Microstructures and mechanical behavior of the bimetallic additively-manufactured structure (BAMS) of austenitic stainless steel and Inconel 625

Bimetallic additively manufactured structures (BAMSs) can replace traditionally-fabricated functionally-graded-components through fusion welding processes and can eliminate locally-deteriorated mechanical properties arising from post-processing.The present work fabricates a BAMS by sequentially depositing the austenitic stainless-steel and Inconel625 using a gas-metal-arc-welding (GMAW)-based wire + arc additive manufacturing (WAAM) system.Elemental mapping shows a smooth compositional transition at the interface without any segregation.Both materials being the face-center-cubic (FCC) austenite,the electron backscattered diffraction (EBSD) analysis of the interface shows the smooth and cross-interface-crystallographic growth of long-elongated grains in the <001> direction.The hardness values were within the range of 220-240 HV for both materials without a large deviation at the interface.Due to the controlled thermal history,mechanical testing yielded a consistent result with the ultimate tensile strength and elongation of 600 MPa and 40 ％,respectively,with the failure location on the stainless-steel side.This study demonstrates that WAAM has the potential to fabricate BAMS with controlled properties.     

Formation and Mechanical Behavior of Body-Centered-Cubic Zr(Hf)-Nb-Ti Medium-Entropy Alloys

The present work investigated the formation and mechanical behavior of body-centered-cubic (BCC) Zr(Hf)-Nb-Ti medium entropy alloys (MEAs), in which three series of alloys, [Zr-Zr₁₄](Zr,Nb)₃, [Ti-Zr₁₄](Ti,Nb)₃, and [Ti-(Hf,Zr)₁₄](Nb)₃, were designed by the cluster formula approach. With increasing the Nb content, the BCC-β structural stability of the [Zr-Zr₁₄](Zr,Nb)₃ alloys would be enhanced, as evidenced by the BCC [Zr-Zr₁₄](Nb)₃ (Zr_83.33Nb_16.67 in atomic percent at. pct) alloy containing a minor amount of ω phase. An appropriate content of Ti addition can further improve the BCC-β stability of [Ti-Zr₁₄](Nb₃) (Zr_77.77Ti_5.56Nb_16.67) alloy without any ω precipitation. The further substitution of Hf/Ti for the Zr could also render the [Ti-Zr₈Hf₄Ti₂](Nb₃) (Zr_44.44 Hf_22.22Ti_16.67Nb_16.67) alloy with a single BCC structure. All these BCC MEAs exhibit prominent mechanical properties, as exemplified by the [Ti-Zr₈Hf₆](Nb₃) (Zr_44.44Hf_33.33Ti_5.56Nb_16.67) with a higher yield strength of 662 MPa, a larger elongation to fraction of 15.2 pct, and a lower Young’s modulus of 71 GPa.

     

Identifying battery aging mechanisms in large format Li ion cells

Large format LiFePO₄-based Li-ion batteries are rapidly becoming available from commercial cell manufacturers. In this paper two types of 10 Ah single cells (one prismatic and another cylindrical) from two manufacturers were tested at room temperature and 60 °C. Both cells suffered severe degradation at 60 °C. The results were analyzed using incremental capacity analysis (ICA) along with other electrochemical techniques. Overall the two types of cells were similar in behavior, despite subtle differences in performance. This study shed some light on the degradation process associated with these two large format LiFePO₄ cell designs with regard to thermal degradation at elevated temperatures. The analysis illustrates a unique capability of using ICA to differentiate cell performance and material utilization in different cell designs.     

Effect of discharge rate on charging a lead-acid battery simulated by mathematical model

To simulate lead-acid battery (LAB) charging has never been an easy task due to the influences of: (1) secondary reactions that involve gas evolution and recombination and grid corrosion, (2) prior end-of-discharge (EOD) and rest conditions; and (3) complexity caused by charging algorithm. In this work, successful results have been obtained with considerations of internal oxygen cycle and gas phase in the valve-regulated lead-acid (VRLA) cells. The success is first attributed to the satisfactory validation of a mathematical model that has been able to simulate discharge regimes with various rates consistently. The model has been subsequently used to simulate a galvanostatic charge regime performed at C/10. The results give a better understanding of the role each electrode played in the polarization, the nature of the polarization (constituted by reaction kinetics and mass transport), and the charging efficiency. We were able to extrapolate the simulation results to rates beyond what the model has been validated for, and the results are still consistent, confirming some experimental observations, notably the maximum charging rate specified by most LAB manufacturers.     

From driving cycle analysis to understanding battery performance in real-life electric hybrid vehicle operation

This paper proposes a methodology and approach to understand battery performance and life through driving cycle and duty cycle analyses from electric and hybrid vehicle (EHV) operation in real-world situations. Conducting driving cycle analysis with trip data collected from EHV operation in real life is very difficult and challenging. In fact, no comprehensive approach has been accepted to date, except those using standard driving cycles on a dynamometer or a track. Similarly, analyzing duty cycle performance of a battery under real-life operation faces the same challenge. A successful driving cycle analysis, however, can significantly enhance our understanding of EHV performance in real-life driving. Likewise, we also expect similar results through duty cycle analysis for batteries. Since 1995, we have been developing tools to analyze EHV and power source performance. In particular, we were able to collect data from a fleet of 15 Hyundai Santa Fe electric sports utility vehicles (e-SUVs) operated on Oahu, Hawaii; from July 2001 to June 2003 to allow driving and duty cycle analyses in order to understand battery pack performance from a variety of EHV operating conditions. We thus developed a comprehensive approach that comprises fuzzy logic pattern recognition (FL-PR) techniques to perform driving and duty cycle analyses. This approach has been successfully applied to EHV performance analysis via the creation of a compositional driving profile called “driving cycle profile” (DrCP) for each trip. The same approach was used to analyze battery performance via the construction of “duty cycle profile” (DuCP) to express battery usage under various operating conditions. The combination of the two analyses enables us to understand both the usage profile of EHV and battery performance in synergetic details and in a systematic manner using a pattern recognition technique.     

A roadmap to understand battery performance in electric and hybrid vehicle operation

This work attempts to bridge laboratory and real-life battery testing data with a comprehensive analysis to provide a coherent approach for a realistic model to simulate battery performance, including life prediction. From electric vehicle field-testing results, we explain how to handle real-life data through driving cycle analysis to establish a scheme of “building blocks” that can be validated by test results obtained in the laboratory. We also show that a simple battery model can be built upon laboratory test data and validated by real-life duty cycles, therefore deriving a more realistic understanding and prediction of battery performance.