Ren-Kae Shiue

The laser heterogeneous lap welding of nickel-plated steel and Cu sheets has been investigated in this study. The YAG (Yttrium-Aluminum-Garnet) laser beam only penetrates the upper Ni-plated steel sheet and cannot weld the bottom Cu sheet due to the low absorption coefficient of the YAG laser beam. Incorporating a blue-light and fiber laser into the coaxial laser beam significantly improves the quality of the weld fusion zone. The fiber laser beam can penetrate the upper nickel-plated steel sheet, and the blue-light laser beam can melt the bottom copper sheet. Introducing the blue-light laser to the coaxial laser beams overcomes the low reflectivity of the bottom copper sheet. The fiber/blue-light coaxial laser continuous welding can achieve the best integrity and defect-free welding. It shows potential in the mass production of the next generation of lithium batteries.

This study attempts to use fly ash as the brazing filler additive to increase the sustainable use of coal-fired power plant by-product materials. The experimental results show that adding 5 wt% fly ash into the Ag paste filler contributes to the interfacial reactions in heterogeneous reactive air brazing (RAB) of the ZrO₂ and Crofer alloy. The Ag-rich phase dominates the brazed zone. The interfacial reaction layers contain oxidation of the Cu-Ti coating layer, Crofer alloy, and the Si/Al-rich oxides from the fly ash particles. The 5% fly ash RAB joint maintained airtightness for 280 h under 2 psig helium at room temperature. When the test temperature was raised to 600 °C for 24 h, the pressure of the joint assembly still did not drop. When the fly ash addition was increased to 10 wt%, the joint assembly was no longer leak-free at room temperature. Many visible voids and cracks exist in the brazed zone and at the ZrO₂/braze and braze/Crofer interfaces. A high volume fraction of the fly ash particles results in many brittle Si/Al-rich oxides in the joint after RAB, and the fracture of these oxides significantly deteriorates the airtightness of the joint. This study shows the feasibility and potential of introducing 5 wt% fly ash particles to the Ag-rich paste filler during the RAB of ZrO₂ and Crofer for airtight applications.

Surface modifications, including laser surface melting (LSM) and micro-shot peening (MSP), were applied to improve the fatigue performance of the Ni-Al bronze (NAB) alloy. LSM could homogenize the NAB into a uniform microstructure with refined columnar grains in the laser-melted zone but introduced residual tensile stress (RTS). The bombardment and generated heat induced by MSP could not entirely remove the granular κ_II precipitates and lamellar κ_III phase in the peened zone of the shot-peened sample (NSP sample) but introduced residual compressive stress (RCS) into the NSP sample under the peening intensity of this work. The results of fatigue tests revealed that the LSM sample had the poorest fatigue performance, but the NSP sample showed the best fatigue performance among the tested samples. The fatigue limit of the NAB alloy was about 325 MPa; meanwhile, the fatigue performance of the LSP (LSM + MSP) sample was equivalent to or a little better than that of the NAB alloy (NBM sample). The RTS and aligned columnar grains accounted for the degraded fatigue resistance of the LSM sample. By contrast, the high RCS and the refined structure were responsible for the improved fatigue strength/life of the NSP sample relative to that of the other samples. The fatigue limit of the NSP sample was as high as 450 MPa. However, the increase in fatigue strength of the NSP sample occurred in service life regions above 3 × 10⁵ cycles.

This study proposes a method to enhance the airtightness of the joint between the ZrO₂ and Crofer alloy using coating technology. With the aid of vacuum sputtering technology, a titanium–copper alloy layer with a thickness between 1.5 μm and 6 μm was first deposited on the surface of ZrO₂ and Crofer, respectively. The chemical composition of the deposited reaction layer was 70.2 Cu and 29.8 Ti in at%. Then, using silver as the base material in the reactive air brazing (RAB) process, we explore the use of this material design to improve the microstructure and reaction mechanism of the joint surface between ceramics and metal, compare the effects of different pretreatment thicknesses on the microstructure, and evaluate its effectiveness through air tightness tests. The results show that a coating of Cu-Ti alloy on the ZrO₂ substrate can significantly improve bonding between the Ag filler and ZrO₂. The Cu-Ti metallization layer on the ZrO₂ substrate is beneficial to the RAB. After the brazing process, the coated Cu-Ti layers form suitable reaction interfaces between the filler, the metal, the filler, and the ceramic. In terms of coating layer thickness, the optimized 3 μm coated Cu-Ti alloy layer is achieved from the experiment. Melting and dissolving the Cu-Ti coated layer into the ZrO₂ substrate results in a defect-free interface between the Ag-rich braze and the ZrO₂. The air tightness test result shows no leakage under 2 psig at room temperature for 28 h. The pressure condition can still be maintained even under high-temperature conditions of 600 °C for 24 h.

This investigation aims to analyze the effect of homogenization heat treatment at 1240 °C for 2 and 6 h on the hardness, distribution, morphology, and chemical composition of the δ-ferrite and sigma phases in 316L stainless steel cast billet. A field emission scanning electron microscope, combined with electron back-scattered diffraction, a field emission electron probe microanalyzer with a wavelength dispersive spectrometer, and a Vickers microhardness tester are applied to identify various phase evolutions in the cast billet. The morphology of the δ-ferrite and sigma phases in the austenite matrix of the 316L cast billet are strongly related to the subsequent hot and cold wire drawings. The homogenization heat treatment is expected to provide a driving force to form spheroid interdendritic δ-ferrite and to minimize the amount of the brittle sigma intermetallic compound in the austenite matrix. The homogenization heat treatment at 1240 °C effectively spheroidized all δ-ferrites into blunt ones in the cast billet. The transformation of δ-ferrite into sigma is dominated by temperature and cooling rate. The fast air cooling after homogenization between 1240 and 850 °C retards the precipitation of the sigma in the δ-ferrite. There are two δ-ferrite transformation mechanisms in this experiment. The direct transformation of the δ-ferrite into sigma is observed in the as-cast 316L stainless steel billet. In contrast, the eutectoid transformation of the δ-ferrite into the sigma and austenite dominates the 316L cast billet homogenized at 1240 °C, with a slow furnace cooling rate.

Two austenitic stainless steel (ASS) plates, 304L and 316L, were cold-rolled (304R and 316R) with a 10% reduction in thickness and then subjected to laser welding. Cold rolling caused slight surface hardening and introduced residual tensile stress into the ASS plates. The susceptibility to stress corrosion cracking (SCC) of the welds (304RW and 316RW) was determined using the U-bend test pieces in a salt spray. To highlight the stress concentration at the weld’s fusion boundary (FB), the top weld reinforcement was not ground off before bending. Moreover, micro-shot peening (MSP) was performed to mitigate the SCC of the welds by imposing high residual compressive stress and forming a fine-grained structure. Cold rolling increased the susceptibility of the 304R specimen to pitting corrosion and intergranular (IG) microcracking. Moreover, pitting corrosion and SCC were found more often at the FBs of the 304RW. The corrosion pits of the peened 304RW (304RWSP) were finer but greater in amount than the those of the un-peened one. The results also indicated that the 316L ASS welds with MSP were resistant to the incidence of pitting corrosion and SCC in a salt spray. The better reliability and longer service life of dry storage canisters could be achieved by using 316L ASS for the construction and application of MSP on it.

In this paper, a novel 95Ag-2.5Ge-2.5Si (in wt %) filler is utilized for brazing yttria-stabilized zirconia (YSZ) electrolytes and commercial Crofer 22H interconnects for solid-oxide fuel cells’ (SOFCs) sealing application. Before brazing, surface metallization is applied on YSZ and Crofer 22H substrates to improve the wetting performance of the filler on YSZ and Crofer 22H substrates. The brazing procedure is performed at 900 °C for 10 min under a high vacuum (~10⁻⁶ torr) to prepare sandwiched YSZ/Crofer 22H brazed coupons. The metallization mentioned above can achieve reactive wetting toward YSZ ceramics. A Si/Ti-rich oxide layer and an Fe-Cr-Si alloying phase are formed at the brazed joints’ YSZ/filler and filler/Crofer 22H interfaces. After exposure to air at 750 °C for 100 h, Cu and Si contents suffer from oxidation and form CuO and SiO₂, respectively, in the brazed zone and the YSZ/filler interface of the joints. The Fe-Cr-Si alloying phase at the filler/Crofer 22H interface is preserved without apparent oxidation. The pressure-drop test results show that the brazed joints’ gas tightness does not deteriorate significantly after thermal aging, which is attributed to the good interfacial integrity of thermal-aged joints.

The investigation aims to analyze the effect of homogenization heat treatment on the quantity, distribution, morphology, and chemical composition of δ ferrite and sigma phase in the 316L austenitic stainless steel cast billet. Microstructures of the cast billet at different locations, center, 0.5 R, R, and the subsequent 2/6-hour homogenization heat treatment at 1240 oC were examined. The microstructure at the radius R was much more uniform and expected a better performance in the following hot/cold forging processes due to the highest cooling rate compared with those at 0.5R and center. The δ ferrite often accompanies the generation of the sigma phase, and the transformation of δ ferrite into the sigma/(austenite) and retained δ ferrite is strongly related to the cooling rate after homogenization heat treatment. The sigma phase is hard and detrimental to the subsequent hot/cold forgings. It must be removed from the homogenization heat treatment before forging. Fast air cooling from 850 oC to room temperature after homogenization heat treatment cannot prohibit the sigma phase formation. Rapid cooling between 850 and 1240 oC after homogenization treatment is required to avoid the sigma phase formation in the 316L stainless steel cast billet.