The heated band width is the most important processing parameter for local postweld heat treatment of tubes and pipes.A correct heated band width must be chosen so that the allowable maximum temperature difference in the soak band is maintained and a sufficient degree of stress relaxation is achieved.In this paper, the effects of the geometrical and the thermal parameters in the postweld heat treatment are investigated using axisymmetric FEM.The most severe condition in which the heat input and insulation are only applied to the outside surface of the pipe has been studied in detail. The effect of the geometrical and the thermal parameters on the through thickness temperature gradients is investigated.Through this study, it is found that the required heated band width is mainly influenced by the wall thickness and the heat transfer coefficient on the internal surface.The computed results show that the Hi number is inadequate for practical use under general conditions.A more general Hi number is proposed as a linear function of the thickness. Further, based on the computed results, the authors propose criteria for the heated band width which includes the effect of the geometrical and the thermal parameters.
In this study, the effect of laser pulse shot number on the hardness of weld metal and strength of welds was investigated. In the repair welding for such micro part as the cutter, laser welding was carried out to investigate the effects of pulse shot number for the hardness and strength of micro welded zone by the methods of changing pulse shot number under the condition that pulse output energy is constant. The following results were obtained in this study. In the case of laser welding under the condition that pulse output energy is 30 J/P and pulse shot number change from 10 to 60 pulse, the hardness of weld metal was increased with increasing pulse shot number. However, the hardness of weld metal is low against the hardness of base metal. The structure of martensite in the weld metal was decreased with increasing the pulse shot number, and pulse shot number is more than 30 pulse, the martensite in the weld metal disappeared from the weld metal. The strength of welds was increased with increasing the pulse shot number, but the rate of increasing at the strength of welded joint were moderated under the condition that pulse shot number is more than 30 pulse.
This study describes laser heat treatment of alloy tool steels SKD11 using pulsed YAG laser. When filler metal SUJ2 is welded to base metal SKD11 which metal is material of rotary cutter, the hardness of weld metal become lower than that of the base metal. In order to improve the hardness of weld metal which become lower than that of the base metal after laser welding, heat treatments using an electric furnace and pulsed YAG laser were carried out after laser welding, then examined the variation of hardness of weld metal after heat treatments. The obtained results are as follows. In the heat treatment using an electric furnace, the hardness of weld metal which become low against the base metal after laser welding become higher than that of the base metal under the conditions of heat treatment holding temperature at 180°C and holding time from 30 s to 120 s, then take out the weld metal from the electric furnace and cooling from furnace temperature to room temperature. In the heat treatment using pulsed YAG laser, after laser welding in succession laser beam radiate at the surface of weld metal, the hardness of weld metal which become low against the base metal after laser welding become the same hardness as the base metal under the conditions of holding temperature at 180°C and holding time from 30 s to 120 s, then cooling to room temperature.
In-flight temperature and velocity of thermal sprayed ceramic particle at various spraying distances were measured and the effect of the temperature and velocity on the flattening behavior of the particle was investigated. Several kinds of ceramic particles, such as an alumina, were sprayed at various spraying distances, and the particles were collected onto a mirror polished substrate. The velocity and temperature of the particles were measured by DPV-2000 system, which is an optical sensing device with on-line monitoring of the individual particle. Both the velocity and temperature of the particles monotonously decreased with increasing of the spraying distance. This tendency is quite similar to that of metallic particle. The transition temperature change of the ceramic particles with spraying distance, however, showed an opposite tendency to that of the metallic particle, namely, the transition temperature rose with an increase of the spraying distance. From the observation results of the collided particles on the flat substrate surface, it was found that the partially melted particles were often observed on the shorter spraying distance. This indicates that in the case of the large particle or the shorter spraying distance, the ceramic particles were melted only on the surface. Coexistence of the melting and non-melting part in a particle makes a viscosity of the particle high, and the higher viscosity restrains the occurrence of the splashing on the flattening. This maybe causes the lower transition temperature at the shorter spraying distance range. Consequently, it must take care that the temperature of thermal sprayed ceramic particle measured by the optical sensing device is only an information on the surface of the particle.
In the stirred zone of friction-stir welds of an aluminum alloy 6N01 plate (formed through extrusion), a zigzag bond line was observed after etching by an aqueous solution of 10% NaOH, when the revolution pitch (tool traveling speed/rotation rate) was greater than a critical value. SEM and TEM observations revealed that the bond line consisted mainly of particular inclusions 0.01-0.2 μm in size. The inclusion was identified as complex oxide involving Al, Mg, and Si based on EDX analyses. The inclusion can be considered to originate from the oxide film that was formed on the plate surface during the extrusion process, since its amount was reduced significantly by removing the as-extruded surface layer through mechanical cutting or grinding. The presence of the bond line had no significant influence on the tensile strength or fatigue strength of the joint, since it was fractured in the heat-affected zone in the tensile and fatigue tests.
In laser welding, monitoring is an important measure to assure the reliability of welding. Various monitoring methods have been proposed in CO2 laser welding. However, the research of monitoring methods in YAG laser welding is still few. We propose here a monitoring method in YAG laser welding. This monitoring method is based on an idea that Ar+ laser irradiates keyhole or molten pool and the intensity of the reflected Ar+ laser is measured by monochromator. The authors investigated the proper irradiation condition of Ar+ laser in this monitoring method. As a result, the signal of reflected Ar+ laser showed the size and oscillation of molten pool under the proper condition. Keyhole behavior was also observed by high speed camera at the same time. By analyzing the high speed video pictures, keyhole profile was detected under the various welding conditions. According to these results, this method is useful to monitor the YAG laser welding.
An electromagnetic acceleration plasma arcjet generator, which is called Magneto-Plasma-Dynamics (MPD) arcjet generator, has a coaxial electrode structure similar to those of conventional thermal arcjet generators. However, their acceleration mechanisms are different ; that is, in MPD arcjet generator, plasma is accelerated by the electromagnetic interaction between the discharge current and the magnetic field induced by it in MW-class input power operations during the discharge, although in thermal arcjet generators the working gas is accelerated aerodynamically through a straight or convergent-divergent nozzle. As a result, the MPD arcjet generator can produce higher-velocity, higher-temperature, higher-energy-density, and larger-area plasma than those of other conventional plasma torches can. These MPD plasma properties are effective for various material processes. This paper deals with a development of MPD arcjet generator for titanium nitride (TiN) reactive coatings. The MPD arcjet generator is equipped with a titanium cathode, and its working gas is nitrogen. The coatings were deposited onto steel substrate. The phase structure and the composition of the coatings were analyzed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), and their Vickers hardnesses were measured. In addition, titanium particles that were generated by MPD arcjet generator ablation process were observed by means of SEM and electron probe micro analysis (EPMA). These analyses showed that the MPD spray process could successfully form dense and uniform titanium nitride coatings. The properties of the titanium nitride coatings were highly sensitive to the titanium cathode diameter and discharge current.
In this work, a feasibility study has been conducted to produce NiTi intermetallic compound layer using mixed powders of Ni and Ti by plasma transferred arc welding. Wear characteristics of the coatings were evaluated by using a modified Okoshi-type wear tester. It was possible to produce intermetallic compound coatings from the simply mixed powders by plasma transferred arc welding. Hardness value reached up to 800 HV in deposited metal. The coating was identified as NiTi, TiFe and TiFe2 intermetallic compounds by an X-ray diffraction analysis. The wear resistance of the coatings was equivalent to that of Stellite No.6 coatings. The coatings show excellent wear characteristics under the condition of lower friction speed. The wear progress of the coatings reveal no sever wear regime and constant, low rate of wear.
Heat conduction and the thermal elastic-plastic analyses were carried out by FEM to clarify the effects of thermal and strain cycles on liquation cracking in HAZ of laser welded joint. The thermal analysis has indicated that the necked zone of weld bead is held at higher temperatures for longer than other regions in HAZ during the weld thermal cycle. Thus, liquation temperature range of grain boundary in the necked zone is considered larger than in other regions. According to the result of the strain analysis during the cooling stage, plastic strain of HAZ in the necked zone showed bigger value compared with that in other regions. In addition, it has also been shown that as the curvature of the necked zone decreases, the principal plastic strain increases. On the basis of these results, the reason why liquation cracking preferentially occurs in the necked zone of HAZ has been considered as follows. When laser welding is carried out, grain boundary near fusion boundary is liquefied due to the eutectic melting of laves cluster during the heating stage. Furthermore, the liquation temperature range of grain boundary in the necked zone was wider than in other regions, and the necked zone is subjected to greater tensile strain during the cooling stage. As a result, liquation cracks preferentially occur in the necked zone of laser weld during welding.
To prevent from the brittle fracture accident of steel structure, various fracture assessment methods were irproved. Most of brittle fractures occur from the cracks such as welding defects at the strain concentrated region. The CTOD design curve specified on WES 2805 : 1997 is suitable for the CTOD estimation of such cracks. However, according to the papers about the fracture analysis of damaged steel structures by large earthquapes, the brittle fracture in the connection zone of the steel structure occurred from small crack sometimes under 1 mm by more than 5% large strain and high strain rate in the order of 0.1/s. And, the strain gradient around the crack is so steep that it is difficult to difine the applied strain for design curve. In such casen, it is not clear the applicability of the CTOD design curve, and strain difinition is also. In this paper, the applicability of the CTOD design curve of WES 2805 : 1997 was dincussed for large strain, steep strain gradient and for high strain rate. 3-dimensional EF-models of beam to column welding connections were made and large strain dynamic analyses were conducted. Especially, under dynamic loading condition, as the temperature raise induced by plastic deformation can't be ignored at the strain concentrated zone, the thermal mechanically coupled analyses were conducted. Based on the rusults of these FE-analyses, new difinition of strain for evaluation was proposed and applicability of CTOD design curve for the crack at the steep strain gradient was confirmed. From these FE-analyses results, the temperature raise at the strain concentrated zone and the crack tip were also obtained. And it can be said that the crack tip temperature of small crack at the column to beam connection zone is almost same as the temperature of the zone difined by the proposed strain for CTOD evaluation in seismic loading condition.
The authors have studied the tensile strength, the fatigue strength and the impact characteristics of the joints of the ferritic spheroidal graphite cast iron produced by the solid state bonding method and found that the mechanical characteristics of the joints were comparable to those of the base metal. With respect to practical uses, it is considered that the brazing method with a brazing sheet is easier than the solid state bonding for bonding of the cast iron. In this study, the ferritic spheroidal graphite cast irons were bonded with Ni-base brazing sheets using the high frequency induction heating apparatus. Those sheets were MBF 15 (Ni-Cr-Si-B system), MBF 30 (Ni-Si-B system) and MBF 60 (Ni-P system) in the thickness of 40μm. Furthermore, effects of bonding time, surface roughness of the surfaces to be bonded, and bonding atmosphere on the tensile strength of the joints were investigated. Main results obtained are as follows: (1) When the cast irons were bonded for 2 and 10 minutes with MBF 15 and MBF 60, the joints were fractured at the bonded zone on the tensile test. When the cast irons were bonded for 30 minutes, however, the tensile strength of the joints was nearly equal to that of the base metal. On the other hand, when the cast irons were bonded for 2, 10 and 30 minutes with MBF 30, all of the joints were fractured in the base metal on the tensile test. (2) When the surface roughness was 2.3μm, the tensile strength of the joints bonded with MBF 15 was nearly equal to that of the base metal. When the surface roughness was 13.9μm, however, all of the joints were tensile-fractured at the bonded zone because small shrinkages were formed at the center of the bonded zone. (3) The tensile strength of the joints bonded in argon was a little lower than that of the joints bonded in vacuum with MBF 15 and MBF 60. However, the strength of the joints bonded in air was decreased considerably because the oxides of Cr were produced in the center of the melting zone. On the other hand, when the joints were bonded with MBF 30 in air, in argon, and in vacuum, all of the joints were tensile-fractured in the base metal. (4) In the case of MBF 15 containing Cr, the high density zone of Cr was formed at the center of the melted brazing metal and Cr2O3 particles were produced by oxygen in air. By the final stage of the bonding, Si was accumulated near the Cr2O3 particles, and they were reduced by the Si, and the SiO2 particles were formed instead of the Cr2O3 particles. (5) When the brazing sheets having 5 levels of the Cr content were used for the bonding, the tensile strength of the joints was decreased with an increase of the Cr content.
In recent years YAG laser is to be applied to welding of car body as light weight construction used thin plates. Lap joint is used generally on welding two thin plates as compared with butt joint. Because groove precision is necessary for butt joint. So, fillet weld has been adopted to lap joint by MAG welding that is conventional welding way. Narrow bead width and deep penetration shape are characteristics of laser welding. But, these characteristics are weak points in case of fillet weld by laser welding, because minute positioning becomes necessary. When it applies laser welding to lap joint, deep penetration shape that is a characteristic of laser welding is able to weld lapped two plates and it does not need positioning precision. In this studies, it picks up fatigue strength of lap joint of thin plates and it examined a rational joining measurement of laser welded joint by an experiment and numerical analysis. As a result, it got some knowledge about joint width of lap joint and it shows a major conclusion to the following. The radius of curvature on weld toe of YAG laser fillet welded joint is larger than that of MAG fillet welded joint and stress concentration on weld toe of YAG laser fillet welded joint is relaxed more than MAG fillet welded joint. So, fatigue strength of lap joint by YAG laser fillet weld is larger than that by MAG fillet weld in case of almost same leg length. Using a mode I stress intensity factor of nonpropergating crack, it was able to predict fatigue strength of lap joint, fillet welded joint by YAG laser and fillet welded joint by MAG in an error of approximately 23%. If it makes joint width of lap joint larger than plate thickness, a stress intensity factor becomes almost constant value. Accordingly, it needs to make joint width larger than plate thickness. Slit length of lap joint hardly cause influence to fatigue strength.
Stress-strain characteristics of S15C carbon steel friction welded butt joints under impact tensile loading are evaluated using the split Hopkinson bar. Round tensile specimens machined from as-welded butt joints of 13 mm diameter are used in both static and impact tension tests. Friction welding is carried out using a brake type friction welding machine under fixed welding conditions. The effect of strain rate up to ε≅700/s on the tensile stress-strain characteristics of both friction welded butt joints and a base material (S15C carbon steel) is studied. It is shown that the flow stress for friction welded butt joints is always higher than that for the base material at low and high rates of strain, but the former are less sensitive to strain rate than the latter. Microhardness measurements are conducted to examine the extent of the heat-affected zone (HAZ) across the weld interface. Metallographic examination reveals that there are zones of different grain structure through the weld region. The increase in the flow stress for friction welded butt joints is attributed to the presence of a locally-hardened region within the HAZ.
Thermal cycling of an electronic assembly produces repeated inelastic strains in solder joints, which may cause thermal fatigue cracks to initiate and grow in a solder. The stress-strain relationship, creep deformation and stress relaxation in 37Pb63Sn, 50Pb50Sn and 90Pb10Sn were measured at various temperatures. Micro-crack initiation and growth behavior in plain specimens of the solders were observed under low-cycle fatigue and creep-fatigue at elevated temperatures. Even at room temperature, strain rate dependency of stress, creep deformation and stress relaxation in the solders were noticeable. Therefore, elements joined by solder are considered expanding freely at elevated temperatures, which results in a large shear strain in solders. Micro-cracks initiated at an early stage and their growth rates dominated the fatigue life of the specimen. The fatigue crack growth rate in 50Pb5OSn was much lower than in 90Pb10Sn and 37Pb63Sn. A slow tensile and fast compressive strain-wave caused a higher crack growth rate than a slow tensile and slow compressive one at elevated temperatures. Cracks 0.1 mm long existed at from 1/20 to 1/10 of the failure life of plain specimens. Above this size the fatigue crack growth rate had a good relationship with the strain intensity factor in different strain ranges. The strain intensity factors of cracks in die bondings were calculated using a low elastic modulus value for the solder. Also, the fatigue crack growth behaviors were calculated using the strain intensity factor values and fatigue crack growth rate of the solders at the maximum temperature of thermal cycling. This simulation gave a good approximation to the measured crack behavior in the thermal fatigue of the die bondings.
It is important to understand the characteristics of material strength and fracture under the dynamic loading like as earth-quakes to assure the integrity of welded structures. The characteristics of dynamic strength and fracture in structural steels and their welded joints should be evaluated based on the effects of the strain rate and the service temperature. On the other hand, the temperature rises so rapidly in structures due to the plastic work done under the high strain rate, that the effect of the temperature cannot be negligible for the dynamic fracture. It is difficult to predict or measure temperature rise history with the corresponding stress-strain behavior. In particular, material behaviors beyond the uniform elongation can't be precisely evaluated, though the behavior at large strain region after the maximum loading point is much important for the evaluation of fracture. In this paper, the coupling phenomena of temperature and stress-strain fields under the dynamic loading was simulated by using the finite element method. The modified rate-temperature parameter was defined by accounting for the effect of temperature rise under the dynamic deformation, and it was applied to the fully-coupled analysis between heat conduction and thermal elastic-plastic behavior. Temperature rise and stress-strain behavior including complicated phenomena were studied beyond the maximum loading point in structural steels and their undermatched joints and compared with the measured values.
Welded joint generally has heterogeneity of strength, material, and fracture toughness. It is important to understand the characteristics of material strength and fracture of welded joint considering heterogeneous effect. Furthermore, material behavior becomes more complicated when the welded joint is subjected to dynamic loading like as earthquake. In this paper, characteristics of strength and fracture of an undermatched joint under dynamic loading is studied by round-bar tension tests and thermal elastic-plastic analyses. The results show that the strength and fracture of the undermatched joints should be evaluated based on the effects of the strain rate and the temperature including temperature rise during the dynamic loading. Tensile strength and yield stress of the undermatched joints increase with strain rate and with decreasing temperature. The strength of the soft layer approaches to that of the base metal when the thickness of the soft layer becomes smaller, and does not depend on strain rate. Finally, it is found that the stress-strain distribution affects the fracture characteristics like such as ductile-to-brittle transition behavior. The fracture characteristics are well precisely explained from the results obtained by numerical analysis.
In this paper, two-dimensional thermal and structural analyses at heat-affected zone (HAZ) of the laser weldment of Inconel 718 alloy were carried out by the finite element code, ANSYS to make clear the mechanism of liquation crack from a mechanical standpoint. According to the heat transfer analysis, the width of liquefied temperature range at the neck region of nail head type penetration of weld bead was larger than that at other regions. This is the cause that liquation crack easily occurred at the neck region of weld bead. Moreover, as a result of structural analysis, both the equivalent plastic strain and the first principal plastic strain at the elevated temperature concentrated at the neck region of weld bead. These strains increased with an increase in welding speed and with a decrease of the radius of curvature at the neck region of weld bead. On the other hand, the occurrence of liquation crack also depended on welding speed and the radius of curvature at the neck region of weld bead, and thus depended on the elevated temperature strain at HAZ. It was followed by structural analysis that the elevated temperature strain should be utilized as a parameter for estimation of occurrence of liquation crack.
This paper deals with a fatigue strength of corner joint without backing plate using experimental data and numerical method. A lack of penetration is usually found at the root of the corner joint. The fatigue strength generally depends on the lack of penetration. From comparing between experimental data of fatigue strength and numerical results, it is shown that a stress intensity factor is useful for predicting the fatigue strength of the corner joint. The stress intensity factor is calculated under condition with changing main factors of corner joint. It is suggested that the plate thickness and the length of lack of penetration are main factors of the fatigue strength of the corner joint.
In the present research, NOx elimination coating photocatalyst was tried to be made by a few kinds of thermal spray processes, and the possibility for the fabrication of coating photocatalyst by thermal spray process was fundamentally investigated. NOx elimination characteristic of the fabricated coatings was evaluated in the simulative polluted environment, and the relationship between the fraction of anatase phase in the coating, the coating thickness and the NOx elimination characteristic of the coating was evaluated. It was possible to leave the anatase phase in the coatings by both gas flame spraying and atomospheric plasma spraying. The fraction of anatase phase, however, was quite limited by these methods. Fairly high fraction of the anatase phase could be left in the coating by using HVOF spraying method. At this moment, HVOF spraying seems to be the best process. The photocatalitic NOx elimination property was recognized in the coatings fabricated by the thermal spraying. The NOx elimination ratio of the sprayed coating reached to the maximum just after the irradiation of the ultraviolet rays and subsequently decreased, just similar to the tendency of the original powder. An effective reaction layer of the coating photocatalyst was observed near the coating surface region, and the thickness of the layer was approximately 25μm in the coating obtained by HVOF spraying.