Laser peening can introduce compressive residual stress on the surface of various materials and, therefore, is effective in enhancing the fatigue strength. This study clarifies whether the laser peening with lower pulse energy comparing to the preceding studies generates compressive residual stress, and whether such stress would account for prolonged fatigue life in the welded zone of high-strength steel HT780. As a result, laser peening condition, which can generate large compressive residual stress on the base metal surface, were selected under the pulse energy of 6, 10 and 20mJ, respectively. With the reduction of the pulse energy, it was observed that the depth of the compressive residual stress tended to decrease. Fatigue lives of the toe of the butt welded joints pretreated by laser peening with the selected conditions were prolonged to the same level of fatigue lives with the pulse energy of 200mJ.
Aluminum (Al) alloy, A6061-T6, plate and stainless steel, SUS304, plate with different thicknesses were butt welded by a friction stir welding (FSW) process to fabricate tailored blanks. The Al plate was thicker than the steel plate. The FSW tool was offset to the Al side and the probe was inserted only into the Al plate. The shoulder of the tool was also plunged only into the Al plate because of the different thicknesses of the plates. The microstructural observation and X-ray CT revealed that small steel fragments were dispersed in Al-side stir zone (SZ) due to the scratching of steel side surface by the probe. The hardness of SZ in Al side was lower than the base metal due to the dissolution of hardening precipitates during welding, where the lowest hardness located near the thermo-mechanically affected zone (TMAZ) in Al side. The tensile failure occurred at TMAZ because of the bonding strength between Al and steel was higher than TMAZ. However, the tensile strength of the dissimilar weld was approximately 38% lower than that of the aluminum alloy base metal due to the softening at TMAZ. Post heat treatment increased the hardness at TMAZ, whilst the tensile strength was still about 33% lower than the base metal.
If the coating of the outer surface of a buried pipeline is damaged by some cause and its cathodic protection is excessive, an electrochemical reaction can produce hydrogen. In order to maintain the load transfer function of a pipeline circumferential weld joint in a hydrogen environment as well, the critical hardness at which a constant strength level is ensured during tensile load was clarified by Slow Strain Rate Testing (SSRT). In cases where the hardness was greater than 320HV, the maximum tensile stress was seen to fall sharply in the hydrogen environment, while in cases where hardness was 320HV or lower, the maximum tensile stress fell approximately only 10% regardless of the welding method. The maximum tensile stress fell sharply when the major material of the pipe was material with a relatively hard structure called lath martensite or bainite. The contribution of upper bainite to the fall of the maximum tensile stress was about 1/3.5 of that of lath martensite. From the results of measurement by EBSD, the greater the material's local misorientation, which is a value corresponding to dislocation density, the more conspicuous the fall of the maximum tensile stress under the impact of the hydrogen. It is assumed that the dislocation that exists in steel is an important cause of hydrogen embrittlement, so the empirical tendency-the harder the material, the higher its hydrogen embrittlement susceptibility-has been revealed to be caused by the fact that the harder the material, the greater its internal dislocation. Judging from the above results, if the hardness is 320HV or lower, a constant strength level is maintained under a hydrogen environment regardless of the welding method or the metallic structure.
The friction stir welding (FSW) is well known as the solid-state joining. Metal materials are not melted during FSW. This paper describes a novel numerical model and calculation method for visualization of material flow during FSW process on A1100 plates. In this numerical model, the particle method as Lagrange approach is mainly employed. In this paper, the numerical model is applied to two FSW processes, and tool's shape change are taken into account. The difference of the material flow around the tool is discussed.