Mitsuo Fujimoto

Effect of contamination on microstructure in friction stir spot welded DP590 steel

Abstract In this study, FSSW of DP590 steel was performed using both bare and TiC/TiN coated Si3N4 tool with Ar shielding, and effect of contamination on the microstructure during FSSW was examined. Defect free welds were successfully obtained at all welding conditions. Si, N and O contaminations were clearly found in the weld when bare Si3N4 tool was used without Ar shielding, but the coated tool and Ar shielding could suppress the contaminations. The contaminations caused higher hardness in the vicinity of the pin hole and reduced the weld strength. The present study showed that the prevention of the contaminations could improve the weld strength, especially the cross-tension strength, in FSSW of DP590 steel using Si3N4 tool.

Microstructural analysis of stir zone of Al alloy produced by friction stir spot welding

Abstract Friction stir spot welding (FSSW), which was recently developed as a spot joining technique, has been applied to a lap joint of Al alloy 6061 sheets, 1 mm in thickness, to clarify its microstructural features of the joint. A nugget shaped stir zone having finer grain size was observed around the exit hole of the probe. Crystallographic texture analyses using EBSD method suggested that the material flow occurred along the rotating direction of the FSSW tool in the wide region including the stir zone. In the periphery of the nugget shaped stir zone, which was characterised by finer grain size than the stir zone interior, no inclusions or precipitates were found on the SEM scale. A softened region was formed around the joint centre, which could be explained as resulting from dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSSW.

Analysis of plastic flow of the Al alloy joint produced by friction stir spot welding

This study examined development of the stir zone and plastic flow of the material during friction stir spot joining of aluminium alloys. The development of the stir zone was discussed through the observation of macro sections of the joints produced at various process times. The friction stir spot joining trial with insert of Au foil to lap surface and the dissimilar lap friction stir spot joining showed that the movement of the lap surface was attributed to the downward plastic flow from the upper plate to the lower plate around the probe. Additionally, it was clarified that the direction of the plastic flow was roughly the same as the direction of the tool rotation. The dissimilar butt friction stir spot joining suggested that the plastic flow occurred not only within the stir zone but also in a larger area below the tool shoulder. The present study revealed that the threads on the probe surface were important for producing the plastic flow of thickness direction during friction stir spot joining.

Microscale evaluation of mechanical properties of friction stir welded A6061 aluminium alloy/304 stainless steel dissimilar lap joint

Abstract Microscale evaluation of the mechanical properties of a friction stir welded A6061/SUS 304 grooved lap joint was performed using a microtensile test and transmission electron microscopy. The microtensile test revealed that ∼62% of the area along which the rotating tool passed the specimen was regarded as the bonded region and that the joint was fractured at the A6061 matrix owing to the formation of very thin interfacial reaction layers. Equiaxed aluminium grains were observed at the interface of the specimen after it was fractured, indicating that the interface deformed only slightly during the microtensile test. It should be noted that although the maximum tensile strength of the joint was approximately the same as that of the base alloy, the proof stress of the joint decreased with the dissolution of the β″ phase in the A6061 aluminium alloy.

Microstructural analysis of the stir zone of Al alloy produced by friction stir spot welding

The present study applied friction spot joining (FSJ), which was recently developed as a lap joining technique of Al alloys, to two sheets of Al alloy 6061, 1 mm in thickness, and then examined the microstructural feature in the weld. The weld had the nugget-shaped stir zone around the exit hole of the probe, and the stir zone exhibited the equiaxed grain structure having finer grain size than that of the base material. The crystallographic texture analysis using electron backscattered diffraction method suggested that the material movement occurred along the rotating direction of the welding tool in the wide region including the stir zone. In the periphery of the nugget-shaped stir zone, which was characterized as the region having the finer grain size than that of the stir zone interior, any inclusions and precipitates were not found in the SEM scale. The weld was softened around the weld centre. The softening could be explained by dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSJ.

Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

Abstract The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.

Effects of process time and thread on tensile shear strength of Al alloy lap joint produced by friction stir spot welding

In automotive applications, friction stir spot welding (FSSW) has been practically used in the construction of aluminium car bodies. In this study, the weld strength and factors governing the weld strength in the friction stir spot welded aluminium alloy 6061-T6 were examined. The weld strength increased with the process time during FSSW up to 3 s, beyond which it decreased. The fracture path changed from the lapped interface into the shoulder edge as the process time increased, and the maximum strength was obtained at the process time when the transition of the fracture path occurred. The cross-sectional microstructure depended on the threads on the probe surface, i.e. the elliptical zone was formed in the stir zone by the threads, but an effect of the thread on the weld strength was hardly found. The present study suggests that the weld strength was strongly related to the size of the well-consolidated region, which was larger than the elliptical zone observed in the vicinity of the exit hole.

Development of Friction Spot Joining

This paper describes the fundamental procedure and joint properties of Friction Spot Joining (FSJ), a new spot welding method for aluminium and other light metal alloys. The process uses frictional heat between a joining tool and work piece as the heat source, and produces a solid phase joining by causing plastic flow of the material. Technological essences of the process including the joining tool, a prototypic FSJ equipment, and process parameters were first fundamentally established. Consequently, based on those results, FSJ systems for practical use have been produced. Furthermore, various properties of the friction spot joint including metallurgical structure, hardness distribution, and tensile strength were investigated.

Refill friction stir spot welding of dissimilar materials consisting of A6061 and hot dip zinc-coated steel sheets

Abstract Friction stir welding (FSW) and friction stir spot welding (FSSW) are being paid much attention in automotive and aerospace industries. Especially, in automotive industry, the light weight body composed of multi materials such as aluminum alloys and high strength steel is strongly desired to satisfy the demand for fuel consumption saving, crash safety and reduce the cost for the car. However, conventional resistance spot welding (RSW) forms intermetallic compounds (IMC) that occasionally result in the brittle joint in the weld interface between aluminum alloys and steel. On the other hand, mechanical fastening such as rivet or self-piercing rivet can join dissimilar material, but it needs additional parts which must increase cost and weight. FSSW is expected to inhibit the formation of IMC because it is solid state joining technique. In addition, Refill FSSW, which is newly developed keyhole-less FSSW process, can bring benefits such as high strength and low scatter joints. Because of those reason, Refill FSSW is expected to contribute to produce aluminum-steel hybrid car body. In this study, configuration of 6000 series aluminum alloy and hot dip zinc-coated steel was attempted with Refill FSSW. Sound joint in terms of its appearance and cross sectional examination was obtained. In the microstructure examination, IMC layer in some micro mater thickness was not observed. Hardness distribution showed its reduction in stir zone and outside of stir zone and minimum hardness on aluminum sheet. In lap shear test, Refill FSSW exceeded JIS standard value for RSW. The fracture modes in lap shear tensile test were the shear fracture and the pull out fracture and the fracture modes seemed to relate to shoulder plunging depth, and every specimen was fractured in aluminum sheet.

Microscale Evaluation of Mechanical Properties and the Interfacial Microstructures of Friction Stir Welded Aluminum Alloy/Stainless Steel Dissimilar Lap Joints

The mechanical properties and interfacial microstructure of slices of friction stir welded aluminum alloy/stainless steel dissimilar lap joints were characterized. In an FSWed A3003 aluminum alloy/SUS304 steel lap joint, the strength on the advancing side was larger than that at the retreating side. TEM observation indicated that a sound joint that fractured at the base metal can be obtained from the stage of the formation of the amorphous layer owing to the mechanical alloying effects before the formation of intermetallic compounds. This lap joining technique was also successfully applied to A6061/T6 aluminum alloy-grooved SUS304 plates. Equiaxed aluminum grains were observed at the interface of the specimen after it was fractured, indicating that the interface deformed only slightly during the microtensile test. It was found that tensile strength of the joint was increased by aging at 433K, considering that precipitation occurred at this temperature. In addition, it was confirmed that the joint heated at 723K for 1.8ks still fractured at the aluminum matrix, assuming that intermetallic layers at the interface would slightly grow in this heating condition.