Shoichi Nambu

Interphase Strain Gradients in Multilayered Steel Composite from Microdiffraction

Multilayered steel composites consisting of alternating martensite and austenite layers and exhibiting a combination of high strength and ductility were successfully fabricated. To understand the microplasticity mechanisms responsible for such exceptional mechanical behavior, 3D X-ray microscopy with a submicron beam size was employed to probe the stress/strain distribution within the top two layers during incremental tensile loading. The 3D depth-dependent strain gradients were monitored in situ near the martensite/austenite interfaces as a function of the load level. It was observed that the strain gradients redistributed during loading. Specifically, large compressive strains developed in the top martensite layer transverse to the loading direction, while small tensile strains were found across the layer interface into the underneath austenite layer.

Friction stir welding of multilayered steel

Abstract This study investigates the mechanical properties and microstructure of friction stir butt welded high strength/ductility multilayered steel consisting of 15 alternating layers of SUS 301 austenitic stainless steel (eight layers) and SUS 420J2 martensitic stainless steel (seven layers) with a total thickness of 1·2 mm. With optimised welding parameters, defect free welds with an ultimate tensile strength (UTS) of 1240 MPa and a fracture elongation of 13% were accomplished. This corresponds to a joint efficiency of 90%. In this case, fracture occurred in the heat affected zone as a result of a very pronounced hardness drop in the martensitic layers resulting from the formation of a large amount of grain boundary precipitates, which were formed at temperatures ∼750°C slightly below Ac1. By applying post-weld heat treatment, the hardness drop in the martensitic layers was removed and the tensile properties were enhanced to UTS of 1310 MPa (95% joint efficiency) and a fracture elongation of 22%.

Investigation on the mechanism of steel/steel solid-state bonding at low temperatures

The solid-state bonding of ultralow-carbon steels was conducted by hot pressing and subsequent isothermal holding at low temperatures ranging from 873 to 923 K. The evolution of the interfacial strength was found to consist of two stages; the first stage, where the increase in interfacial strength is rapid and significant, and the second stage, where the increase is gradual. The evolution of strength in the first stage primarily takes place in contact regions produced by hot pressing. In the second stage, on the contrary, the evolution seems to result from the increase in the contact regions due to the shrinkage of voids. A molecular dynamics simulation was performed to clarify the atomic behaviour at the interface during the first stage. The results revealed that the disordered atomic arrangement caused by compression was rearranged with increasing isothermal holding time, leading to improved coherency between the contact regions and increased interfacial strength.

Effects of compressive strain on the evolution of interfacial strength of steel/nickel solid-state bonding at low temperature

ABSTRACT Solid-state bonding between ultralow-carbon steel and pure nickel was conducted by hot pressing with various compressive strain ranging from 5 to 15% and subsequent isothermal holding at 923 K. It was found that the interfacial strength of contact area is accounted for by the evolution of the intrinsic strength of the interface and the amount of plastic energy dissipation at the crack tip during interface fracture. The compression induces severe deformation around the interface and consequently inhibits the plastic energy dissipation during interface fracture. In the first stage of isothermal holding, the residual strain around the interface on the steel side is reduced by recovery process, which concurrently decreases in the yield stress of the area adjacent to the interface. This promotes plastic energy dissipation of the area, leading to a significant increase in interfacial strength in the first stage.

Development of a bonding interface between steel/steel and steel/Ni by ultrasonic welding

ABSTRACT Ultrasonic welding is a solid-state welding technique that can bond materials at a relatively low temperature and pressure. In this study, steel/steel and steel/Ni combinations were successfully bonded by ultrasonic welding, and the development of the bonding interface was examined. The bonding strength was obtained by a lap shear test and increased with welding time, as did the fraction of bonded area observed by SEM. The bonding process sequence was investigated by SEM and electron backscatter diffraction (EBSD) analysis of a cross-section at the bonding interface. It was revealed that abrasion is caused by oscillation to form small particles consisting of steel and Ni and that the particles are grown and subsequently flattened with welding time. Bonding is achieved by the flattened particles spreading along the bonding interface without any voids.

Evolution of bonding interface during ultrasonic welding between steel and aluminium alloy

ABSTRACT Ultrasonic welding (USW), a solid-state bonding technique, was applied to join ultra-low carbon steel and Al5052 aluminium alloy. The evolution of bonding strength and microstructure at the bonding interface during ultrasonic welding was investigated with an emphasis on the early stage before the formation of intermetallic compound. Initially, adhesive wear starts when steel and Al alloy have sliding contact, and thereafter, a thin layer of Al is attached onto the surface of the steel while the wear of Al base metal is continued owing to the sliding. As the welding time increases, bonding sections are gradually formed with the increase of interfacial temperature owing to the sliding friction, whose process is illustrated in this article.

Three-dimensional quantification of texture heterogeneity in single-crystal aluminium subjected to equal channel angular pressing

Abstract A three-dimensional crystal plasticity finite element method (3D CPFEM) modelling of a real equal channel angular pressing (ECAP) process for investigating the mechanical properties and texture evolutions of single-crystal aluminium has been developed for the first time. The challenge of modelling such a severe plastic deformation via 3D CPFEM is how to accurately predict the deformation mechanism under the complicated contact conditions between a billet and a die. The validation by comparison with experimental observations demonstrates that the developed 3D CPFEM ECAP model is able to precisely capture the deformation characteristics at the microscale. Furthermore, this research clarified the previously remaining disputes such as the microstructural formation mechanism in the deformed area and the deformation nature in the plastic deformation zone. It is also the first time to extensively discuss the orientation-dependent deformation feature of the ECAP-processed billets, including morphology, lattice rotation angle and grain refinement.

Crack Growth Characteristics of Pure Copper for Smart Stress Memory Patch

A new sensing method called “smart stress memory patch”, which could estimate the maximum stress, the stress amplitude and the fatigue cyclic number simultaneously using Kaiser effect of Acoustic Emission (AE) and crack length of this patch, was developed. In this study, the crack growth characteristics of this patch was evaluated. Pure copper was used for this patch because its good corrosion resistance, stable crack propagation and so on. Two kinds of samples which were rolled and electrodeposited copper were prepared to investigate the effect of microstructure on crack growth behavior. Fatigue test was performed under constant stress amplitude to evaluate the crack growth behavior using the relationship between stress intensity factor range and crack propagation rate. The scattering in fatigue crack growth was also investigated to obtain the relationship between crack length and the fatigue cyclic number including two-sided 95% confidence interval. The effect of thickness and grain size on the scattering was discussed. Finally, good crack growth behavior was obtained and the fatigue cyclic number could be estimated by this patch.

Development of Smart Stress Memory Sensor Using AE Kaiser Effect

Reliability of structures is an important task to ensure the ease and safety of our life, and further development of non-destructive evaluation for structures such as bridges and tunnels is required. Some fatigue sensors that consist of sacrificed specimen have been developed to evaluate the fatigue damage of structures such as fatigue cyclic number and residual lifetime. However, these fatigue sensors can be used only when the applied stress amplitude is known. We tried to develop a new smart stress memory patch that measured both maximum stress and number of fatigue cycles simultaneously using Kaiser effect of Acoustic Emission (AE) and crack length. In this study, the characteristics of the smart patch was evaluated. Pure copper was used for this sensor because its good corrosion resistance, stable crack propagation and detectability of AE near yield point. Fatigue test was performed under the constant stress amplitude to evaluate the crack propagation behavior using the relationship between stress intensity factor and crack propagation rate. The obtained curve between crack length and number of fatigue cycles by these crack propagation behavior was in good agreement with experimental results. AE measurement after some fatigue tests was performed and AE was detected at the applied fatigue stress. These results demonstrated that number of fatigue cycles and the maximum stress could be measured by this fatigue sensor.

Evaluation of Mechanical Properties of Alumina Green Compact during Sintering at Relative Low Temperature

Recently, ceramics was used extensively as structural materials and ceramics components became larger and more complex. Fracture sometimes occurs during firing because of large and complex shape, and this fracture interrupts manufacturing process. The simulation of sintering has been studied to prevent this fracture. However, it was difficult to simulate fracture process because there was little data on strength of green compact. It is necessary to measure strength during sintering in order to perform a useful simulation. In this study, we measured strength of two kind of alumina green compact during sintering. Three point bending test at elevated temperature was performed and strength was estimated at each temperature. A model for strength at relative low temperature was also proposed using the temperature dependence of specific surface area. Furthermore, fracture toughness test was performed and the relationship between strength and fracture toughness was obtained. Strength at relative low temperature increased with temperature. Fracture toughness was proportional to strength at the temperature range where materials demonstrated brittle fracture manner. Strength of each alumina was analyzed using this model.