower tool rotation rate was not bonded because there was no intermixing between the upper and lower sheets. The maximum shear failure load increased with the increasing tool rotation rate, and reached a maximum value at 1400 rpm, which can be ascribed to the increasing area and effective length (d) of stir zone (SZ). The experimental observations showed the presence of the intermetallic compounds (Al2Cu and AlCu3) in the SZ. It was concluded that the intermetallic compounds, accompanied by the material crushing, increased the hardness of the SZ.

Structural, Optical, Electrical, and Dielectric Properties of the Spray-Deposited WO 3 Thin Films

Abstract: Studies on effect of the substrate temperature on physicochemical properties of WO3 thin films prepared using spray pyrolysis technique have been presented. Raman spectra of the film shows presence of W-O-W network with stretching and bending vibrations which revealed monoclinic structure of WO3 which is confirmed by XRD studies. XPS studies show that films are sub-stoichiometric and O/W ratio is 2.87, with W present in two valence states W+5 and W+6 with ratio of 0.21. Smallest crystallite size (28 nm) is observed for the film deposited at 425 °C, and on either side crystallite size is larger. Optical studies show band gap energy 2.6 eV and NUV, blue and green photo-emissions from WO3 films. Scanning electron micrographs depict wired network of the WO3, and AFM shows rough nature of the films. The thermo-emf is found to be linearly changing with temperature difference and decreases with increase in the substrate temperature.

Synthesis and Characterization of Silver Nanoparticles and Silver Inks: Review on the Past and Recent Technology Roadmaps

Abstract: The synthesis of silver nanoparticles for silver ink formation has attracted broad interest in the electronic part printing and semiconductor chip industry due to the extraordinary electrical and mechanical properties of these materials. The preparation of silver nanoparticles through a physical or chemical reduction process is the most common methodology applied to obtain nanoparticles with the required size, shape and surface morphology. The chemical solution or solvent carrier applied for silver ink formulation must be applied simultaneously with the direct writing technique to produce the desired adherence, viscosity, and reliable performance. This review paper discusses the details concerning the past and recent advancement of the synthesis and characterization of silver nanoparticles and silver ink formation. A review on the advantages of various sintering techniques, which aim to achieve the electrical and mechanical properties of the required printed structure, is also included. A brief summary concerning the recent challenges and improvement approaches is presented at the end of this review.

Modeling of Directional Solidification of Columnar Grain Structure in CMSX-4 Nickel-Based Superalloy Castings

Abstract: The paper presents the analysis of numerical simulation of the Bridgman directional solidification process performed on CMSX-4 rods. The numerical simulation was studied applying the ProCAST software. The constitutive law parameters of the normal Gaussian distribution were used to describe the nucleation process. The coefficients of the equation were determined and used to calculate the growth rate of dendrite tip. The analysis of the as-cast microstructure was carried out with the use of Aphelion software in order to determine the average area of grains and their quantity. The experimental verification of both nucleation and grain growth coefficients used for the simulation of the directional solidification process confirmed that the model was correct and described well the investigated process of directional solidification using the Bridgman method.

Microstructural Evolution and Deformation Behavior of a Hot-Rolled and Heat Treated Fe-8Mn-4Al-0.2C Steel

Abstract: The microstructural evolution following tensile deformation of a hot-rolled and heat treated Fe-8Mn-4Al-0.2C steel was studied. Quenching in the range of 750-800 °C followed by tempering at 200 °C led to a ferrite-austenite mixed microstructure that was characterized by excellent combination of tensile strength of 800-1000 MPa and elongation of 30-40%, and a three-stage work hardening behavior. During the tensile deformation, the retained austenite transformed into martensite and delayed the onset of necking, thus leading to a higher ductility via the transformation-induced plasticity (TRIP) effect. The improvement of elongation is attributed to diffusion of carbon from δ-ferrite to austenite during tempering, which improves the stability of austenite, thus contributing to enhanced tensile ductility.

Effect of Preheating in Hybrid Friction Stir Welding of Aluminum Alloy

Abstract: The controlled energy input into the system by introducing an extra heat source to enhance the material flow along with reduction of the plunging force remains a potential area of considerate for the development of hybrid friction stir welding (FSW) process. Hence, the effect of preheating on the weld joint properties is evaluated using plasma-assisted friction stir welding (P-FSW) process for joining aluminum alloy. A comparative study of mechanical and macro-microstructural characterizations of weld joint by FSW and P-FSW has been performed. Transverse tensile strength of weld joint is approximately 95% of base metal produced by P-FSW and is 8% more than conventional FSW welds. The effect of preheating enhances material flow and dissolution of fine oxide particles by plasma arc results in increase of strength and marginal modification of deformation behavior. The preheating brings uniformly distributed hardness in weld zone and the magnitude is higher in the advancing side with overall increase in average hardness value. Grain sizes are much finer due to the pinning effect of Al2O3 particles that retarded grain growth following recrystallization during P-FSW and thus led to more pronounced reduction in grain size and relatively brittle fracture during tensile loading of welded joint. Overall, the influence of preheating acts quite homogeneously throughout the structure as compared to conventional FSW. However, the results reveal that the development of P-FSW is still in initial stage and needs to improve in various aspects.

Electrical and Optical Properties of Nickel- and Molybdenum-Doped Titanium Dioxide Nanoparticle: Improved Performance in Dye-Sensitized Solar Cells

Abstract: Nanocrystalline undoped and doped TiO2 particles with different concentrations of nickel and molybdenum (1-7%) was synthesized using the hydrothermal method followed by characterization using standard analytical techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-Vis spectroscopy The XRD analysis shows no change in crystal structure of TiO2 after doping with different concentration of Ni and Mo which indicates the single phase polycrystalline material The SEM analysis shows the partial crystalline nature of undoped and doped TiO2 and TEM analysis shows the particle size were in the range of 7-11 nm for Ni and 9-13 nm for Mo-doped TiO2 The electrical and photovoltaic properties of the undoped and newly synthesized Ni- and Mo-doped TiO2 were studied The ac analysis shows that the dielectric constant e′ and dielectric loss tan δ decreases with increase in frequency and become independent at higher frequency ranges The dielectric property decreases with increase in dopant concentration which provides the valuable information about conduction process At low frequency, the mechanism of ac conductivity was found to be same as that of dc conduction As the frequency increases, the magnitude of complex impedance decreases indicating the increase in ac conductivity It was observed that the impedance increases with the corresponding increase in the dopant concentration Under simulated solar illumination with an optimum content of Ni and Mo involves in TiO2, the amount of dye absorption increases resulting in the gradual increase in photovoltaic current and hence improvement in the cell efficiency of dye-sensitized solar cell (DSSC) from 623 to 672% and 623 to 716% by increasing the dopant concentration of Ni and Mo from 0 to 5%, respectively, was achieved