Friction stir spot welding of dissimilar aluminium alloys using the three-flat tool is carried out in order to investigate the effects of probe geometry on mechanical properties. The failure loads in A5754/A6111 lap joints made using a three-flat 0.5 mm no-thread tool were always higher than the joints made using a three-flat 0.5 mm threaded tool. Material flows due to the shear deformations were observed in the stir zone of A5754/A6111 and A5052/A6061 lap joints made using the three-flat tools. In addition, the bonded and stir zone widths in A5052/A6061 lap joints made using a three-flat 0.7 mm no-thread tool (in which the arc lengths of the triangular vertex region in the cross-section of probe were shortened) were always smaller than the joints made using a three-flat 0.5 mm no-thread tool. Therefore, it is suggested that there are pressures in a horizontal direction which are produced by the triangular vertex regions on the probe.
In fusion welding, gravity makes molten metal flow downward and it sometimes causes irregular shaped weld bead and weld defects such as undercut and overlap. This problem makes high-quality and high-deposition rate welding difficult especially in horizontal and overhead positions. To solve this problem, Dr. Manabe and others have proposed the basic concept of "Electromagnetic Controlled Molten Pool Welding Process (ECMP)", which controls the molten metal flow and the bead shape by using upward electromagnetic force, which is generated by giving the magnetic field perpendicular to the unidirectional current in the molten pool. Based on this concept, the ECMP method which uses magnetic field coaxially to the welding torch was already put into practical use in horizontal and vertical position welding with a high deposition-rate. To expanding the application of ECMP process, we have paid attention to a penetration welding without backing material represented by pipe welding, gravity causes a serious influence on the penetration bead shape which determines the weld joint quality in many cases. For this reason, if the penetration bead shape can be controlled by ECMP process, the industrial application will be expanded. So, we propose a new ECMP method using the opposed magnetic poles straddling the weld line, and the possibility of this ECMP method was examined for overhead and flat position welding. As a result, in flat position welding even under excessive heat input condition which molten metal hangs down and undercut occurs, the molten metal was lifted up by the upward electromagnetic force and the penetration bead shape was improved remarkably by adjusting the conditions of magnetic field and wire heating current, etc. Furthermore, in the most difficult overhead position welding, we could prevent undercut defect by adjusting the above-mentioned conditions and welding torch angle, and we could obtain proper penetration bead shape.
Ultrasonic welding phenomenon of pure aluminum sheets was observed using high speed video camera. The dynamic vibration behaviors of the edges of a welding tip, and a pair of aluminum sheets (upper specimen, lower specimen) were analyzed from the captured images using digital image correlation method. The following results were obtained. The welding process was comprised of following three stages. First, the upper specimen vibrated accompanying with small slipping between the edge and the upper specimen. The upper specimen thereby moved to one side along the vibration direction (Welding process I). In the second process, the vibration amplitude of the upper specimen decreased, while the friction between the edge and the upper specimen relatively increased (Welding process II). Then, the vibration amplitudes of the upper specimen and the lower specimen increased and the welding region plastically deformed (Welding process III). Joint strength increased with the welding time. In the fracture surface observation, partially welded regions were observed in the welding time corresponding to the welding process I, and growth of the welded regions was confirmed in the welding process II. The maximum strength was obtained in the welding process III, while adhesion to the welding tip and breakage of welded joints caused by compressive plastic deformation of welding region occurred in some specimens. The edges of a welding tip penetrated to the upper specimen with friction and plastic deformation of the upper specimen during the welding processes I and II. The penetrated depth saturated in the welding process II and the tip indentation forms on the surface of upper specimen.
It was clarified that the promotion of the crystallization of Chromium carbide during welding solidification by increasing C content was effective to decrease the solidification cracking susceptibility of fully austenitic weld metal with a large amount of Phosphorus. The decrease of solidification cracking susceptibility by increasing carbon content in Fe-(0.01-0.7)%C-25%Cr-50%Ni-0.1%P weld metal was confirmed using transverse varestraint testing and restraint weld cracking test. Also, γ/M7C3 eutectic phases were observed in the neighborhood of solidification crack on 0.7%C weld metal. In order to solve the mechanism of the decrease of solidification cracking susceptibility by the crystallization of Chromium carbide, a numerical model to predict the behavior of microsegregation during welding solidification was developed with notable features considering the competition of the crystallization between M23C6 and M7C3 eutectic phases and the coupling with thermodynamic database. It was found that the decrease of solidification cracking susceptibility on fully austenitic weld metal with 0.1% Phosphorus by increasing carbon content caused by the acceleration of the decrease of the amount of microsegregations by the promotion of the Cr based carbide crystallization.
Dissimilar material joining is a key process in manufacturing multimaterial products. The objectives of this research is to produce and evaluate dissimilar metal joints of SPCD low carbon steel and A5052 aluminium alloy with sandwiching polyethylene terephthalate (PET) plastic for both joint strength and galvanic corrosion. Laser-Assisted Metal and Plastic (LAMP) joining method could produce the sandwich structure, owing to availability of either metal-side or plastic-side laser irradiation. The maximum tensile shear load achieved 5500 N in the obtained dissimilar metal joints of 30 mm width, where both the metal sheets were elongated. Typical Transmission Electron Microscope (TEM) pictures of the jointed areas demonstrate that the steel or the aluminum was tightly bonded to PET through each metal-oxide film in nanoscale size. Moreover, it was reveled that aluminum galvanic corrosion of the obtained joints was drastically suppressed in an immersion test using salt agar, compared with conventional laser welds of carbon steel and aluminum alloy.
This paper describes the examination of the insert piece shape for the joint of a low carbon steel (LCS) fixed specimens using a brass insert piece by an autocompleting friction welding method, that method was developed by authors. The weld faying surface of the LCS specimen had a 10mm diameter, and the joint was made with a friction speed of 27.5rps (1650rpm) and a friction pressure of 90MPa. Then, the effect of the thickness and that at the bottom of the grooves (groove bottom thickness) for the brass insert piece on the joining phenomena and joint properties were investigated. The joint, which was made with an insert piece thickness of 4mm and a groove diameter of 11mm, had the circumferential shear fracture during the friction process at the groove bottom thickness of 0.95mm or below. However, some joints did not have the circumferential shear fracture although other joints had its fracture at a groove bottom thickness of 1.15mm or above. The joint efficiency of those joint had scatter, although that increased with increasing the groove bottom thickness and one of the joint had 100% joint efficiency at 1.45mm. That is, the joint, which had reliably the circumferential shear fracture with the 100% joint efficiency, was not obtained at an insert piece thickness of 4mm. To reliably obtain the joint which had the circumferential shear fracture with the 100% joint efficiency, the effect of the insert piece thickness was investigated. The joint efficiency increased with increasing insert piece thickness, and the scatter of that was decreased. When the joint was made with an insert piece thickness of 8mm and a groove diameter of 11mm, those had reliably the circumferential shear fracture with the 100% joint efficiency at the groove bottom thickness of 0.70 and 0.75mm. Also, when the joint was made with an insert piece thickness of 6mm and a groove diameter of 12mm, the joint had also reliably the circumferential shear fracture at the groove bottom thickness of 0.70mm. Therefore, a joint with 100% joint efficiency, which was made with the LCS with the brass insert piece, should be made with opportune brass insert piece shape of that thickness, the groove diameter, and the groove bottom thickness. In conclusion, a dissimilar joint was able to make by an autocompleting friction welding method.
Fine K-type thermocouple is fabricated by the non-contact discharge welding, which is developed by our group, and is examined comparing with a K-type thermocouple of 0.65mm in diameter. An alumel wire and a chromel wire of 50μm diameter are fixed to butt their tips with the crossing angle of ca. 0.35rad. A tungsten needle is positioned about 50μm above the tips. High voltage of 5kV is repeatedly applied to the needle for 0.5s to generate discharge between the needle and the tips. The tips are welded after the end of 7th discharge. Three different kind of discharge are observed by the completion of welding. Thermoelectromotive force (EMF) of the fine thermocouple is almost the same with that of the thermocouple of 0.65mm diameter in the temperature range below 643K, though EDX analysis indicates that hot junction and wires near the hot junction are partly oxidized. Hot air heated at 354K is blew to the hot junctions of fine thermocouple and 0.65mm thermocouple at the same time, and variation of their EMF is recorded with a digital oscilloscope. The heat response rates, which is defined by the initial increasing rate of EMF, of the fine thermocouple and the 0.65mm thermocouple are 870K/s and 22K/s, respectively.
In the present paper, the mechanism of mixing of metal vapor in arc plasma is discussed though the dynamic observation of spectra of helium, iron, chromium and manganese during stationary TIG welding of stainless steel and pure metals. Radiation from the arc is sent to a monochromator through optical lens and spectroscopic images are captured with 500 fps by a high-speed digital video-camera. Spectra of metal elements exist locally in the arc plasma due to dependence of plasma temperature and also intensive region of each metal spectrum depends on kinds of metal elements. The most part of metal vapor produced from the weld pool surface are carried on the cathode jet and then are swept away towards surroundings of the arc. However, if the driving force like diffusion in plasma is large, some metal elements can get across the cathode jet and then can be carried on the circulation flow towards tungsten electrode. Therefore, a part of metal vapor can be carried on this circulation flow towards tungsten electrode and then metal vapor can exist in the arc plasma.
Retained austenite affects the low-temperature tensile properties of Fe-14Cr-(4∼9)Ni weld metal through strain-induced martensite transformation. The strengthening process caused by strain-induced martensite transformation is evaluated in terms of work-hardening-rate. By means of this result, the effect of retained austenite on elongation is interpreted. Strength-ductility balance is found to be strongly influenced by retained austenite at low temperature.
Improvement of local tensile properties in cast aluminum alloy by friction stir processing (FSP) was examined. Small tensile specimens were machined out from nine locations inside of the stir zone manufactured by the FSP condition of tool rotation speed 1000 rpm and tool travel speed 500 mm/min, and tensile strength and ductility were locally evaluated. Although the plate as casted condition showed very brittle manner with only 2 or 3% fracture ductility, some of specimens machined from the stir zone achieved more than 20% of fracture ductility with local necking. However, the retreating side was not improved, and its fracture behavior was brittle similarly with the plate as casted condition. Fractography revealed that the ductility in cast aluminum alloy was controlled by the coarse brittle second phase, and an improvement of ductility resulted from the crashed brittle second phase to be fine. The stir zone, which was manufactured by tool travel speed 1000 mm/min, showed higher ductility than that by tool travel speed 500 mm/min. The retreating side was also improved in fast travel speed of 1000 mm/min. The mechanism to achieve an improvement of fracture ductility by FSP was discussed from the viewpoint from heat input.
The brittle crack arrestability is highly important in welded joints of heavy gauge steel plates used for large container ships. From the viewpoint of material design, it is possible to intentionally change the material properties of thick plates in the thickness direction. In this research, the brittle crack arrestability of thick steel plates with heterogeneous properties in the plate thickness direction were investigated in order to clarify the optimum distribution of properties in the thickness direction, which is necessary in the development of thick steel plates with excellent brittle crack arrestability. In the heavy gauge steel plate manufactured by lower temperature rolling, texture favorable for the arrstability was developed at the center of plate thickness (1/2t), and the brittle crack arrest position displays a split-nail shaped fracture surface morphology. When compared under identical Kca conditions, steel plate which has higher arrstability at the 1/2t position than at the 1/4t position, and display the split-nail fracture surface morphology, show higher brittle crack arrest performance under a condition of no temperature gradient, in which steel materials are actually used, than plates in which arrstability is lower at the 1/2t position than at the 1/4t position. The above-mentioned phenomenon can be explained approximately if the split-nail fracture surface morphology which is characteristic of steel plates with high arrstability at the center of thickness and the decrease in the stress intensity factor at the crack tip accompanying crack progress are considered.
It was clarified that the promotion of the crystallization of rare earth monophosphide during welding solidification by increasing Nd content was effective to decrease the solidification cracking susceptibility of fully austenitic weld metal with a large amount of Phosphorus. The decrease of solidification cracking susceptibility by increasing Nd content in Fe-18%Cr-15%Ni-0.1%P weld metal was confirmed using transverse varestraint testing and restraint weld cracking test. Also, NdP crystallization during GTA welding solidification on 0.5%Nd weld metal was confirmed by in-situ observation using synchrotron radiation. In order to solve the mechanism of the decrease of solidification cracking susceptibility by the crystallization of rare earth monophosphide, a numerical model to predict the behavior of microsegregation during welding solidification was developed with notable features considering the competition of the crystallization between NdP and M23C6 eutectic phases and the coupling with modified thermodynamic database. It was found that the decrease of solidification cracking susceptibility on fully austenitic weld metal with 0.1% Phosphorus by increasing Nd content caused by the restraint of the increase of Phosphorus amount in liquid phase by NdP crystallization.
New 18-8 high strength austenitic stainless steel with 0.1%P-0.2%Ti-0.2%Nb for super-heater and re-heater tubes in ultra-super critical power plants has been developed using the precipitation hardening of fine intragranular precipitates such as globular M23C6 carbides, MC type carbides and needle like FeTiP distributed inside grain. The solidification cracking susceptibility of the new 18-8 heat resistance steel was investigated by varying the Creq/Nieq and phosphorus amount. It was clarified that the solidification cracking susceptibility of new 18-8 heat resistance steel decreased by increasing Creq/Nieq to increase the amount of δ ferrite crystallization and promote the progress of welding solidification by (Nb,Ti)C crystallization in terms of microsegregation behavior using the numerical model.