Recently, the production of products using pure titanium has increased in Japan, and the welding for the pure titanium has attracted the attention. Concerning the welding of titanium, there are many research reports on the oxidation of weld zone in the welding process. However, the research report on practical welding procedure is scarce. In this study, weld property, which arose when the pure titanium was welded by TIG, was examined. As a result, the following was clarified: (1) The melting of the surface is easy to advance further than the penetration to the depth direction. And the weld zone of the shallow penetration is productive. (2) The undercut is hard to generate in the high-speed welding in comparison with a stainless steel 304. But, if there is an undercut, the crack is productive even in the shallow undercut. Therefore, it is proposed that the following countermeasures will be effective for the welding procedure: (1) The plate thickness of the root part should be made at about 1 mm because of the sufficient penetration. (2) Helium-argon mixed gas, in which argon is included at about 20%, should be used as a shielding gas. (3) The joint should have the root gap and be welded using the filler rod. And the knife-edge electrode should be used too.
The inspection of the weld zone is required, because the welding product of the pure titanium increases. The rediography is effective for the detection of defects inside the weld zone. However, the exposure chart for deciding the testing conditions of the radiography on the titanium has not been announced officially yet. First, the exposure charts of the pure titanium were made for this study. And it is examined that the relationship between defects detected by non-destructive testing and mechanical strength. Thus, the following is clarified: (1) The radioparency rate of the titanium is between aluminum and steel. (2) The deformability of weld part is very much decreased by the oxidation state of ash color. But the decrease of the tensile strength is a little. (3) The effect of the undercut to weld part is nearly equal to the oxidation state of ash color. (4) The penetration shortage is greatly made to lower tensile strength and deformability.
In order to enable that butt welding of 0.3 mm ultra thin aluminum sheets by using commercially available TIG welders, the authors has conducted a series of experiments. In the experiments 2%ThO2-W electrode of small diameter in DCEN polarity and two thick copper plates contact with base metals are used. The welding speed of 1500-6000 mm/min was attained. The effects of the remodeled nozzle are also studied at the speed higher than 6000 mm/min in the previous papers. This paper describes the new type of electrode shapes and the inclination of the torch. First the experiment with the electrode tip of conical shape with angle from 45° to 120° are conducted and the welding speed up to 6000 mm/min can be obtained, but the welding speed of 12000 mm/min can be done only for the electrode of conical angle 30°. The experiments with the electrode tip of conical and hemisphere shape show that the shape with a certain ratio of conical and spherical shape is suitable for the high spped 12000 mm/min welding. For the inclination of the torch we found it is better to decline so that the tip comes backward. By the combination of inclined torch and the electrode with a small spherical shape, high speed welding can be attained for the wide range of welding current.
In the former reports, it was discussed that a TIG arc behavior was controlled by a CO2 laser when the traveling speed of a base metal was fast and the arc length was long. In this case, the bead width was different, although the experimental conditions were same except the traveling direction of the base metal. In this report, the behavior of the TIG arc only was investigated using high speed video images when the traveling direction was changed. Furthermore, the reasonable spot of CO2 laser was examined for stabilization of the arc. In this case, the backward direction meant that the traveling direction was same as the direction of a tungsten electrode. The opposite direction was called the forward direction. The main experimental conditions were as follows. The TIG arc current, the traveling speed of a SUS304 base metal, Ar gas flow rate and the distance between the base metal and a tungsten electrode were 100 A (DCEN), 150 mm/s, 1×10-3 m3/s and 15 mm, respectively. When the base metal moved to the backward direction, a straight arc column bent smoothly at the point which was near to the base metal, and moved straightly to an anode spot. This spot existed in the circular molten pool and the shape of the pool did not changed. In this case, the bending point scarcely changed its position. On the other hand, in the case of the forward direction, the shape of the bending arc column, the position of the bending point and the shape of the melted part were different from ones of the backward direction. It was considered that arc stiffness at the bending point of the backward direction arc and preheat effect of arc plasma were less than the ones of the forward direction arc.
Hollow Cathode Arc (HCA) is a low-pressure arc discharge, which is formed in vacuum by supplying a very small quantity of gas from the tip of hollow cathode. The purpose of this work is to elucidate the HCA characteristics as a welding heat source. In the present work, arc voltage-current characteristics of HCA and its melting characteristics have been discussed. As a result, it is made clear that the HCA process is very sensitive to arc parameters, such as gas flow rate, arc length, electrode shape, and ambient pressure. The results of the present work are summarized as follows. (1) The voltage of HCA increases with the increase of electrode inner diameter and the decrease of Ar gas flow rate. (2) The penetration in HCA melting is extremely deep and huge under the condition that Ar gas flow rate is low and arc length is long. (3) The penetration in HCA melting is sensitive to the minor elements in base metal such as sulfur. Even in the low sulfur material, a deep penetration is formed under high speed and high current conditions. The above results suggest that HCA method is promising as the heat source of welding in space or welding on the earth.
In order to obtain high quality welded joints, the ultra-narrow gap GMA welding (UNGW) process has been developed in which the arc moves up-to-downwards along the groove wall. In this process, the arc up-to-down oscillation pattern depends on the wire melting tip behavior that is controlled by low frequency pulse current waveform below 10 Hz. In the low frequency pulse arc welding condition, it can be suggested that the wire-melting behavior is in the non-steady state. Firstly, the one-dimensional heat coduction FDM analysis is proposed for the non-steady wire-melting behavior in GMA welding. For the numerical analysis, both values of the equivalent anode melting potential (Φ) and superheat temperature of droplet(ΔT) must be given, however there are significant experimental uncertainties in these values reported by many researchers. Therefore, in constant welding current, some pairs of these values are searched so that the wire extension calculated by the FDM analysis agrees with the measured one. As the results, the correlation between Φ and ΔT for each welding current can be found out in MIG and MAG arc welding. Secondly, unsteady wire melting behavior in low frequency pulse arc welding is simulated by the numerical analysis using the Φ-ΔT correlation. As the remarkable behavior in the simulation results, it can be found that the wire extension almost does not change for a few times under the transition period from the peak current to the base current. The wire melting rate by Joul’s heat energy stored in the wire extension during peak current period produced the delayed time. Finally, the high-speed camera and current data sampling system are used to measure the unsteady wire melting behavior, and the simulated results are compared with the measured results in UNGW. The measured locus of melting wire tip shows good agreement with the calculated one. From the results, the proposed one-dimensional heat conduction FDM analysis is effective for the prediction of the suitable arc up-to-down oscillation pattern in UNGW.
The Modified Embedded Atom Method (MEAM) is applied to dimers of several materials whose parameters are published by Baskes. For some materials, the MEAM is found to return fatal errors which never arises in bulk systems. The problem of the fatal error is found to be because of a formalism which calculates the background electron density, and the usage of an original formalism is found to prevent the problem. Since the dimer is the most non-bulk system, this problem has to be considered in the application of MEAM to non-bulk systems. The MEAM calculation has been retried for all of the dimers by using the original formalism. The results by the MEAM are compared with density functional theory (DFT) calculations. The usage of the original formalism returns the consistent results for most of the dimers. This suggests that the original formalism has to be used for the background electron density calculation, if the MEAM would be applied to non-bulk systems. It would also suggest that the MEAM parameters should be modified for some materials (Mo, Cr, C).
The good mechanical and corrosion resistance properties of duplex, ferritic-austenitic steels depend critically on the phase balance. While typical commercial wrought products display 50/50 ferrite/austenite, this can be significantly, and adversely, affected by a weld thermal cycle. Work was carried out to derive a predictive system for phase balance in single pass weld metals. Single run beads were produced on UNS S31803 steel using the SMAW, TIG and submerged arc welding processes, and varying consumable compositions, to give a range of deposit compositions and microstructures. Weld metal cooling rates were recorded. The Austin-Rickett equation was used to describe transformation behaviour, and comparison was made between anstenite contents measured and calculated from an equilibrium value at 1000°C. Austenite contents of single pass weld metals from about 5 to 95% were predicted from their composition and the cooling cycle experienced. At the 50% level, predicted values were within about ±10% of the measured value. From the predictive model, the change in austenite content due to varying the cooling time from 1200 to 800°C was assessed. The change in austenite content is expected to be greatest in weld metals having an equilibrium austenite content of 50%. Weld metals having higher or lower equilibrium austenite contents are less sensitive to variations in welding conditions.
A study was carried out to determine the effect of rare earth metal (La, Ce) addition on the HAZ cracking susceptibility of cast alloy 718 welds. The cast alloy 718 containing rare earth metal was melted in an alumina crucible heated in ambient argon by an induction generator and then poured into a copper mold. The contents of La and Ce addition were varied from 0 to 0.7 mass%. HAZ cracking susceptibility of the La and Ce-added cast alloy 718 in TIG and laser welds were evaluated by using self-restraint U-type hot cracking test and bead-on-plate test, respectively. The values of grain size and aspect ratio of grain in the La, Ce containing specimens were much less than those of rare earth metal-free specimens. Compared with the Ce-added specimen, the value of cumulative crack length was significantly lower for the La-added specimens. The La and Ce addition up to 0.3 mass% was found to have a beneficial effect to reduce the HAZ cracking susceptibility. Furthermore, no HAZ cracking was observed for the specimen containing 0.2 to 0.3 mass% La. However, when the amount of La and Ce addition exceeded 0.3 mass%, the HAZ cracking susceptibility contrarily increased.
The present work was undertaken to examine the mechanism in the improvement of HAZ cracking susceptibility of cast alloy 718 welds by addition of rare earth metals (La, Ce). The HAZ cracking susceptibility of La, Ce-added specimens have been discussed in terms of the intergranular liquation, microconstituents and impurity segregation at grain boundaries. The isothermal liquation test result suggested that, in the case of specimens with up to 0.3 mass% La and Ce, the reduced HAZ cracking susceptibility could be attributed to the decrease in intergranular liquation. The microscopic observation revealed that the cause of reduced intergranular liquation in La and Ce containing specimens was responsible for the reduction in the amount of laves cluster and sulfur segregation at grain boundaries due to grain-refining and sulfur-scavenging effects of La and Ce. Furthermore, the increased HAZ cracking susceptibility and grain boundary liquation at over 0.3 mass% La and Ce addition was due to the increase in low-melting point Ni7La2 and Ni2Ce present at grain boundaries.
High nitrogen-bearing stainless steel (HNS) is attractive material due to its superior corrosion resistance, mechanical properties as well as resource saving type material. HNS of about 1 mass% in nitrogen content manufactured by a pressurized ESR was tried to joint by conventional arc welding. Because of the nitrogen supersaturation at molten state, blowholes were easily formed in the weld metal. A new welding method was proposed by using high Cr-Mo Steels as welding consumables which had relatively large nitrogen solubility at molten state in order to suppress the blowhole generation in the weld metal. The evaluation of pitting corrosion resistance of weld metal was also investigated with taking into account the solidification mode. The welding method by using high Cr-Mo steels as the welding consumables and by controlling the dilution rate to hold the nitrogen content in molten metal less than equilibrated solubility of molten metal with a nitrogen gas at 1×105Pa was found to be effective to suppress the blowholes generation in the weld metal. The critical pitting corrosion temperature (CPT) of the weld metal portion in 6%FeCl3 solution dropped compared with that of the base metal because of microsegregation of alloy elements. Controlling the composition of the weld metal so as to solidify with FA mode, the degree of CPT drop could be minimized compared with that of other solidification modes because of nitrogen segregation enriched in austenite in which Cr, Mo were relatively depleted. The alloy elements compositions at the portion where Cr and Mo contents were minimized in the weld metal could be estimated by Thermo-calc and compared with the results of EPMA analysis. Tensile strength of the weld metal portions were 700∼900MPa and those were about 20% lower value than that of the parent metal because of the reduction of nitrogen content.
High nitrogen-bearing stainless steels (HNS) are attractive materials because of their superior localized corrosion resistance, mechanical properties and also resource saving type compositions. HNS in which nitrogen content reached to about 1 mass% were manufactured by pressurized ESR, and an attempt was made to weld them. In the heat affected zone (HAZ) of GTAW, only nitride precipitation or nitride precipitation and ferrite growth were observed. The precipitated nitrides were determined to be Cr2N containing a small amount of Mo and Fe by TEM/EDS analysis. The pitting corrosion potential and crevice corrosion potential in the artificial seawater of the HAZ, in which Cr2N precipitated, degraded comparing with the base metal. The time-temperature-precipitation (TTP) curves of HNS were obtained. It was found that for 18 Cr-14 Ni-2.5 Mo-0.7N steel, in the heating temperatures ranging from 1073K to 1473K, the critical pitting corrosion temperature (CPT) in the 6% FeCl3 solution decreased after heating 1 s, and for 23 Cr-4 Ni-2 Mo-1 N steel, CPT decreased raging from 1173K to 1373K after heating 2 s. The TTP curves of the HNS were estimated by taking account of the incubation period and the precipitates-growing period. The former period was derived from the kinetics theory of solid-solid nucleation at grain boundary, and the latter period was estimated on the assumption that the growing rate of Cr2N was controlled by one dimensional diffusion of Cr inside the grains. The estimated total growing period was roughly consistent with the experimental period at which CPT began to decrease. CPT decrease was successfully predicted by using additivity rule (linear summation rule of the thermal cycle during welding). Cr2N precipitation could be prevented by CO2 laser welding due to its high cooling rate. The CPT of the CO2 laser welded joint of the HNS with high Cr-Mo steel insert metal was 348K, and its tensile strength was 920 MPa.
Resistance spot heating which has a cooling rate of approximately 5000 K/s was carried out for high strength steel sheets with ultra-fine grained microstructure to investigate the characteristics of the welds. The steel sheets have a low carbon equivalent in order to retain good weldability and realize high strength by means of their ultra-fine grained microstructure. Though ferrite grains in the HAZ coarsened, the HAZ softening was restrained due to the formation of tiny dispersed MA constituents from γ phase at the boundary of the ferrite grains. Though the constitution of the nugget was martensite, the nugget hardness was restrained due to the low carbon content. Therefore the resistance spot welding joint of the steel sheets might have good tensile shear strength and good cross tension strength without the temper treatment. The prior austenite grain size was also examined for the steel sheets (the initial ferrite grain size was approximately 0.8 μm) and annealed steel sheets (the initial ferrite grain size was approximately 30 μm). The results indicate that the prior austenite grain size depends upon the initial ferrite grain size. The prior austenite grain size at the fusion boundary also decreased as the initial ferrite grain size decreased.
We have investigated an effect of vanadium or aluminum into titanium alloys on the formation of Kirkendall voids in titanium region in diffusion bonds between titanium and molybdenum. The influence of bonding conditions on the joint stregnth has been also investigated. The bondings were performed under bonding temperature range from 1023 to 1473 K and bonding time range from 0 to 250 ks in a vacuum of 8 mPa. Main results obtained are as follows. The Kirkendall voids were observed in the bonded zone of the Ti-4.04at%Al/Mo joint. On the other hand, when Ti alloys had more than 1.89 at%V, the void were not observed at the bonded zone of the Ti-V alloys/Mo joint. The void fraction on the bonded zone of the Ti-V alloys having the vanadium content less than 0.95 at% were decreased with increasing the vanadium content. The concentration of vanadium in the vicinity of the voids was higher than the average concentration of the Ti-V alloy containing less than 0.95 at%V. When the bonding time was 1.8 ks, the joint strength of the joint of TB340B/Mo was maximum value of 200 MPa. The tensile strength of joints of the Ti-4.04 at%Al/Mo and Ti-1.89 at%V/Mo reached about 340 MPa. On the other hand, the joint strength was increased with increasing the vanadium content of the Ti-V alloys having vanadium content range from 0.11 to 3.64 at%.
The stress behavior in adhesive scarf joints of dissimilar adherends subjected to tensile impact loads are analyzed in an elastic region using three-dimensional finite element method (FEM). FEM code employed is DYNA3D. An impact load is applied to a joint by dropping a weight. The end of the upper adherend is fixed and the other end of the lower adherend is subjected to an impact load. The effects of scarf angles and combination types of the adherends, the adhesive thickness and Young’s modulus of the adhesive on the stress behavior at the interfaces are examined. It is found that the maximum value of the maximum principal stress σ1, appears at the interface between the adhesive layer and the lower adherend, and it is independent of the combination types of the adherends. In the cases of scarf angles of 52.47, 60 and 75 degrees among the seven cases, the maximum principal stress σ1 and each stress distributions at the interfaces become to be more homogeneous. The value of the maximum principal stress σ1 decreases as Young’ modulus of the adhesive decreases and the adhesive thickness increases. In addition, experiments are carried out to measure the strain response of the adhesive scarf joints subjected to impact tensile loads using strain gauges. A good agreement is seen between the tendencies of the analytical and experimental results.
This paper deals with experimental data and a predictive method of the welding longitudinal shrinkage which is important in the development of the high accuracy production system of the large welding structures. The longitudinal shrinkage can be calculated from the inherent force, namely Tendon Force, which is related to the total of the inherent strain. First the method to measure the longitudinal shrinkage with the error less than 1/100 mm was invented and then the experimental values of the longitudinal shrinkage and the inherent strain were obtained under various welding conditions. Next, the theoretical equation was proposed to calculate the longitudinal shrinkage from the inherent strain. As the results, the validity of the theoretical equation combining the welding heat input with Tendon Force obtained from the inherent strain in the weld direction is proved. And, the possibility to predict the longitudinal shrinkage is shown by the comparison between longitudinal shrinkage values predicted from Tendon Force and experimental values.
In this study, software for 3-dimensional image reconstruction from two stereo images was developed. The software was applied to ductile and brittle fracture surfaces of duplex stainless steel and the validity was evaluated. The procedure for 3-dimensional image reconstruction was as follows; first, digitizing two micrographs of standard and oblique image by an image scanner, second, searching homologue points by using template matching algorithm to calculate the elevation from measured parallax, and finally displaying reconstructed 3-dimensional images on a monitor. Fractographic analysis for ductile and brittle fracture surfaces of ferritic-austenitic duplex stainless steel was performed by means of the developed software. The reconstructed images obviously reveal the elongated dimple rupture which originates from inclusions in ductile fracture and cleavage cracks which are arrested by austenite grains and re-initiate from other regions in brittle fracture. The reconstruction was also applied to a brittle fracture surface of ferritic stainless steel tested at 77K. It can be seen that cleavage steps appearing as river patterns are detectable and reconstructed when their elevation is greater than 2μm at the scanning resolution of 150 dpi and 1μm at the resolution of 300 dpi in this study.
Although a fabrication possibility of TiO2 coating phototcatalyst by a thermal spray process has been indicated, little has been known about the existence and distribution of anatase phase in the sprayed coating. It is necessary to observe precisely the microstructure of TiO2 coating photocatalyst for the improvement of the photocatalytic property of the coating. In this paper, to clarify the distribution of both rutile and anatase phases in the coating, microstructure of TiO2 coating was analyzed. Furthermore, the effect of adsorbent addition to TiO2 on the transformation temperature and NOX elimination property was also investigated. The sprayed particles passed away from the HVOF flame were collected in the water and were observed by the Laser-Raman spectroscopic analyses. In the relatively big particles in diameter, melting occurred only on the outer surface of the particle and the anatase phase was effectively retained at the inside of the particle. In the small particles, on the other hand, particles were fully melted and anatase phase completely transformed into rutile. Thus, the coating fabricated by HVOF spraying was constructed by both anatase phase region and anatase/rutile composite region. The transformation temperatures for TiO2, TiO2+Ca3 (PO4)2 and TiO2+Fe2O3 powder were 1173 K, 1273 K, and 1098 K, respectively. By the addition of the adsorbent to TiO2, transformation temperature increased in TiO2+Ca3 (PO4)2 system. This is beneficial for the retaining of anatase phase during and after the spraying. From the measurement results of NOX elimination property of the coating, the coating with adsorbents showed higher adsorption ability of NO2. It was revealed that the coating made by HVOF spraying has fairly high NOX elimination property among the other practical coatings, such as a solution spreading.