Zhenguang Liu

Bio: Zhenguang Liu is an academic researcher from Jiangsu University. The author has contributed to research in topics: Microstructure & Materials science. The author has an hindex of 7, co-authored 24 publications receiving 137 citations. Previous affiliations of Zhenguang Liu include Northeastern University (China).

Role of hot rolling procedure and solution treatment process on microstructure, strength and cryogenic toughness of high manganese austenitic steel

Abstract: In this study, the relationship between mechanical properties and microstructure of high manganese steels processed by different rolling practice and solution temperatures were systematically studied in terms of strength and toughness using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM) techniques. The experimental results suggested that the solution treatment is an effective method to improve Charpy impact toughness at −196 °C from ~68 J to ~120 J, while the strength decreases. The grain size coarsened with the increase of solution temperature. However, the change in strength and toughness was small. The role of grain coarsening on strength and toughness was weak. The stability of austenite at low temperature was high as the microstructure only consisted of austenite. The fracture mode during low-temperature impact process was ductile and the main deformation mechanism was mechanical twinning

Effect of secondary peak temperature on microstructure and toughness in ICCGHAZ of laser-arc hybrid welded X100 pipeline steel joints

Abstract: Intercritically reheat-coarse grained heat-affected zone (ICCGHAZ), as a brittle zone in double-pass welding, its toughness is affected by the peak temperature of the secondary thermal cycle. To elucidate the effect of secondary peak temperature on the microstructure and impact toughness of ICCGHAZ in laser-arc hybrid welded X100 pipeline steel joints, thermal simulation studies were carried out. The experimental results indicated that the microstructure of ICCGHAZ consisted of tempered martensite and products of secondary thermal cycle along the prior austenite grain boundaries. The products of secondary thermal cycle were necklace-type M-A constituent, necklace-type lath martensite and granular bainite + acicular ferrite at 740 °C, 790 °C and 840 °C, respectively. Based on experimental observations, transformation mechanism of ICCGHAZ at different secondary peak temperatures was proposed. The higher half zone(790℃∼Ac3) in ICCGHAZ of the laser-arc hybrid welded joint has a higher impact energy than the base metal. Consequently, microhardness difference between the products of secondary thermal cycle and matrix was reduced such that the stress concentration and crack nucleation rate was decreased during impact. The lattice orientation was more dispersed and fraction of high angle grain boundaries was increased at 840 °C, which suppressed crack propagation. The method of pipeline steel by laser-arc hybrid welding can effectively reduce the width of low toughness zone in ICCGHAZ and increase the impact energy of the welded joints.

Intercritical tempering enables nanoscale austenite/ε-martensite formation in low-C medium-Mn steel: A pathway to control mechanical properties

Abstract: In a 0.05C-5.4Mn-0.2Si-0.8Cr-wt% steel intercritically tempered at 700 °C for 50 min a new hexagonal closed-packed phase (e-martensite) appears on former reversed austenite by an air-cooled process. The tensile strength of the tested steel increased from ~ 840 MPa after intercritical tempering at 650 °C for 50 min to ~ 1002 MPa with the tempering temperature increased to 700 °C, and the product of strength and elongation was improved from 19.9 GPa % to 21.6 GPa %; moreover, the impact energy at − 40 °C was decreased from 143 J to 68 J, and the yield ratio was reduced from 0.84 to 0.50. This result was attributed to the weakening of TRIP effect in existence of e-martensite. The partitioning of alloying elements at different intercritical tempering conditions and the calculation of SFE (stacking faults energy) indicated that e-martensite was produced in the ~ 50–100 °C temperature range while cooling in air after tempering at 700 °C for 50 min e-Martensite affected the stability of the reversed austenite and the fracture mode. The fracture mode was changed with the increased tempering temperature. The results reveal that a multiphase low-C medium-Mn steel can be produced via intercritical tempering accompanied by a unique phase transformation process. A high-temperature and long-time tempering condition will lead to a thermally-induced e-martensite transformation due to lower SFE possibly associated with more homogenized (or less amount of) alloying elements in austenite, and affects the mechanical behaviors of medium Mn steel.

Impact of Reversed Austenite on the Impact Toughness of the High-Strength Steel of Low Carbon Medium Manganese

Abstract: We elucidate the relationship between the volume fraction of austenite and the Charpy impact toughness in a medium-Mn steel in terms of microstructural evolution with impact temperature. Different from retained austenite in the matrix after direct quenching, sub-micron lath-shaped morphology-reversed austenite in medium-Mn steel was produced by intercritical annealing. We found that reversed austenite steadily affected the fracture mode; only ductile fractures and dimples decreased with decreasing impact temperature. After the impact fracture test, the content of reversed austenite in the matrix increased slightly with a decreasing impact temperature due to the stability of the austenite grains caused by recrystallization of α′ martensite. Reversed austenite slightly decreased during the impact process with a decreasing impact temperature.

Misorientation mapping for visualization of plastic deformation via electron back-scattered diffraction.

Abstract: The ability to map plastic deformation around high strain gradient microstructural features is central in studying phenomena such as fatigue and stress corrosion cracking. A method for the visualization of plastic deformation in electron back-scattered diffraction (EBSD) data has been developed and is described in this article. This technique is based on mapping the intragrain misorientation in polycrystalline metals. The algorithm maps the scalar misorientation between a local minimum misorientation reference pixel and every other pixel within an individual grain. A map around the corner of a Vickers indentation in 304 stainless steel was used as a test case. Several algorithms for EBSD mapping were then applied to the deformation distributions around air fatigue and stress corrosion cracks in 304 stainless steel. Using this technique, clear visualization of a deformation zone around high strain gradient microstructural features (crack tips, indentations, etc.) is possible with standard EBSD data.

Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

Abstract: Laves phases with their comparably simple crystal structure are very common intermetallic phases and can be formed from element combinations all over the periodic table resulting in a huge number of known examples. Even though this type of phases is known for almost 100 years, and although a lot of information on stability, structure, and properties has accumulated especially during the last about 20 years, systematic evaluation and rationalization of this information in particular as a function of the involved elements is often lacking. It is one of the two main goals of this review to summarize the knowledge for some selected respective topics with a certain focus on non-stoichiometric, i.e., non-ideal Laves phases. The second, central goal of the review is to give a systematic overview about the role of Laves phases in all kinds of materials for functional and structural applications. There is a surprisingly broad range of successful utilization of Laves phases in functional applications comprising Laves phases as hydrogen storage material (Hydraloy), as magneto-mechanical sensors and actuators (Terfenol), or for wear- and corrosion-resistant coatings in corrosive atmospheres and at high temperatures (Tribaloy), to name but a few. Regarding structural applications, there is a renewed interest in using Laves phases for creep-strengthening of high-temperature steels and new respective alloy design concepts were developed and successfully tested. Apart from steels, Laves phases also occur in various other kinds of structural materials sometimes effectively improving properties, but often also acting in a detrimental way.

A review on dissimilar laser welding of steel-copper, steel-aluminum, aluminum-copper, and steel-nickel for electric vehicle battery manufacturing

Abstract: The electric vehicle (EV) battery systems are complex assemblies of dissimilar materials in which battery cells are connected using several thousand interconnect joints. Every single joint influences the functionality and efficiency of the whole battery system, making the joining process crucial. Laser welding is considered a desirable choice for EV battery manufacturing due to its non-contact nature, high energy density, precise control over the heat input, and ease of automation. However, incompatible thermos-physical properties of dissimilar materials used in battery tabs and interconnectors pose a significant challenge for achieving complete metallurgical bond. Furthermore, the formation of undesirable weld microstructures such as hard and brittle intermetallic compounds (IMCs) substantially undermines the structural, electrical, and thermal characteristics of battery joints. This paper reviews the fundamental difficulties and latest developments in dissimilar laser welding of steel-copper, steel-aluminum, aluminum-copper, and steel-nickel, some of the potential joint combinations in EV battery pack manufacturing. The weld microstructure and common metallurgical defects, as well as mechanical and electrical properties of joints are discussed. In addition, the effects of laser welding process parameters on the joint properties and the applicability of various interlayers and coatings in laser welding of battery materials are assessed.