Tensile properties of the semi-solid state in solidifying aluminum alloys

Hot tearing is one of the most serious defects encountered in aluminum alloy castings. During solidification of aluminum alloys, the localized region of solidified alloys is submitted to thermally induced strains that can be lead to severe solidification defects, such as shrinkage porosity and hot tearing. The formation of hot tearing is related to the development of local stress or thermal strains. It is such a complicated phenomenon that a full understanding has not been achieved yet, though it has been extensively investigated for decades. Therefore, in order to further understand this complicated phenomenon and establish the mathematical models of hot tearing, it is necessary to obtain the accurate mechanical property data in the mushy zone of alloys. In response to the demand for this purpose, a newly experimental apparatus has been used to perform tensile measurements of aluminum alloys during solidification. Therefore, the tensile properties measurements of the mushy zone in A356 alloy have been carried out. The fracture surfaces and microstructures of the hot tearing samples have been examined by optical microscopy and scanning electron microscopy. The results show that the yield stresses are increasing with the increase of the solid fraction. When the solid fraction is close to one, they will keep stable to a certain value. According to the analysis, the yield stresses will change with the evolution of solid fraction, which is in accordance with the Boltzmann Function.     

High-Temperature Mechanical Behavior and Fracture Analysis of a Low-Carbon Steel Related to Cracking

Cracking in continuously cast steel slabs has been one of the main problems in casting for decades. In recent years, the use of computational models has led to a significant improvement in caster performance and product quality. However, these models require accurate thermomechanical properties as input data, which are either unreliable or nonexistent for many alloys of commercial interest. A major reason for this lack of reliable data is that high-temperature mechanical properties are difficult to measure. Several methods have been developed to assess the material strength during solidification, especially for light alloys. The tensile strength during solidification of a low carbon aluminum-killed (LCAK; obtained from Tata Steel Mainland Europe cast at the DSP plant in IJmuiden, the Netherlands) has been studied by a technique for high-temperature tensile testing, which was developed at Sumitomo Metal Industries in Japan. The experimental technique enables a sample to melt and solidify without a crucible, making possible the accurate measurement of load over a small solidification temperature range. In the current study, the tensile test results are analyzed and the characteristic zero-ductility and zero-strength temperatures are determined for this particular LCAK steel grade. The fracture surfaces are investigated following tensile testing, which provides an invaluable insight into the fracture mechanism and a better understanding with respect to the behavior of the steel during solidification. The role of minor alloying elements, like sulfur, in hot cracking susceptibility is also discussed.     

Rheological behavior of Al-Mg-Si-Cu alloys in the mushy state obtained by partial remelting and partial solidification at high cooling rate

This work investigates the mechanical behavior of two aluminum alloys in the mushy state, the alloy AA6056 and an alloy based on mixing AA6056 and AA4047. These alloys have been studied to give insight into the susceptibility to hot tearing, which occurs during laser welding of AA6056 with 4047 filler wire. Two types of isothermal tensile tests have been conducted: (1) tests during partial remelting and (2) tests after partial solidification at a high cooling rate. Results show that the maximum tensile stress increases with increasing solid volume fraction. Both materials exhibit visco-plastic behavior for solid fractions in the range 0.9 to 0.99, except for a critical solid fraction of 0.97, where the semisolid material also shows minimum ductility. The stress levels observed for the remelting experiments are larger than those found for partial solidification experiments at the same solid fraction due to the influence of the microstructure. The influence of temperature and strain rate on the maximum stress is described by using a constitutive law that takes into account the fraction of grain boundaries wetted by the liquid.     

Simulation of Stresses during Casting of Binary Magnesium-Aluminum Alloys

A viscoplastic stress model is used to predict contraction forces measured during casting of two binary Mg-Al alloys. Force measurements from castings that did not hot tear, together with estimates from data found in the literature, are used to obtain the high-temperature mechanical properties needed in the stress model. In the absence of hot tearing, the simulation results show reasonably good agreement with the measurements. It is found that coherency of the semisolid mush starts at a solid fraction of about 0.5 and that the maximum tensile strength for the Mg-1 and 9 wt pct Al alloys at their final solidification temperatures is 1.5 and 4 MPa, respectively. In the presence of hot tearing, the measured stresses are generally overpredicted, which is attributed to the lack of a fracture model for the mush. Based on the comparison of measured and predicted stresses, it is also shown that coupling of the stress model to feeding flow and macrosegregation calculations is needed in order to accurately predict stresses in the presence of hot tearing.     

Contraction of aluminum alloys during and after solidification

A technique for measuring the linear contraction during and after solidification of aluminum alloys was improved and used for examination of binary and commercial alloys. The effect of experimental parameters, e.g., the length of the mold and the melt level, on the contraction was studied. The correlation between the compositional dependences of the linear contraction in the solidification range and the hot tearing susceptibility was shown for binary Al-Cu and Al-Mg alloys and used for the estimation of hot tearing susceptibility of 6XXX series alloys with copper. The linear thermal contraction coefficients for binary and commercial alloys showed complex behavior at subsolidus temperatures. The technique allows estimation of the contraction coefficient of commercial alloys in a wide range of temperatures and could be helpful for computer simulations of geometrical distortions during directchill (DC) casting.     

Research on the Effect of Pouring Temperature on Hot-Tear Susceptibility of A206 Alloy by Simulation

Cast alloys with wide solidification ranges are prone to hot tearing. This study deals with prediction of hot tearing location and its intensity by computer simulation. The simulation was performed at different pouring temperatures on A206 aluminum alloy. As superheat increases, the critical fraction solid time increases which means the alloy is more susceptible to hot tearing. These theoretical predictions are in complete accordance with experimental results.     

Prediction of hot tearing tendency for multicomponent aluminum alloys

It is of practical importance to be able to predict the hot tearing tendency for multicomponent aluminum alloys. Hot tearing is one of the most common and serious defects that occurs during the casting of commercial aluminum alloys, almost all of which are multicomponent systems. For many years, the main criterion applied to characterize the hot tearing tendency of an alloy system was based on the solidification interval. However, this criterion cannot explain the susceptibility-composition relation between the limits of the pure base metal and the eutectic composition. Clyne and Davies correlated the susceptibility-composition relationship in binary systems based on the concept of the existence of critical time periods during the solidification process when the structure is most vulnerable to cracking. The Scheil equation was used in their model using constant partition coefficient and constant liquidus slope estimated from the phase diagram. In the current study, the authors followed Clyne and Davies’ general idea, and directly coupled the Scheil solidification simulation with phase diagram calculation via PanEngine, a multicomponent phase equilibria calculation interface, and extended the model to higher order systems. The predicted hot tearing tendencies correlated very well with the experimental results of multicomponent aluminum alloys. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA.     

Hot tearing susceptibility of Fe-20.96 Cr-2.13 Ni-0.15 N-4.76 Mn-0.01 Mo duplex stainless steel

The hot tearing susceptibility of a Fe-20.96Cr-2.13Ni-0.15N-4.76Mn-0.01 Mo duplex stainless steel was investigated using method of constrained solidification shrinkage in one dimension.An apparatus for realtime measuring the contraction stress and temperature during solidification was developed,which can achieve the in-situ observation of melting and solidification and avoid the large temperature gradient of casting under the condition of pouring.The results show that the contraction stress increases significantly when the core temperature of casting reaches the liquidus temperature.The contraction stress is released when the core temperature of casting reaches 1456°C.At this temperature,the hot tearing susceptibility of duplex stainless steel is the largest.With decreasing the core temperature to 1363°C,the slope of contraction stress increases,which is related to the ferrite-to-austenite transformation.     

Hot tearing of ternary Mg−Al−Ca alloy castings

Ternary Mg−Al−Ca alloys are the base of a few new creep-resistant, lightweight Mg alloys for automobiles. Hot tearing in Mg−xAl−yCa alloys was studied, including Mg−4Al−0.5Ca, Mg−4Al−1.5Ca. Mg−4Al−2.5Ca, Mg−4Al−3.5Ca, Mg−5Al−2.5Ca, and Mg−6Al−2.5Ca, by constrained rod casting (CRC) in a steel mold—with a movable pouring cup to keep solidification therein from interfering with the rising tension in the rods. The hot tearing susceptibility, based on measured crack widths and crack locations, decreased significantly with increasing Ca content (y) but did not change much with the Al content (x). An instrumented CRC with a steel mold was developed to detect the onset of hot tearing by monitoring the tension in the rod during casting and the temperature near the cracking site. It was further improved by reducing the rod diameter to detect hot tearing earlier, at a higher temperature, and with a clear peak in the load curve. To further understand the hot tearing susceptibility of these alloys, the secondary phases, eutectic content, solidification path, and freezing range were examined. Alloy Mg−4Al−0.5Ca had the widest freezing range and the lowest eutectic content and was most susceptible to hot tearing, while alloys Mg−4Al−3.5Ca and Mg−6Al−2.5Ca were the opposite. Mg−4Al−0.5Ca had the widest freezing range (183 °C) because its solidification path led to the formation of Mg₁₇Al₁₂ from the liquid at a very low temperature (440°C). The application of the results to die casting was discussed. G. CAO, formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

Physical modeling of the deformation mechanisms of semisolid bodies and a mechanical criterion for hot tearing

The mechanical response of a semisolid body to an applied, uniaxial strain rate has been expressed as a function of strain by modifying an existing analysis based on an idealized representation of the microstructure. An existing mechanical criterion for hot tearing of the semisolid body has been adapted to the deformation mechanisms. The resulting hot tearing model shows that the strength of the body depends on the strain, the viscosity of the intergranular fluid, the solid fraction, the isothermal compressibility of the fluid, the surface tension of the liquid, the limiting liquid-film thickness for viscous flow and a parameter m, which describes microstructure. The effect of each parameter on the mechanical response and the onset of hot tearing has been examined for ranges of values relevant to aluminum alloys and the direct-chill (DC) casting process. The parameter testing has shown that the mechanical response predicted by the model agrees well with some experimental data for both the mechanisms of fracture and the parameters that govern the process. An adjustment of unknown model parameters to experimental data would permit use of the model as a constitutive law and a fracture criterion for numerical modeling of hot tearing during the solidification of Al alloys by DC casting.     

Simulation of Semi-Solid Material Mechanical Behavior Using a Combined Discrete/Finite Element Method

As a necessary step toward the quantitative prediction of hot tearing defects, a three-dimensional stress–strain simulation based on a combined finite element (FE)/discrete element method (DEM) has been developed that is capable of predicting the mechanical behavior of semisolid metallic alloys during solidification. The solidification model used for generating the initial solid–liquid structure is based on a Voronoi tessellation of randomly distributed nucleation centers and a solute diffusion model for each element of this tessellation. At a given fraction of solid, the deformation is then simulated with the solid grains being modeled using an elastoviscoplastic constitutive law, whereas the remaining liquid layers at grain boundaries are approximated by flexible connectors, each consisting of a spring element and a damper element acting in parallel. The model predictions have been validated against Al-Cu alloy experimental data from the literature. The results show that a combined FE/DEM approach is able to express the overall mechanical behavior of semisolid alloys at the macroscale based on the morphology of the grain structure. For the first time, the localization of strain in the intergranular regions is taken into account. Thus, this approach constitutes an indispensible step towards the development of a comprehensive model of hot tearing.     

稀土Pr对Al-Zn-Mg-Cu-Zr合金组织和性能的影响

采用铸锭冶金法制备含稀土元素Pr的Al-Zn-Mg-Cu-Zr合金,并通过金相分析以及拉伸性能、晶间腐蚀和剥落腐蚀性能的测试研究价格相对低廉的Pr对Al-Zn-Mg-Cu-Zr合金显微组织、力学性能和腐蚀性能的影响.结果表明,添加稀土元素Pr能影响合金铸态组织中第二相的析出,并显著抑制合金在变形和热处理过程中再结晶的发生,在保持合金的强度及弹性模量的同时,改善合金抗晶间腐蚀和剥落腐蚀的性能,并提高合金的塑性.     

Influence of grain refinement on hot tearing in B206 and A319 aluminum alloys

Aluminum-copper (Al-Cu) and aluminum-silicon-copper (Al-Si-Cu) alloys are among the most common aluminum casting alloys. Aluminum alloy B206 is a relatively new Al-Cu alloy with high strength and ductility at room and elevated temperatures, while A319 is an Al-Si-Cu alloy with good strength and excellent wear resistance. However, despite their advantages, when these alloys are cast via the permanent mold casting (PMC) process, they show a high susceptibility to hot tearing. Grain refinement has shown promise as a means to reducing hot tears in aluminum alloys. In this study, Ti-B grain refiner was used to investigate the effect of grain refinement on hot tearing in B206 and A319 aluminum alloys during permanent mold casting. The results suggest that Ti-B additions significantly reduced hot tearing in B206 and A319. Grain sizes were also seen to reduce significantly in both alloys with addition of Ti-B grain refiner. However, Ti-B grain refiner had a diverse effect on alloy grain morphology, as a dendritic morphology in B206 was transformed to a more globular one, while in A319, the grain structure remained dendritic.     

Hot Tearing Modeling: A Microstructural Approach Applied to Steel Solidification

Hot tearing during solidification processes has been deeply investigated in past and recent years through testing, modeling, and development of a number of macroscopic hot tearing criteria. The objective is predicting the crack occurrence during industrial solidification processes, which, in the steel production, are mainly ingot and continuous casting. The present work is inspired by the criterion proposed in the work of Bellet et al.[1] called CBC criterion, from which the methodological approach and experimental data used for calibration, related to nine carbon steels, have been derived. The proposed hot tearing criterion adopts as parameters: primary and secondary arm spacing, the mechanical resistance near the solidus temperature, the solidification parameters G (gradient) and v (dendrite tip velocity), the brittle range extension in the dendritic front and the temperature of formation of manganese sulfides. The new formulation is an attempt to substitute to brittle temperature range and steel content, appearing in the CBC criterion, the dendritic structure characteristics, in the aim of: (a) moving toward a generalized expression of the cracking index applicable to different steel classes; (b) introducing the dependence of the crack susceptibility on the cooling conditions. The agreement of the new hot tearing index values with the experimental ones is of the same kind as that of the CBC criterion, indicating that the parameters and the dependences adopted in the new criterion make a sense. Further study and experimental work are needed to assess the influence of the microstructure morphology on the hot cracking sensitivity and to check the suitability of the approach to a wider range of steel compositions.     

Hot Tear Susceptibility of Al-Mg-Si-Fe Alloys with Varying Iron Contents

Hot tear susceptibility in cast Al-0.52Si-0.34Mg-xFe 6060 aluminum alloys was investigated using a hot tearing test apparatus to simulate hot tearing in DC casting. The test apparatus has two cast bars, one that is used to measure the load response and one which is fixed at both ends to restrain thermal contraction so that hot tearing can be observed and rated where it occurred. The iron (Fe) content, ranging from 0.02 to 0.5 wt pct, was seen to have a major influence on the load response during solidification and the tear rating of these alloys. The findings are discussed in terms of Rappaz-Drezet-Gremaud (RDG) model sensitivity analysis and related to the effect of Fe content on the morphology and prevalence of the β-Al₅FeSi and α-AlFeSi intermetallic phases and their influence on the coherency and coalescence of the microstructure.     

Effect of Hot Rolling on Grain Refining Performance of Al–5Ti–1B Master Alloy on Hot Tearing on Al–7Si–3Cu Alloy

It has been known experimentally that TiAl₃ acts as a powerful nucleant for the solidification of aluminum from the melt; however, a full microscopic understanding is still lacking. To improve microscopic understanding, hot rolling technique has been performed to the Al–5Ti–1B alloy and the effect of shape and size of the particles on grain refinement has been studied. The effect of hot rolling of Al–5Ti–1B master alloy on its grain refining performance and hot tearing have been studied by OM, XRD, and SEM. Hot rolling improves the grain refining performance of this master alloy, which is required to reduce hot tearing in Al–7Si–3Cu alloy. The improvement in grain refining performance of Al–5Ti–1B master alloy on rolling is due to the fracture of larger TiAl₃ particles into fine particles during rolling. The presented results illustrate that the morphology of TiAl₃ particles alter from the plate-like structure in the as-cast condition Al–5Ti–1B master alloy to the blocky type after rolling due to the fragmentation of plate-like structures. The grain refining response and effect on hot tearing of Al–7Si–3Cu alloy have been studied with as-cast and rolled Al–5Ti–1B master alloys. The results display hot-rolled master alloys revealing enhanced grain refining performance and minimizing hot tear tendency of the alloy at much lower addition level as compared to as-cast master alloys.     

Hot Tearing Characteristics of Binary Mg-Gd Alloy Castings

Hot tearing characteristics of Mg-xGd (x = 1, 2, 5 and 10 wt pct) binary alloys have been studied in a constrained rod casting apparatus attached with a load cell and data acquisition system. The onset temperature of the hot tearing was identified from the force drop in the force–temperature–time curve, and the corresponding onset solid fraction was obtained from the fraction solid–temperature curve derived using Scheil non-equilibrium solidification model. The results indicate that the onset solid fraction for the hot tear decreased as the Gd content increased. The susceptibility defined by the total tear volume measurements by the X-ray micro-tomography technique indicates that the susceptibility increased with increase in Gd content to reach a maximum at 2 pct and then reduced with further increase in Gd to reach a minimum with 10 pct Gd. The high susceptibility observed in Mg-2 pct Gd was attributed to its cellular or columnar grain structure, which facilitated easy tear propagation, high strain at the onset with little amount of remaining liquid. In contrast, the lowest susceptibility of Mg-10 pct Gd was related to its equiaxed grain structure, which effectively accommodated the strain during solidification by reorienting themselves and the ability of the Gd-rich liquid to partially or completely refill the tear at the end of solidification. The results also indicate that the increase in mold temperature [723 K (450 °C)] significantly reduced the total crack volume and hence reduced the susceptibility, which was attributed to the increase in the hot spot size and lesser total stain at the hot spot region.     

Effect of Silicon Concentration on the Dendrite Coherency Point in Al–Si Binary Alloys

Dendrite coherency is important to the formation of the solidification structure. The coherency point is a temperature at which the microstructure starts to bridge and develop some mechanical resistance. It is still too early in the solidification process for hot tearing to develop. Dendrite coherency point (DCP) characteristics in Al–Si binary alloys have been studied by double thermocouples method during solidification process. The results indicate that the DCP and solid fraction at DCP are decreased with an increase in silicon concentration. As for the unrefined Al–xSi (x = 1, 3, 5, 7 and 9 wt%) system alloys, the solid fraction at DCP varies from 0.14 to 0.38 and the corresponding dendrite coherency temperature varies from 598.6 to 653.8 °C. In addition, there is an approximate nonlinear relationship between DCP and silicon concentration. For the binary Al–Si hypoeutectic alloys, the change of DCP is not obvious by the grain refinement and modification treatment for the melt.     

增压涡轮用K424高温合金组织特征及热裂倾向性

针对K424精密铸造增压涡轮叶片出现的热裂问题,采用组织观察和计算模拟的方法,分析增压涡轮用K424合金特性以及涡轮叶片铸造中出现热裂的原因,并对比K418合金提出合金热裂倾向性的影响因素以及减少热裂的建议.结果表明,铸造增压涡轮热裂倾向性与合金特性及铸件特性等有关.K424合金中Al和Ti元素含量较高,导致合金中共晶组织含量多且尺寸大,与K418合金相比,热裂倾向性较大;另一方面,由于铸件叶片位置厚度小且曲率变化大,易形成应力集中,导致热裂.为减小热裂倾向性,需控制K424合金中Al和Ti元素含量并选择合理的工艺参数.