Showing papers by "Hiroyuki Kokawa published in 2005"

Characteristics of the kissing-bond in friction stir welded Al alloy 1050

Abstract: Initial oxide layer on the butt surface fragments during friction stir welding (FSW) often remaining as a faint zigzag-line pattern on the cross section. When remnants of the oxide layer often adversely affects the mechanical properties in the weld, it is called as “kissing-bond”. The present study systematically examines the effect of oxide array on the bend property in the root region of friction stir (FS) welded Al alloy 1050 by transmission electron microscopy (TEM) to clarify the identity of “kissing-bond”.

Novel production for highly formable Mg alloy plate

Abstract: The principle and advantages of multi-pass friction stir processing (FSP) for the production of a highly formable Mg alloy, and some convincing experimental results are reported in this paper. FSP is a solid state processing technique which involves plunging and traversing a cylindrical rotating FSP tool through the material. FSP achieved grain refinement and homogenization of the as-cast microstructure in Mg alloy AZ91D. Multi-pass FSP produced a fine homogeneous microstructure having a grain size of 2.7 μm throughout the plate. The plate containing this FSPed microstructure exhibited fracture limit major strains six times larger than the diecast plate in the fracture limit diagram (FLD). The present study shows that multi-pass FSP is an efficient production method for a large-scale plate of a highly formable Mg alloy.

Microstructural characterisation of stir zone containing residual ferrite in friction stir welded 304 austenitic stainless steel

Abstract: Friction stir welding was applied to a 2 mm thick 304 austenitic stainless steel plate The microstructural evolution and hardness distribution in the weld were investigated The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively, which are typically observed in friction stir welds in aluminium alloys The hardness of the SZ was higher than that of the base material and the maximum hardness was observed at the TMAZ The higher hardness at the TMAZ was attributed to high densities of dislocations and subboundaries Microstructural observations revealed that the ferrite was formed along grain boundaries of the austenite matrix in the advancing side of the SZ It is suggested that the frictional heat due to stirring resulted in the phase transformation of austenite to ferrite and that upon rapid cooling the ferrite was retained in the SZ

Low-temperature growth of high-quality lead zirconate titanate thin films by 28 GHz microwave irradiation

Abstract: Pb(ZrxTi1−x)O3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The elevated temperature generated by microwave irradiation to obtain the perovskite phase is only 480 °C, which is significantly lower than that of conventional thermal processing. X-ray diffraction analysis indicated that the PZT films crystallized well in the perovskite phase. A scanning electron microscopy image showed that the film has a spherulite grain structure and most of the grains are approximately 2 μm in size. The average values of the remanent polarization, coercive field, dielectric constant, and loss of the PZT films are 40μC∕cm2, 50 kV/cm, 1100, and 004, respectively. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures.

Weld decay-resistant austenitic stainless steel by grain boundary engineering

Abstract: This paper presents an example of grain boundary engineering (GBE) for improving intergranular-corrosion and weld-decay resistance of austenitic stainless steel. Transmission and scanning electron microscope (TEM and SEM) observations demonstrated that coincidence site lattice (CSL) boundaries possess strong resistance to intergranular precipitation and corrosion in weld decay region of a type 304 austenitic stainless steel weldment. A thermomechanical treatment for GBE was tried for improvement of intergranular corrosion resistance of the 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small roll-reduction. The corrosion rate was much smaller in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface. The optimized 304 stainless steel showed an excellent resistance to weld decay during arc welding.

Microstructures in Friction Stir Welded 304 Austenitic Stainless Steel

Abstract: Friction stir welding was applied to 304 austenitic stainless steel. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-grains. Electron microscopic observations revealed that ferrite and sigma phase were formed in austenite matrix in the SZ depending on the cooling rate during FSW.

Nitrogen desorption by high-nitrogen steel weld metal during CO2 laser welding

Abstract: Nitrogen desorption by high-nitrogen steels (HNSs) containing 0.32 and 0.53 pct nitrogen during CO2 laser welding in an Ar-N2 gas mixture was investigated and the obtained data were compared with those for arc welding and at the equilibrium state predicted by Sieverts’ Law. Although the nitrogen content in the weld metal during CO2 laser welding was lower than that in the as-received base material in all conditions, the nitrogen desorption was larger in the top part of the weld metal than in the keyhole region. The nitrogen desorption in the Ar atmosphere was less during CO2 laser welding than during arc welding. With the increase in nitrogen partial pressure, the nitrogen content in the weld metal sharply increased during arc welding, but only slightly increased during CO2 laser welding. The nitrogen absorption and desorption of the HNS weld metal were much smaller during CO2 laser welding than during arc welding.

Effect of grain boundary character distribution on stress corrosion cracking behavior in austenitic stainless steels

Abstract: The effect of grain boundary character distribution (GBCD) on intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels in high temperature water was verified experimentally. GBCD control using the strain annealing method increased the fraction of low- S coincidence site lattice (CSL) boundaries and the segmentalized network of random grain boundaries in austenitic stainless steels. The fractions of low- S CSL boundaries of GBCD controlled steels were 75–85%, while those of uncontrolled steels were 60–70%. Creviced bent beam tests were conducted at 561 K in pure water containing 8 ppm dissolved oxygen for stress corrosion cracking (SCC) evaluation. The tests revealed that GBCD control suppressed IGSCC initiation or propagation and that cracks were predominantly propagated along random grain boundaries. It is considered that induced lower- S CSL boundaries result in high resistance to IGSCC.

Microstructures and properties of friction stir welded 304 austenitic stainless steel

Abstract: Friction stir welding (FSW) is a solid phase joining process which makes use of friction and was developed in 1991 by TWI (The Welding Institute) of Britain.1 This welding process received attention as a joining process to solve the problems associated with fusion welding of Al alloys and active research into the applications of this technique has been progressed by numerous research institutions, at home and abroad. As a result, this process was made fit for practical use in less than ten years of development and has been applied extensively in the manufacture of aerospace equipments,2 rolling stock3 and automobile components.4

Corrosion Properties in Friction Stir Welded 304 Austenitic Stainless Steel

Abstract: A 304 austenitic stainless steel was friction stir welded using polycrystalline cubic boron nitride (PCBN) tool. Relationship between corrosion properties and microstructure was examined in the weld using several corrosion tests and microstructural observation techniques. The stir zone (SZ) had better corrosion properties than the base material (BM). The corrosion tests showed that the intergranular corrosion was developed in the heat-affected zone (HAZ) compared to the BM and SZ although the grain boundaries were not severely corroded. TEM/EDS analysis revealed that Cr depletion zone near grain boundary in the HAZ was shallow and narrow. Friction stir welding (FSW) suppressed sensitisation in the HAZ, which could be explained by short duration at sensitisation temperatures during welding. On the other hand, many grain boundaries were deeply corroded in the AS, where the corrosion resistance was significantly degraded. The microstructural observation revealed that sigma phase was formed in the AS during FSW. Sigma formation produced the wide and deep Cr depletion zone with the minimum Cr content less than 12 wt % in the vicinity of the grain boundary in the AS, which severely deteriorated the corrosion resistance in the AS.

Electrical Properties and Microstructures of Sol-Gel-Deposited Lead Zirconate Titanate Thin Films Crystallized by 28 GHz Microwave Irradiation

Abstract: Pb(ZrxTi1-x)O3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The crystalline phases and microstructures as well as the electrical properties of the microwave-irradiated PZT films were investigated as a function of the elevated temperature generated by microwave irradiation. X-ray diffraction analysis indicated that the PZT films crystallized well into the perovskite phase at an elevated temperature of 480°C by microwave irradiation. Scanning electron microscopy images showed that the films had a granular grain structure and most of the grains were approximately 1.5 µm in size. With increasing elevated temperature from 480°C to 600°C by microwave irradiation, the breadth of grain boundaries of the films became narrow and the remanent polarization of the films increased slightly. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures in a short time.

Kissing-Bond Characteristics in a Friction Stir Welded Aluminum Alloy by Transmission Electron Microscopy

Abstract: Initial oxide layer on the butt surface fragments during friction stir welding (FSW) and then often remains as a faint zigzag-line pattern on the cross section. When remnant of the oxide layer often adversely affects the mechanical properties in the weld, it is called as "kissing-bond". The present study systematically examines effect of oxide array on bend property in the root of friction stir (FS) welded A1 alloy 1050 by transmission electron microscopy (TEM), and then clarifies identity of the kissing-bond.