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。     

镁合金热裂行为研究进展

热裂是合金铸造过程中最常见的严重铸造缺陷之一。随着镁合金材料应用范围的不断扩大和复杂薄壁铸件的增加,热裂成为镁合金商业化应用的瓶颈。全面阐述了国内外对镁合金热裂行为的测试方法,归纳出了镁合金热裂形成机制,以及镁合金中热裂倾向定量评价方法等。根据多年的深入研究结果详细总结了镁合金热裂的影响因素。同时,还介绍了镁合金热裂的理论预测方法和最新数值模拟模型。为建立稳定、统一的热裂评价标准提供了有实际价值的参考。     

镁合金热裂行为研究的测试方法探讨

针对镁合金的特点，就合金热裂行为研究的测试方法进行了实验研究和分析。用热裂行为测试系统测量合金凝固过程中的温度、线收缩和收缩应力，用铸造热分析系统分析铸造凝固条件下的相析出，这为研究合金的热裂形成机理提供了重要的实验手段。采用热裂环法可以有效地评价合金的热裂倾向性，而等长度试棒法(无论金属型还是砂型)则不适于测试镁合金。     

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。     

铝合金铸件中的凝固缺陷形成机理及预测

孔洞与热裂是铸件在凝固过程中最常遇到的典型缺陷。两者均形成于合金凝固末期,与液相补缩不足相关,但又有各自的形成机理。铝合金熔体对氢气有很大的溶解度,凝固过程中又因为溶解度的剧降而析出。孔洞是液相中过饱和的气体压力与凝固收缩引起的压力降共同作用的结果。热裂则是在凝固末期由于铸件收缩受阻而产生的应力以及液相补缩不足而导致的,其不仅与合金性质、铸造条件有关,并且受铸件形状的影响。文章基于近年来该方面的理论研究成果,总结了关于孔洞与热裂的形成机理以及几种目前所应用的经典预测模型,并对这几种模型的理论基础以及所考虑的关键参数进行了分析与讨论,在此基础上提出了未来新模型的研究方向和亟需解决的关键问题。     

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.     

瓦楞辊中频感应淬火热裂的计算机动态仿真

用有限元方法建立了瓦楞辊中频感应淬火模型,分析了瓦楞辊各参数变化时对热应力分布的影响,探讨了热处理过程中楞齿变形及热裂的原因并计算机仿真.     

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。     

薄带连铸镁合金的研究与思考

综述了单辊铸造和双辊铸造镁合金薄带的研究进展与产业化现状,提出了存在的问题,分析了影响镁合金薄带浇铸速度的因素,指出,改善浇铸系统的传热能力是提高镁合金薄带浇铸速度的关键;立式双辊连铸系统具有更高的传热能力,镁合金薄带连铸应借鉴立式双辊连铸钢带技术.     

金属型重力铸造高强度铝铜合金

目的 针对铝铜合金热裂敏感性高、难以进行金属型铸造的难题,研究其合金成分、铸造工艺、组织与性能的关系。方法 首先充分分析铸造铝铜合金的凝固机制与特点,在ZL205A合金基础上,设计了新型合金成分以及两种铸造试样模,经过熔炼、铸造与热处理,获得砂型铸造与金属型铸造试样,进行力学性能测试、组织分析,并观察试样的抗热裂性。结果 两种铸造试样都获得了较高的力学性能,铸态及T6态全部为等轴晶组织,成功避免了热裂问题,并提高了合金的力学性能;金属型铸造试制了油马达铸件。结论 证明了通过铝铜合金的成分优化设计,以及含钪复合细化剂的合理匹配,采用金属型铸造获得等轴晶组织,进而避免粗大枝晶组织,是克服热裂问题的关键,从而实现金属型铸造高强度铝铜合金。     

A modified hot tearing criterion for direct chill casting of aluminium alloys

Using the good criteria to predict hot tearing is very important during DC casting of aluminium alloys. Among all the hot tearing criteria, a fracture-mechanics based SKK criterion proposed by Suyitno et al. has made considerable improvements in the hot tearing prediction. However, its obtained hot tearing susceptibility (HTS) evolution during solidification is also not completely consistent with real industrial production circumstances, especially when approaching the solidus temperature. In this paper, some further modifications are made based on the SKK criterion to emphasise the important effect of solid bridging/grain coalescence on hot tear propagation. It is proved that the HTS evolution in freezing range predicted by the modified hot tearing criterion is in good agreement with casting practice.     

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.     

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.     

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.     

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.     

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。     

稀土Mg-Gd系合金的研究现状与展望

Mg-Gd系镁合金因满足航空航天材料轻量化、高温强度的要求而受到极大关注。简述了稀土Mg-Gd系合金的研究现状。从合金氧化夹杂、流动性能、Gd在合金中的偏析以及合金的热裂性等方面阐述了浇铸法制备Mg-Gd系合金的铸造性能。通过统计比较了近年来在Mg-Gd基础上设计出的合金在不同状态下的强度、塑性数据。阐述了合金在低温、室温下的强度与抗蠕变性能、高温压缩性能和短时强度等高温力学性能及各自的断裂机制。最后,叙述了该合金系的耐腐蚀性能,并展望了该合金系的研究前景。