Metal additive manufacturing (AM) technologies are attracting attentions not only as a forming process but also as microstructure controlling processes. In powder bed fusion (PBF) AM, crystal orientations can be controlled by scanning strategies of energy beam. To optimize microstructures, computer simulations for predicting microstructures play very important roles. In this work, we have developed simulation programs to explain the mechanism of the crystal orientation control. First, we simulated the shape of melt pool by analyzing the heat transfer using apparent heat conductivity when the penetration of laser beam through keyholes was taken into consideration because of the evaporation and accompanying convections. It was assumed that the primary crystal growth direction can be determined by the temperature gradient, and the crystals grow keeping the growth direction as generally recognized. The shapes of simulated melt pools agree well with experimental observations. The modified cellular automaton simulations successfully reproduced two typical textures with different preferential orientations along the building directions of 〈001〉 and 〈011〉 when the bidirectional scanning with and without a rotation of 90° was accomplished between the layers.
Fig. 8 Comparison of analysis results (lower side) with experimental results (upper side: after Ishimoto et al.1）) of scan strategy X.
In this study, three-layered porous aluminum consisting of AC4CH (porosity: 70％) / pure aluminum (80％) / A6061 (75％) / and AC4CH (70％) / pure aluminum (70％) / A6061 (70％) were fabricated by a sintering and dissolution process. When plateau stress of each layer was significantly different, such as three-layered porous aluminum with varying porosities, exhibited clear multiple plateau regions corresponding to the deformation of each layer. In contrast, when plateau stress of each layer had a close value, such as three-layered porous aluminum with homogeneous porosities, exhibited no clear multiple plateau regions.
Fig. 4 (a) Deformation behavior and (b) stress-strain curves of three-layered porous aluminum (Sample A) and corresponding uniform porous aluminum.
The grain size effects on the hydrogen embrittlement susceptibility of pure Ni and Ni-20Cr alloy were investigated. The hydrogen embrittlement susceptibility was evaluated by tensile testing under electrochemical hydrogen charging. Relative elongation, defined as the elongation under hydrogen charging divided by elongation in air, increased with increasing grain size in pure Ni (the grain size was in the range of 11-22 µm). In contrast, the relative elongation of Ni-20Cr alloy increased with decreasing grain size from 13 to 1.8 µm. Correspondingly, intergranular fracture was suppressed by grain coarsening in pure Ni and grain refinement in the Ni-20Cr alloy. In addition, the intergranular fracture surface in pure Ni showed curved slip lines, and in the Ni-20Cr alloy showed straight line marks. These fractographic features imply that the mechanisms of the hydrogen-assisted intergranular crack growth were different in pure Ni and Ni-20Cr alloy and this can be attributed to the difference in stacking fault energy.
Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds have not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fractured surface and intergranular fractured surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at the crack tip.
Illustration of the method used to evaluate hydrogen embrittlement resistance and the relationship between diffusible hydrogen content, tensile shear strength and fracture surfaces.
Electrodeposition of Zn was performed on an Fe electrode at a current density of 50-5000 A·m−2 and a charge of 4 × 104 C·m−2 in an unagitated zincate solution at 313 K containing 0.62 mol·dm−3 of ZnO, 4.0 mol·dm−3 of KOH or NaOH, and organic additives. The effects of KOH and NaOH on the deposition behavior of Zn in solution containing organic additives and the microstructure of the deposits were investigated. In solution containing a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), the current efficiency for Zn deposition at high current density region of 1000 to 5000 A·m−2 to produce the glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A·m−2, Zn deposition approaches the diffusion limitation of ZnO22− ions. With additions of PQ and QA, the diffusion of ZnO22− ions was significantly suppressed, and the degree of suppression was smaller with KOH than that with NaOH. The polarization resistance at 200 A·m−2 investigated by AC impedance revealed that the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH. Since the suppression effect of additives on the Zn deposition is smaller with KOH than that with NaOH, the current efficiency for Zn deposition at high current density region is larger with KOH. The upper limit of current density to produce the glossy films was smaller with KOH than that with NaOH, and the spongy thin films were partially observed on the platelets crystals obtained at high current density in KOH solution. The content of C resulting from the additives in deposited Zn was smaller with KOH. These phenomena are attributed to the adsorption ability of PQ and QA onto the cathode being smaller with KOH.
Fig. 6 Current efficiency for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]