基于流变学模型的铸钢试件凝固过程应力应变数值模拟

建立了铸件凝固过程热应力模拟的一维流变学本构方程,并且对一端受约束一端带热节的铸钢试棒进行了数值模拟,进而研究了热裂机理。模拟结果表明,随着凝固过程的进行,在热节处粘塑性（Ｂｉｎｇｈａｍ）体的应变急剧增大而弹性应变急剧减小。浇注温度和砂型初始温度越高,热节处粘塑性应变越大,而热应力越小。并且热节处表观粘度随凝固进行逐渐增大,在凝固末期急剧增大。因此粘塑性应变决定了热裂的产生,并且热裂发生在凝固末期。     

铸钢件凝固过程三维温度场热应力场的数值模拟与缩孔（松）热裂的判定

以带凹槽圆筒形铸钢件为对象，应用有限元结构分析软件ＡＢＡＱＵＳ，建立了凝固过程三维温度场和热应力场的分析软件，提出了铸钢件缩孔（松）和热裂和判据，并指出采用高导热能力和低弹性模量芯砂是防止铸钢件热裂的有效措施。计算机预测缩孔（松）和热裂倾向性与实际浇注结果一致。探讨了用呋喃树脂砂芯生产铸钢件产生热裂的机制。     

砂型条件下铸件凝固过程热裂形成的流变学探讨

讨论了铸件凝固过程在枝晶间液体宏观流动补缩停止时，热裂形成的流变学过程。指出热裂起源于宾汉体，受“类表面张力”作用而扩展。理论分析结果与实验研究结果［４～７］均认为，塑性应变能够反映热裂的倾向性是一致的。     

Zn对Mg-9Al合金凝固行为的影响

研究了Zn元素对Mg-9Akl合金的凝固行为，尤其是对热裂倾向性的影响。通过热分析手段考察合金的凝固特征。用热裂环法评价合金的热裂倾向性，引入了热裂倾向性（HSC）因子的概念。并通过金相观察、扫描电镜和电子探针对合金的凝固、热裂行为进行了研究。研究发现，在凝固过程中An,Al元素由于晶间偏析而在晶界富集。Zn元素的加入，增加了晶界低熔点相的量，并降低了其熔点，从而明显增加了合金的热裂倾向性。Mg-9Al-xZn合金的热凝产生于凝固后期，呈沿晶断裂。加入Zn元素对Mg-9Al合金的晶粒有细化作用。     

Zn和RE对Mg-9Al合金热裂倾向性的影响

用热裂环法考察合金的热裂倾向性，并对合金的冷却曲线和铸态组织进行了分析。Zn加入Mg—9Al合金降低了合金的凝固结束温度，平均每增加0．1％，合金的凝固结束温度降低0．7℃。Zn促进了Mgl7Al12相在晶界的析出，降低了凝固结束温度，并显著增加了合金的热裂倾向性。RE对Mg—9Al合金的热裂倾向性和凝固结束温度影响很小。但当Zn含量超过0．8％时，RE的加入可以显著降低Mg—9Al-Zn合金的热裂倾向性。     

AZ91合金的热裂行为研究

介绍了一种新的研究合金热裂行为的方法．通过精确记录合金凝固过程中的应力和温度信息，并结合金相手段，对AZ91合金的热裂行为进行了研究．实验结果表明，在实验条件下AZ91合金最可能产生热裂的温度为实际凝固结束温度424.2℃，凝固后期共晶组织的出现是AZ91合金产生热裂的直接诱因．     

热裂的故障树分析

研究认为热裂是铸造生产中产生的一种故障，首次利用工业预测技术中最具实际用途的故障树分析技术，建立了热裂故障树。对所建故障树进行了定性分析及定量计算，指出了热裂产生的主要原因及发生概率，为铸件质量的科学管理和铸件缺陷的微机帮助诊断提供一种有效方法。     

热裂的故障树分析

研究认为热裂是铸造生产中产生的一种故障，首次利用工业预测技术中最具实际用途的故障树分析技术，建立了热裂故障树。对所建故障树进行了定性分析及定量计算，指出了热裂产生的主要原因及发生概率，为铸件质量的科学管理和铸件缺陷的微机帮助诊断提供一种有效方法。     

晶界状态对合金热裂倾向的影响

进行了定向凝固Ａｌ－Ｃｕ合金和Ｒｅｎｅ１２５合金晶界状态分析，并与合金热裂倾向性试验结果对比，揭示了晶界状态对合金热裂倾向的影响。     

化学成分对定向凝固Al—Cu合金热裂倾向的影响

本文提出了热裂倾向评定的半定量等级标准，并进行了不同成分Ａｌ－Ｃｕ合金定向凝固热裂试验。理论计算与试验结果比较，证实了化学成分对定向凝固合金热裂倾向的影响符合凝固过程的收缩补偿理论，以及柱晶组织对形成晶间搭桥的影响。     

Effect of grain boundary fraction on castability of a directionally solidified nickel alloy

The effect of grain boundary (GB) fraction on hot tearing during directional solidification was explored. The increase of GB fraction was found to reduce the hot tearing tendency. The eutectic melt is finely dispersed and increasingly discontinuous at the GBs when the GB fraction is increased. The change of eutectic melt at the GBs is related to the reduced concentration of GB elements or impurities. The better castability is attributed to the uniform distribution of strain due to the presence of more GBs and the stronger GB cohesion because of the larger bridging areas.     

Hot tearing studies in AA5182

One of the major problems during direct chill (DC) casting is hot tearing. These tears initiate during solidification of the alloy and may run through the entire ingot. To study the hot tearing mechanism, tensile tests were carried out in semisolid state and at low strain rates, and crack propagation was studied in situ by scanning electron microscopy (SEM). These experimentally induced cracks were compared with hot tears developed in an AA5182 ingot during a casting trial in an industrial research facility. Similarities in the microstructure of the tensile test specimens and the hot tears indicate that hot tearing can be simulated by performing tensile tests at semisolid temperatures. The experimental data were compared with existing hot tearing models and it was concluded that the latter are restricted to relatively high liquid fractions because they do not take into account the existence of solid bridges in the crack.     

Effects of grain refiner additions (Zr,Ti–B) and of mould variables on hot tearing susceptibility of recently developed Al–2 wt-%Cu alloy

Abstract

The hot tearing susceptibility of the new Al–2 wt-%Cu based alloys prepared using Zr and Ti–B additions was tested using the constrained rod casting mould under different mould variables. The 206 alloy type was used to evaluate the results obtained from this new alloy. It was found that the hot tearing susceptibility of the alloys under investigation decreases proportionally as the mould temperature is increased; thus, the hot tearing susceptibility of the Al–2 wt-%Cu and 206 alloys decreases from 21 for both the alloys to 3 and 9 respectively as the mould temperature is increased from 250 to 450°C. This beneficial effect of elevated mould temperatures may be attributed to a reduction in the contraction strain rate and in the porosity level. Grain refinement additions of Ti–B or Zr–Ti–B enhance the hot tearing resistance of the Al–2 wt-%Cu to a significant level.     

Hot tearing behavior of NZ30K Mg alloy under progressive solidification

Progressive solidification is usually considered an effective strategy to reduce the hot tearing susceptibility of a cast component.In this study,special constrained plate castings with progressive changes in cross-section were designed,which enabled progressive solidification.The hot tearing behavior of a newly developed NZ30 K Mg alloy(Mg-3.0 Nd-0.2 Zn-Zr,wt.%)was studied under progressive solidification using various mold temperature distributions and constraint lengths.Of these,a homogeneous mold temperature distribution is found to be the best option to avoid hot tearing,followed by a local low mold temperature distribution(with a chiller),then a gradient mold temperature distribution.Unexpectedly,compared with the homogeneous mold temperature distribution,adding a chiller does not provide any further reduction in the hot tearing susceptibility of the NZ30 K Mg alloy.A high mold temperature and a short constraint length increase the hot tearing resistance of cast Mg alloys.Progressive solidification is not a sufficient and necessary condition to avoid the formation of hot tearing.The two key factors that determine the occurrence of hot tearing under progressive solidification are the maximum cooling rate and the constraint length.Decreasing these values can reduce the incidence of hot tearing.     

Hot-tearing in Aluminium Copper Alloys

An experimental technique has been developed to investigate hot tearing. In this technique either the load or strain built up during casting could be measured as a function of the mould wall temperature. Experiments were carried out in Al—Cu alloys. Although there was considerable scatter in the results, it was concluded that strength began to build up in the casting when either the liquid volume fraction reached about 4% or when the eutectic temperature was reached. It was also concluded that the initial crack or hot tear occurred in a thin film of liquid or in the solid just below the eutectic temperature. It is suggested that the crack occurs in the liquid when the alloy composition is less than the most susceptible hot tearing composition and in the solid for higher compositions. The crack occurs in the liquid when the liquid volume fraction reaches about 2%. A numerical analysis was made of the heat flow in the casting so that corrections could be made to the thermocouple readings and a technique was developed to calculate the heat transfer coefficient from temperatures measured near the mould wall.     

浇注温度对间接挤压铸造Al-5Cu合金的影响

浇注温度作为间接挤压铸造的一个重要工艺参数，对Al—5Cu合金的热裂倾向和力学性能有一定的影响。试验中凋节了不同的浇注温度，通过热裂率和热裂系数的结果可知，适当的提高浇注温度有利于减小热裂倾向，获得组织致密的铸件。试验结果表明，在750℃时挤压铸件抗拉强度为273、8MPa，该温度下的铸件经过T6热处理后其抗拉强度为450、0MPa；伸长率在T4状态下为17．6％。     

熔体氢含量对Al-Cu合金热裂行为的影响

Al-Cu合金具有较宽的凝固间隔,并且由于供给不足容易出现热裂和气孔缺陷。采用约束棒铸造（CRC）模具研究了氢含量对Al-xCu合金热裂敏感性（HTS）的影响。通过分析熔体中不同氢含量的Al-xCu合金的热裂敏感性值、断裂形态和微观结构,研究了孔隙形成对热裂行为的影响。结果表明,随着熔体氢含量的增加,合金在凝固后期由于晶粒粗化和液相供给不足,热裂敏感性明显增加。提出了一种基于孔隙率的热裂形成机制,以解释孔隙率和热裂之间的相互作用。     

GH3128合金的高温流变行为与组织演变规律

通过对GH3128合金进行热模拟压缩试验,研究了该合金在变形温度950~1150 ℃、应变速率0.01~10 s^-1及应变量30%~70%条件下的流变特征。通过绘制合金流变应力曲线,并基于Arrhenius模型建立了GH3128合金的本构方程。在此基础上,获取了变形量30%~65%的材料加工图,并结合GH3128合金完全再结晶条件图,明确了合金在高温变形过程中组织演变同塑性变形参数之间关系。此外,通过对碳化物的金相分析,探明了合金在热变形过程中碳化物的演变规律。结果表明:GH3128合金热加工激活能约为305 kJ/mol,合理的加工区域为:变形温度1050~1100 ℃,应变速率0.1 s^-1左右。此时合金内碳化物基本回溶,组织再结晶充分,晶粒尺寸可控制在10 μm以下。     

A macro- and micromechanics investigation of hot cracking in duplex steels

The relationship between microstructure and high temperature ductile tearing in duplex stainless steels has been investigated. Several grades were considered corresponding to different chemical compositions, different volume fractions and morphologies of the ferrite and austenite phases and different oxide inclusion contents. The high temperature cracking resistance has been quantified using both the essential work of fracture (EWF) and the fracture strain. The EWF discriminates the different grades of duplex steels and the different microstructures in terms of hot tearing resistance better than does the fracture strain. Metallographic characterization reveals that damage preferentially nucleates near inclusions at the austenite/ferrite boundary. Voids grow inside the ferrite until they coalesce. Damage develops more rapidly when increasing either the mismatch of rheology between the phases, which was evaluated by micro-scale strain measurements, or the inclusion content. The cracking resistance is related to the plastic work performed in the fracture process zone whereas the fracture strain depends on the damage kinetics. Both processes involve length scales related to the morphology and to the microstructure dimensions. Guidelines for improving the hot cracking resistance of duplex steels are formulated.