RESEARCH

Dissimilar joints (especially, Fe/Al joint) for functionally graded materials

Functionally graded materials, including their characterization, properties and production methods are a new rapidly developing field of materials science. These materials do not contain well distinguished boundaries or interfaces between their different regions as in the case of conventional composite materials. Because of this, such materials possess good chances of reducing mechanical and thermal stress concentration in many structural elements, which can be developed for specific applications.

Recently, we have obtained metallic functionally graded materials (MFGM) from dissimilar joint (Fe/Al) produced by friction stir welding (FSW) with an offset of the tool towards the DP steel. The SZ of the joint shows three distinctively different layers dissimilar to typical joint, which affect mechanical/electrochemical behavior of the joint. A more significant issue associated with the joining of aluminum and steel alloys is the different corrosion rate.

We have proposed comprehensive method to analyze the resulting microstructure and the corresponding mechanical/electrochemical properties based on experimental and numerical approaches. This provides sufficient possibilities to apply the method for real-life cases, even the ones that involve more complex multi-materials.

Related papers

1. Sam Yaw Anaman, Hoon-Hwe Cho (Corresponding author), Hrishikesh Das, Jong-Sook Lee, Sung-Tae Hong; Microstructure and mechanical/electrochemical properties of friction stir butt welded joint of dissimilar aluminum and steel alloys, Materials Characterization, Vol. 154, pp. 67-79, 2019

2. Sam Yaw Anaman, Hoon-Hwe Cho (Corresponding author), Hrishikesh Das, Sung-Il Baik, Sung-Tae Hong, Jong-Sook Lee; Galvanic corrosion assessment of friction stir butt welded joint of aluminum and steel alloys, International Journal of Precision Engineering and Manufacturing-Green Technology, https://doi.org/10.1007/s40684-019-00183-5

3. Sam Yaw Anaman, Solomon Ansah, Hoon-Hwe Cho (Corresponding author), Jong-Sook Lee, Sung-Tae Hong; Experimental and numerical studies on the galvanic corrosion of an electrically assisted pressure joint of stainless steel and Ni-based superalloys, Corrosion Science, under review

Mechanical reliability and impedance of lithium-ion batteries (LIBs)

Understanding lithiation-induced volume changes in LIBs is important, since those can result in large mechanical stresses, leading to severe damage of the anodes through delamination, cracking and pulverization, and thereby causing a rapid battery capacity fade. Continuum mechanics-based simulation, such as finite element (FE) analysis, enables a deeper understanding of the lithiation process, and a FE model coupled with experimental results is a very powerful tool for exploring this complex process.

Consequently, to investigate the lithiation-induced volume change and the corresponding severe mechanical deformation, a rigorous numerical is developed to predict the mechanical/diffusional behavior of porous metal-based LIBs considering hydrostatic stress gradient effect on lithium ion diffusion. The computed strains are compared with in operando XRD-based strain measurements to verify the validity of the FE model.

Under high charging rate conditions, a slower diffusion near the surface can lead to accumulation of lithium ion near the surface and even higher compressive stress, hence a runaway instability resulting in “surface locking”. To understand this phenomenon, the FE analysis is employed by incorporating various governing equations, which enables to find physically relevant diffusion equation when the surface locking phenomenon occurs during lithiation.

These studies allow analyzing the mechanical behavior of LIBs and opens the door to understanding and predicting the mechanical degradation (or failure) of electrodes due to the very large lithiation-induced volume change.

Related papers

1. Hyeji Park, Hoon-Hwe Cho, Kyungbae Kim, Kicheol Hong, Jae-Hun Kim, Heeman Choe, David C. Dunand; Surface-oxidized, freeze-cast cobalt foams: Microstructure, mechanical properties and electrochemical performance, Acta Materialia, Vol. 142, pp. 213-225, 2018

2. Hoon-Hwe Cho (Corresponding author), Matthew P. B. Glazer, David C. Dunand; Modeling of stresses and strains during (de)lithiation of Ni₃Sn₂-coated nickel inverse-opal anodes, ACS Applied Materials and Interfaces, Vol. 9, pp. 15433-15438, 2017

3. Hoon-Hwe Cho, Matthew P. B. Glazer, Qian Xu, Heung Nam Han, and David C. Dunand; Numerical and experimental investigation of (de)lithiation-induced strains in bicontinuous silicon-coated nickel inverse opal anodes, Acta Materialia, Vol. 107, pp. 289-297, 2016

Ultra-high-temperature materials with novel and advanced structures

Three-dimensional (3D) woven metal structures, fabricated from Cu or Ni–Cr wires, provide minimal loss in stiffness for thermo-structural applications. Such cellular materials are of interest for extreme environments, since increased permeability allows for active cooling and results in material use, at elevated service temperatures and stresses.

FE analysis is employed to predict the creep deformation of the 3D woven structure with transient liquid phase (TLP) bonding using the experimental architectures obtained from X-ray nano-tomography. The computed creep strain rates are compared with experimentally measured data. The effects of TLP bonding and wires on the creep strain rates are studied by artificially adding or removing those in the 3D woven structure.

The obtained results propose an optimized topology of 3D woven structure having innovative high-temperature properties. This reveals the importance of continuum mechanics-based simulation which enables various case studies. The insight gained by these studies may serve as a design guideline in fabricating 3D woven structure with a connected-pore size gradient, which enhances spontaneous, efficient fluid transport and the corresponding active cooling at elevated temperatures.

Related papers

1. Hoon-Hwe Cho (Corresponding author), Dinc Erdeniz, Keith W. Sharp, David C. Dunand; Experimental and modeling study of compressive creep in 3D-woven Ni-based superalloys, Acta Materialia, Vol. 155, pp. 236-244, 2018

2. Hoon-Hwe Cho, Yu-chen Karen Chen-Wiegart, and David C. Dunand; Finite element analysis of mechanical stability of coarsened nanoporous gold, Scripta Materialia, Vol. 115, pp. 96-99, 2016