Solidification cracking and liquation cracking susceptibility of four kinds of alloy 617 commercial filler metals were evaluated by Varestraint test, in order to clarify a fundamental effect of each solute element in the alloy. Fracture surface of cracking in alloy 617 multipass welds by GTA welding exhibited a possibility of occurrence of both solidification cracking and liquation cracking. Brittle temperature ranges (BTR) of the liquation cracking at each alloy were almost same, however the BTRs of solidification cracking indicated significant difference. Complex carbide composed carbon, titanium and niobium was formed in the weld metal in the case of the smallest BTR for the solidification cracking, in other words alloy 617 including a lot of titanium and niobium indicated a good susceptibility for the solidification cracking. Numerical simulation of solidification segregation of alloy 617 was carried out to reveal the solidification behavior at terminal stage of the alloy. The solidification segregation analysis suggested that solidification cracking susceptibility was attributed the carbide formation depending on the solidification segregation of carbon, titanium and niobium. In the alloy containing titanium and niobium, the carbide formation occurred at the terminal stage of the solidification. Therefore, solidification completed temperature increased because the amount of carbon in residual liquid decreased after the carbide formation.
Laser ultrasonic technique (LUT) is applicable for measurement under high temperature condition because of the non-contact and remoteness. In this study, LUT was applied to detection the welding defect such as solidification crack and/or lack of penetration at single bevel groove during Gas Metal Arc (GMA) welding process. Nanosecond pulse laser (1064nm, 60mJ/pulse 100Hz) was used to generate ultrasonic, and was scanned using a galvanometer mirror. Nd:YAG laser interferometer (532nm, 1W) was used for detection the surface micro vibration caused by arrival of ultrasonic wave. After acquiring the B-scope, which is composed of multi transmit- and receive-points, the synthetic aperture focusing technique (SAFT) is applied. SAFT image obtained by in-process measurement was evaluated to confirm the presence or absence of welding defect indication. As a result, it was possible to indicate the solidification crack and the lack of penetration in the joint even during the welding process. Furthermore, it was able to indicate the penetration depth change (2mm difference) due to the welding condition change during process. These results suggest that in-process measurement using LUT is very effective technique for detecting weld defects.
In this study, fatigue crack propagation under the condition where the superimposed stress history with different frequency components was given intermittently was investigated by numerical simulation and fatigue crack propagation test. The numerical simulation of fatigue crack propagation used in this study can consider the opening and closing behavior of fatigue crack and implement stress history extraction algorithm that contributes to fatigue crack propagation under superimposed stress history with different frequency components. An advanced fracture mechanics approach based on the RPG (Re-tensile Plastic zone Generating) stress criterion to identify the effective stress intensity factor range was applied to estimate the fatigue crack propagation history. Fatigue crack propagation test under superimposed stress history containing different frequency component appearing intermittently were also conducted.Validity of our applied numerical simulation method was investigated by comparing numerical simulation results with experimental ones. As a result, the validity of our proposed numerical simulation procedure to estimate the fatigue crack propagation history under such a complicated applied stress history was confirmed.
Heat source characteristics in the arc welding process such as temperature distribution of arc plasma, the current and heat input densities to the base metal is very important since they are one of the upstream factors of the process. Many studies have reported the distribution of temperature and metal vapor concentration in welding arc plasma by emission spectroscopy, and the distribution of heat input density to the base metal by split copper anode plate method. These studies generally require the axisymmetric assumption of the arc plasma. However, actual welding arcs are not limited as axisymmetric phenomenon. For example, tilted TIG arc welding is often performed due to the visibility and some reasons, resulting in a non-axisymmetric phenomenon in production sites. In order to measure such target, a tomographic approach is necessary. In this study, multidirectional measurement system with rotary split copper anodes was constructed to measure the non-axisymmetric distribution of the current and heat input densities of tilted TIG arc plasma. Integral values are measured from 4 directions and converted into a two-dimensional distribution by the image reconstruction method. It was experimentally revealed that as the tilting angle increases, the maximum value of current and heat input density decreases and the low density region of the distribution spreads toward the aiming direction. Although the measurement method using split anode including this study possesses the problem of arc deflection the insulating gap of split part, we found the fitting process on the measured data is useful to reduce this effect.
For lightweight materials such as aluminum and magnesium alloys, joining technology called “multi-material” technology is required to achieve weight reduction. When joining dissimilar materials in arc welding, there are problems such as the formation of intermetallic compounds. FSW can solve these problems. However, FSW has a problem that defects such as root flow occur due to changes in bonding conditions. In order to suppress defects, it is necessary to optimally control the material flow and to stir the materials to be joined in a coordinated manner, and the shape of the welding tool directly affects this. In this study, we tried to produce a healthy friction stir weld of aluminum and magnesium alloy sheet using a newly designed tool called “double-spiral tool” that can increase the volume of the stir zone around the tool. did. The materials used were A5083 aluminum and AZ31 magnesium alloy sheets with a thickness of 3mm. Two types of tools were used: normal type with M4 to M3 screws (single-spiral tool) and double-spiral type with the same pitch and double lead length. The latter can transport theoretically twice as much metal along the tool axis compared to the former. From this, it can be expected to prevent welding defects by increasing the material flow rate. In this study, we designed and manufactured a “double-spiral tool” with an effect of increasing the material flow rate, and demonstrated that it leads to improved reliability of FSW joints made of dissimilar aluminum and magnesium alloys. It became clear that the double spiral tool effectively enhanced the agitation action and the area of the agitation zone was stacked more densely. The tensile test at room temperature also showed that the double spiral tool has a high merit, that is, high tensile strength.
Cold cracking sensibility is severe problem in the welding fabrication of large-scale steel structures. It is well-known that diffusible hydrogen content in the weld metal strongly influences this cracking. In the recent years, a novel welding process using special welding torch was researched and developed. By using this process, amount of hydrogen content in the weld metal can be highly reduced than the conventional process. In this study, the effect of cold cracking prevention and required preheating temperature reduction by new welding process were confirmed. The window type restraint weld cracking test was applied. Both base metal and welding material were 780MPa class high-tensile steel. During the test, the preheat and interpass temperatures were controlled strictly. Moreover, the temperatures level was changed from 50 to 0℃. As a result, the cold cracking (transverse crack) was occurred with the conventional method under 50℃ temperature controlling. In contrast, there is no crack with the developed one under 50 and 25℃. When the controlled temperature was lowered to 0℃, many cracks were observed with the developed process. From the above, the effect of cold cracking prevention by the novel welding process for reducing diffusible hydrogen were confirmed and about 50℃ preheating temperature reduction effect were demonstrated. These test results were confirmed the consistency with past research results. A very good agreement was obtained when comparing the test results and the past research results of cold cracking control indexes in terms of root cracking and multi-pass weld transverse cracking. Therefore, the validity of this study was demonstrated.