Showing papers by "Hiroyuki Kokawa published in 2021"

Additively manufactured high strength and ductility CrCoNi medium entropy alloy with hierarchical microstructure

Abstract: The CrCoNi medium entropy alloy (MEA) is a technologically intriguing material showing superior cryogenic mechanical properties. In this work, we fabricated near full-dense hierarchical CrCoNi MEA via selective laser melting (SLM) and achieved superior yield strength of 860 MPa at 77K that surpasses that of conventionally fabricated counterparts, in addition to reasonable ductility. The strength and ductility increase as temperature drops, reaching a 1340 MPa ultimate tensile strength and a 47% elongation at 77K. EBSD, TEM, and in-situ synchrotron X-ray diffraction (SXRD) tensile tests disclose that the high strength stems from the hierarchical microstructure composed of high-density dislocations-formed cellular structures and low-angle grain boundaries (LAGB) within the complex heterogeneous columnar grains, and further validated by theoretical calculation. In-situ SXRD tensile tests and post-deformation TEM and EBSD reveal that nano-twinning response in the SLM-built CrCoNi MEA is suppressed by both high-density LAGB and near growth texture. The results demonstrate that SLM is a viable technique for fabricating dense hierarchical CrCoNi MEA and suggest a design strategy to improve mechanical properties further.

Microstructural control of the interface layer for strength enhancement of dissimilar Al/Cu joints via ni addition during tig arc brazing

Abstract: Dissimilar metal joining between Al and Cu is effective for reducing the weight and cost of electrical components. In this study, dissimilar lap joining of pure Al to pure Cu with an Al-Ni filler material was conducted using tungsten inert gas (TIG) arc brazing, and the effect of Ni on the joint strength associated with the microstructure of the intermetallic compound (IMC) layer at the dissimilar interface was examined. The addition of Ni effectively increased the interfacial strength of the joints. Regardless of the addition of Ni, the joints fractured in the thick Al2Cu layer formed at the Al/Cu interface. However, the Ni addition reduced the thickness of the IMC layer and led to the formation of Al7Cu4Ni particles in the weakest Al2Cu layer. Both the thickness reduction and reinforcing Al7Cu4Ni particle formation are thought to contribute to the increase in joint strength of the Al/Cu dissimilar interface.

Effect of Tool Geometries on “Heat-Input” During Friction Stir Welding of Aluminum Alloys

Abstract: Many previous studies proposed equations and models to estimate the heat-input of FSW, but their verifications would be still insufficient. In this study, the heat-input was calorimetrically measured during FSW of aluminum alloys with various welding tools having different shapes, and the effect of tool geometries on heat-input was examined. The measured heat-input increased with increasing shoulder diameter, which was in good agreement with many previous works. Interestingly, the heat-input strongly depended on the probe diameter and length. An attempt to correlate the heat-input with the tool surface area was made, virtually showing that the heat-input linearly increased with increasing “effective surface area” defined as (half of shoulder surface area + probe column surface area + probe tip surface area). This result experimentally suggested that the effect of probe dimension on heat-input was unexpectedly high. Moreover, an empirical equation estimating the heat-input during FSW of aluminum alloys from the welding parameters and the tool geometry was successfully established.