Materials Science Forum Vol. 1016

Abstract: Recently, mechanochemical multifunction cavitation (MC-MFC) was developed to improve the corrosion resistance of the magnesium surface. MFC is a technology that combines water jet peening and ultrasound cavitation. MC-MFC is a technology that adds phosphoric acid to water. It can improve the corrosion resistance by forming a phosphate film on the Mg surface. Conventional anodic oxidation, plating, and chemical vapor deposition can improve corrosion resistance by forming a film on the Mg surface, but it is difficult to improve characteristics such as compressive residual stress on the surface. MFC-treated surfaces have previously imparted various properties such as imparting compressive residual stress necessary to improve the fatigue strength to Al alloys and Cr-Mo steels. In this study, the effect of film formed on MC-MFC processed surface on compressive residual stress was investigated.

Abstract: Grain refinement tendency of pure Fe and Fe-X (X=5~10at%Al, 2~10at%Ti) alloys produced by laser powder bed fusion (LPBF) process was investigated. Pure Fe, Al, and Ti powders were dry mixed and cubic samples were built from the mixtures. The microstructure analysis revealed that (1) the microstructure of pure Fe consisted of equiaxed grains having an average diameter of 1.7 μm with fine iron oxide particles. (2) Fe-5 and 10at%Al alloys showed coarse columnar grains. (3) Fe-2at%Ti shows a mixture of fine equiaxed and columnar shape grains. (4) the microstructures of Fe-5at%Ti and 10at%Ti alloys are fully equiaxed, and grain refinement tendency was confirmed with increasing Ti content. Ti(N,O) oxi-nitrides are efficient in reducing the grain size because of the low lattice misfit with the ferrite matrix. Additionally, the effectiveness of Ti(N,O) particles as grain refiners was confirmed by building samples using TiN powder mixed with Fe-10at%Al and Fe-2at%Ti. While these alloys alone are coarse grained, a dispersion of Ti(N,O) particles achieved a fine-grained microstructure.

Abstract: Mg-Li alloys have much better formability than traditional Mg alloys. However, the corrosion resistance of Mg-Li alloys are low due to Li addition and cold-worked Mg-Li alloys sustain exfoliation corrosion, which is serious problem for practical use of Mg-Li alloys [1]. According to our previous report, the elongated grains may be the cause of the exfoliation corrosion. In this study, the exfoliation corrosion behavior of Mg-14mass%Li-1mass%Al (LA141) alloy processed by equal channel angular extrusion (ECAE) is characterized to see the corrosion morphology of the samples introduced more grains than rolling. The specimens processed by rolling showed only elongated grains and the ones processed by ECAE (Route A) at 2 and 4 times showed elongated grains and partially equiaxial grains and in the samples fabricated by ECAE (Route B_c) at 2 and 4 times, equiaxial grains were found by optical microscopy. In corrosion examination, the specimens of rolling and ECAE (Route A) developed exfoliation corrosions and the samples of ECAE (Route B_c) didn’t sustain exfoliation corrosion. This results were consistent with our previous report. Then the corrosion behavior of rolled and ECAEed samples were focused. The evaluation of the time until crack initiation and the number of cracks were employed to see the corrosion morphology of the samples

Abstract: Mg alloys are very attractive materials for transportation industry due to their toughness and lightness. Recycling Mg alloys is desired for energy saving that otherwise would be required to produce its primary metal. However, secondary produced Mg tends to contain a few impurity elements that deteriorate its corrosion resistance. For example, contamination of Mg alloy by Cu induces second phase of Mg₂Cu and it works as strong cathode, resulting in the corrosion rate rapidly increasing. It was previously reported that the corrosion resistance of Mg with impurity Cu was remarkably improved by addition of alloying element Zn. Addition of Zn into Mg formed MgZn₂ phase and incorporated Cu into MgZn₂ phase instead of Mg₂Cu formation. In this way, since Zn serves to improve the corrosion resistance of Mg, Mg alloy with high Zn concentration may form a lot of MgZn₂ and may have better corrosion resistance even with high Cu concentration. In this work, the corrosion behavior of Mg-6mass%-1mass%Al (ZA61) with different Cu content up to 1mass% was investigated. As a result, ZA61-1.0Cu had much lower corrosion rate compared to Mg-0.2%Cu and the corrosion rate was almost the same as that of pure Mg.

Abstract: To reach high demands of a stainless steel surface quality the location of a slab edge is optimized utilizing multiphysical finite element (FE) analysis. The slab edge forms in roughing process when the longitudinal edge of the stainless steel slab moves parallelly towards the center of a transfer strip surface due to several rough and edge rolling passes. Strip spreading and location of the slab edge are managed by edge rolling process which is accomplished concurrently with roughing. Deformation resistance has a significant role characterizing the strip spreading and material flow in the roll bite, thus experimental material compression testing was carried out and the results fitted to the Hensel-Spittel equation. Multiple edger roll profiles were designed, and the most feasible details of the roll profile were iteratively utilized for the new profiles. In this way the location of the slab edge was optimized closer to the edge of the transfer strip by developing a new edger roll profile and resetting edge rolling passes according to results of FE-simulations. To mimic an industrial-scale roughing process an automated pass schedule control was developed in the FE-model. Therefore, multipass simulations require only a pass schedule data to run simulation.

Abstract: Simple figures illustrate the basic concepts: orientation, Euler angles, Euler space, orientation density function, pole density function. The iteration that decisively influenced the development of orientation analysis follows directly from the relationship between the two density functions. The minimum principle defines the initial function and the structure of the iteration. Using model orientation density function, we prove that this kind of orientation analysis is extremely effective.

Abstract: Electrical vehicles (EV) offer the automotive industry the potential to meet future emission targets by developing large battery systems. These battery systems require several thousand single battery cells to be connected together. The battery cells are complex assemblies of dissimilar materials with very low thicknesses, which presents a significant challenge during the joining process, especially welding. We have investigated the performance of laser beam welding (LBW), as well as pulsed arc welding (PAW) for joining 0.3mm thickness nickel coated copper to 0.7mm thickness mild steel. The parametric study for good quality lap welds based on high tensile strength, was performed. The weld microstructure was investigated using optical, as well as scanning electron microscopy (SEM). The mechanical performance of the weld samples was characterized through tensile testing and micro hardness measurements to establish the microstructure property relationship. The maximum tensile strength measured for specified weld geometries was 660N for LBW and 496N for PAW. A significant increase in the hardness was measured in the weld nugget due to the formation of Cu-Fe composite microstructure

Abstract: This paper presents experimental studies on the post-fire performance of concrete-filled steel tube (CSFT) columns under uni-axial load. The structural responses and axial load capacity of CSFT columns after exposure to elevated temperatures are investigated and discussed. All of the specimens are 750 mm in height, the nominal cross-section of the specimen is 150 mm x 150 mm, and have cylinder compressive strength of 18 MPa. The primary test parameters to be measured during the uni-axial compression test are wall thicknesses of the square tube (3.0 mm, 4.5 mm and 6.0 mm) and three different exposure to elevated temperatures (400°C, 600°C and 800°C). The results showed that the load-axial shortening relationship of the CSFT columns have a linear elastic response up to 80-90% of axial load capacity. After the axial load capacity is reached, the load-axial shortening curves are rarely becoming a nonlinear manner. It is also shown that the axial load capacity and ductility of the post-fire test columns are decreased significantly compared to the columns at ambient temperature, depending mainly on the elevated temperature. In addition, by comparing the axial load capacity of the test results with those obtained from the ACI design equation, the comparison results indicate that calculation formula in ACI code unconservative predicts the axial load capacity of the CSFT columns after exposure to elevated temperatures. Finally, the residual strength ratios are modified to both strength of concrete and steel tube under ambient temperature, and analyzed to evaluate the effect of post-fire behavior on the axial capacity of CFST columns.

Abstract: Bending behavior of a new thermomechanically processed low-alloy steel containing 0.40 wt.% carbon has been investigated. The processing included laboratory hot rolling to 10 mm thick strips followed by direct quenching to different quench-stop temperatures followed by slow furnace cooling to room temperature stimulating hot strip mill processing. The final microstructures were upper and lower bainite with yield strengths of a ~700 and ~1200 MPa, respectively. Local microstructures were characterized using a field-emission scanning electron microscope, microhardness profiles were measured, and bendability was determined using three-point brake press bending. The minimum applicable punch radius for a defect-free bend was 28 mm (2.8 times thickness) for the high-strength lower bainitic microstructure, while it was much smaller, i.e. 20 mm (2.0 times thickness) for the lower strength upper bainitic microstructure. Fractographic examination of the cracked surfaces revealed a more ductile fracture behavior for the upper bainitic microstructure.

Abstract: The hot compression tests of 06Cr19Ni9NbN steel were conducted at strains rate of 0.005-5s^-1 and temperature of 900-1200 °C on Gleeble1500 thermal mechanical simulation tester. Based on stress-strain data, processing maps of the steel were established. According to the results of processing maps, the optimal process parameters of hot compression were obtained, which lies in the temperature range of 1000-1200°C and strain rate of 0.005-0.1s^-1. And then, the process of plane strain compression of 06Cr19Ni9NbN steel was investigated and carried out at the temperature of 1000-1200 °C and reduction ratio of 10%-50%. After the hot compression tests, the room temperature tensile tests were carried out. The results indicated that the grain size and the mechanical properties gradually become stable when the reduction ratio increases to 30%, 34% and 40% at 1200 °C, 1100 °C and 1000 °C, respectively. Finally, a new model was presented to describe critical forging penetration efficiency, which is significant to optimize the steel forging process. Furthermore, the calculated results based on this new model were consistent with experimental results, indicating that the model is suitable to predict the critical FPE for the steel.