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Low alloy TRIP steel is expected to be applied to automobile bodies because of its high strength, high ductility, and excellent impact properties and press formability. It has been reported that the low alloy TRIP steel of hydrogen embrittlement resistance is improved by utilizing the hydrogen storage characteristics of highly stable retained austenite. Therefore, for the purpose of increasing the volume fraction of retained austenite, it was produced at various cooling rates below the martensite transformation start temperature. As a result, the volume fraction of retained austenite increased, and then the effect of hydrogen embrittlement decreased. The matrix phase and retained austenite is refined with decrees of the cooling rate. It is considered that the size and surface area of the retained austenite also affected the improvement of hydrogen embrittlement resistance.

Spinning is a type of plastic forming in which a tool such as a roller is pressed against a rotating plate or tube to gradually deform the shape and obtain a product with the same shape as the forming die. This processing method has the advantage that it can be processed seamlessly but has the drawback of causing internal defects due to deformation. In this study, the purpose is to obtain basic knowledge about the internal defect generation mechanism in spinning, and to perform cold and hot spinning with high diameter reduction under various conditions on Al-Mg-Si alloy tube. From the experimental results, it was confirmed that the inner defects increased as the diameter reduction ratio increased. It was considered that the main cause of the occurrence of inner defects was that, at a high diameter reduction ratio, the amount of processing was large, so that the increase of material flow led to increase of wall pressure.

Various high strength steel sheets for weight reduction and safety improvement of vehicles have been developed. TRIP-aided steel with transformation induced plasticity of the retained austenite has high strength and ductility. Conventional TRIP-aided steels are subjected to austempering process after austenitizing. Generally, elongation and formability of TRIP-aided steel are improved by finely dispersed retained austenite in BCC phase matrix. The finely dispersed retained austenite and grain refinement of TRIP-aided steel can be achieved by hot rolling with heat treatment. Therefore, the improvement of mechanical properties of TRIP-aided steel is expected from the manufacturing process with hot rolling and then isothermal transformation process. In this study, thermomechanical heat treatment is performed by combining hot rolling and isothermal holding as the manufacturing process of TRIP-aided steel sheets. The complex phase matrix is obtained by hot rolling and then isothermal holding. Although the hardness of the hot rolled and isothermal held TRIP-aided steel is decreased, the volume fraction of retained austenite is increased.

The 7000 series aluminum alloys suffer from intergranular fracture (IGF) that limits the use of the alloys, although they have highest strength among aluminum alloys. The types of IGF can be classified into two categories: (i) with smooth fracture surface showing practically no plastic deformation that takes place in hydrogen embrittlement and stress corrosion cracking, and (ii) with shallow and fine dimples on the fracture surface showing localized plastic deformation inside precipitate free zones. In this study, attempts have been made to suppress the IGF of both types by (a) controlling precipitate microstructure on grain boundaries by quench control and (b) controlling grain boundary morphology by strain induced boundary migration. The IGF of type (i) (hydrogen embrittlement) was successfully suppressed both by the two controlling processes.

Behavior of hydrogen in tensile-deformed Al-9mass%Mg and Al-5.8mass%Zn-2.4mass%Mg alloys was investigated by means of hydrogen microprint technique, HMPT, a method to visualize the microscopic location of hydrogen evolution from specimen surface as silver particles. Both in the two alloys, surface relief was formed at most grain boundaries by the stretching, while hydrogen evolution was observed at some grain boundaries. The evolution of hydrogen was discussed with parameters such as the angle between grain boundary on the specimen surface and tensile direction, the angle between grain boundary on the surface and slip line inside the grain, the height of the surface relief, and maximum gradient of the surface relief. The results indicated that the shear deformation along grain boundary caused transportation of hydrogen atoms with gliding dislocations to the surface, breakage of surface oxide film. In the Al-Zn-Mg alloy, it was suggested that the preferential deformation in the precipitate free zone was attributed to hydrogen evolution.

The use of duplex stainless steels (DSSs) is steadily increasing. For many uses where joining is needed, gas tungsten arc welding (GTAW) is one of the most important joining methods for DSSs. Since hydrogen embrittlement (HE) occasionally occurs in DSSs, understanding the relationship between the extent of HE and the welding condition is crucial to prevent HE. In this research, the effect of the heat input of GTAW process on the microstructure and the extent of HE in a UNS S31260 (JIS SUS329J4L) has been investigated. For this purpose, three samples have been prepared with diffrent velosity. All the samples have been cathodically hydrogen-charged, and then subjected to tensile test at a strain rate followed by fractography observation. Thermal desorption spectroscopy (TDS) has been carried out on the samples welded at low and average velosities. The results showed that tensile properties of the welded specimens were lower than those of base metal due to coarsening of the matrix ferrite grains and loss in the fraction balance of ferrite and austenite phases in the weld metal zone, where fracture took place.

Tensile properties at room and elevated temperatures of AZX612 alloy sheets processed by a kind of lateral extrusion method namely Friction-Assisted Extrusion (FAE) were investigated. The FAE was developed to control the texture, and carried out at temperatures ranging from 250 to 350°C with an extrusion ratio of 4 from the as-rolled condition. The results showed that FAE changes the basal texture of the as-rolled sheet into that inclined by about 15˚ against the extrusion direction and raises the intensity of the texture. It was observed that the significant microstructure refinement from as-rolled condition of 10.8μm to 4.7μm after FAE due to dynamic recrystallization at the extrusion temperature of 250°C. The 0.2% proof stress of the FAEed sheets at room temperature became significantly smaller than that of the as-rolled sheet in the extrusion direction but became larger in the transverse direction, resulting in the larger anisotropy. This can be understood by the activity of basal slip. The anisotropy of the tensile properties disappeared at a temperature of 300°C and an initial strain rate of 3.3×10-4s-1. In addition, the elongation was improved from 72% of the as-rolled sheet to 152% at maximum of the FAEed sheets in the extrusion direction. This improvement was attributable to superplastic flow based on grain boundary sliding.

Spinning is a processing method in which a metal material such as a plate or pipe is pressed against a roller tool while rotating material with a forming die. Hot spinning processing is used for the production of high pressure hydrogen storage container. As an aluminum liner material for a high pressure hydrogen storage container, an Al-Mg-Si alloy is used from the viewpoint of hydrogen embrittlement resistance and strength. However, wrinkles and cracks generated during the formation of the liner material by hot spinning may cause breakage of the container. In this study, hot spinning is performed on Al-Mg-Si (A6063) alloy by changing processing conditions and processing temperature. After that, the effect of processing conditions and processing temperature on surface texture and wall thickness change was investigated. The results are followings, if the number of passes is large, the unevenness of the inner surface is small, but the length of the processed portion increases and the wall thickness decreases. On the other hand, when the temperature is high, the unevenness on the inner surface become large, the length of the worked portion hardly increases, and the wall thickness increases.

Spinning processing is used to produce the seamless liner of high-pressure hydrogen gas container. Currently, Al-Mg-Si alloys are applied for the hydrogen high-pressure gas container, where the formation of surface defects such as wrinkles and cracks on the inner surface of the liner is a crucial problem. In this study, spinning is performed on Al-Mg-Si (A6063) alloy by changing processing conditions. Then, surface defects and texture of test pieces affected by processing conditions. The results are followings, in the larger number of spinning pass, good surface properties can be obtained on the contacting surface of the roller. On the other hand, in the smaller number of pass, irregularities on the inner surface, the length of the processed portion, and the reduction of thickness are decreased. The decrease of irregularities on the inner surface may be caused by a small number of continuous deep drawing processes without decreasing thickness.The irregularities on the inner surface of test pieces are reduced by using the forming die.

Paper has been retracted. Welding parameters such as current intensity, voltage, number of passes can affect the mechanical properties of the weld. In this paper the effect of these parameters on structure and mechanical properties of welded A105 and A106 steels has been evaluated. According to the mechanical and microstructure test results, increasing in welding pass number causes reduction in grain size and increasing in average hardness of HAZ. Also inter-pass slag inclusion defect occurred in high number of passes.

Hydrogen in aluminum has been known to be the cause of blister and pore. Some aluminum alloy is susceptible to stress corrosion cracking, which is based on intergranular cracking arisen from hydrogen embrittlement. The behavior of hydrogen in aluminum has not been fully understood yet. Hydrogen gas plasma enables to introduce high hydrogen concentrations into specimen without Al (OH)3 layer on the surface of specimen. In this paper, we have investigated the behavior of hydrogen in a plasma charged aluminum by means of thermal desorption spectroscopy, a method to evaluate the amount and trap states of hydrogen. Cold-rolled pure aluminum were annealed, electro-polished and charged with hydrogen gas plasma. Immediately after hydrogen gas plasma charging, TDS tests were performed under ultra-high vacuum. The hydrogen desorption spectrums obtained by TDS tests had three peaks corresponding to the co-diffusion of hydrogen-vacancy pair, dislocation and pore. Compared to a sample without charging, in a plasma charged sample, the amount of hydrogen trapped in vacancies especially increased.

The 7000 series alloys have the highest strength in the aluminum alloys, but lower fatigue properties than 2000 series alloys. Thus, 7000 series alloys are not applied to a large proportion of the aircraft components. However, the mechanism for this has not been elucidated yet. In humid air, hydrogen embrittlement based on intergranular cracking has been known to occur in 7000 series alloys. To date, in order to explain the difference in the fatigue crack growth behavior in the two series alloys, the effect of the test environment on the fatigue crack growth of the two series alloys has been investigated in this study. A 7075-type as well as 2024-type alloy with relatively coarse equi-axed grains was T6-tempered and subjected to fatigue crack growth test in humid and dry environments. Crack growth rate at low ΔK level seemed to be larger in the 7075-type alloy than the 2024-type alloy in the humid air, when assessed by means of gradually decreasing K method. In order to clarify this result, crack growth rate of the two alloys was assessed by means of gradually increasing K method as well as decreasing K method. Crack growth rate of the 7075-type alloy in moist air was concluded to be the largest in consistent with the previous study. Thus, the large fatigue crack growth rate of the 7075-type alloy is attributable to hydrogen embrittlement.

Tungsten has higher melting point than any other metals. Therefore, tungsten is applied to the electrode for resistance welding. The electrodes for resistance welding of tungsten are mainly used for fusing joining because they have high heat resistance and are hard to be alloyed with work metal pieces. However, the cracks on electrode surface occur during cyclic welding processes. In this study, to reveal the relationship between the elevated temperature deformation characteristics and fracture mechanism of recrystallized and unrecrystallized tungsten, tensile tests were carried out with varying strain rate and testing temperature. At 400°C, the total elongation at a strain rate of 1.4×10-1 s-1 in the recrystallized tungsten decreased more than the total elongation at the other strain rate. At 600°C, total elongation increased by approximately 10%, if the strain rate was increased from 1.4×10-3 s-1 to 1.4×10-1 s-1. Then, the recrystallized tungsten at the strain rate of 1.4×10-1 s-1 had more necking than that of 1.4×10-3 s-1. The may be caused by an environmental embrittlement due to oxygen in the high temperature atmosphere.

Currently liners of high-pressure hydrogen storage container for fuel cell vehicles are manufactured from the 6061 aluminum alloy pipes through spinning at elevated temperatures. Since the surroundings of the containers are reinforced with a large amount of high-cost CFRP, the use of 6066 or 6069 aluminum alloy with higher strength than 6061 is demanded to lower the cost of the container. However, the formability of these aluminum alloys at elevated temperatures has not been elucidated yet. In this study, tensile deformation characteristics of 6066 and 6069 aluminum alloys at temperatures ranging from 25 to 550°C were investigated. The total elongation of 6066 aluminum alloy was higher than that of 6069 aluminum alloy at 450°C. This may be caused by the lower volume fraction of constituent particles. The flow stresses of the two alloys were almost the same, and were decreased with increasing testing temperature. The increase in elongation and decrease in strength observed in the two alloys were attributable to dynamic recovery.

The substitution of Si with Al in 0.2%C-1.5%Si-1.25%Mn-0.2%Cr ultrahigh strength transformation-induced plasticity (TRIP)-aided martensitic (TM) sheet steel improves galvanization. The effect of Al content on the microstructure and formabilities of the TM steel was therefore investigated. Replacement of Si with Al maintained the high volume fraction of the retained austenite and the high stretch-formability and stretch-flangeability, whereas it decreased the tensile strength. Complex addition of Si and Al yielded the best formabilities with 1.5 GPa tensile strength grade. The superior formabilities of Si-Al bearing TM steel were attributed to the strain-induced transformation of the metastable retained austenite and the relatively soft lath-martensite structure matrix. The former leads to plastic relaxation of the localized stress concentrations, thus suppressing void formation.

Behavior of hydrogen in a tensiledeformed Al 9m assMg alloy was investigated by means of hydrogen microprint technique, HMPT, a method to visualize the microscopic location of hydrogen emission from specimen surface as silver particles. Hydrogen emission was observed on some of grain boundaries. The amount of surface relief and the maximum gradient across the grain boundary with hydrogen emission were larger than those without hydrogen emission. Most grain boundaries with hydrogen emission were from 61 to 75 degree to tensile direction and nearly parallel to slip lines. The results suggest that slip deformation parallel to and close to grain boundary plane caused hydrogen transport with moving dislocations to the surface, breakage of the surface oxide film on the grain boundary, and then the emission of the hydrogen atoms. [doi:10.2320/jinstmet.JC201411]

In the present study, we investigated the hydrogen embrittlement susceptibility of Al-4%Cu-1.5%Mg alloys subjected to several heat treatments by means of SSRT tensile test and humid gas stress corrosion cracking(HG-SCC) test. For SSRT tensile test, the tensile test pieces were cut from cold-rolled sheets of 1mm thickness. The test pieces were solution-treated at 500 °C for 1h, water-quenched and aged at 140oC for 72h or 360h. SSRT tensile test was performed in two environments, humid air (HA) and dry nitrogen gas (DNG) at a strain rate of 1.39×10-6s-1. Fracture surfaces were observed with a scanning electron microscopy(SEM). For HG-SCC test, compact tension(CT) test pieces were cut from hot-rolled plate of 6mm thickness. The CT test pieces were solution-treated, water-quenched and aged at 190 °C for 9h, 50°C for 96h or 140°C for 72h. HG-SCC test was carried out based on High Pressure Institute of Japan standards; HPIS E103:2018. The pre-cracked CT specimens with stress loading were kept for 90 days in two environments, HA and DNG. After 90 days, in order to observe whether cracks propagated due to HG-SCC, the specimens were loaded up to fracture rapidly, followed by SEM observation. Tensile properties obtained by SSRT tensile tests were almost the same in two environments. Also, fracture surfaces were not affected by test environments. Moreover, in HG-SCC tests, crack propagation was not observed at each test conditions. Therefore, Al-4%Cu-1.5%Mg alloy had high-resistance to hydrogen embrittlement.