A novel self-alignment process was developed using liquid surface tension for future assembly of electronic or optoelectronic devices. It could be achieved for the material with low surface tension, e.g., resin materials by using 3-D pads for making the positioning boundary and mounting chips in the opposite direction for releasing the force acted on the liquid bumps. The principle and characteristics are described and the relationship between process parameters and joint geometry were examined. The possibility of self-alignment process was verified by analytic numerical method and scaled-up experiment. A self-alignment accuracy was examined experimentally and show that it became less than 0.4μm. It can provide a useful information on various parameters involved in the joint geometry and optimal design guideline to generate the proper profiles.
Ablation phenomena when the fourth harmonics of Nd: YAG laser is irradiated to an aluminum substrate during picoseconds were simulated using the modified molecule dynamics method that Ohmura and Fukumoto have developed. Scattering velocity of ablation particles were displayed by vectors, and both molten pool and slip planes in the material were visualized. At the same time, the angle, size and velocity distributions of the scattering particles, both potential and kinetic energies per ablation atom, and ablation energy were examined quantitatively. Authors have already clarified that there are two types in ablation form under constant laser fluence. One is explosive ablation and the other is relatively calm ablation. The former occurs when pulse width is extremely short. The simulation results showed that the angle and size of scattering particles depend on the ablation forms. These differences appear because ablation energy in the explosive process is much larger than that in the relatively calm ablation, that is, kinetic energy per ablation atom becomes very large when the pulse width is extremely short. Velocity of particles also depends on the ablation forms, but the tendency is comparatively weak. Scattering angle in both ablation forms has almost a normal distribution, but the standard deviation in the explosive ablation is much smaller.
The spatter is cooled during scattering by the air. So, many spatters change from the melt state to the solidification state. Spatter does not adhere to the base metal and peripheral equipment in the solidification state. But, the possibility of adhering to them rises, when the spatter melts. In other words, the adhesion of the spatter is influenced by its cooling condition, that is, the scattering route and velocity of the spatter. Therefore, in order to clarify its adhesion behavior, it is very important to estimate its scattering behavior. Then, is this study, the scattering behavior of the spatter is CO2 gas shielded arc welding is measured and the scattering behavior and terperature of the spatter are simulated. As the results of examining 300 spatters, it is indicated that the initial velocity of the spatter which arise from welding point is about 10 m/s or less and the initial angle of elevation is 70 degree or less. In addition, it is indicated that the Reynolds number of scattering spatter is 7 from 2 and the drag coefficient is 30 from 10. Moreover, when scattering locus of the spatter are simulated by substituting the relationship between drag coefficient and Reynolds number into the momentum equation of the spatter, the results agree well with the experimental results.
In our first paper, we have proposed the monitoring method using reflected Ar+ laser beam in YAG laser welding. It has been shown that this method was effective to monitor the changes of the welding conditions in YAG laser welding. It is also very important to detect the welding defects in YAG laser welding. In this paper, signals of the light and acoustic emission in addition to that of reflected Ar+ laser were measured at the same time. We investigated changes of these monitoring signals due to welding defects, such as lack of penetration, underfill by a wide gap and misalignment in butt joints. Main results are as follows; 1) The signal of the light emission was suitable to detect the penetration was full or partial. 2) The reflected Ar+ laser signal was suitable to detect the underfill due to a wide gap and the misalignment. 3) Only one signal was changed when these welding defects were occurred. While various signals were altered at same time when the welding conditions were varied. 4) The signal of the acoustic emission was not changed by these welding defects.
Measuring arc length is important to obtain good welding quality regardless of the torch deviation from the base metal and the molten pool. Therefore, it is necessary to detect the arc behavior in the transient state in addition to the steady state. For this purpose, this paper proposes the neural network model which outputs the present wire extension from the previous wire extension, welding current and wire feed rate in every sampling period. The structure of this neural network was determined by considering the differential equation which describes the wire melting phenomena. At the same time, the arc length is calculated by the equation of voltage drop in the extended wire and an approximate expression describing the arc characteristics. To confirm the validity of this system, fundamental experiments were carried out. The arc was directly observed and recorded as image data using a high speed camera. The output data from the neural network were compared with the measured data which were obtained from every captured image. It was found that the system can be applied as an arc sensor because of good responses in the transient state and no steady state error.
The present work was undertaken to examine the mechanical properties of lanthanum containing-cast alloy 718 laser welds. The 0.3 mass% lanthanum-added specimen (03 LaWP) was welded by CO2 laser welding in argon gas atmosphere. The commercial cast alloy 718 produced by the sand mold casting was used in as-received condition and at the heat treatment to produce plates with three different levels of grain size, and they were also welded for comparison. All post-welding heat treatments of the joints were carried out in a vacuum furnace. The mechanical properties of the welded joints were evaluated from tensile properties measured in the temperature range of 298 K to 1073 K. The tensile strength of lanthanum added-cast alloy 718 laser welds decreased with increasing the test temperature. The tensile strength of 03 LaWP specimen was equal to or greater than that of base metal at any test temperatures. Compared with the commercial cast alloy 718 welds, 03 LaWP specimen had good tensile properties at room and high temperatures. Detailed fractography confirmed that the failure occurred in the base metal in all cases. The cause of improved tensile properties of lanthanum containing-cast alloy 718 laser welds could be due to grain size strengthening-effects.
Recently, high strength steel becomes more and more important material from the viewpoint of lightening of the structures and decreasing the amount of welding. It is important to know the behavior of material steels after plastic deformation because many structures need plastic deformation for material steels in the manufacturing process of steel product. On the other hand, high strength steel plate for commercial use is mainly composed of Martensite microstructure by low carbon content in order to balance weldability and strength. So far, the relation between stress and strain after plastic deformation has been investigated only for Ferrite or the mixture of Ferrite, Pearite and Bainite structure. In this paper, by using 780 MPa steel plate, behavior of deformation of Martensite material after plastic deformation including the repeated patterns is surveyed comparing of behavior of Ferrite steel plate. Main results can be summarized as follows, 1) In case of large prestrain, over 3%, configuration of stress-strain curve is decided by the last prestrain condition even in repeated pattern. 2) Uniform elongation in the tensile properties vanishes after 6% pretension: This phenomenon is caused by many dislocations in the Martensite structure. 3) Baushinger effect becomes greater, as prestrain to reverse direction increases up to 2%. This amount corresponds to stress constant range of virgin material. It can be thought that piling-up of dislocations propagates all over the structure by taking 2% strain. 4) Baushinger effect ratio α0.2 of HT780 is approximately 0.3 under sufficient large reverse prestrain. 5) Configuration of stress-strain curve after prestrain in both direction hardly change by aging heat treatment (250°C). This is caused by difficulties of movement of dislocations because of mutual binding due to the tangle of many dislocations characteristic of Martensite transformation. 6) HT780 steel composed of Martensite is hardly changed in condition of damage by strain during practical use. This characteristic nature is unlike Ferrite steel.
An extraordinary property by refining the grain size to nanocrystalline level has become of great concern in the field of materials science. An explosion of the activity in this subject results from the success in both the fabrication of nanostructured powder and the process development for the powder. In this study, microcrystalline Ni coating was fabricated by HVOF spraying of mechanically alloyed Ni powder. Both the mechanical property and the thermal stability of the coating obtained were evaluated. The powder characteristic with milling period and mass of methanol addition in the mechanical alloying was fundamentally investigated. To gain the extremely finer grain size in the powder, it was revealed that the attritor milling was more effective than the ordinary rotational milling. Moreover, finer grain size in the powder was given under the conditions of smaller amount of methanol addition and longer milling period. The micro hardness of the coating made from the MA powder was approximately 32% higher than that of the coating made from the conventional powder. As the trace of the microcrystalline property of the MA powder was recognized in the coating, it was revealed that the grain growth in the powder was effectively suppressed during spraying in the HVOF process. From the measurement results of the micro hardness for the heated coating under the various heating conditions, it was confirmed that the coating microstructure has fairly high thermal stability.
The tensile strength and energy absorption of dissimilar metal friction welds between 6061 aluminum alloy and SUS304 stainless steel at high rates of loading are determined using the split Hopkinson bar. Round tensile specimens machined from as-welded butt joints of 12 mm diameter are used in both static and impact tension tests. Friction welding is conducted using a brake type friction welding machine under two different welding conditions. The effects of welding parameters and loading rate on the tensile properties are investigated. It is demonstrated that the tensile properties are greatly affected by the welding conditions, and are slightly enhanced with increasing loading rate. Macroscopic observations reveal that the fracture mode of the friction welded butt joints varies with loading rate, depending on the welding conditions. Microhardness measurements are performed to examine the extent of the heat-affected zone (HAZ) across the weld interface. The slight enhancement in the tensile properties with increasing loading rate is primarily due to the strain rate dependence of the thermally- softened 6061 aluminum alloy base material.
Many investigations have been reported for the mechanism of friction welding process of similar materials. However, the mechanism has not been clarified precisely up to now. A series of studies has been carrying out to clarify the joining mechanism of friction welding process. As for the first step, the present paper describes the joining phenomena in the first phase of friction welding process, i.e., friction stage. The materials joined were mild steels (same materials), and a brake type friction welding equipment was used throughout this experiment. Main conclusions are obtained as follows. (1) When the weld faying surfaces of both substrates contacted each other, the friction torque rapidly increased and maintained at the nearly constant. Following this, the friction torque reached to the initial peak torque. (2) The wear of both surfaces started at the periphery portion (outer surface) of joint, and moved to the center portion (center axis). Then, seizure and joining began at the center portion, and they extended toward the periphery portion. (3) The friction torque reached to the initial peak torque when the welded interface was joined completely and upsetting of both substrates started. (4) The joining models at the first phase in wear and the seizure stages were made from the observation results. As a conclusion, the first phase of friction stage was composed of the wear and the seizure stages.
In the previous report 1, the authors have clarified the joining mechanism of the first phase of friction stage during friction welding process. The present paper describes the effects of various conditions on friction torque in the first phase during friction stage for joining of carbon steels. The various conditions included friction pressure, friction speed, weld faying surface diameter and yield strength properties of both substrates, and pre-heat of them. The following are concluded. (1) The initial torque and the elapsed time for initial torque to it were affected by friction pressures and friction speeds described above for joining of same yield strength properties and weld faying surface diameter substrates. That is, the initial torque increased with increasing the friction pressure and it decreased with increasing the friction speed, and the elapsed time for initial torque decreased with increasing the friction pressure and the friction speed. (2) In spite of the weld faying surface diameter in both substrates, the initial torque and the elapsed time for initial torque hardly changed. (3) In spite of the yield strength property in both substrates, the initial torque and the elapsed time for initial torque hardly changed. (4) The initial torque and the elapsed time for initial torque decreased with increasing of pre-heat temperature. Especially, the wear stage rapidly decreased when pre-heat temperature exceeded approximately 473 K (200°C).
Oxidation of HVOF sprayed 316 L stainless steel coatings was studied experimentally. Oxygen content in the sprayed coatings was analyzed and its dependence on several spray parameters such as spraying distance, mixture ratio of fuel to oxygen, and composition of atmospheric gas on the substrate was studied. The oxygen content in the original powder was about 0.03 wt%, which typically increased to 0.3 wt% in the HVOF sprayed coatings under standard spraying conditions. Reduction of spray distance significantly increased the oxygen level due to the excessive heating of substrates by the flame. By using a nitrogen-gas shield attached to the substrate, it was revealed that the oxidation during flight is around 0.2 wt%. Control of oxidation by attaching a gas shroud to the HVOF nozzle has been attempted and oxygen content below 0.15 wt% has been achieved so far under a fuel-rich combustion condition while maintaining deposition efficiency over 73%. Coating porosity, however, increased from 0.5 to 2.5 vol% by using the gas shroud.