Hiromichi T. Fujii

Effect of post-GBE strain-sensitisation on corrosion resistance of grain boundary engineered 304 austenitic stainless steel

Control of grain boundary microstructure for grain boundary engineering (GBE) is effective to prevent intergranular corrosion by disconnection of corrosive random boundary network resulting from the introduction of coincidence site lattice boundaries during the frequent formation of annealing twins in bulk austenitic stainless steels. Since the practical use of austenitic stainless steels often includes straining and heating processes, such as forming, annealing and welding, the processes after the GBE treatment could degrade the optimised grain boundary microstructure and the high corrosion resistance of grain boundary engineered (GBEed) austenitic stainless steels. Therefore the present study examined the effects of post-GBE strain-sensitisation on grain boundary microstructure and corrosion resistance of a GBEed 304 austenitic stainless steel produced by a single-step thermomechanical processing for GBE. The results showed that the grain boundary microstructure was changed apparently and the corrosion resistance gradually decreased in the GBEed steel with an increase in post-GBE strain followed by sensitisation, but the GBEed steel maintained significantly higher corrosion resistance during the post-GBE strain-sensitisation than the equivalently strain-sensitised base steel.

Thermal Transients During Processing of 3003 Al-H18 Multilayer Build by Very High-Power Ultrasonic Additive Manufacturing

Previous investigations suggested a gradient in bond microstructure along the height of a “build” made by very high power ultrasonic additive manufacturing—a rapid prototyping process that is based on ultrasonic seam welding. The bonding of foils is associated with the occurrence of dynamic recrystallization at the interfaces between them. To understand heating patterns across the build that may be responsible for such microstructure evolution, temperatures from different interface regions were recorded simultaneously during the fabrication of a 3003 Al-H18 multilayer build under a given processing condition. Thermal transients were observed over multiple interfaces of the build during welding of each layer. The temperatures were the highest for the layer processed and were found to diminish beneath with each subsequent layer. Such maximum temperatures also depended on the height at which the new layer was bonded. The occurrence of transients across the build is rationalized based on heat being conducted away from the processed layer.

Advanced solder TSV filling technology developed with vacuum and wave soldering

This research investigated advanced filling technology, different from existing technologies, for the purpose of 3D layering on electronic circuits. Filling with molten solder causes a pressure difference between the upper and lower part of the wafer, to overcome surface tension of the through via holes, and then due to pressure difference molten solder is filled into the TSV. The wafer thickness was 100–200μm with holes of diameter 20∼30μm. The TSVs were formed by deep reactive ion etching (DRIE). A wetting layer of Ti/Cu or Au was sputtered on the wall of the TSVs. Due to pressure differences between upper and lower parts, the molten solder filled into the Through Silicon Via (TSV). Vacuum Pressure was between 0.02MPa and 0.08MPa. The filling speed was under 3 seconds, much higher than conventional methods. Cross-sectional micrographs were taken with a field emission second electron microscope (FE-SEM).

Effect of post-GBE strain on weld decay resistance of grain boundary engineered 304 austenitic stainless steel

Abstract Weld decay, a severe type of intergranular corrosion in the heat affected zone (HAZ), is a common and serious problem during welding of austenitic stainless steels. Grain boundary engineering (GBE) has been attracting attention as an effective method to prevent weld decay. Remarkably high resistance to weld decay has been reported for grain boundary engineered (GBEed) austenitic stainless steels. In practical applications, however, deformation and welding after the GBE process during manufacturing may alter the grain boundary character distribution and the resistance to weld decay. This study examined the effect of strain plus welding on the intergranular corrosion resistance in the weld HAZ of GBEed 304 austenitic stainless steel. The results show that the GBEed steel retained a much higher corrosion resistance than as received and non-GBEed steel during the same strain plus welding process, even though for both steels the corrosion resistance exhibited a gradual decrease with increasing strain.

Microstructural Evolution in Dissimilar Joint of Al Alloy and Cu during Ultrasonic Welding

Dissimilar joints between 1050 Al alloy and Cu were prepared by ultrasonic spot welding technique to understand the joint characteristics. The joint strength was evaluated by tensile shear strength test. The joint strength increased with increasing joining energy and the joint produced with sufficiently high energy was fractured at the base metal region of Al alloy. The interface microstructure in Al alloy consists of severely deformed region due to ultrasonic vibration. In addition, the fine and equiaxed grains were observed near the joint interface in the specimens fractured at the base metal. These characteristics were significantly different from the microstructure in the bulk region of Al alloy. In contrast, the microstructure in Cu was hardly changed around the interface after ultrasonic welding. Additionally, thin intermetallic compound layer with the thickness of 40 nm was found to be formed at the joint interface in the specimens fractured at the base metal. Peak temperature during ultrasonic welding was found to be approximately 480 K at the interface, which was measured using embedded thermocouple.

Friction stir welding of grain boundary engineered 304 austenitic stainless steel

“Weld decay” which is intergranular corrosion in the heat-affected zone is a conventional and momentous problem during welding of austenitic stainless steels. The intergranular corrosion is attributed to sensitization. Sensitization by chromium depletion due to chromium carbide precipitation at grain boundaries in austenitic stainless steels cannot be prevented perfectly only by previous conventional techniques, such as reduction of carbon content, stabilization-treatment, local solution-heat-treatment. It has been reported that grain boundary engineering is effective to suppress weld decay by increase in frequency of coincidence site lattice boundaries so as to disconnect random boundary network. Friction stir welding has been also confirmed to be effective to reduce the degree of sensitization in weld decay region due to small thermal affect. In this study, the simultaneous or synergy effect of the two technologies on prevention of weld decay was examined by friction stir welding of grain boundary engineered 304 austenitic stainless steel. The corrosion test demonstrated that the combination of grain boundary engineering and friction stir welding shows a significant suppression of weld decay.

Friction stir welding of single crystal austenitic stainless steel

The practical success of friction-stir welding (FSW) is necessitating a more fundamental understanding of the underlying physical processes. As a result, an approach involving experiments with single crystals deserves particular attention. In this work, electron backscatter diffraction (EBSD) technique was applied to track grain structure evolution and texture formation during FSW of single crystal austenitic stainless steel. Three initial orientations of the single crystal were investigated: //welding direction (WD), //WD and //WD. In all cases, the microstructural development was found to be a complex process. Far from the tool probe, at relatively low temperatures, the microstructural changes commenced from a formation of deformation-induced boundaries. Their boundary traces were often aligned with close - packed {111} crystallographic planes as well as with tool probe column surface. Approaching to the tool probe, the deformation-induced boundaries decreased in space and increased in misorientation producing a specific lamellar-type grain structure. All these observations suggested a development of continuous recrystallization. In the vicinity of the tool probe, however, temperature raised thus promoting grain boundary migration. This eventually produced fine recrystallized grains containing annealing twins. This process was interpreted in terms of discontinuous recrystallization.