Showing papers in "Transactions of JWRI in 2012"

Dissimilar Welding of Titanium Alloys to Steels

Abstract: This review summarizes the dissimilar welding of titanium alloys to steels over a period of ten recent years, involving the welding processes which are used for the dissimilar welding of titanium alloys to steels, the mechanical properties of the joints and the discussion for the forming mechanism of the interface. Reducing the formation of brittle intermetallic compound (IMC) is a key requirement in the dissimilar welding of titanium alloys to steels, because the strength of the welding joints depends on the presence of IMC.

Oxygen Solid Solution Strengthened Pure Titanium Powder Materials

Abstract: The applications of Ti and its alloys are limited to high-performance products because of expensive cost and poor plastic formability. In order to develop a cost-effective processing route for pure Ti and its alloys, pure Ti powder was used as raw material and consolidated by different powder metallurgy routes in this study. Warm compaction and cold compaction were employed to consolidate Ti powder and spark plasma sintering (SPS) was used as reference method. The obtained compacts were subsequently hot extruded. The microstructure and mechanical properties of the hot-extruded pure Ti were evaluated. It was found that the samples prepared by warm compaction showed a higher tensile strength of 973.6 MPa, a better elongation of 26% and a higher hardness of 389.8 Hv comparing with the other two methods. Solid solution strengthening of oxygen was the main reinforcement mechanism for the sample prepared by warm compaction in this study. The strengthening effect of oxygen was calculated as 769.8 MPa / mass.%.

Laser Welding of Magnesium Alloys

Abstract: Aiming at establishing a reliable welding technique for magnesium alloys, the laser weldability of different types of magnesium alloys has been investigated using a high-brightness disc laser. It was found that high quality joints could be fabricated in the case of wrought type alloys when a certain defocusing level was optimized for the employed high-brightness laser beam of the newly developed diode-pumped solid-state laser. Porosity formation in the case of die-cast type alloys was extensively studied, and it was revealed that the gases entrapped in the base material during the die-casting process grew much larger. It was also found in the case of powder metallurgy alloys that the base material produced an extremely porous weld metal due to the gases entrapped in the production processes. While a significant reduction in porosity formation could be realized by the insertion of extruded alloy sheet in butt welding of die-cast alloys, the higher pressure levels applied in the production of the powder metallurgy type alloys made fusion welding of this alloy very difficult.

Effects of Ag Content on the Mechanical Properties of Bi-Ag Alloys Substitutable for Pb based Solder

Abstract: As one of the candidate materials for the high-temperature-resistant joining materials which substitute from the conventional Pb-rich solder, Bi-Ag solder alloys were used to investigate the characteristics. The mechanical properties have been investigated on lead-free Bi-Ag alloys using tensile tests to clarify the important characteristics for solder alloys having reflow resistant relatively high melting temperature solders. The tensile strength gradually increased with increase of Ag content, however, the elongation showed a minimum at eutectic Bi-2.5Ag and it only slightly increased with increase of Ag content up to 11%Ag for quenched alloys. On the other hand, air cooled Bi-11Ag alloy showed improved elongation about 20%, the value is much higher than that of quenched one, about 3%. The hypereutectic Bi-11Ag solder can be used as a solder material having relatively good wettability and mechanical properties, in particular it showed elongation of 20% under air cooled condition.

Fatigue Life Enhancement of Fillet and Butt Welded Joints after Laser Peening

Abstract: Laser peening which introduces compressive residual stresses on surfaces is effective in enhancing fatigue lives of welded components, because tensile residual stress due to welding is one of the most prominent factors reducing the fatigue lives. Structural steels are widely used for bridges, buildings, etc.; however, the effects of laser peening on the structural steels are not well established, especially on the welded zones. In this study, the welded zones of a box-welded joint were treated by laser peening and the residual stress was examined. Moreover, fatigue lives were evaluated for box-welded joints and butt welded joints with and without laser peening. Results obtained are summarized as follows: (1) Laser peening resulted in a change in the residual stress from tensile to compressive around box welding zones of a fillet welded rib-plate. (2) Laser peening can dramatically extend the fatigue lives and increase the fatigue strength of fillet and butt welded joints.

A Study of Impact Toughness of Intercritically Reheated Coarse-grain Heat Effected Zone of Two Type X80 Grade Pipeline Steel

Abstract: The impact toughness of intercritically reheated coarse-grain heat-effected zone (ICCGHAZ) of ordinary X80 pipeline steel and X80 pipeline steel with excellent deformability with (Bainite +Ferrite) dual-phase microstructure was studied by means of thermal simulation technique、 microscopic analysis method and impact toughness testing, especially discussed the embrittlement mechanism of impact toughness and microstructure of ICCGHAZ. The result indicate that impact toughness of the pipeline steel with excellent deformability and the ordinary pipeline steel have the same variation in different welding second thermal cycle peak temperature conditions. When the second peak temperature was in two phase (α+γ) temperature range, ordinary X80 pipeline steel and X80 pipeline steel with excellent deformability all show local embrittlement of ICCGHAZ. The local embrittlement in critical coarse grained region was caused mostly by grains growth and the increase in the number and size of Martensite /Austenite-constituent (M/A). KEY WORDS: (X80 pipeline steel with excellent deformability), (ordinary X80 pipeline steel), (intercritically reheated coarse-grain heated-affected zone), (impact toughness), (M-A constituent)

Numerical Analysis of GTAW Arc Behaviors in External Axial Magnetic Field

Abstract: In welding process, it is widely accepted that grain refinement is an effective way to improve the properties of welding joints. When fine equiaxed grain was formed in the microstructure of welding joints, the tendency of welding crack would be decreased, and mechanical properties would be improved [1] . In seeing this, both metallurgical methods, by adding alloying elements in the welding pool, and mechanical methods, by mechanically stirring or vibrating the welding pool, have been broadly investigated to refine grains. In practice, a lot of methods, such as mechanical vibration [2-3] , ultrasonic vibration [4-5] , pulse current [6-7] , electromagnetic stirring, has been implemented to stir or vibrate the welding pool and thus refine the grains. During all of such mechanical methods, electromagnetic stirring distinguished itself by its lower investment and applicability. Generally, research on electromagnetic stirring welding concentrates on three aspects listed as arc behaviors, analysis of microstructure & properties, and model of welding arc & pool. In all of three concerns mentioned above, arc behaviors are so general and basic for understanding the mechanism of electromagnetic stirring, because arc behaviors are responsible for the heat transfer, heat distribution, penetration and vibration of weld pool. So, research on arc behaviors should be stressed and given a priority. Many researchers have done a lot of work about welding arc and acquired many great achievements [8-14] , even though it is a very long and hard journey. In present paper, MHD equations are built in order to describe the GTAW arc behavior at external constant axial magnetic field. With the help of CFD software FLUENT, the GTAW arc behaviors are studied and discussed. The arc pressure is measured and the results are used to validate the model. 2. Experimental system Fig.1 shows the experimental system used in this work. We made an exciting coil, and a DC power provides the exciting current, and then the coil generates the axial magnetic field. The arc pressure is investigated by the data collecting card and the pressure sensor which is connected with the hole in the center of the water-cooled copper.

Ohmic Contact Mechanism of Titanium-based Electrodes on n-type Gallium Nitride

Abstract: Electrical properties at the interface between n-type gallium nitride (GaN) and Ti-based contact layers formed by radio-frequency magnetron sputter deposition under various conditions were investigated to clarify the mechanism to achieve ohmic contacts. TiN contacts deposited using N2 gas are non-ohmic. Therefore, formation of TiN adjacent to GaN is not a necessary condition for achieving ohmic properties. On the other hand, Ti contacts deposited using Ar gas are ohmic in the as-deposited state, even though a layer of Ti2N is formed between GaN and Ti during the deposition. It is also shown that Ti deposition on undoped GaN produces ohmic contacts. The nitrogen vacancies increased in the sub-interface of GaN are essential for ohmic properties. The interfacial reaction between Ti and GaN to form nitrogen vacancies is affected by the partial pressure of N2 during the deposition.

Microstructure and Mechanical Properties of Friction Stir Welded Pure Cu Plates

Abstract: The process window for the friction stir welding of fully annealed commercial pure Cu was determined at an applied load of 1000kg, which includes a welding speed that ranged from 200 to 800 mm/min, and a rotation speed that ranged from 500 to 1300 rpm. In the stir zone of sound welds, a remarkably refined microstructure with the average grain size of 6.9 Πm can be obtained by decreasing the rotation speed to 1000 rpm. However, the grain size becomes larger than the base metal when the rotation speed is higher than 1300 rpm. The stir zone may contain a structure composed of a high density dislocation cell or large developed annealing twin. The joints welded at a rotation speed less than 1300 rpm fractured in the base metal. However, the joints welded at a rotation speed higher than 1300 rpm fractured in the stir zone. The mechanism of the mechanical property changes in the Cu welds was proposed and clarified from the viewpoint of microstructural evolution.

Formation of Periodic Nanostructures on Titanium Dioxide Film by Femtosecond Laser Irradiation

Abstract: Periodic nanostructures formation on Titanium dioxide (TiO_2) film by scanning of femtosecond laser beam spot is reported. Periodic nanostructures, lying perpendicular to the laser electric field polarization vector, were formed on TiO_2 film by femtosecond laser irradiation for 1mm/sec at laser fluence from 0.35 J/cm^2 to 065J/cm^2. For 0.5mm/sec, they were formed at laser fluence from 0.35 to 0.55J/cm^2. Periodic nanostructures were also formed on large areas of the film. The period of Periodic nanostructures was 200nm.

Transient Welding Distortion of the Thick-wall Pipes Circumferentially Welded by All-position Narrow Gap TIG Welding

Abstract: In the paper, the thick-wall pipes of 304L stainless steel used in nuclear power station are circumferentially welded with an all-position narrow gap TIG welding machine and the transient welding distortions were automatically measured. Conclusions show that the axial welding distortion is strongly related with the filling of the weld groove and heat input, and the axial welding distortion occurs mainly in the two-thirds filler depth of the weld groove from the inner surface, while the axial distortion barely exists in the rest of the weld groove. As the heat input is critical to axial shrinkage, the axial shrinkage could be controlled by continuous welding with less heat input.

Influence of Root Gap and Tack Weld on Transverse Shrinkage during Welding

Abstract: Welding distortion is caused by the local shrinkage produced during the welding thermal cycle. The local shrinkage can be separated into four components, namely transverse shrinkage, longitudinal shrinkage, transverse bending and longitudinal bending. In this research, the influence of the root gap on the transverse shrinkage of welded joints is closely examined from the aspect of closing or opening of the root gap with the movement of the welding torch. Since the contact phenomena between the two surfaces forming the root gap are important, an interface element is introduced to a thermal elastic-plastic FEM. The butt welding model and fillet welding model are used to study the influence of gap size and the pitch of tack welding. Once the relation between the size of the gap and the transverse shrinkage is obtained, it can be introduced to the elastic FEM using the inherent deformation to predict the welding distortion of large structures.

Visualizations and Predictions of Welding Arcs

Abstract: This paper gives an instance of recent progress which has led to the capability of the visualization and prediction of welding arcs, though the dynamic observations of spectral image by a high speed digital video camera with a monochromator and also the calculations by numerical models. The dynamic observations lead to visualizations of metal vapor behavior in arc plasma and, for example, show the temperature minima at arc axis in GMA welding. Finally, we discuss plasma physics in welding arcs through numerical calculations, using the basic conservation equations of mass, energy, momentum, current and electron density of plasma physics. There is close interaction between the electrode, the arc plasma, the weld pool, and also the metal vapor, which constitute the welding process, and must be considered as a unified system.

Study on Burn-through Prediction of In-service Welding

Abstract: The pressure of media flowing in the pipe is one of the most important factors that has greatly influence on burn-through during in-service welding. The burn-through phenomenon was studied on the radial displacement with numercial simulation method supplemented by means of actual welding tests under different inner pressure. The results show that the radial displacement of the node beneath the molten pool increases with the increasing temperature of welding. When the temperature is close to the peak temperature, the deformation of the node has a mutation. Futhermore, the mutation degree becomes larger with higher inner pressure. Under a specific temperature field, the radial displacement of the node with maximum temperature at the inner wall changes linearly with the increasing pressure at the first stage, but when the inner pressure is up to a fixed value, the trend cannot be continued, and a mutation appears and indicts that the buru-through should occur if we don’t take preventive measures. So the radial displacement can be seen as the burn-through criterion of in-service welding.

Strengthening Mechanism of TiNi Shape Memory Sintered Alloy using Elemental Mixtures of Pre-alloyed TiNi Powder and TiO_2 Particles

Abstract: The mechanical properties and the strengthening mechanism of the extruded and heat-treated titanium-nickel (TiNi) shape memory alloys (SMAs) prepared by powder metallurgy (P/M) were investigated. TiNi pre-alloyed powder was mixed with titanium dioxide (TiO_2) particles by a planetary ball mill, and consolidated by spark plasma sintering (SPS) at a sintering temperature of 1173 K in vacuum (6 Pa). Subsequently, the SPSed TiNi alloy compacts were extruded at a pre-heated temperature of 1373 K, and shape memory heat-treated at a holding temperature of 773 K. Ti_4Ni_2O phases formed by reaction between the TiNi matrix and the additive TiO_2 particles caused an increase of the soluted Ni content of the matrix, and resulted in a decrease of the martensitic transformation temperature of the TiNi matrix. Consequently, the extruded and heat-treated TiNi powder alloy with 1.0 vol.% TiO_2 particles showed a remarkably high plateau stress of 524 MPa and ultimate tensile strength (UTS) of 1298 MPa at room temperature (298 K), respectively. It also revealed a good shape recovery rate of 89.3 % when 8% tensile strain was applied.

Virtual Tryout Technologies for Preparing Automotive Manufacturing

Abstract: It is important to reduce the weight of the automotive and to shorten the development period for reduction of CO2 discharge. For these purpose, virtual tryout technologies have been introduced automotive industry. Every process from stamping to assembly for car development has introduced CAD design and numerical simulation such as forming simulation, dimensional accuracy simulation, and so on. In this paper, sheet metal forming simulation and related technologies are presented as a one example of virtual tryout technologies for preparing automotive manufacturing A total stamping simulation system, which enables advanced study on formability of stamping parts and accuracy of assembled parts, has been developed. It can be applied from vehicle body parts design phase to production preparation phase. The developed system is consisting of 1) simulation technologies to enable parts designers to evaluate function and formability of a part, 2) simulation technologies to enable high precise prediction of dimensions of stamping parts and 3) simulation technologies to obtain parts with high dimensional accuracy after connection of stamping parts with hemming. The system can reduce vehicle development period drastically without reducing dimensional quality of parts.

Numerical Analysis of Hot Cracking in Welded Pipe Structure

Abstract: Masakazu SHIBAHARA*, Takuya IWAMOTO**, Takahiro OSUKI***, Kazuhiro OGAWA***, Hisashi SERIZAWA**** and Hidekazu MURAKAWA**** * Graduate school of engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku Sakai 599-8531, Japan ** Graduate student, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku Sakai 599-8531, Japan *** Sumitomo Metal Industries, Ltd, 1-2, Huso-cho, Amagasaki 660-0891, Japan **** Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Study on Transient Buckling Behavior of Thin Plate Structures during Welding Process

Abstract: For the fabrication utilizing welding method, welding distortion is always one of most complicated problems on welded structure. Furthermore, for the thin plate welded structure, the buckling type distortion is the most prevalent mode of welding distortion compared with other distortion modes. Considering the large deformation theory, the buckling type distortion of welded structure during and after the welding is investigated. In this study, the transient buckling behavior during the welding is studied by means of Thermal Elastic Plastic FE analysis. In 1953, Masubuchi

Impact Strength of Sn-Ag-Cu Solder Joints in Electroless Ni-P/Au Plating in Laser Reflow Soldering †

Abstract: The laser has been utilized as a alternative heat source for the soldering process because of its unique properties such as localized heating, rapid rise and fall in temperature, non-contact heating and easily automated process. However, there is limited discussion of the laser reflow soldering and a lack of information exists about the performance of the resultant joints. In this study, the impact strength of Sn-Ag-Cu solder joints in electroless Ni-P/Au plating in laser reflow soldering was investigated using a ball impact test. As a result, it has been made clear that the impact strength of the solder bumps was affected by the heat source for the soldering process.

Wear Behavior of Network-Structured Carbon Nanotube Coating on Titanium Substrate

Abstract: This study investigated the tribological property and wear behavior of pure titanium (Ti) plate coated with un-bundled multi-walled carbon nanotubes (MWCNTs). The network-structured MWCNT films were formed on Ti substrate, and their tribological properties were examined by the ball-on–disk wear test equipment under a dry sliding condition. SUS304 stainless steel ball was used as a counterpart material in this test. The mean friction coefficient of the Ti plate coated with MWCNTs was remarkably lower and stable compared to the as-received pure Ti plate without any coating films. SEM-EDS analysis showed the network-structured MWCNT films obviously remained after a wear test for 3.6 ks in sliding and no seizure phenomena with the SUS304 ball. The above excellent tribological performance was due to CNTs self-lubricant properties, their bearing effects and the strong metallurgical bonding between Ti plate and MWCNT films by annealing.

Numerical Analysis of the Straightening Process of Thin Plate Structures by Elastic FEM based on the Inherent Strain Method

Abstract: Welding distortion is practically an expected feature in shipbuilding, with a few exceptions; when thick plates are used, for example. Nowadays, the trend is to use thinner plates in order to reduce weight, aiming to reduce waste of fuel. However, thin plates are drastically affected by welding distortion, this finally represents a significant - unnecessary expenditure of money and time on straightening the structure. Here it is necessary to realize that even with the most effective welding process, welding distortion in thin plates appears, so it is unrealistic to think about free of distortion lightweight welded structures. Although there have been gradual movements to optimize the straightening process, still many question remain to be answered. In this paper, an Elastic Finite Element Model based on Inherent Strain Method is developed in order to study the process of straightening deformed plates. Some techniques, usually used in the practice, are numerically evaluated and their effectiveness compared. Finally, useful recommendations that aim to the optimization of the process are drawn.