The increasing contents of alloy elements, such as P and Cu, can lead to cracks in the welded joints of weathering steel during conventional welding processes. Friction stir welding (FSW) provides an effective method to join weathering steel at a lower heat input. In the present study, two high-strength weathering steels, high aluminum weathering steel (S-AL) and high phosphorus weathering steel (S-P), were joined using FSW at a relatively low temperature, below 700°C. The microstructure of the welded joints was observed by scanning electron microscope (SEM) and EBSD, the micro-hardness was also measured across the various weld regions. The mechanical properties were then assessed by digital image correlation (DIC). The results suggest that the joints with no defects were obtained for both materials (S-AL and S-P). The microhardness results show slight softening in the stir zone (SZ) of S-AL. Conversely, for S-P, there is noticeable softening in the thermal-mechanical affected zone (TMAZ) and hardening in the SZ. The joint efficiency of the S-AL and S-P reached 100% and 97.1%, respectively. The fatigue strengths of S-AL base metal (S-AL-BM) and its FSW joints (S-AL-FSW) are nearly identical with only a 0.7% difference. Although the fatigue strength of S-P-FSW is 4.7% lower than that of S-P-BM due to softening, the effect is not significant, which also indicates good welding performance.

Silicon nitride is one of the inexpensive and attractive tool materials for friction stir welding (FSW) of steel material. The improvement of silicon nitride tool life has been studied by controlling welding conditions and the tool geometry. In this study, the tool life of a hemispherical silicon nitride tool tilted toward the retreating side during FSW of steel plate was investigated. The defect-free welded SPHC plate was obtained using the hemispherical silicon nitride tool under the optimum welding condition. The tool life was evaluated at tool rotation speeds of 400, 500 and 700 rpm. At 400 rpm, the tool broke at a welding distance of 100 mm. At 700 rpm, the tool wear was observed at the welding distance of 3.0 m, and the tool broke before reaching the welding distance of 6.0 m. In contrast, at 500 rpm, tool wear or tool breakage was not observed up to the welding distance of 10.0 m. The tool life of the hemispherical silicon nitride tool strongly relates to the welding condition. The temperature of the hemispheric silicon nitride tool was 1078 K at the tool rotation speed of 500 rpm. Tool wear depends on the high temperature strength of the silicon nitride. The use of a simple shape such as the hemispherical tool allows FSW to perform at relatively low temperatures, which prevents tool wear and tool breakage.

Friction stir welding (FSW) was applied to a 10 mm-thick plate for the Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy. A sound FSW joint was obtained successfully without macro-defects such as groove-like defects and tunnel holes. However, small pores with diameters of 1–5 μm were formed owing to the wear of the FSW tool during the FSW. The decrease in the heat input suppressed the tool wear. Consequently, the distribution of small pores was limited to the border of the stir zone at the advancing side under smaller heat input conditions. The stir zone of the FSW specimen produced at 125 rpm showed a higher tensile strength of 759 MPa owing to the grain refinement and the high elongation of 50% compared with the base metal. In addition, the stir zone exhibited a remarkable fatigue life of 9,723 cycles. This was higher than that of the base metal (8,908 cycles). Grain refinement occurred by discontinuous dynamic recrystallization (DDRX) via high-angle boundary bulging and direct nucleation in the high-dislocation area. The increase in the heat input suppressed the DDRX owing to the promotion of dynamic recovery.

In this study, neutron diffraction mapping was performed on linear friction welded joints of a 12 mm thick high-phosphorus weathering steel (SPA-H) to evaluate the distribution of residual stress, dislocation density and crystallographic orientation. Linear friction welding (LFW) was conducted under two applied pressures (100 MPa and 250 MPa). The welded interface primarily consisted of refined ferrite with minor retained austenite and martensite, suggesting that peak temperatures during welding exceeded the A₁ point (the eutectoid transformation temperature) and induced reverse transformation to austenite. However, the joint produced at 250 MPa exhibited a lower welding temperature. At the weld region, grains near the specimen surface were elongated along the oscillation direction (OD), whereas equiaxed grains appeared at the center in both thickness and width directions. Inhomogeneous microstructural distributions were observed near the interface along OD. Both joints exhibited high tensile residual stresses in all directions at the weld center, while compressive residual stresses developed near the surface in the direction perpendicular to the weld interface. The applied pressure had minimal influence on the overall residual stress distribution trend within the tested welding conditions. Dislocation density at the weld interface was higher than that in base metal, and the increase was more pronounced under higher applied pressure. This is attributed to suppressed dynamic recovery caused by the lower welding temperature at higher pressure. Finally, strong texture formation was observed at the welding interface due to plastic flow during welding. The applied pressure had only a limited effect on texture development.

In this study, linear friction welding is applied to join JIS-S45C medium carbon steel with ferrite and pearlite structures at temperatures above and below the A₁ point. Additionally, low-strain-rate tensile tests are conducted both in air and with a cathodic hydrogen charge to evaluate the hydrogen-embrittlement susceptibility of the linear friction-welded joints under both joining conditions. Results of hydrogen thermal-desorption analysis show that the hydrogen-charging conditions in this study simulated atmospheric corrosion conditions. The joining zone of the above- A₁ joint comprises fine martensite and ferrite, whereas that for the below- A₁ joint comprises ultrafine ferrite and cementite. In air tensile tests, both joints fractured in the base-metal region, thus suggesting the high reliability of the joints. In the hydrogen-charged tensile test, the above- A₁ joints exhibit premature fracture at the joining zone. By contrast, the below- A₁ joints exhibit base-metal fractures, thus suggesting that the joints are highly reliable in a hydrogen environment. Fracture-surface observations show that the above- A₁ joints exhibit cleavage fractures in the martensite-dominated region. Tensile tests on heat-treated martensite S45C specimens show that their fracture strength decreased significantly in a hydrogen environment. Therefore, the joint fracture is due to the significant decrease in the fracture strength of martensite formed in the above- A₁ joints in the hydrogen environment. The linear friction-welded medium carbon steel joints below the A₁ temperature can ensure reliability even in a hydrogen environment.

Cold spot joining (CSJ), a new joining method using resistance heating, can join metals at solid-state low temperatures without melting, unlike conventional resistance spot welding (RSW), and therefore can solve the problems associated with fusion welding. In this study, 980 MPa-class ultra-high strength dual-phase steel sheets were joined to clarify the difference between CSJ and RSW joints. Since CSJ uses plastic flow to expand the joining diameter, it was confirmed that a larger joining diameter can be obtained and that the joint strength is also higher than that of RSW. Furthermore, since CSJ can be joined at low temperatures, it is possible to avoid the generation of spatter and to produce sound joints that prevent the reduction in joint volume and void defects caused by spatter. In addition, the distribution of HAZ softened regions in joints fabricated by CSJ was investigated, and the effect of the crack propagation direction during cross tensile tests on the fracture morphology was discussed.

A new solid-state bonding technique using local resistance heating was employed for dissimilar bonding of 304 stainless steel to aluminum alloy 5052. The effects of the bonding parameters, including the bonding temperature and time, on joint strength and interfacial microstructure were examined. The joints were successfully produced without macroscopic deformation of base metals within a short time. The joint strength increased to 82 MPa with bonding temperature up to 723 K, corresponding to the joint efficiency of 30%, but significantly decreased at 773 K due to the formation of a thick intermetallic compound (IMC) layer. On the other hand, the joint strength increased with bonding time up to 30 s, beyond which it was saturated. Fracture propagated inside the IMC layer and Al near the interface, and through the unbonded interface during tensile tests. The relationship between thickness of the IMC layer and the joint strength showed that a thin IMC layer (< 2 µm) enhanced the joint strength by reducing unbonded interfaces, whereas thicker IMC layer (≥ 2.5 µm) deteriorated the joint strength due to the brittleness of IMC layer. These findings demonstrate that this new bonding technique enables dissimilar metal bonding with short processing time and minimal deformation of the base metals.

This study investigated low-temperature linear friction welding (LFW) of Ti-17 alloy, performed below the β transus temperature (applied pressure: 550 MPa), and compared its microstructures and mechanical properties with conventional LFW joints fabricated above the β transus temperature (applied pressure: 50 MPa). Both joints were sound without joining defects. At an applied pressure of 50 MPa, the joint interface consisted of equiaxed β grains with a grain size of 5 μm, and Vickers hardness measurements indicated significant softening at the interface region. In contrast, at 550 MPa, the joint interface contained ultrafine equiaxed β grains having a grain size of 0.7 μm with a small fraction of α phase, exhibiting hardness comparable to the base material. Tensile tests showed that at 50 MPa, fracture occurred at the softened joint interface region, whereas at 550 MPa, fracture occurred in the base material, indicating the improved joint efficiency of about 100%. In cyclic fatigue tests of specimen at 550 MPa, the joint fractured at the interface but exhibited a fatigue life nearly equivalent to that of the base material. In cold dwell fatigue tests, the joint at 550 MPa exhibited the base material fracture, and the fatigue life was extended by a factor of 3.2 compared to the base material. These results demonstrate that the low-temperature LFW joint (550 MPa) possesses superior mechanical properties compared to conventional LFW joints (50 MPa). This improvement highlights its potential applicability to high-performance aerospace components such as blisks, where enhanced strength and fatigue resistance are critical.