The effect of nitrogen on weld solidification cracking susceptibility of Fe-25%Cr-22%Ni-P-N fully austenitic stainless steels was experimentally investigated. Maximum crack length in Trans-Varestraint test slightly increased with increasing nitrogen content. Moreover, nitrogen content also did not affect the cracking susceptibility in self-restraint type weld crack test. The observation of liquid Sn quenched microstructure revealed that the addition of nitrogen caused microstructural transition from cell to dendrite, decreasing microsegregation of chromium and phosphorus during solidification. We concluded that brittleness temperature range was not enlarged by nitrogen addition, because the increment related to microsegregation of nitrogen itself and the decrement related to the suppressed microsegregation of chromium and phosphorus canceled each other out.
In the fabrication of steel structures, welding is widely utilized to join the materials. Due to the welding, distortions are inevitably generated, and these distortions may cause problems in accuracy or labor costs. In this research, to establish an analysis method which can predict the welding distortions in complex large-scale structures, we proposed a new analysis method based on the Idealized Explicit FEM. In the proposed method, an algebraic multigrid method was introduced to the Idealized Explicit FEM to achieve an efficient analysis in realistic structures. The proposed method was applied to the prediction of the welding distortion in the base structures of the construction machine. The number of welding passes was 28. The predicted and measured distortions were compared. As a result, it was demonstrated that the proposed method has a high analysis accuracy. The analysis finished within the realistic time within 35 hours. The influence of welding sequence on the deformation was also investigated by changing welding sequence. The result indicated that the welding sequence may have considerable effect on the welding distortion and is necessary to be investigated in advance of the production.
Tensile-shear test blanks were fabricated using high strength and low carbon steel sheets with three different strength levels by a resistance spot welding (RSW) procedure. The nugget sizes were changed by controlling the welding process parameters, resulting in the nugget sizes of 3√t, 4√t and 4.7√t, where t was the sheet thickness. Subsequently, tensile-shear static and fatigue tests were conducted to investigate the effects of strength levels of steels and nugget sizes on the mechanical properties of the welds. The tensile strength increased with increasing strength levels and nugget sizes. However, the fatigue strengths of the welds with the nugget sizes of 4√t and 4.7√t were nearly comparable irrespective of the strength levels of steels. That was because fatigue crack propagation life was dominant in the total fatigue life. In the welds with 3√t nugget size, the steel with higher strength level exhibited lower fatigue strength. It could be attributed to the lower bonding strength along the corona bond in the high strength steel.
The purpose of this paper is to examine the influence of fracture toughness and non-welded part in submerged arc weld (SAW) on deformation capacity of pre-assembled H-shaped beam to column joint. For this purpose, higher toughness (about 60J) and lower toughness (about 20J) SAW joints were prepared. In addition, improved beam end scallop details, BS_P (billet insertion), BM (mini scallop), and BF_5 (height of fillet:5mm), were proposed. The rate of increase of strength and the deformation capacity of these four type details were examined by experiment. From the results of experiments, it is revealed that brittle fracture at beam end flange is likely to occur at small deformation when the field welding beam to column joint has BS (composite scallop) detail and non-welded part of low toughness SAW. On the other hand, using higher toughness SAW and shortening non-welded SAW parts delayed brittle fracture and raised deformation capacity. Moreover, using proposed improved beam end scallop details buffered stress at the bottom of scallop and improved deformation capacity. Therefore, the authors showed following the method for improving deformation capacity of field welding beam to column joints. 1) Improving toughness of SAW 2) Using proposed improved beam end scallop details 3) Shortening the length of non-welded of SAW
Narrow pitch spot welding causes decrease in nugget diameter due to shunting to existing- welds under conventional resistance spot welding. This study aims to develop new resistance spot welding technology for ensuring required nugget diameter regardless of weld spacing. Adaptive control based on real-time feedback of heat quantity was applied for welding of mild steel sheets. However, expulsion occurred from steel sheet surface around the contact edge with electrode when weld spacing was shortened to 10mm. To solve this problem, a new adaptive control method was developed. The method includes 2-stage adaptive control, in which heat quantity is controlled independently in each stage to obtain proper nugget diameter. It is effective to set relatively low heat quantity compared to conventional method in the 1st stage to ensure current pass between steel sheets, and then to apply enough current-flow to obtain proper nugget diameter in the 2nd stage. This method can decrease shunt current flow to existing-welds, and excess increase in welding current and over-heating around contact edge with electrode was suppressed. Appropriate nugget diameter can be consequently ensured without expulsion from steel sheet surface even with 10mm weld spacing.
The influence of magnesium contents on cathode spot (CS) behavior in aluminum AC TIG welding was investigated by High-Speed Video Camera (HSVC) observation with the maximum frame rate of 500,000fps. The number of CS in A5052 case was larger than that in A1050, leading to the smaller average current per spot of 6.9A comparing with 16.7A in A1050. The larger current per spot is considered to cause the CS higher velocity. The average velocity of CS on liquid surface of A5052 was 110±37m/s far lower than that in A1050 case. The central area, where the CS did not exist, had a radius of 2.0mm and expanded over EP time. The existence of magnesium in A5052 led to the increase in the cathode spot number inside the weld pool. The predicted mechanism could be the more easy evaporation of magnesium than that of aluminum.
We investigated the effect of laser-irradiating conditions—specifically laser spot size, laser power density and laser weaving. Three laser spot widths of 10, 4 and 2mm were applied by changing the optical-lens and fiber-cable combination to investigate the effects of the laser power density and laser spot width. The weaving-irradiating method was applied with narrow laser widths of 4 and 2mm. The effects of the laser-irradiating condition were obtained based on high-speed imaging during welding and cross-sectional observation. Stable laser irradiation by a 10-mm laser spot width provided a lower power density than the critical value of 35W/mm2 and a lack of fusion. Weaving laser irradiation by a 4- or 2-mm laser spot width provided a higher power density, reduced the large lack of fusion and achieved a large penetration of base metal. The ratio between the laser beam-spot width and gap width (WL/WG ratio) affect the base-metal fusion significantly. A sound WL/WG ratio promoted base-metal fusion by providing a uniform and stable molten-pool temperature, whereas a small WL/WG ratio maintained a smaller fusion area because of the sudden temperature drop and temperature fluctuation of the molten pool.
The joint of the self-induced fusion solidification bonding of aluminum is obtained by the isothermal fusion and the isothermal solidification by the reaction diffusion. The reaction diffusion between the dissimilar solid interfaces requires the fracture of the oxide film on the interface at the initial stage of the bonding process. The periodic motion of the interface promoted the fracture of the oxide film by the impact at the bonding interface. Increase of the frequency of the impact also will promote the fracture of the oxide film. The joint of the self-induced fusion solidification bonding under bonding pressure control was investigated with the periodic motion of the different frequency. The initial stage of the bonding process occurred in the short time by high frequency motion. Flash of the liquid from the interface occurred at the fixed impacts under a stable maximum pressure, regardless of the change of the frequency. High frequency motion reached the fixed number of the impact in the short time. However, middle frequency motion obtained the self-induced fusion solidification bond with maximum tensile strength, maximum bonding area ratio and maximum displacement of the bond. These results suggest that the deformation of the bonding interface promotes the self-induced fusion solidification bonding process.
Varestraint test is one of the most-used methods to evaluate weld solidification cracking susceptibility. However, standard about the detailed test method and the evaluation method has not been clearly defined. Thus, it is required to standardize the methods of Varestraint test and the evaluation indexes in order to compare the results tested by each researcher. In this study, round-robin test of transverse-Varestraint test with GTAW was carried out under identical specimens and test conditions using five test machines to clarify the influence of test machine and human factor on the test result. Crack number and crack length were measured using a scanning electron microscope (SEM) and an optical microscope. 6 evaluators measured the crack number and the length. The maximum crack lengths measured by the evaluators were nearly the same. In contrast, there were big differences in the crack numbers and the total crack lengths. Thus, the influence of the human factor on the maximum crack length was relatively low and that on the crack number and the total crack length was high. This should be caused by different judgment depending on the evaluators for the crack opening and the propagated crack tip. Using SEM was an adequate measuring method for the crack length since the measurement using an optical microscope tended to cause false judgment of wrinkle as cracks. Maximum crack length must be appropriate evaluation index for solidification cracking susceptibility compared to the total crack length and the crack number.
We investigated the melting of a base metal, molten pool growth, and joint creation during vertical welding via hot-wire-laser welding. Three laser-weaving conditions were investigated by changing the weaving frequency and waveform to study the effects of the irradiation duration near the groove surface. In addition, high-speed and cross-sectional imaging were performed to investigate the heating and melting processes on the groove surface during hot-wire laser welding. The irradiation duration near the groove surface in a cycle had a marked effect on the melting of the groove surface. The combination of a 5Hz laser frequency with an exponential waveform led to a longer duration near the groove surface during a cycle and realized improved fusion compared with the other combinations with a 15Hz laser frequency and a sine waveform. The laser beam reflected from the molten pool surface was the main source of heat for melting the groove surface. Hot-wire feeding provided a continuous and efficient supply of melted material and a stable heat input on the groove surface via the reflected laser beam.
Advantageous effect on cross tension strength (CTS) of resistance spot welds by “pulsed current pattern” has been evaluated assuming nugget diameter variation which unexpectedly causes deterioration of joint strength of ultra-high strength steel (UHSS) in auto assembly lines. Pulsed current pattern is newly developed resistance spot welding method consisting of main welding for forming nugget and multiple combinations of short-time cool time and short-time high-current post-heating (pulsed current) to realize time-effective change in brittle nugget. For quantitative evaluation of nugget diameter variation and post-heating effects, various welding experiments and numerical simulations are conducted by various main welding current without or with 1- or 2-time pulsed current using 1180MPa grade steel sheets. The experimental results clarify 2-time repetitions of pulsed current ensures plug failure and high CTS over 10kN at wider range of nugget diameter than without and even with 1-time pulsed current. Numerical calculation and cross-sectional microstructures suggest that post-heating with too small nugget diameter overheats nugget and partially remelts phosphorus-rich area due to solidifying segregation leading to low CTS, but repetitive pulsed current can lower peak temperature to appropriate range to avoid deterioration of CTS. Therefore, pulsed current pattern is expected to realize high quality joint even when nugget diameter is not insufficient due to instability of welding processes.
This paper investigates the mechanical characteristics during friction stir spot welding when cutting tools are used with consideration made to tool shapes. Friction stir spot welding tools with cutting edges similar to a cutting tool were used in this study, and the rake angles of the tools were changed from negative to positive. The failure load in the tensile shear test of an A5052/A6061 friction stir spot lap joint made at a tool rotational speed of 1500rpm was the highest when using a tool with a negative rake angle, and was almost the same when using tools with rake angles of 0º and positive angles. The thrust force and torque were similar to the results of the tensile shear test. These were the highest when using a tool with a negative rake angle, and were almost the same when using tools with rake angles of 0º and positive angles. These results indicate whether it is possible to easily generate and discharge chips during probe penetration. Regarding this, it seems that the rake angle of the tool significantly affects the generation and discharge of the chips. These results suggest that the mechanical characteristics during friction stir spot welding can be controlled by changing the tool geometry.
We have developed a joining process which we named disc friction joining (DFJ). DFJ is a solid state butt joining method that utilizes a rotating disc that is sandwiched and compressed by two plates to be joined. Frictional heat generated at the contact interfaces effectively softens the materials to be plastically deformed by the compression force applied, yielding a joint interface once the disc is withdrawn and the plates make contact. The feasibility of DFJ was examined by joining commercial pure aluminum and polyethylene terephthalate (PET) plates. These materials were successfully joined, and the joint exhibited a tensile fracture stress of 17.1MPa at room temperature.
The temporal evaluation in the distribution of additives inside the electrode was clarified by the cross-section observation of the electrode after the arc discharge and the numerical simulation. The effect of the adsorption energy of additives on the electrode lifetime was also investigated by the numerical simulation. During the arc discharge, additives whose work function is lower than tungsten are gradually consumed from the electrode tip due to evaporation of additives on the electrode surface and diffusion of additives inside of the electrode. When the operating temperature of the electrode exceeds the melting point of tungsten, the electrode reaches the lifetime by melting and deforming. Furthermore, by setting an appropriate adsorption energy, the two dimensional computational model including diffusion and evaporation of additives obtains the electrode lifetime that agrees with the experimental result. In addition, the larger the adsorption energy of additives inside of the electrode is, the longer the electrode lifetime is.
The microstructure formation process in duplex stainless steel was simulated using a multiphase field (MPF) method. The dendrite solidification of a primary δ phase and the growth of an acicular γ phase by solid-state transformation were simulated, and the calculation results were found to nearly correspond to those of the experimental microstructure obtained using a liquid-tin quenching method. The MPF method is considered to be highly effective in designing duplex stainless steels and in evaluating the cooling-rate dependence of the microstructure morphology in welding solidification.