In this investigation, a new approach based on the energy minimization method is proposed to analyze the mechanism of the metal transfer (globular and spray modes) in the GMAW process. Energies due to the surface tension, gravity, electromagnetic and drag forces exerted on the pendant drop are minimized and the shape of molten drop in the equilibrium state is calculated without presumed geometry. The energy due to the electromagnetic force is derived using the Maxwell stress. Effects of welding conditions on the shape of the pendant molten drop are analyzed. Results show that the proposed approach can be applied to both globular and spray modes. When compared with other approaches such as force balance and pinch instability models, the results of proposed approach show similar trends and predict the available experimental results more accurately.
Ductile cast iron (FCD450) and stainless steels (SUS304, SUS430) were friction-welded. Heat content of each joint was measured by calorimetry and tensile test was made. No defect was observed in joints of cast irons. In the present study, a simplified model has been proposed for calculating the heat input rate in dissimilar joints of friction welding using one dimensional thermal conductivity equation. If the burn-off rate is taken into consideration, a correlation between this heat input rate and tensile property of joints examined appeared to exist. In addition, the values obtained by modeling and measured ones were in good agreement.
Electrode life in spot welding is governed by the nugget formability of welded material and the expandability of electrode face diameter. The authors studied the effects of coating layer and electrode face diameter on nugget formation in two-stack welding using zinc and iron-zinc coated sheets. Here minimum welding current is difined as the current where nugget diameter is 4√t of sheet thickness (t). In both coating types, minimum welding current increases as coating-layer zinc contentt increases. Therefore, the greater the coating layer, the higher the minimum welding current. This effect is apparent in 0-1.4 μm coating but rapidly diminishes beyond. When coating thickness is constant, minimum welding current of zinc coated sheet is greater than that of iron-zinc coated sheet. The minimum welding current of iron-zinc coated sheet increases with zinc content in coating layer. This effect is weak up to 60% Zn but pronounced above 60%. Effects of coating thickness and type on minimum welding current increase with increase of electrode face diameter. The effect of coating type correlates with coating layer melting point. Analysis of faying surface conditions shows that the lower the melting point, the faster the softening and melting of coating layer, and that the true contact area rapidly increases from the initial power application to the start of nugget formation. In single-layer coating, coating layer melting point is considered to govern the increase in true contact area and affect minimum welding current, while increase in coating thickness and electrode face diameter enhances these effects. Similar assumption applies to multi-layer coating, though qualitatively, by combining the effects of melting point and thickness of each coating layer, without considering structural differences in the coating layer between single-and multi-layer coating.
Using Al-Zn alloys and pure Zn as a filler metal, we attempted to solder high strength aluminum alloys containing magnasium of more than 2.5% with the aid of ultrasonic vibration. The filler metal potency to solder the aluminum alloys was evaluated and a new soldering process was developed to increase the tensile strength of the joint. The following results were obtained. Although the aluminum alloys can be soldered with the Al-Zn alloy filler metals, the soldered joints does not have high tensile strength because of the grain boundary segregation of silicon in the filler metal and formation of voids in solederd layer. When pure zinc is used as a filler metal, soldering during isothermally holding causes the grain boundary segregation of magnesium which results from the base metal dissolution into the filler metal, and leads to decreasing the tensile strength of the joints. However, applying ultrasonic vibration during cooling stage reduces the dissolution of the base metal and suppresses the grain growth of α and β phases, followed by increasing the tensile strength of the joints. It was revealed that ultrasonic vibration apparently promotes the eutectic reaction between a base metal and a filler metal.
This paper deals with the resistance spot weldability of steel to aluminum alloy sheets using an intermediate layer of aluminum clad steel. Five types of the clad sheets with various steel/aluminum thickness ratios were produced by hot rolling. The mechanical properties of the clad sheets changed with the thickness ratio and ranged between those of the steel and the aluminum sheets. The peel strength of the steel/aluminum interfaces was as high as more than 25 N/mm. Materials used in spot welding were 0.8 mm thick EDDQ steel sheets, three types of 1.0 mm thick aluminum alloy sheets and the clad sheets mentioned above. Spot-weldability, including suitable welding current, nugget diameters, tensileshear strength and the thicknesses of the intermetallic compound layers formed at the interfaces of the clad sheets, changed with the thickness ratio of the clad sheets. From these results, it was concluded that spot weldability was affected by the thickness ratio of the clad sheets. Spot weldability was also affected by the alloying elements of aluminum alloy sheets. The tensile shear strength and the nugget diameters varied by using various types of aluminum alloy sheets.
This study deals with shielded metal arc welding and CO2 gas shielded arc welding, measuring the force to remove the adhering spatter on the surface of base metal by using different filler metals and by changing the conditions of the surface of base metal. Rolled steel for general structure (SS400) was used as base metal. A high titanium oxide type electrode and a low hydrogen type electrode were used for shielded metal arc welding, a solid wire and a flux-cored wire were used as filler metal for CO2 gas shielded arc welding respectively. In order to examine the relationship between the condition of the surface of base metal and the force to remove spatters, the base metal that the surface was ground by the electric grinder was used for experiments, the one was not ground by an electric grinder, and the another was coated with an anti-spatter compound. Whichever filler metal was used, the spatter adhered to the surface of base metal was located within 100 millimeters from the weld line. In those cases, the force to remove the adhering spatter can be measured mostly below 98 N. The spatter needed more than 98 N to remove was located especially within 20 millimeters from the weld line. No matter when the scale was on base metal or not, the difference of the force to remove the spatter was small. When we used the base metal coated with anti-spatter compound, in some cases we found the spatter on the base metal, and in other cases we didn't. When we found the spatter it was located within 40 millimeters from the weld line and the force to be removed can be measured below 20 N.
Resin composite type vibration damping steel sheet (VDSS) is composed of two skin steel sheets and the core damping resin. To spot-weld VDSS, an electrical contact must be needed between the two skin sheets by using a shunting circuit because the core resin is an electrically insulator. In this mothod of spot welding, it is considered that time to exclusion of resin between upper and lower skin sheets affects both weld quality and working efficiency. In this study, the relationship between time to exclusion of resin and material-, welding-conditions were clarified by investigating the mechanism of resin exclusion in shunt welding. The proposed mechanism was considered as follows : In the initial stage of welding, the temperature elevated first around the electrode which has curved tip face and the core resin started to melt. As electrode-skin sheet contact area is gradually enlarged by heat generation, finally this brings about the contact between upper and lower skin sheets. In this model, time to exclusion is proportional to melting point of resin and inversely proportional to the square of shunt current. The core resin thickness and the radius of electrode face curvature affect exclusion time as a parameter of electrode-skin contact diameter. This model was well corresponded with the experimental results. From these results, it was revealed that the thin resin thickness as material conditions and the short distance between welding and shunting points as welding conditions were important to obtain good weldability.
In order to spot-weld VDSS without providing a shunting curcuit, electro-conductive property is needed to the core damping material. The method to give electro-conductivity is addition of metal particles into the resin which has diameter nearly equal to the resin thickness to ensure direct contacts between skin sheets. As particles were increased or resin thickness became thinner under the same particle content, the resistance between skin sheets was reduced. Nickel particle exhibited the best conductivity of all which were tested. In the case of the small amount of Nickel particle, pin-hole defects were observed and these defects were peculiar to the welding of VDSS because welding current was concentrated to an specific Nickel particle with relatively low electrical resistance. The defect was caused by growing the melting area by joule heating to the outer surface of skin sheet. Decreasing the voltage between skin sheets is effective to prevent pin-hole defect formation. From the practical point of view, it is needed that decreasing the resistance between skin sheets or controlling the welding voltage. As for the resistance, it was showed that initial resistance between skin sheets was useful for the virtual index in material production. Using a sufficient conductive VDSS, time to exclusion of resin was quite short, and weld defect was not observed.
The measured resistance between skin sheets was compared with the calculational results of steel substrate resistance, particle resistance and contact resistance. It was found that the resistance between skin sheets was mainly composed of contact resistance because contact area between particles and skin sheet was very small. From the calculational results of the distribution of welding current and voltage in the skin sheet, it was considered that their distribution spread widely outside of the contact area between electrode and skin sheet comparing to ordinary steel sheet. Maximum value of voltage and current was observed at the edge of contact area. It seems reasonable to suppose that welding current and voltage are directly related to the time to exclusion of resin and the occurrence of pin-hole defects respectively. Therefore the effective factors to both current and voltage were investigated in detail. In the case of high resistance between skin sheets, the distribution of welding voltage spread widely within the skin sheet and welding current becomes lower. This fact was confirmed to cause the harmful effect to the weldability. Also the size of work piece affected weld quality. In paticular, the smaller the size of work piece became, the higher welding voltage loaded and the pin-hole defects occurred. Futhermore, since 90% of current flows outside of contact area, it was considered that these materials were seemed to have many shunting points.
Gas pressure welding has been widely used as a field welding process of rail and steel bar for concrete reinforcement. However, in the first place, a reasonable inspection method for gas pressure welds is seldom practiced. So we have developed a method of "bulge removal by hot shearing" which is effective for quality inspection of welded joints. The present report explains the results of examining this method for quality estimation of welds. The results may be summarized as follows : (1) The quality of gas pressure welds depends on local area expansion factor (α) and weld temperature (T). As these values increase, the quality of weld gets better. (2) The fracture observation in fracture test helps to evaluate the quality of weld. (3) Disappearance of flat fracture is owing to the carbon content of the base metal. Carbon is cansidered to reduce the oxides in interface.
The dissimilar spot welding of aluminum and steel sheet with insert of aluminum clad steel sheet was investigated. The study was conducted considering the application to the fabrication of automobile body, which effective to decrease its weight. To clarify the properties of intermetallic compounds to be formed at the steel and aluminum bonding interface, the mechanical properties were measured using the balks of four kinds of intermetallic compound in Fe-Al binary system. The mechanical properties of spot welded joint of aluminum and steel sheet with or without insert of aluminum clad steel sheet were measured using cross tension test and the microstructure of welded joint by this process were observed. The intermetallic compounds of FeAl and Fe3Al had comparatively higher ductility and the Fe2Al5 and FeAl3 had high hardness and low ductility, so that the Fe2Al5 intermetallic compound were formed at spot welded zone of the aluminum and steel resulting in the deterioration of strength. The dissimilar metal spot welded joint by using insert of aluminum clad steel sheet had excellent tensile strength in cross tension test and fractured in the base metal of aluminum alloy. Two nuggets of aluminum to aluminum and steel to steel were obtained independently by this process. These two nuggets and clad interface by hot rolling contributed to keeping the joint strength. It was confirmed the dissimilar welded joint of aluminum and steel having the strength equal to that of similar welded of aluminum alloy were obtained using this process.
An intelligent robot system in which the working procedure and the moving trajectory of a robot can be generated based on the design data of the assembled objects and the working specification is needed for the flexible manufacturing. It is easy to calculate the position of the endeffector and the robot posture based on each joint angle of the robot. But, it is difficult to calculate each joint angle of the robot based on the position of the endeffector and the robot posture. Moving trajectory of the robot is determined in the area where the robot can move and the posture can be settled in each position of the endeffector. In this report, generation system of moving trajectory based on the movable posture map is proposed. Movable posture map is generated easily based on the posture calculated by each joint angle. Movable posture map is expressed by maximum posture angle and minimum posture angle for the distance from a basis joint of a robot to a endeffector. By using this movable posture map, moving trajectory and posture on its trajectory can be selected easily in the movable posture range of a robot.
Si3N4 was brazed to Si3N4 with Cu-Ti alloys containing Ti content up to 50 at% at 1373 K for 1.8 ks in vacuum, where pressureless sintered Si3N4 were used. The strength of Si3N4/Si3N4 joints were measured by 4-point bending test at room temperature and elevated temperature. The microstructures and elemental distribution and reaction phases in the joining layers were investigated by means of electron probe micro-analyser and X-ray diffract-meter. The main results obtained are summarized as follows. (1) The addition of Ti content from 5 to 20 at% to the alloys effectively increases the strength of around 200 MPa for Si3N4/ Si3N4 joint. The strength of the joint extremely rises with further addition of Ti, and reaches the maximum of 500 MPa or more. (2) Ti in the alloys reacts with Si3N4 and forms TiN and Ti5Si3 for Ti content from 5 to 50 at%. The formation of the compounds at the interface between Si3N4 and the alloy imparts the superior strength to the joint, and the distribution of Ti, Si3 in the alloys gives rise to the improvement of the mechanical properties of the Si3N4/Si3N4 joints. (3) The Si3N4/Si3N4 joint brazed with Cu-34 at% Ti alloy represents the high strength of 500 MPa up to testing temperature of 773 K, and the degration of strength with further increasing the temperature.
SiC was brazed to SiC with Cu-Ti alloys containing Ti content up to 50 at% at 1373 K for 1.8 ks in vacuum, where pressureless sintered SiC were used. The strength of SiC/SiC joints were measured by 4-point bending test at room temperature and elevated temperature. The microstructures and elemental distribution and reaction phases in the joining layers were investigated by means of electron probe micro-analyser and X-ray diffract-meter. The main results obtained are summarized as follows. (1) The strength of SiC/SiC joint increases with increasing Ti content in Cu-Ti alloys for Ti content up to 34 at%, and shows the maximum of 208 MPa at Ti content of 34 at%. (2) Ti in the alloys with Ti content up to 15 at% reacts with SiC and forms TiC, Ti5Si3 and TiSi. Ti in the alloys with Ti content from 20 at% to 25 at% reacts with SiC and forms TiC, Ti3SiC2, Ti5Si3 and TiSi. Ti in the alloys with Ti content of 34 at% or more reacts with SiC and forms TiC, Ti3SiC2, TiSi and TiSi2. (3) The formation of TiC and Ti5Si3 at the interface between SiC and Cu-Ti alloy prevents the direct reaction of copper with SiC, and reduces the thickness of the brittle copper penetrating layer. The formation of thick carbide layer of Ti3SiC2 is attributable to the maximum strength of SiC/SiC joint. (4) The SiC/SiC joint brazed with Cu-34 at% Ti alloy represents the strength of 208 MPa up to testing temperature of 723 K, and the degration of strength with further increasing the temperature.
The solid state bonding of pressureless-sintered SiC to SiC using 20 μm Ti foil was conducted at bonding temperatures ranging form 1373 to 1673 K for 0.3 to 72 ks in vacuum. For a constant bonding time of 3.6 ks, the granular TiC next to Ti and a mixture of Ti5Si3Cx+TiC phases next to SiC were formed. Increasing the bonding temperature in 100 K intervals from 1473 K to 1773 K, the Ti5Si3Cx single phase, Ti3SiC2 phase and TiSi2 phase sequentially appeared. Fracture shear testing was used to measure bonding strength. The bonding strength was found to increase up to a maximum of 153 MPa at 1473 K. At higher temperatures it decreases to 54 MPa at 1573 K. At still higher temperatures bonding strengh again increases. The highest strengh of 250 MPa was measured at 1773 K. This variation of bonding strength with bonding temperature will be correlated with the microstructure observed at the interfase of the joints. The SiC/Ti joint with a duplex phase of Ti3SiC2+TiSiz shows the stable strength up to testing temperature 973 K.
The temper embrittlement occurring in HAZ of 2 1/4Cr-1Mo steel was studied in the tempering temperature-time range of 774 to 900 K and 0.2 to 10000 hours. The transition temperature, TrE was measured on each specimen, and the temper embrittlement was detected as the rise of TrE. The temper embrittlement were classified into four types. The first and second types of embrittlement appear in the earlier stage of tempering, such as 775 K for 1 hour, and 100 hours, respectively. They differ from "the low-temperature temper embrittlement" which has ever been observed. The secondary hardening occurring during tempering seems to play an important role to induce those types of embrittlement. The third type appears in the tempering in a log time (825 K for 100 to 1000 hours). This type is identical with "the high-temperature temper embrittlement (reversible temper embrittlement) ". The fourth type arises in the final stage of tempering (825 K for 1000 to 10000 hours). Little work has been made to distinguish this type from the third type, and hence little is known about its characteristics. The temperature-time ranges in which those four types of embrittlement arise were shown in a diagram. The range where no embrittlement arises was also pointed out. Impact test carried out below the temperature of TrE produced an intergranular fracture in the specimen causing the third type of embrittlement, and a transgranular one in those for the first, second and fourth types of embrittlement.
Low temperature sensitization (LTS) behavior in the weld metal of Type308 stainless steel was investigated in this study. Three kinds of Type308 stainless steels, of which carbon contents were 0.04%, 0.06% and 0.08%, were used for this study. TIG welding method was adopted to make the weld metals. Weld metals were subjected to the sensitizing heat treatment in the temperature range between 773 K and 1073 K. The degree of sensitization were examined by the EPR method and the Strauss test. Chromium carbide was absorbed to precipitate at δ/γ grain boundaries in the as-welded weld metals Corrosion test results have shown that the higher carbon content in the weld metal is, the earlier sensitization yields in it. Sensitization in weld metals is found to occur faster than in those solution heat-treated at 1273 K prior to sensitizing heat-treatment. This fact suggests that preexisted chromium carbides have an effect to accelerate sensitization. That is, it is apparent that LTS phenomenon occur even in the weld metal. Moreover, sensitization in the weld metal has occurred in much shorter time than in HAZ, which is attributed to the preferential precipitation of chromium carbide at δ/γ grain boundaries in the weld metals.
Diffusion bonding between a commercial pure tantalum and an austenitic stainless steel SUS304 was performed in a vacuum of 8 mPa at temperatures ranged from 600 to 1200°C. The direct bonding of the tantalum to the stainless steel, resulted in extremely low joint strength and formed intermetallic compounds such as FeTa, Fe7Ta3, NiTa2, NiTa, Ni2Ta, Ni3Ta, Ni8Ta and Cr2Ta in the bond zone of the joint. To examine the influence of Cr, C, Fe and Ni of alloying elements of the stainless steel to the joint strength, the diffusion bondings were carried out between the tantalum and a pure chromium, the tantalum and a carbon steel (C: 0.42 mass%), the tantalum and a pure iron and the tantalum and a pure nickel. The effect of the insert metals of Fe, Ni and Cu foil on the joint strength between the tantalum and the stainless steel has been also studied. Main results obtained are as follows : (1) The joint of the tantalum to the chromium was not enough to machine to tensile test pieces due to the forming of the intermetallic compound of Cr2Ta in the bond zone. (2) The joint of the tantalum to the carbon steel and the joint of the tantalum to the iron formed intermetallic compounds such as FeTa, Fe7Ta3, TaC and Ta2C, and FeTa and Fe7Ta3 in the bond zone, respectively. These joint strength, however, increased with the bonding temperature and attained to about 150 MPa at maximum the bonding temperature of 900°C. The joint strength was found to decreased at the higher bonding temperature than 1000°C. The joints could not be machined to tensile test pieces at the bonding temperature than 1100°C. (3) The intermetallic compounds such as NiTa2, NiTa, Ni2Ta, Ni3Ta and Ni8Ta in the bond zone were formed in the bond zone of the joints of the tantalum to the nickel. The maximum joint strength of 225 MPa was obtained at the bonding temperature of 900°C. (4) The joint strength of the tantalum to the stainless steel with Cu foil as an insert metal increased with the bonding temperature. The maximum value of 265 MPa was obtained at the bonding temperature of 1050°C, which was almost the same as that of the tantalum base metal.
The effect of tungsten in duplex stainless steels on the precipitation behavior of inter-metallic compound and its mechanism were investigated to clarify the advantage of tungsten alloying for the weldment properties. The fine precipitates of sigma phase were observed in the electron micrograph of simulated heat affected zone (HAZ) of the 25%Cr-7%Ni-0.3%N steel containing 4%Mo, while the fine precipitates of chi phase were observed in that containing 3%Mo and 2%W. The tungsten addition delayed the sigma phase precipitation in aged steel at 850°C to 950°C for 600 s. In the steel containing below 8% tungsten aged at 800°C to 900°C sigma phase and austenite phase were identified, while in the aged steel containing over 12% tungsten sigma, chi and austenite phase were identified. Using these results and chemistry data of each phase obtained with electron probe microanalyzer (EPMA), the phase diagram at 900°C was derived experimentally. Chi phase precipitated with the lower contact angle to the matrix than sigma phase, therefore chi phase was considered to nucleate preferentially. However, the growth rate of chi phase can be low due to lower diffusion rate of tungsten. It was considered that tungsten can delay the sigma phase precipitation by the mechanism that tungsten should promote the nucleation of other phase than sigma, which has lower growth rate comparing sigma phase.
We investigated the effect of internal restraint on the grain growth behavior in the heat-affected zone of austenitic stainless steel and referred to the plastic-strain increasing process. The following results were obtained in this study. The amount of the plastic strain which generated at the heat-affected zone during heating stage in the welding thermal cycle is increased with the increase of the specimen width. However, the amount of the plastic strain is saturated with the specimen width increasing. The dislocation density in the heat-affected zone heated to 1200°C in the synthetic welding cycle is almost similar to that in the unheated specimen. It seems that the effect of the plastic strain added below 1200°C in the heat-affected zone on the grain growth is equivalent to that of the plastic strain added at room temperature on the grain growth being heated by synthetic welding cycle.
The behavior of the superficial oxide film at diffusion-bonded interfaces of Al alloys including Mg, Si, Cu, and Zn has been investigated by TEM observations to explain the effect of the alloying elements on the bondability of the Al alloy. Oxides remaining at the bond interface at bonding temperatures around 873 K could be classified by Mg content CMg : amorphous oxide films at CMg≤0.01%, crystalline particles of Al2MgO4 at 0.3%≤CMg<1%, and crystalline particles of MgO at 1%<CMg. The particles of Al2MgO4 and MgO were very fine about 10-100 nm in size. Alloying elements Si, Cu and Zn have only slight effect on the formation of crystalline oxide particles, but constituted precipitates which formed preferentially at the bond interface compared with the matrix and grain boundary. Precipitates observed were Mg2Si, CuAl2 and MgZn2, the size of which were very large compared with the crystalline oxide particles. The joint of the alloys with CMg≥0.3% had tensile strength significantly higher than those with CMg≤0.01%, suggesting that the morphology of the oxide is a controlling factor in the bond formation in the diffusion bonding of the Al alloy; i.e., the continuous oxide film is much more detrimental to the bond formation at the interface than the dispersed oxide particle.
The effects of the bonding temperature on the interfacial phase and bond strength of diffusion-bonded joints have been investigated mainly by TEM observations for commercial aluminum alloys containing Mg: Al-Mg-Si alloy 6063 (0.5%Mg), Al-Mg alloy 5005 (0.8%Mg), and Al-Zn-Mg alloy 7N01 (1.15%Mg). At the bond interfaces of these alloys, amorphous oxide films were observed, which altered gradually to crystalline oxide particles, as the bonding temperature was increased. The bonding temperature at which the amorphous oxide film disappeared lowered with increasing the Mg content. As the area occupied by the amorphous oxide film on the bond interface was decreased, the tensile strength of the joints of all the alloys increased significantly, suggesting that the amorphous oxide film was a major factor interfering with the increase in the bond strength. The crystalline oxides at the bond interfaces of 6063 and 7N01 alloys were identified as Al2MgO4 and MgO at all bonding temperatures employed, respectively. In contrast, at the bond interrf ace of 5005 alloy, both the crystalline oxides were observed depending on bonding temperature Tw ; i.e., Al2MgO4 and MgO at Tw <853 K, and MgO at Tw≥853 K. In addition, particulate intermetallic compounds much larger than the oxide particles were precipitated preferentially at the bond interface (Mg2Si for 6063 alloy joints and MgZn2 for 7N01 alloy joints). Although these intermetallic compounds precipitated at the bond interface seem to act an initiation site of dimples on the fractured surface, they can be considered to have much less influence on the bond strength than the amorphous oxide film, since solution treatments for dissolving these precititates resulted in only slight improvement in the tensile strength of joints.
A new stress improvement method which reduces tensile stress on inner and outer surface near butt weld joint of piping is described. The procedure of this method is as follows. Circular region on both side of the weld line is heated from outside at the same time to make temperature difference between weld line and heating region. The heating region and weld joint do not cool actively after completion of heating. A tensile strain is produced on the inner surface near weld joint to produce plastic deformation by thermal stress when the circular region is heated. The tensile stress after cooling is decreased by this plastic deformation. The authors show the mechanism and effect of this method by FEM analysis. The influence of parameters of this method, such as heating width, heating location and heating conditions (intensity and time), on the residual stress is studied by FEM analysis. This method is actually applied to the weld joints of 4BSch40 (O.D.=114.3 mm, t=6.0 mm), 4BSch160 (O.D.=114.3 mm, t=13.5 mm) and 20B (O.D.=508.0 mm, t=8.0 mm) stainless steel pipe and the change of residual stress is measured. The measured welding residual stress on inner surface near GTA weld joint is more than +200 MPa. The residual stress of inside surface near weld joint decreases and changes to compression after treated by this method in all joints.
Stable crack resistance (R) curves were frequently used for assessing the integrity of structures containing defects in loading regimes. In that case, R curves are treated as characteristics of materials, but it is known that R curves are actually affected by various factors. The effect of strength mis-match on stable crack growth resistance is discussed in this paper. In order to compare the R curve of bimaterial materials with that of homogeneous ones, compact type tests are carried out. 3-dimensional FEM analyses taking into account the stable crack growth are also executed using the result of performed experiments. In the case of over-matched specimens (high strength region including a notch are surrounded with low strength region), dCTOD/da apparently increases comparing with homogeneity. In the case of under-matched specimens (opposite of the previous case), dCTOD/da apparently decreases comparing with homogeneity. Strength mis-match affects on CTOD(J)-R curves. On the other hand, the equivalent plastic strain at stable crack tip obtained from 3D analyses may be generally independent of the strength mis-match during ductile fracture process.
It was clarified that strength mis-match induces a significant effect on stable crack growth resistance in the previous paper. Extensive experimental and numerical works should be done for establishing a method for assessing a defect in heterogeneous material such as weld joints. This paper describes the effects of strength mis-match and plastic restraint on equivalent plastic strain at the stable crack tip. The CTOD-R curves are experimentally obtained using homogeneous and bimaterial compact specimens produced by diffusion bonding. In the under-matched bimaterial specimens, the ratios of the width of low strength region including a notch to the thickness (H/t) are 1/4 and 1/2. The shallow (a/W=0.15) and deep (a/W=0.5) notched specimens are tested to study the effect of plastic restraint on ductile crack behavior. A 3-dimensional analysis is carried out to get the strain near the crack tip using the results of performed experiments. The equivalent platic strain is expected to be a characteristic value independent of the existence of strength mis-match in the ductile fracture process. In the range of stress triaxiality obtained in the present works, the equivalent plastic strain does not change during the stable crack growth irrespectively of the difference in stress triaxiality. The load-load displacement curve obtained from the stable crack growth simulation using the equivalent plastic strain agrees well with the data obtained from the compact test.
The previous report has clarified that the quality of gas pressure welds is influenced by local area expansion factor (α) and weld temperature (T) and that, moreover, the fracture observation helps to evaluate the quality of welds. In this paper we report on a study about the quality estimation mechanism and the reliability of this inspection method. The conclusions are as follows; (1) Bright or black flat fracture is found around the forced break fracture of gas pressure welds in which are observed line flaws on the surface. (2) Line Haw, if bonding work is poor, is caused by hot shearing. (3) The micro-observation results of bonding interface agree with the fracture form. (4) The transition from flat fracture to crystalline fracture is influenced by α and T. (5) It is impossible to detect light coarse welds, which present a gray flat fracture, by this inspection method. However, such welds have enough strength for practical use.
It is well known that the cleavage fracture toughness of steels deteriorates by pre-straining. Quantitative prediction of fracture toughness after pre-strain was examined in this work. Based on the local fracture criterion approach, the cleavage fracture toughness of steels can be described in terms of the cleavage fracture strength and the yield strength of the materials. Since the experimental cleavage fracture strengths before and after pre-strain were almost constant, deterioration in fracture toughness can be attributed to the work hardening behavior of the base steel. The predicted fracture toughness successfully agreed with the experimental results. The effect of the yield ratio on the deterioration in cleavage fracture toughness was also investigated. The experimental results in fracture toughness showed larger deterioration in lower yield ratio steel and showed the saturation under the relatively high pre-strain. These tendencies could be qualitatively described with the numerical analysis based on the local fracture criterion approach.
In the previous report, an approximate model which describes the mechanism of bulge removal by hot shearing was presented. But the details of stress distribution and plastic deformation mechanism in this process which have influence on the occurrence of a linear defect could not be known experimentally because of its high temperature. In this report, the problem of determining temperature and stress histories, stress distributions during gas pressure welding of steel and subsequent hot shearing is solved on the basis of coupled thermal and mechanical energy balances using the finite element method. A nonlinear heat transfer analysis and a thermo-elastic-plastic stress analysis are performed simultaneously taking into account the temperature dependence of the material properties, heat losses due to convection and radiation, and phase transformation. Tensile stress is confirmed to be generated just under the edge in hot shearing which opens the joint when the welding condition is not suitable. The computational results are compared with experimental ones, showing good correlation between them.
The nitriding process of solid state Ti-alloy by RF plasma was fundamentally investigated. Based on the results obtained in the nitriding process, reactive plasma spraying of Ti powder was carried out to fabricate the thick nitride coatings. The wear resistance of the nitride coatings was also evaluated by the conventional wear test. The results obtained are summarized as follows: (1) A linear relationship was recognized in the relation between thickness of the nitride layer and square root of nitride time. Therefore, the thermal plasma process seemed to be a diffusion dominating process. (2) The diffusion rate of nitrogen in the Ti-alloy: Rd was estimated from the experimenal results obtained, and the plasma spraying of Ti powder was conducted under the deposition rate of less than Rd. By using this process, it was possible to fabricate the nitride coating with high deposition rate. (3) It revealed that the nitriding degree of the coating could be changed by controlling the coating deposition rate. Based on this results, functionally gradient coating, whose compositional configuration changed from coating/substrate interface to coating surface, was fabricated by controlling the deposition rate. (4) From the results of wear tests, it was found that the wear resistance property of the nitride coating obtained was equivalent to that of nitride layer on the Ti-alloy.
Representative electric parts, LSI packages are turning to more fine pitch and more high pin count based on the demands of package size reduction which is coming from needs of smaller electric components. This reserch work has originated from high density LSI package surface mounting, including microjoining of leadframe on printed circuit board. In surface mounting of electric parts, solder past mass reflow line is applied to microjoining of fine pitch leadframe even less 0.5 mm pitch package outer lead on account of easy process controll and massproduction. But leadframe materials tend to smaller lead width and thinner by fine pitch photochemical etching technology propagation. This reserch work focused on reliability of fine pitch leadframe made with copper metal and Fe-42 mass% metal alloy surface mounted with solder past through long run aging. The result of this study have shown that fine pitch surface mount with solder paste (min. 0.3 mm pitch) is capable. But after high temperature aging (150°C×1000 h in air), Pb-60Sn eutectic microstructure of solder at leadframe microjoined is segregated to Pb rich and Sn rich large grain. And copper leasds become thin by high temperature reaction with solder. It was determined that copper was damaged in high temperature circumstance.