Showing papers in "Transactions of JWRI in 2011"

Finite Element Analysis of Welding Distortion in a Large Thin-plate Panel Structure

Abstract: Welding distortion not only negatively affects the dimensional accuracy but also significantly degrades the quality and performance. Although many researchers and engineers have made great efforts to control and reduce welding deformation, it is still a difficult engineering problem. In the current study, welding distortion in a large thin-plate panel structure was predicted by means of elastic FEM based on inherent strain theory and interface element formulation. The welding distortions in the thin-plate model computed by large deformation theory and by small deformation theory were compared. In addition, the influences of welding procedure and assembly sequence on the final distortion were examined numerically.

Residual Stresses in Flash Butt Welded Rail

Abstract: The residual stresses on the surface of a rail joined by flash-butt welding are measured using a hole-drilling method and the thermal elastic-plastic deformation process considering the phase transformation are simulated using FEM code JWRIAN. The residual stresses measured by experiments and computed by FEM have an acceptable agreement in accuracy. The simulation results show that the phase transformation in the cooling stage has a significant effect on the residual stress distribution and its values. The tensile or compressive state for residual stresses near the welding section strongly depends on the phase transformation which occurred during the cooling process. The biaxial tensile stresses are produced at the web near the fusion line of the flash butt welded rail. The biaxial tensile stresses increase the risk of cracks. Although the Martensite phase transformation during the cooling process at top and bottom near the fusion line produces compressive stress, it increases the brittleness at the same time. Therefore, the proper heat treatment after welding is still necessary in order to improve the toughness.

Mechanical Properties and Strengthening Mechanism of Pure Ti Powder Composite Material Reinforced with Carbon Nano Particles

Abstract: The strengthening mechanisms of titanium-carbon black (CB) nano particle composite materials were investigated quantitatively in the present study. The titanium matrix composite (TMC) specimens were prepared via a powder metallurgy (P/M) route including the wet coating process for composite titanium (Ti) powders with un-bundled CB nano particles, consolidation by spark plasma sintering (SPS) and subsequent hot extrusion. The extruded TMC, with carbon content of 2.66 at.%, exhibited excellent tensile strength of 837 MPa in 0.2%YS, 899 MPa in UTS and 18.7% in elongation at ambient temperature. In this composite, some of additive CB nano particles were dissolved into the Ti matrix as a solid solution element. The other CBs and Ti matrix were chemically reacted when sintering, and resulted in the in-situ formed titanium carbide (TiC) compounds, which were uniformly distributed throughout the Ti matrix. These TiC dispersoids promoted the dynamic recrystallization during hot extrusion process, and resulted in the Ti grain refinement. Consequently, carbon solid solution hardening, TiC dispersion strengthening and grain refinement enhanced the mechanical performance of the TMC. In particular, the effect of TiC reinforcements, accounting for about 60% of the whole strengthening behavior, played the most significant role.

Cyclic Oxidation Behavior of CoNiCrAlY Coatings Produced by LVPS and HVOF Processes

Abstract: CoNiCrAlY bond coatings for thermal barrier systems were produced by low vacuum plasma spray (LVPS) and high velocity oxy fuel (HVOF) on the nickel base superalloy, IN738LC. High temperature cyclic oxidation resistance of coated samples was performed in 1100�` for 100 cycles including 1h holding at high temperature and cooling to room temperature by air flow. Oxidation kinetics were determined by weight gain measurement of samples in each cycle, microstructural characterizations of coatings and oxide scales were investigated by scanning electron microscope equipped with EDS, and X-ray diffraction methods. The results proved better cyclic oxidation of CoNiCrAlY-HVOF coatings at the initial stage of oxidation and better oxidation behavior of LVPS coatings at continuation. The enhanced cyclic oxidation resistance of the HVOF coating at the initial stage was due to the nucleation of �. -Al2O3 during the coating process and formation of continuous protective oxide layer at the initial stage of oxidation, but at continuation, large grain oxide, unsuitable adherence and existence of some microcracks in the TGO cause to easily spallation and severe oxidation in the HVOF samples was observed.

Investigation of Interface Layer Failure and Shear Strength of CMT Brazed Lap Joints in Dissimilar Materials

Abstract: In order to save fuel consumption by reducing the weight of automobile body, the use of aluminum alloys has a great advantage. However, how to join aluminum alloys with steels becomes a big problem in the assembly lines. Cold metal transfer (CMT) is a promising joining process for steel/Al dissimilar materials. To evaluate the shear strength and to investigate the failure modes of CMT brazed lap joints of dissimilar materials, both experimental observation and numerical simulation are performed. A numerical model for the interface layer and for the failure criteria of the interface layer between steel and aluminum is developed. The interface layer of CMT brazed lap joint can be modeled by the interface element. The failure stress and failure energy at the interface element are proposed as the failure criteria for the prediction of shear strength of CMT lap joints. If steel sheet thickness becomes thicker, stress distribution and concentration at interface layer elements have some change and shear strength at the interface layer can be improved. Then the failure occurring at the interface element may transfer to the fusion line at the side of the aluminum alloy sheet.

The Effect of Solid Solutionizing Ti Element on Microstructural and Mechanical Properties of Extruded Cu-40Zn-Ti Ternary Alloy

Abstract: The effects of solid solutionizing treatment and following hot extrusion on microstructural and mechanical properties of the extruded brass alloy (Cu-40Zn) with a small addition of titanium (Ti) were investigated in the present study. Cu-40Zn with 0.5 mass% Ti (Cu-40Zn-0.5Ti) alloy ingot was prepared by a casting process. This cast alloy consisted of �. -�� duplex phase structures. Furthermore, Cu-40Zn-0.5Ti alloy contained coarse Cu2TiZn intermetallic compounds (IMCs) with 10-30 �� m diameters. The IMCs were completely soluble in the both �. and �� phases by heat treatment at 973 K for 15 min. Cu-40Zn-0.5Ti alloy was pre-heated at 973 K for 15 min in solid solutionizing, and immediately extruded to fabricate a rod specimen of 7 mm diameter. The extruded specimen contained fine precipitates, having a mean particle sizes of 0.5 �� m diameter, which were dispersed in both phases, not coarse Cu2TiZn IMCs. In particular, since the grain growth of the �. phase was inhibited by the pinning effect of the above fine precipitates at the grain boundaries, �. phase consisted of fine grains. The extruded specimen consisted of remarkably fine and uniform �. -�� phases with an average grain size of 2.14 �� m. The tensile properties of the extruded specimens showed an average value of yield strength: 304 MPa, ultimate tensile strength: 543 MPa, and 44 % elongation. The extruded specimens revealed suitable strengths and good ductility. The high strengthening mechanism of the wrought brass alloy was mainly due to the grain refinement of �. and �I phases by the fine precipitates derived from solid solutionizing Ti elements in the matrix.

Numerical Model of Gas Metal Arc with Metal Vapor for Heat Source in Welding

Abstract: In gas metal arc (GMA) welding, an arc discharge is applied for melting and joining metals. An electric arc is established between a base metal cathode and a consumable wire anode. By the high heat flux from the arc plasma, a droplet forms at the tip of wire and a weld pool forms at the base metal. Arc plasma is composed of large amounts of metal vapor from the droplet and the weld pool. From past studies, it is known that a mixture of metal vapor affects the properties of the arc plasma, such as electrical conductivity and radiative emission coefficient. Numerical models need many assumptions and time for complicated calculations. Therefore, to reduce assumptions and calculation time, a simplified model of the GMA welding arcs ignoring metal transfer is built. In the present model, special account is taken of the amounts of metal vapor, enthalpy of droplet and wire melting rate. These are calculated simultaneously. And then, the present model assumes a steady state, and that arc length is constant. Therefore, the wire feed rate is equal to the calculated wire melting rate. And then, the wire feed rate and the wire melting rate are balanced to keep arc length constant. We study the effects of metal vapor on the heat source properties of GMA welding arcs by using this model. The calculated mole fraction distribution is in agreement with the observed optical image separating bright regions at the arc center, dominated by the metal vapor, and dark regions at the outer arc. The highest temperatures occur at the edge of the arc core. Higher welding currents lead to faster melting and feed rates of consumable wire because of larger heat inputs, such as thermal conduction from the arc plasma and ohmic heating at the wire. For balance of larger heat inputs and faster feed rates of the wire, temperatures of the tip of wire, namely droplet, are kept constant though higher welding currents. This result is experimentally known, however this model enable it to be understood as a physical phenomenon.

Evaluation of Plasma Ion Beam Sputtered TiN / TiAlN Multilayers on Steel for Bio Implant Applications †

Abstract: Layers of 1.2 μm thick TiN / TiAlN were deposited onto 316L stainless steel substrates by a plasma ion beam sputtering technique. The layers were sputtered in pure Ar and N2 with the ion beam voltage of 1 keV and ion beam current of 30mA. Polycrystalline nature with FCC structure was identified from XRD analysis. The presence of Ti, Al, C and N elements on the surface of the film was confirmed by the XPS survey spectrum taken on the surface of the coatings. The Ti2p region can be decomposed into several contributions corresponding to the different oxidation states of titanium. The columnar nature of growth was observed from the cross sectional HRTEM analysis. The nano-hardness of the TiN/TiAlNcoatings were found to be 30 GPa. The superior corrosion resistance in simulated body solution environment was observed for the multilayer coated specimen.

Micro-Structural Observation of the Bonding Interface between Au Wire and a Platinum Electrode

Abstract: The Au wire/Pt bonding pad interface mechanism using ultrasonic bonding was studied in this paper. Following experiments were investigated After the bonding process or thermal aging test at 235 for 300 h, the Au-Pt interface was observed by UHR-SEM, TEM and electron diffraction. Intermetallic compound layer was not formed. Even under thermal aging less than 235 , Au-Pt interface keeps the same structure. These results show that Au-Pt bonding has strong thermal stability.

High Temperature Creep Properties of Sn-3.5Ag and Sn-5Sb Lead-free Solder Alloys

Abstract: In order to acquire relevant creep characteristics such as stress exponent and activation energy, creep strain tests were conducted on Sn-3.5Ag and Sn-5Sb systems of solder alloys in the intermediate temperature regime from 353 K to 453 K corresponding to the homologous temperatures�O =0.715:*0.917 and�O =0.699:*0.897 for the two alloys, respectively. It was found that an apparent creep-activation energy of both Sn-3.5Ag and Sn-5Sb solder alloys had a change point at�O =0.82 under the higher stress of 9.8 MPa. At the lower stress of 4 MPa, the change point was found only for Sn-5Sb alloy. Moreover, under the lower stress level, the stress exponent of both solder alloys are approximately 1, while under the higher stress region (more than 9.8 MPa), the stress exponents of both solders are 7 or more. Determining factors of creep deformation are discussed in relation to the temperature and stress region. It was considered that the deformation of both solder alloys is dominated by the rate of the dislocation climb/dislocation glide in the higher temperature and stress region.

Modeling of Fume Formation Process in Arc Welding

Abstract: In order to clarify fume formation mechanism in arc welding, quantitative investigation based on understanding of interaction among the electrode, arc and weld pool is indispensable. A fume formation model consisting of heterogeneous condensation model, homogeneous nucleation model and coagulation model has been developed and coupled with the GMA welding model. A series of processes from evaporation of metal vapor to fume formation from the metal vapor was totally investigated by employing this simulation model. This paper aims to visualize the fume formation process and clarify the fume formation mechanism theoretically through numerical analysis. Furthermore, the reliability of the simulation model was also evaluated through comparison of the simulation result with experimental result. As a result, it was found that most part of the fume was produced in downstream region of the arc originating from the metal vapor evaporated mainly from the droplet in argon GMA welding. This kind of the fume was constituted of particles with size of several tens nm. On the other hand, if the metal transfer becomes unstable and the metal vapor near the droplet diffuses directly toward the surroundings of the arc not getting on the plasma flow, the size of particles reaches several hundreds nm.

Environmentally Friendly Low Temperature Solid State Micro-joining

Abstract: The present review describes the low temperature solid state micro-joining applied to advanced electronics packaging. Four topics are introduced. The first one concerns the trend of semi-conductor packaging. The second one is the room temperature bonding carried out under an ultra high vacuum condition. The third one is the mechanism of micro bump bonding applied to the electronic assemblies. The bump bonding is usually called "flip chip bonding (FCP)" and is applied to the chip scale packaging (CSP), which is very important for three dimensional packaging. The bump deformation behavior is visualized by numerical simulation. The bond area growth mechanism (slip and folding mechanism) is numerically analyzed. The last one is the ultrasonic bonding between Au wire and Al pad. The Al pad has a very strong thin oxide film on the surface. The breakdown of the oxide film is discussed based on the idea of the slip and folding mechanism. The effect of ultrasonic vibration on the low temperature solid state micro-joining process is also discussed.

In-situ Observation Techniques of Solidification and Phase Transformation during Welding

Abstract: In order to respond to request for the direct observation of microstructure formation during welding, “in-situ observation techniques” are outlined in the present paper. They include the high-temperature laser scanning confocal microscopy (LSCM), the in-situ SEM which consists of the microstraining system compatible with EBSD measurements, the in-situ SIM, the spatially resolved X-ray diffraction (SRXRD), the time-resolved X-ray diffraction (TRXRD) and the hybrid system with LSCM and TRXRD. They can be applied to the analysis of microstructural changes for improving control of weld properties. When combined with additional experiments and modelling, these techniques enable a deeper understanding of the kinetics of phase transformations.

Electron Excitation Effect of Pure Mg Surface on initial Corrosion Phenomenon

Abstract: Magnesium (Mg) has the lowest negative standard electrode potential (-2.363V) among industrial metals. This means that corrosion phenomena easily occur when Mg specimens contact with other metals because a galvanic cell is formed at the interface between two materials due to their large potential difference. The conventional method to improve corrosion resistance of Mg alloys is anodic oxide film formation and/or chemical conversion treatment. However, these methods are expensive and chemically treated Mg products are hard to recycle. In this study, a new corrosion protection method by electron excitation to change the surface potential of pure Mg was investigated. The surface potential variation of pure Mg was measured by using Scanning Kelvin Probe Force Microscopy (SKPFM) to determine the anodic/cathodic area at the interface between different phases in the local galvanic cell. The electronically excited area by electron beam irradiation of a Scanning Electron Microscope (SEM) showed a lower surface potential than that of the as-polished original surface. SEM observation results indicated corrosion resistance at the electronically excited area of a pure Mg specimen was improved. This observation is also supported by salt water immersion tests and Atomic Force Microscope (AFM) analysis results as they indicate fewer corrosion products at the electronically excited area.

Particle Size Effects and Mechanical Properties of Alumina Dental Crown Fabricated by Stereolithography

Abstract: Ceramic dental crowns have a low risk of metallic allergies and are esthetically pleasing and so are actively investigated and developed in worldwide medical industries. In this study, three-dimensional dental-crown models composed of alumina and glass composite were fabricated successfully by using stereolithography. These precursors were dewaxed and sintered in air. Alumina bending test specimens of 1.2×4×20 mm in dimensions were fabricated by using the similar materials and processes. The highest bending strength of alumina ceramics with La₂O₃-B₂O₃-Al₂O₃-SiO₂ glasses was about 400 MPa. These glass components were coated on the alumina specimens to close micro cracks on the surface.

Variation of Photoconductive Properties of Titanium Dioxide Film by Femtosecond Laser Irradiation

Abstract: Titanium dioxide (TiO2) films were irradiated with a femtosecond laser to alter their photoconductive properties. The TiO2 films irradiated with the femtosecond laser were darkened without changing the topography of the TiO2 film surface. The electrical resistance of the films as a function of time were measured under visible light illumination. The transient electrical resistance decreased as time was increased after turning on the light, and then it increased as time was increased after turning off the light.

Effect of Heat Cycle in Multi-layer Welding on Charpy Absorbed Energy in Low and High Toughness Steel

Abstract: In this study, the heat cycle is determined in the case of multi-layer welding using three-dimensional heat-conduction analysis. The relationship between the heat cycle of multi-layer welding and the absorbed energy is defined by simulating various heat cycles of multi-layer welding by performing the synthetic HAZ test using steel with a relatively high Charpy absorbed energy and steel with a relatively low Charpy absorbed energy. On the basis of this relationship, the authors have studied the effects of heat cycle conditions on the Charpy absorbed energy of weld interfaces. In the results, the Charpy absorbed energy increased more than in the case of single-pass welding in some heat cycle in the case of multi-layer welding, both in high-toughness steel and low-toughness steel. The Charpy absorbed energy of the weld interface was greatly susceptible to the achieved temperature after the maximum achieved temperature, and the range of the temperature after the maximum temperature was achieved in which the Charpy absorbed energy was found to be different depending on the steel toughness.

Dissimilar Lap Welding of Ni-based Metallic Glass and Stainless Steel Foil by Fiber Laser Beam

Abstract: Dissimilar joining of Ni60Nb15Ti15Zr10 metallic glass foils to type316L stainless steel foils has been investigated using a fiber laser welding method. Dissimilar joints without cracking were obtained at the condition of laser power from 20 W to 35 W with constant welding speeds of 100 mm/s and argon gas shielding. Partial crystallization of metallic glass weld was detected by XRD over 30 W of laser power. Fracture loads of dissimilar joints increased with increasing laser power, but excess laser power at 35 W reduced the joint strength due to crystallization of the welds.

High Functional Alumino-Silicate / Polymer Nanocomposite

Abstract: Towards Innovating of advanced clay based - polymer nanocomposite with improved properties, this research is proposed and conducted. This research project is a phase of big research theme towards overcoming the difficulties and shortcomings of the previous studies for each of the experimental work (mixing, processing) and fundamental analysis (characterization, modeling, mechanical analyses) to produce controlled nanocomposites with required properties. This research is devoted to fundamental study for predicting the aspects of nanoclay particles – polymer interaction during processing and final product. In this regards a new approach and model are applied for predicting and innovating modified mechanical properties for producing a new nanocomposite. Several types of both nanoclay and polymer materials are used. Design, modeling, processing, characterization, testing, fundamental analyses and mechanism are considered regarding the influences of the some other controlling parameters such as interfacial properties and thermal effect. The study depended mainly on the directional fracture energy approach.

High Rate Diamond Deposition by Cooled Acetylene/ Oxygen Combustion Flame

Abstract: Since the combustion flame method using acetylene/oxygen is a high rate diamond synthesis process can be conducted in open air, this process is thought to be useful for low cost diamond production. In this study, to develop a method for diamond synthesis on a thick and/or complicated shape article, diamond depositions by water-cooled and air-cooled combustion flames were carried out. It was proved that diamonds could be deposited on 3 and 5 mm thick 304 stainless steel substrates without melt down of the substrate. Besides, in the case of 15 l/min in air flow rate, diamond like hexahedron and octahedron particles could be synthesized on the substrate without meltdown of the substrate even in the condition without substrate bottom cooling.

Joining of AlN Ceramic Bonded Carbon with AlN and W

Abstract: To obtain light and tough materials with high thermal conductivity, ceramic bonded carbon (CBC) composites consisting of carbon particles and thin ceramic boundaries were developed. The AlN ceramic bonded carbon (AlN/CBC, 20vol% AlN) could be densified by spark plasma sintering or hot-pressing, and exhibited light weight (~2.34 Mg/m 3 ), high bending strength (~100 MPa), and high thermal conductivity (~170 W/mK). AlN/CBC could also be joined strongly with ceramic (AlN) and some metals (W) due to its unique microstructure. These results suggest that the AlN/CBCs have large potential to be used as structural components and heat sinks for electrical and plasma facing devices.

Rewriting Process of Lower Electrical Resistance Lines on TiO₂ Film using Methods of Whitening with CW Fiber Laser and Darkening with Femtosecond Laser

Abstract: * Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ** Graduate school of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan *** Osaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku,, Osaka 536-8553, Japan **** School of Science and Engineering, Kinki University, Higasi-Osaka, Osaka 577-8502, Japan ***** Institute for Laser Technology, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan

Monitoring of Fatigue Failure in Steel Plate Deck by FSM

Abstract: For investigating the applicability of FSM (Field Signature Method) on fatigue failure monitoring, a series of fatigue tests on steel plate decks (4,625×2,200(mm)) was carried out by using a wheel load traveling test machine. From the results of the experiments, the position where the fatigue cracks initiated could be accurately specified. The initiation of minuscule cracks, which could not be confirmed by visual inspection, was specified. The direction of propagation and the length of fatigue cracks could also be specified. The results have shown that the fatigue failure could be detected adequately and with high accuracy in a real-sized steel plate deck.

Mechanical Fatigue Behavior under Macroscopically Elastic Stress Cycles Predicted by a Crystal Plasticity FE Analysis

Abstract: Predicting the mechanical fatigue phenomena of materials subjected to cyclic stresses, the mechanisms on generation and accumulation of inelastic deformation has to be clarified. In this study, a numerical study based on crystal plasticity FE modeling is conducted to evaluate its applicability for the mechanical simulation under uniaxial extension condition through the comparison with the experimental result of a carbon steel (SM400B). Then, the model is applied for the simulation under the symmetrical cyclic loading condition to discuss the effects of inclusions on the fatigue behavior at the stress level lower than the yield stress, i.e macroscopically elastic condition.