A 10 kW power fiber laser welding is supposed to produce sound deep penetrations, but to occasionally generate welding defects. The objectives of this research are to investigate penetration, to clarify welding defect mechanism and to develop defect prevention procedure in bead-on-plate welding of Type 304 austenitic stainless steel plates with a 10 kW fiber laser beam. The penetration depth reached 18 mm at the maximum. However, underfilling and humping weld beads were formed at 6 m/min or higher welding speeds under the conventionally-focused and tightly-focused conditions, respectively. At 3 m/min or lower welding speeds, porosity was generated at any fiber laser spot diameter. With respect to the underfilling, spatter generation was influenced mainly by a strong shear force of a laser-induced plume and was widely reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was decided by several dynamic or static factors such as melt volume, melt flow, surface tension and solidification. Its suppression was effective in expansion of the bead width at defocused-laser positions or reduction of melt volume out of keyhole inlet under full-penetration conditions. Concerning porosity, X-ray transmission observation images demonstrated that pores were formed not only at the tip of the keyhole but also at the middle part. The keyhole behavior was stabilized by using nitrogen shielding gas, which led to the porosity prevention. Consequently, a 10 kW laser power could produce sound welds of 18 mm depth, but the reduction procedures of welding defects were required for the production of high-quality welds on the basis of understanding their formation mechanism in 10 kW high-power fiber laser welding.
Friction Stir Welding (FSW) succeeded in producing high quality dissimilar welds with AA5083 and A6N01 by evaluation of the microstructure and the root bend testing. A6N01 with a wide optimum range of welding condition should be placed on the retreating side to weld sound joint between AA5083 and A6N01. The optimum welding condition of FSW for the dissimilar alloys between AA5083 and A6N01 was wider than that of AA5083. In the opposite orientation, A6N01 on the advancing side can hardly flow into AA5083 on the retreating side in front of the tool. As the pores on inappropriate welding condition were observed, large pores on a lower tool rotation speed were different from small discontinuous pores on a higher tool rotation speed.
The temperature of metal droplets is essential for clarifying the phenomenon of metal droplet transfer and the melting behavior of wire; also, it governs the emission of fumes. On the other hand, in-situ measurement of the temperature of a metal droplet formed at the tip of a wire during welding was difficult. Hence, this temperature was obtained in many experiences of measurements by such a way that several numbers of metal droplets were collected in a calorimeter to measure the amount of heat content of metal droplet and the heat was converted to temperature. With this way, however, the reliability of the measurement is not necessarily high because the heat loss of the metal droplet during the time when detaching from the wire tip and entering into the calorimeter has to be estimated properly. In this research, two-color pyrometry has been conducted to obtain the temperature of metal droplets, in which metal droplets have been photographed by high speed camera during arc welding, two wave lengths (950 and 980 nm) of light in the infrared range have been selected from the thermal radiation light emitted from the metal droplet at the instant of arc extinguishment by using an imaging spectroscope, and the temperature has been obtained from the intensity ratio of the two waves of light. Consequently, in CO2 arc welding, it has been revealed that the constricted arc causes high heat input density locally at the arc root portion of a metal droplet and thereby the arc root portion exhibits a higher temperature. By contrast, in MAG (80%Ar-20%CO2) arc welding, it has been disclosed that because the arc covers metal droplets, the temperature distribution in a metal droplet is relatively uniform and the average temperature is lower than in CO2 arc welding.
Recently, the thermal power plants tend to operate under at higher temperature and pressure steam conditions for the CO2 reduction. The modified 9Cr-1Mo steels are used in the ultra super critical power plant because of the excellent creep property. However, the toughness of base metal decreases after long-term thermal aging at operation temperature. Moreover, it paid attention to the toughness of the weld metal because toughness of weld metal is the lowest in the MIG weldment. In this study, metallurgical factors of toughness of the modified 9Cr-1Mo steel weld metals by thermal aging at operation temperature were investigated. And the evaluation method of toughness by the electrochemical measurement used with 5% sulfuric acid aqueous solution was investigated. The weld metal that received post weld heat treatment (PWHT: 1023 K, 5.4 ks) decreased toughness by thermal aging in 873 K, 31.5 Ms. However toughness of the welds after thermal aging were recovered to the same level of as PWHT welds when the thermal aged welds received the same thermal history as PWHT. A lot of large Laves phase was observed in the weld metal after the thermal aging. However almost of the Laves phase dissolved in parent phase by the same thermal history as PWHT. Therefore, it seemed that the decrease and the recovery of toughness was mainly determined by the behavior of precipitation of Laves phase. Peak of the current density (Ip) in the electrochemical measurement was appeared in the weld metals of precipitated a lot of large Laves phase. It seemed that appearance of Ip was caused by the dissolution of the Laves phase. The good correlation between toughness and Ip was observed. Therefore, it can be said that toughness of the weld metals suppose to be predict using Ip of electrochemical measurement.
A friction stir spot welding process, in which a rotating tool without a probe was employed, was applied to a lap joint of aluminum plate to low carbon steel plate. The thicknesses of both plates were 0.5 mm. In this process, the rotating tool of 5 mm diameter, rotating at 18000 rpm, was plunged into the aluminum plate at a rate of 2 mm/s, and then kept at a maximum plunged depth of 0.05–0.35 mm for 0–2 s (dwell time). In the weld obtained by this process, a hole due to the impression of the penetrated tool probe was not formed, although a slight depression by the tool plunging remained. At tool plunge depths of 0.1 mm or over, it was possible to weld the two plates. The maximum tensile failure load of 454N was obtained at a plunge depth of 0.1 mm and a dwell time of 1.5s. Its joint was fractured at an almost constant load along the periphery of the depression, leaving a part of the aluminum plate on the steel plate surface. Based on the observation of the weld interface microstructure and metal flow of aluminum in the weld, controlling factors of the joint strength were discussed.
Tandem beam brazing with the aluminum filler metal (BA4047) was conducted in order to develop the fluxless laser brazing technique of aluminum alloy (AA6022) to galvanized steels (GA and GI steels). Laser powers of tandem beam and offset distance of preheating beam from the root to the steel base metal were varied. Sound braze beads could be obtained by optimizing the preheating and main beam powers under the offset distances of 0-1mm. A small amount of zinc remained at the braze interface between galvanized steels and the braze metal. The reaction layer consisting of Fe-Al intermetallic compounds was also formed at the steel interface, and the thickness of reaction layer could be predicted during laser brazing (thermal cycle) process based on the growth kinetics with the additivity rule. The metal flow analysis of the melted filler metal on joints revealed that wettability and spreadability of the filler metal on GI steel joint were superior to those on GA steel joint. The fracture strength of lap joint attained approx. 55-75% of the base metal strength of aluminum alloy. It was concluded that fluxless laser brazing could be successfully performed by using tandem beam because the zinc coat layer acted as brazing flux.
In order to investigate the solidification cracking susceptibilities of SUS347H weld metal containing high niobium and carbon contents, it is important to analyze the crystallization behaviors of not only δ and γ phases but also niobium carbide during welding solidification. In the present study, the behavior of the phase selection for γ, δ phase and niobium carbide on Fe-18%Cr-0.2%C-1%Nb-(5-12)%Ni weld metals during welding solidification was investigated by the developed in-situ observation technique of welding solidification process using synchrotron radiation. On the other hand, to evaluate the micro segregation in liquid phase on SUS347H weld metal during welding solidification, which has influence on the solidification cracking susceptibility, the numerical model to calculate the micro segregation in liquid phase considering the crystallization of γ, δ and niobium carbide was developed. As a result, the validity of the developed numerical model was supported by the comparison with experimental results by in-situ observation of welding solidification process on Fe-18%Cr-0.2%C-1%Nb-(5-12)%Ni weld metals. Furthermore, it was found that the solidification cracking susceptibilities of Fe-18%Cr-0.2%C-1%Nb-(5-12)%Ni weld metals predicted by the developed numerical model coincided with the experimental results evaluated by trans varestraint test.