2021年, 第18卷, 第2期　
刊出日期：2021-03-28

  

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Research & Development
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  Grain selection and growth orientation of prior-β phase for Ti-6-4 during additive manufacturing: insights from a modeling perspective

  Wei-zhao Sun, Fei-hu Shan, Nan-fu Zong, Hong-biao Dong, Tao Jing

  《中国铸造》英文版. 2021, 18(2): 83-93. https://doi.org/10.1007/s41230-021-9002-8

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  The microstructure of Ti-6-4 components produced by additive manufacturing suffers from the coarse and elongated prior-β grain, which leads to a decrease of the tensile behavior and the occurrence of anisotropy. To understand and control the grain evolution, a multiscale simulation is applied to investigate the relationship between the grain selection, growth orientation, and the molten pool morphology with the different deposition layer numbers and processing parameters. The accuracy of the simulation is validated by experiments in both qualitative and quantitative ways. Results show that when the grain with unfavorable orientation loses the competitive growth with its neighbors, there will be a great chance that the blocked grain is eliminated in the following layer-and-layer deposition, which leads to the increase of the grain width. The size of the molten pool increases remarkably as the layer number increases, which lays a heavy burden on the stability of the molten pool. The analytical relationship between the molten pool morphology and the grain growth orientation is also deduced. The flat molten pool causes the grains with the <001> direction close to the building direction to have greater survival potential. Besides, decreasing the line power energy shows little effect on the stability of the molten pool and the grain growth orientation, especially when the deposited layer number is large. The revealing mechanisms will help in understanding and further controlling the grain evolution.
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  Distinctions of dendritic behavior influenced by constant pressure and periodic pressure

  Shan Shang, Zhi-peng Guo, Zhi-qiang Han, Xin-yu Zhang, Yi-nuo Cheng, Jun Li

  《中国铸造》英文版. 2021, 18(2): 94-100. https://doi.org/10.1007/s41230-021-0149-0

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  The distinctions of dendritic morphology and sidebranching behavior when solidified under atmosphere pressure, constant pressure which is higher than atmosphere pressure (hereinafter referred to as constant pressure) and periodic pressure were investigated using 3-D phase field method. When growing at atmosphere pressure, side branches (secondary dendritic arms) are irregular. When solidified under constant pressure with a relatively high value, side branches are much more luxuriant, with more developed high-order side branches. When applied with periodic pressure, resonant sidebranching happens, leading to many more regular side branches and the smallest secondary dendritic arm spacing (SDAS) in the three cases. The significant difference in dendritic morphology is associated with tip velocity modulated by total undercooling including pressure and temperature undercooling. In the case of constant pressure, tip velocity increases linearly with total undercooling, and it varies periodically in periodic pressure case. The different variation trend in tip velocity is the reason for the distinct dendrite growth behavior in different cases. Unlike the phenomenon in constant pressure case where the dendrite grows faster with higher pressure, the dendrite grows slower under periodic pressure with higher amplitude, resulting in less developed primary dendrite and side branches. This is influenced by tip remelting due to low undercooling or even negative undercooling. It is revealed that the accelerated velocity of tip remelting increases with the decline of undercooling. The greater the amplitude of periodic pressure, the faster the tip remelting velocity during one period. This is the reason why the average tip velocity decreases with the rise of amplitude of periodic pressure.
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  Confluence and cold shut computation based on time field in casting simulation

  Yong-shuai Feng, Dun-ming Liao, Tao Chen

  《中国铸造》英文版. 2021, 18(2): 101-109. https://doi.org/10.1007/s41230-021-9008-2

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  Numerical simulation technology has been widely used in the foundry industry to analyze and improve casting processes. During the casting filling process, many filling-related defects form easily at the confluences of liquid metal streams. The main filling-related defects are cold shut defects. To calculate the positions of casting defects, the characteristics of liquid metal confluences were analyzed. The flow front of liquid metal was captured by the volume-of-fluid algorithm to obtain a time field, which was used to calculate the time derivatives of the liquid front position and the confluences of liquid metal streams. To distinguish small confluences from the main confluences, the concept of confluent scale was developed, which was used to filter the small confluences based on a threshold. The calculation process was demonstrated through the post-processing of numerical simulation. A "W" shaped casting and a steering wheel casting were calculated to validate the accuracy of the method developed in this study. The positions of cold shut defects were predicted by calculating the confluences of liquid metal streams. The method was proved to be practical by comparing the calculation results with the positions of cold shut defects in an end cover casting. The computation of confluences and cold shut defects can improve the analysis efficiency and provide assurance for the optimization of a casting process plan.
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  Internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio

  Jing-an Yang, Hou-fa Shen

  《中国铸造》英文版. 2021, 18(2): 110-117. https://doi.org/10.1007/s41230-021-0141-8

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  Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center.
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  Effect of melt cooling rate on glass transition kinetics and structural relaxation of Vit1 metallic glass

  Wei Zhang, Qing-chun Xiang, Ying-dong Qu, Qing-feng Li, Ying-lei Ren, Ke-qiang Qiu

  《中国铸造》英文版. 2021, 18(2): 118-123. https://doi.org/10.1007/s41230-021-0129-4

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  The thin ribbons and the bulk cylindrical rods with diameters of 2 mm and 10 mm of the Vit1 metallic glass (MG) were prepared by the single roller melt spinning method and the copper mold injection casting method, respectively. The cooling rates of the samples during melt solidification were evaluated. The glass transition behaviors of three groups of MG samples with different solidification cooling rates were studied by differential scanning calorimetry (DSC) at different heating rates. The effects of melt cooling rate on the glass transition kinetic parameters such as apparent activation energy (E) and fragility parameter (m) of the Vit1 MG were studied using the Kissinger and the Vogel-Fulcher-Tammann (VFT) equations. Additionally, the structural relaxation enthalpy (ΔH_rel) of three groups of MG samples was quantitatively analyzed by DSC through multi-step temperature rise and fall measurements. Results show that the melt cooling rate (R) has a significant effect on the glass transition kinetics and the structural relaxation of the Vit1 MG. As R decreases in the order of magnitude, the glass transition temperature (T_g), E, m, and ΔH_rel of the Vit1 MG gradually decreases. Furthermore, in the range of the experimental cooling rates, E, m, and ΔH_rel all have an approximately linear relationship with lgR.
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  Effect of ultrasonic field treatment on degassing of 2024 alloy

  Zheng Jia, Bing Yu, Qing Lan, Tong Wang, Li Fu, Yu-lin Ma, Qi-chi Le, Jian-zhong Cui

  《中国铸造》英文版. 2021, 18(2): 124-130. https://doi.org/10.1007/s41230-021-9001-9

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  The 2024 aluminum alloy was prepared with different ultrasonic processes. Effects of ultrasonic treatment parameters including ultrasonic power, treatment time, treatment temperature, and frequency resonance, as well as C₂Cl₆ degasser on degassing of the 2024 aluminum alloy were investigated. Results indicate that increasing ultrasonic power at the same ultrasonic treatment time can improve the degassing effect. The optimum degassing efficiency can be obtained under the resonant ultrasound condition. With the combination of 1% C₂Cl₆ addition and 150 W ultrasonic treatment for 40 s, the hydrogen content of the alloy is decreased by 52.9%. At the same time, the tensile strength and elongation are increased by 28.3% and 92.3%, respectively, and the yield strength is slightly increased by 6.7%. The degassing mechanism is also discussed.
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  Effect of ageing treatment on microstructures, mechanical properties and corrosion behavior of Mg-Zn-RE-Zr alloy micro-alloyed with Ca and Sr

  Yu Fu, Chen Liu, Hai Hao, Yong-dong Xu, Xiu-rong Zhu

  《中国铸造》英文版. 2021, 18(2): 131-140. https://doi.org/10.1007/s41230-021-0146-3

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  Effects of ageing treatment on the microstructures, mechanical properties and corrosion behavior of the Mg-4.2Zn-1.7RE-0.8Zr-xCa-ySr[x=0, 0.2 (wt.%), y=0, 0.1, 0.2, 0.4 (wt.%)] alloys were investigated. Results showed that Ca or/and Sr additions promoted the precipitation hardening behavior of Mg-4.2Zn-1.7RE-0.8Zr alloy and shortened the time to reaching peak hardness from 13 h to 12 h. The maximum hardness of 77.1±0.6 HV for the peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy was obtained. The microstructures of peak-aged alloys mainly consist of α-Mg phase, Mg₅₁Zn₂₀ phase and ternary T-phase. The Zn-Zr phase is formed within the α-Mg matrix, and the Mg₂Ca phase is formed near T-phase due to the enrichment of Ca in front of the solid-liquid interface. Furthermore, fine short rod-shaped β'₁ phase is precipitated within the α-Mg matrix in the peak-aged condition. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy exhibits optimal mechanical properties with an ultimate tensile strength of 208 MPa, yield strength of 150 MPa and elongation of 3.5%, which is mainly attributed to precipitation strengthening. In addition, corrosion properties of experimental alloys in the 3.5wt.% NaCl solution were studied by the electrochemical tests, weight loss, hydrogen evolution measurement and corrosion morphology observation. The results suggest that peak-aged alloys show reduced corrosion rates compared with the as-cast alloys, and minor additions of Ca and/or Sr improve the corrosion resistance of the Mg-4.2Zn-1.7RE-0.8Zr alloy. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy possesses the best corrosion resistance, which is mainly due to the continuous and compact barrier wall constructed by the homogeneous and continuous second phases.
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  Effect of harmonic magnetic field and pulse magnetic field on microstructure of high purity Cu during electromagnetic direct chill casting

  Lei Bao, Da-zhi Zhao, Yin-ji Zhao, Yong-hui Jia, Xuan Wang, Qi-chi Le

  《中国铸造》英文版. 2021, 18(2): 141-146. https://doi.org/10.1007/s41230-021-0087-x

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  The effects of two types of magnetic fields, namely harmonic magnetic field (HMF) and pulse magnetic field (PMF) on magnetic flux density, Lorentz force, temperature field, and microstructure of high purity Cu were studied by numerical simulation and experiment during electromagnetic direct chill casting. The magnetic field is induced by a magnetic generation system including an electromagnetic control system and a cylindrical crystallizer of 300 mm in diameter equipped with excitation coils. A comprehensive mathematical model for high purity Cu electromagnetic casting was established in finite element method. The distributions of magnetic flux density and Lorentz force generated by the two magnetic fields were acquired by simulation and experimental measurement. The microstructure of billets produced by HMF and PMF casting was compared. Results show that the magnetic flux density and penetrability of PMF are significantly higher than those of HMF, due to its faster variation in transient current and higher peak value of magnetic flux density. In addition, PMF drives a stronger Lorentz force and deeper penetration depth than HMF does, because HMF creates higher eddy current and reverse electromagnetic field which weakens the original electromagnetic field. The microstructure of a billet by HMF is composed of columnar structure regions and central fine grain regions. By contrast, the billet by PMF has a uniform microstructure which is characterized by ultra-refined and uniform grains because PMF drives a strong dual convection, which increases the uniformity of the temperature field, enhances the impact of the liquid flow on the edge of the liquid pool and reduces the curvature radius of liquid pool. Eventually, PMF shows a good prospect for industrialization.
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  Microstructure evolution and mechanical properties of Mg-12Zn-2Y alloy containing quasicrystal phase fabricated by different casting processes

  Ji-guang Liu, Bu-ke Dong, Xiao-gang Fang, Bin Li, You-wen Yang, Tian-yu Wu, Rui-dong Ren, Shu-sen Wu

  《中国铸造》英文版. 2021, 18(2): 147-154. https://doi.org/10.1007/s41230-021-0132-9

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  Although icosahedral quasicrystal phase (denoted as I-phase) has been verified as an outstanding reinforcing phase, the mechanical properties of quasicrystal-reinforced Mg-Zn-Y alloys fabricated by traditional casting processes are still unsatisfactory due to the serious segregation of intermetallic compounds. In this study, the microstructure and mechanical properties of Mg-12Zn-2Y alloy fabricated by different casting processes, including permanent mold casting, squeeze casting and rheo-squeeze casting with ultrasonic vibration, were systematically investigated and compared. The results show that massive, large-sized I-phase and Mg₇Zn₃ phase gather together in the permanent mold cast sample, while the squeeze casting process leads to the transformation of I-phase into fine lamellar morphology and the amount of Mg₇Zn₃ decreases. As to the rheo-squeeze casting process, when the ultrasonic vibration is exerted with power from 800 W to 1,600 W, the α-Mg grains are refined and spheroidized to a large extent, and the lamellar spacing of the eutectic structure is significantly reduced, accompanied by some tiny granular I-phase scattering in the α-Mg matrix. However, when the ultrasonic power continuously increases to 2,400 W, the eutectic structure becomes coarse. The best mechanical properties of the rheo-squeeze cast alloy are obtained when the ultrasonic power is 1,600 W. The microhardness, yield strength, ultimate tensile strength and elongation are 79.9 HV, 140 MPa, 236 MPa, and 3.25%, which are 44.1%, 26.1%, 25.5%, 132.1% respectively higher than the corresponding values of the squeeze casting sample, and are 47.6%, 44.3%, 69.8%, and 253.3% respectively higher than the corresponding values of the permanent mold casting sample.
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  Effects of laser shock processing, solid solution and aging, and cryogenic treatments on microstructure and thermal fatigue performance of ZCuAl₁₀Fe₃Mn₂ alloy

  Guang-lei Liu, Yu-hao Cao, Lu-xin Shi, Meng-jie Zhang, Zhi-qiang Ye, Ling Zhao, Jian-zhong Zhou, Nai-chao Si

  《中国铸造》英文版. 2021, 18(2): 155-162. https://doi.org/10.1007/s41230-021-0072-4

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  The materials used in variable temperature conditions are required to have excellent thermal fatigue performance. The effects of laser shock processing (LSP), solid solution and aging treatment (T6), and cryogenic treatment (CT) on both microstructure and thermal fatigue performance of ZCuAl₁₀Fe₃Mn₂ alloys were studied. Microstructure and crack morphology were then examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The result showed that, after being subjected to the combination treatment of T6+CT+LSP, the optimal mechanical properties and thermal fatigue performance were obtained for the ZCuAl₁₀Fe₃Mn₂ alloy with the tensile strength, hardness, and elongation of 720 MPa, 300.16 HB, and 16%, respectively, and the thermal fatigue life could reach 7,100 cycles when the crack length was 0.1 mm. Moreover, the ZCuAl₁₀Fe₃Mn₂ after combination treatment shows high resistance to oxidation, good adhesion between the matrix and grain boundaries, and dramatically reduced growth rate of crack. During thermal fatigue testing, under the combined action of thermal and alternating stresses, the microstructure around the sample notch oxidized and became loose and porous, which then converted to micro-cracks. Fatigue crack expanded along the grain boundary in the early stage. In the later stage, under the cyclic stress accumulation, the oxidized microstructure separated from the matrix, and the fatigue crack expanded in both intergranular and transgranular ways. The main crack was thick, and the path was meandering.