双辊铸造热裂形成及工艺控制

作为铸造过程中最常见且较严重的缺陷,热裂对双辊铸坯的质量具有重要的影响。在对热裂形成机理进行总结与分析的基础上,结合双辊铸造过程中热裂的形成及其主要影响因素,从工艺上对热裂的控制进行探讨与综述,提出在双辊铸造热裂研究中有待加强的方面。     

Experimental and numerical analysis of hot tearing susceptibility for Mg–Y alloys

The influences of Y content and initial mold temperatures on the hot tearing susceptibility (HTS) of binary Mg–Y alloys were investigated using a constrained rod casting (CRC) apparatus, which is equipped with a load cell and data acquisition systems. The hot crack formation was monitored during CRC experiments. The experimental results show that HTS first increases with increase in the Y content, reaches the maximum at about 1.5 wt% Y and then decreases with further increase in the content of Y. The severest of hot tearing is found in Mg–1.5 wt% Y alloy which is due to its large columnar grain structure, wide solidification range, and small amount of eutectic. The resistance to hot tearing is apparently improved by increasing the initial mold temperature. ProCAST simulation software was used to predict the hot tearing of Mg–Y alloys in CRC. The simulation results show a good agreement with the experimental measurements. The numerical simulations will be helpful and valuable to optimize the alloy composition and casting parameters to minimize the hot tearing defects.     

Hot tearing mechanisms of B206 aluminum–copper alloy

In this study, the mechanisms of hot tearing in B206 aluminum alloy were investigated. Castings were produced at three mold temperatures (250 °C, 325 °C and 400 °C) and with two levels of titanium (0.02 wt% and 0.05 wt%) to investigate the effects of cooling rate and grain refinement. A constrained-rod casting mold attached to a load cell was used to monitor the contraction force during solidification and subsequently determine the onset temperature of hot tearing in B206. The corresponding onset solid fraction of hot tearing was estimated from the solid phase evolution of α-Al in B206 using in situ neutron diffraction solidification analysis. Hot tears were found to occur at solid fractions ranging from 0.81 to 0.87. Higher mold temperatures significantly reduced hot tearing severity in B206 but did not alter the onset solid fraction. In contrast, additions of titanium to B206 were effective at eliminating hot tears by transforming the grain structure from coarse dendrites to finer and more globular grains. Finally, in situ neutron diffraction solidification analysis also successfully determined the solid phase evolution of intermetallic Al₂Cu during solidification, which in turn, provided a better understanding of the role of Al₂Cu in the development of hot tears in B206.     

Study of hot tearing of A206 aluminum alloy using Instrumented Constrained T-shaped Casting method

The hot tearing susceptibility of A206 aluminum alloy was investigated using Instrumented Constrained T-shaped Casting method and the effect of the casting temperature on hot tearing was studied. The Instrumented Constrained T-shaped Casting apparatus enabled real-time measurements of the contraction load developed in the casting and the temperature variations during solidification as a function of time. Critical temperatures and points during solidification of the castings were extracted from these data. The contraction load developed at the coalescence point of the castings was identified as a comparative criterion for predicting the hot tearing susceptibility of the alloys which could be utilized even when no visual tearing had occurred. The results showed that hot tearing susceptibility increased with the casting temperature. This was associated with reduced cooling rate, increased solute segregation and more localized hot spot formation at the T-junction area. Increase in the casting temperature also increased the grain size which may in turn have affected the initiation of the hot tears. The visual observations were further validated with radiographic tests.     

汽车增压器涡轮用铸造高温合金热裂研究进展

采用高温合金浇注汽车增压器涡轮时,叶片极易产生热裂,热裂的存在严重影响了铸造高温合金在增压涡轮上的使用。介绍了几种目前国内广泛使用的汽车增压器涡轮用铸造高温合金,对铸件热裂的形成机理进行了综述,重点探讨了铝、钛、碳、锆和铪等元素对铸造高温合金热裂倾向性的影响。综述了合金凝固方式和铸型性质、浇注条件、铸件结构、浇注系统等铸造工艺参数对热裂的影响,并提出了防止热裂产生的措施。     

Effect of Zn addition on hot tearing behaviour of Mg–0.5Ca–xZn alloys

The influence of Zn addition (0, 0.5, 1.5, 4.0 and 6.0 wt.%) on hot tearing behaviour of Mg–0.5 wt.% Ca alloy was investigated using a constrained rod casting (CRC) apparatus. The effects of mould temperature and grain refinement on the hot tearing susceptibility (HTS) were studied. Hot tears were observed with 3D X-ray tomography and the tear volumes were quantified. Results show that the Zn addition increases the HTS of Mg–0.5Ca alloys. At a mould temperature of 250 °C, all alloys investigated except Mg–0.5Ca–6Zn alloy show severe HTS. An increase in the mould temperature from 250 °C to 450 °C did not reduce the HTS in Mg–0.5Ca–1.5Zn and Mg–0.5Ca–4Zn alloys. Among all the investigated alloys, Mg–0.5Ca–4Zn alloy exhibits severe HTS as it completely broke away from the sprue–rod junction. The HTS of alloys was well correlated with the susceptible temperature range (ΔT_s). An increase in ΔT_s increased the HTS. The hot tears propagated along the grain boundaries through liquid film rupture. Grain refinement by Zr addition improved the hot tearing resistance of Mg–0.5Ca–4Zn alloy as the fine grain structure facilitated the easy feeding of liquid into the last area of solidification and accommodated the developed strain more effectively.     

Effect of Al content on hot tearing behaviour of Mg–xAl–2Ca–2Sm alloys

Mg–xAl–2Ca–2Sm (x?=?3, 5, 9 and 15) alloys were tested using an ‘L’-shaped sand mould serving as a hot tearing testing system. The experimental results showed that the solidification range of the Mg–xAl–2Ca–2Sm alloys first decreased and then increased as the Al content was increased. Furthermore, by increasing the Al content, the dendritic arms of the α-Mg phase become more developed, and the hot tearing tendency of the Mg–xAl–2Ca–2Sm alloys increased. In addition, the variety of precipitated phases was seen to be affected by the Al content and the tendency for hot tearing depended on the precipitated phase. The tendency of the Mg–xAl–2Ca–2Sm alloys for hot tearing first decreased and then increased with increasing Al content.     

A study on the numerical simulation of thermal stress during the solidification of shaped castings

In order to study the development of thermal stress and to predict the hot tearing and residual stress of shaped casting, two models were used to carry out the stress analysis of the two stages of solidification. The rheological model [H]–[H|N]–[N|S] was used for the quasi-solid zone while the thermo-elasto-plastic model was used for the period after solidification. Coupling the thermal analysis based on the finite different method with the stress analysis based on the finite element method, a FDM/FEM integrated system of thermal stresses analysis during the solidification process was developed. After experimental verification, the system was put into practical application. The analysis results during the quasi-solid zone show that the visco-plastic strain is an important factor for the occurrence of hot tearing. The hot tearing of a case steel casting and the residual stresses and deformation of a hydro-turbine blade steel casting were analyzed and predicted using the system. The simulation and the practical results were basically in agreement.     

A study on the numerical simulation of thermal stress during the solidification of shaped castings

In order to study the development of thermal stress and to predict the hot tearing and residual stress of shaped casting, two models were used to carry out the stress analysis of the two stages of solidification. The rheological model [H]–[H|N]–[N|S] was used for the quasi-solid zone while the thermo-elasto-plastic model was used for the period after solidification. Coupling the thermal analysis based on the finite different method with the stress analysis based on the finite element method, a FDM/FEM integrated system of thermal stresses analysis during the solidification process was developed. After experimental verification, the system was put into practical application. The analysis results during the quasi-solid zone show that the visco-plastic strain is an important factor for the occurrence of hot tearing. The hot tearing of a case steel casting and the residual stresses and deformation of a hydro-turbine blade steel casting were analyzed and predicted using the system. The simulation and the practical results were basically in agreement.     

Effects of Y and Zn/Y on hot tearing susceptibility of Mg–Zn–Y–Zr alloys

The effects of Y and Zn/Y on hot tearing susceptibility (HTS) of Mg–6.5Zn–xY–0.5Zr (x?=?4, 9, 12 and 18) and Mg–5Zn–13.5Y–0.5Zr alloys were investigated herein. The results illustrated that HTS of the investigated alloys decreased in the following order: Mg–6.5Zn–4Y–0.5Zr?>?Mg–6.5Zn–18Y–0.5Zr?>?Mg–6.5Zn–9Y–0.5Zr?>?Mg–6.5Zn–12Y–0.5Zr?>?Mg–5Zn–13.5Y–0.5Zr. The results also showed that HTS of the α-Mg-based alloy containing only LPSO phase was lower than that of the alloy containing only W-phase and (I+W) or (W+LPSO) mixed secondary phases. This was attributed to the coherence relationship between LPSO phase and α-Mg, and the bridging effect of LPSO phase.     

铸造镁合金热裂行为的研究进展

热裂是铸造镁合金成形过程中常见的缺陷,该缺陷的产生极大限制了高性能镁合金的工业应用。了解热裂形成机理、掌握热裂评估方法和影响因素对科学选材和批量应用具有重要的理论和实践价值。总结了现有的热裂理论、试验评估方法、热裂影响因素以及大部分镁合金热裂行为的评估结果,期望能为研究同行提供参考。强度理论、液膜理论、晶间搭桥理论、凝固收缩-补偿理论、裂纹形成功理论和冲击应力理论指导了热裂评估设备的设计,并为研究合金成分、凝固条件和微观组织等因素对热裂行为的影响提供了理论基础。现有的热裂数据并非采用同一评估方法获得,很难进行横向比较。未来热裂研究应尽量统一评估所采用的设备方法,以便建立镁合金相关的数据库,进而为大数据科学选材提供可能。     

34Mn2V钢的热加工缺陷

34Mn2V钢坯经火焰切割下料，再经热冲压制成氧气瓶，因氧气瓶底凹面出现热撕裂而造成大量废品。通过试验跟踪和解剖分析，研究了下料坯端面裂纹和半成品瓶底部相应缺陷的本质。结果表明，切割下料时在切割面热影响层发生部分奥氏体化；当切割热消失后，奥氏体化的金属即受到周围冷金属的冷却作用形成马氏体组织。因此，瓶底的局部热撕裂是由下料坯切割时产生的温度梯度和相变引起的。采用预热切割、切割后缓冷和下料坯温度降至环境温度前立即入炉退火都能避免热影响层的开裂。     

Y含量对MgZn9YxZr0.5合金热裂敏感性的影响

基于Clyne-Davies模型,对MgZn9YxZr0.5(x=1,2,4,6,质量分数/%,下同)合金热裂敏感性(CSC)进行预测;采用X射线衍射(XRD)和扫描电子显微镜(SEM)分别对MgZn9YxZr0.5合金的显微组织与断口区域形貌进行观察,并采用"T"型热裂模具测试系统,采集MgZn9YxZr0.5合金凝固收缩应力随温度(或时间)的变化曲线。结果表明:CSC预测值与裂纹体积实测值具有相同的变化趋势,其热裂敏感性从高到低的顺序是:MgZn9Y1Zr0.5>MgZn9Y6Zr0.5>MgZn9Y2Zr0.5>MgZn9Y4Zr0.5;当w(Y)≤4%时,枝晶干涉点温度与热裂纹萌生温度随Y含量增加而降低,而当w(Y)=6%时,枝晶干涉点温度与热裂纹萌生温度随之升高。随Y含量不同,MgZn9YxZr0.5合金凝固过程中析出相类型、含量以及α-Mg枝晶倾向的改变被认为是影响晶界裂纹萌生、扩展和热裂的主要微观机制。     

铸造Al-Cu合金凝固缺陷研究现状

铸造Al-Cu合金的凝固缺陷严重影响了铸件的性能,控制或消除凝固缺陷对提高铸件成品率有重大意义。综述了铸造Al-Cu合金在工程中出现的常见凝固缺陷,如偏析、热裂、显微疏松、缩孔等。重点分析了各类缺陷的形成机理与特点,从合金化、熔铸工艺、热处理工艺、数值模拟等角度提出了减少铸造Al-Cu合金凝固缺陷的方法。     

水玻璃石英砂和呋喃树脂石英砂对铸钢件热裂的影响

热裂是铸钢件生产中最严重的缺陷之一,防止和控制热裂是铸造工作者的共同目标。本文探讨了型砂对铸钢件热裂的作用,指出呋喃树脂石英砂和水玻璃石英砂对热裂的不同影响。     

铸件热裂缺陷预测及其工艺优化

以解决铸件热裂为目的,以生产中遇到的典型铸件为例,利用ProCAST铸造模拟软件,对不同铸件结构和不同浇注工艺条件下的铸件热裂缺陷形成倾向进行了系统研究,得到了优化后的铸件结构和铸造工艺,并进行了生产验证,其结果基本吻合.     

Study of slant fracture in ductile materials

Slant fracture is widely observed during crack growth in thin sheet specimens made of ductile materials, providing a good case for investigating three-dimensional criteria for mixed-mode ductile fracture. To gain an understanding of slant fracture events and to provide insight for establishing a slant fracture criterion, stable tearing fracture experiments on combined tension-torsion (nominal mixed-mode I/III) specimens and nominal Mode I Arcan specimens made of Al 2024-T3 are analyzed using the finite element method under three-dimensional conditions. Two types of finite element models are considered for the study of slant fracture: (a) combined tension-torsion specimens containing stationary, flat and slant cracks subject to loads corresponding to the onset of crack growth, and (b) stable tearing crack growth with slanting in a nominal Mode I Arcan specimen. Analysis results reveal that there exists a strong correlation between certain features of the crack-front effective plastic strain field and the orientation of the slant fracture surface. In particular, it is observed that (a) at the onset of crack growth in the combined tension-torsion experiments, the angular position of the maximum effective plastic strain around the crack front serves as a good indicator for the slant fracture surface orientation during subsequent crack growth; and (b) during stable tearing crack growth in the Mode I Arcan specimen, which experiences a flat-to-slant fracture surface transition, the crack growth path on each section plane through the thickness of the specimen coincides with the angular position of the maximum effective plastic strain around the crack front.     

An investigation on solidification path and hot tearing tendency of Mg−2Zn−3Y−xAl alloys

Based on the improved Clyne‐Davies’ model (CSC_T), the hot tearing tendency of Mg?2Zn?3Y?xAl (x = 0, 0.5, 1, 2, wt.%) alloys were predicted. The concretion path, the solidification characteristic temperature, the dendrite coherent solid fraction were studied by the double thermocouples thermal analysis. The curve of solidification contraction stress with temperature (or time) was measured with a “T” type hot tearing permanent‐mold. The experimental results show that the modified Clyne‐Davies’ model prediction values are in good agreement with the experimental results. Al₂Y phase can be used as the heterogeneous nucleation core of α‐Mg matrix and refine the α‐Mg grain size. It is found that Al₂Y increases, grain refinement is more obvious, dendrite coherence is postponed, and the hot tearing susceptibility of the alloy decreases significantly with increasing aluminum content.     

Deformation and tearing of circular plates with varying support conditions under uniform impulsive loads

Transient dynamic finite element analysis of circular plates with varying support configurations under uniform single square wave form impulsive load has been carried out in FEA package ANSYS. Experimental results of Teeling-Smith and Nurick [The deformation and tearing of thin circular plates subjected to impulsive loads. Int J Impact Eng 1991;11(1):77–91] and Nurick et al. [Tearing of blast loaded plates with clamped boundary conditions. Int J Impact Eng 1996;18(7–8):803–27] for the onset of thinning and tearing at the boundary of clamped circular plates subjected to uniformly loaded air blasts have been used to compare and validate the numerical simulation and procedure. The Mode II failure with respect to clamped circular plates has been simulated using a rupture strain criteria. Mode III failure or plastic shear sliding, has been considered using a shear strain failure criteria as proposed by Wen and Jones for plates. A stiffness reduction scheme has been proposed to decide on the initiation and progression of tearing in conjunction with suitable failure model under Modes II and III. The evolution of deflections, plastic zones, rupture zones and failure modes under the blast loading conditions are found to match well with the experimental results. The validated numerical model has further been used to study the effect of plate thickness on the deformation and tearing response of the circular plates subjected to impulsive loads. The deformation, tearing and shock absorption response of clamped circular plates under uniform impulsive loads with ring support of varying edge configurations at the boundary have also been numerically studied. Further, the response of circular plate–tube combination with varying boundary support configurations has been studied. The plate has been considered at the mid-span of the tube of length equal to the plate diameter with the ends of the tube modelled as clamped. The numerical model has been used to study the effect of tube thickness variations on the deformation and tearing response of the circular plate under shock loads. The response of tube–plate combinations under uniform impulsive loads with ring support at the plate–tube junction have also been numerically studied.