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.     

Thermal strain in the mushy zone related to hot tearing

A volume-averaged two-phase model addressing the main transport phenomena associated with hot tearing in an isotropic mushy zone during solidification of metallic alloys has recently been presented elsewhere along with a new hot tearing criterion addressing both inadequate melt feeding and excessive deformation at relatively high solid fractions. The viscoplastic deformation in the mushy zone is addressed by a model in which the coherent mush is considered as a porous medium saturated with liquid. The thermal straining of the mush is accounted for by a recently developed model taking into account that there is no thermal strain in the mushy zone at low solid fractions because the dendrites then are free to move in the liquid, and that the thermal strain in the mushy zone tends toward the thermal strain in the fully solidified material when the solid fraction tends toward one. In the present work, the authors determined how variations in the parameters of the constitutive equation for thermal strain influence the hot tearing susceptibility calculated by the criterion. It turns out that varying the parameters in this equation has a signiicant effect on both liquid pressure drop and viscoplastic strain, which are key parameters in the hot tearing criterion. However, changing the parameters in this constitutive equation will result in changes in the viscoplastic strain and the liquid pressure drop that have opposite effects on the hot tearing susceptibility. The net effect on the hot tearing susceptibility is thus small.     

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.     

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.     

Nanoparticle-Induced Superior Hot Tearing Resistance of A206 Alloy

Al-Cu alloys (such as A206) offer high strength and high fracture toughness at both room and elevated temperatures. However, their widespread applications are limited because of their high susceptibility to hot tearing. This article presents a nanotechnology approach to enhance hot-tearing resistance for A206. Specifically, γ-Al₂O₃ nanoparticles were used, and their effects on the hot-tearing resistance of the as-cast Al-4.5Cu alloy (A206) were investigated. While it is well known that grain refinement can improve the hot-tearing resistance of cast Al alloys, the current study demonstrated that nanoparticles can be much more effective in the case of A206. The hot-tearing susceptibilities (HTSs) of A206 alloy and its Al₂O₃ nanocomposite were evaluated by constrained rod casting (CRC) with a steel mold. Monolithic A206 and M206 (the Ti-free version of A206) alloys with the B contents of 20, 40, and 300 ppm from an Al-5Ti-1B master alloy addition were also cast under the same conditions for comparison. The results showed that with an addition of 1 wt pct γ-Al₂O₃ nanoparticles, the extent of hot tearing in A206 alloys was markedly reduced to nearly that of A356, an Al-Si alloy highly resistant to hot tearing. As compared with grain-refined A206 or M206, the hot-tearing resistance of the nanocomposites was significantly better, even though the grain size was not reduced as much. Microstructural analysis suggested that γ-Al₂O₃ nanoparticles modified the solidification microstructure of the eutectic of θ-Al₂Cu and α-Al, as well as refined primary grains, resulting in the enhancement of the hot-tearing resistance of A206 to a level similar to that of A356 alloy.     

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.     

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.     

Experimental and Theoretical Studies of the Hot Tearing Behavior of Al-xZn-2Mg-2Cu Alloys

The effect of Zn addition on the hot tearing susceptibilities of non-refined Al-xZn-2Mg-2Cu (x = 2-12 wt pct) alloys was investigated via direct crack observations and load response measurements. The obtained experimental results were compared with the predictions made using a modified Rappaz–Drezet–Gremaud (RDG) hot tearing model. Both the minimum crack width and load at the non-equilibrium solidus (NES) temperature (which served as a good indicator of hot tearing response) were observed at a Zn concentration of approximately 4 wt pct, and the formation of cracks was highly correlated with the predictions made via the modified RDG hot tearing model (although the obtained relationship critically depended on the magnitude of fraction solid at which solid coalescence was expected to occur). Furthermore, it was confirmed from the load development pattern that the addition of Zn into the matrix of Al-xZn-2Mg-2Cu alloys promoted the formation of coalesced networks, which decreased their corresponding coalescence fraction solids.

     

Hot tearing criteria evaluation for direct-chill casting of an Al-4.5 pct Cu alloy

Predicting the occurrence of hot tears in the direct-chill (DC) casting of aluminum alloys by numerical simulation is a crucial step for avoiding such defects. In this study, eight hot tearing criteria proposed in the literature have been implemented in a finite-element method simulation of the DC casting process and have been evaluated. These criteria were based on limitations of feeding, mechanical ductility, or both. It is concluded that six criteria give a higher cracking sensitivity for a higher casting velocity and that five criteria give a higher cracking sensitivity for the center location of the billet. This is considered in qualitative accordance with casting practice. Seven criteria indicate that use of a ramping procedure (lower casting speed during start-up phase) does not make a significant difference. However, in industrial practice, this is a common procedure, needed for avoiding hot cracking. Only one criterion is in qualitative accordance with casting practice, but it fails to quantitatively predict the hot tearing occurrence during DC casting.     

Hot plane strain compression testing of aluminum alloys by channel-die compression

A thoroughly tested, high-temperature channel-die compression (CDC) rig is described for simulating hot plane strain compression of metallic alloys up to 500 °C. The equipment is currently used to characterize the flow stress and microstructure evolution in hot-rolled Al alloys. It has been validated by several tests involving (1) metallographic analysis of deformed samples; (2) flow stress comparisons with the same, or similar alloys deformed in conventional uniaxial or plane strain compression; and (3) microstructure and texture measurements. The use of modern lubricants enables one to obtain accurate flow stresses and true plane strain deformations that are homogeneous over 80 pct of the sample. The equipment also features rapid heating and cooling systems to minimize thermally-induced microstructure changes. Some results on high-temperature slip systems, hot deformation textures, and microstructures, and the behavior of constituent particles are outlined to illustrate the advantages of the technique.     

7050铝合金半连铸过程应力场及开裂倾向

7050铝合金在半连铸生产过程中发生热裂和冷裂的倾向很高,不但影响了产品的质量和生产效率,还可能导致生产事故.工厂常采用试错法以找到最优的工艺参数,但这种方法成本高且效率低.运用数值模拟的方法再现铸造过程中各物理场的变化情况,已成为优化铝合金熔铸工艺非常重要的研究手段.本文通过将温度场、流场和应力场进行直接耦合,对7050铝合金的半连铸过程进行了数值模拟研究.结果显示,在糊状区沿铸锭宽度方向的应力和应变分量最大,特别是在起始铸造阶段,因而最容易在起始阶段产生垂直于宽度方向的热裂纹.冷裂与铸锭内应力集中有关,根据计算可知铸锭在冷却至200℃时冷裂倾向最大.由实际裂纹所处的部位及所需的临界尺寸可以推测,该冷裂纹极有可能是糊状区产生的热裂纹在低温时失稳扩展而形成的.      

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

A Study on Hot Tearing Susceptibility of Al–Cu,Al–Mg,and Al–Zn alloys

The present investigation deals with the hot tearing susceptibility of A206, A518, and A713 alloys. The hot tearing tests of the mentioned alloys were conducted at three different pouring temperatures using sand mold casting. Metallic cores designed to facilitate constrained radial contraction of the aforementioned alloys were used for casting. Macroscopic cracks were found in all the samples except in A518 alloy. It was observed that pouring temperatural and grain size have significant effect on crack susceptibility. Among the investigated alloys, A713 was found to be extremely prone to hot tearing. The microstructure characteristics of the alloys were studied using optical and scanning electron microscopy. Relationships between the pouring temperature, grain size and crack lengths of the alloys were also established.     

A New Criterion for Prediction of Hot Tearing Susceptibility of Cast Alloys

A new criterion for prediction of hot tearing susceptibility of cast alloys is suggested which takes into account the effects of both important mechanical and metallurgical factors and is believed to be less sensitive to the presence of volume defects such as bifilms and inclusions. The criterion was validated by studying the hot tearing tendency of Al-Cu alloy. In conformity with the experimental results, the new criterion predicted reduction of hot tearing tendency with increasing the copper content.     

Effects of Recovery Behavior and Strain-Rate Dependence of Stress–Strain Curve on Prediction Accuracy of Thermal Stress Analysis During Casting

Recovery behavior (recovery) and strain-rate dependence of the stress–strain curve (strain-rate dependence) are incorporated into constitutive equations of alloys to predict residual stress and thermal stress during casting. Nevertheless, few studies have systematically investigated the effects of these metallurgical phenomena on the prediction accuracy of thermal stress in a casting. This study compares the thermal stress analysis results with in situ thermal stress measurement results of an Al-Si-Cu specimen during casting. The results underscore the importance for the alloy constitutive equation of incorporating strain-rate dependence to predict thermal stress that develops at high temperatures where the alloy shows strong strain-rate dependence of the stress–strain curve. However, the prediction accuracy of the thermal stress developed at low temperatures did not improve by considering the strain-rate dependence. Incorporating recovery into the constitutive equation improved the accuracy of the simulated thermal stress at low temperatures. Results of comparison implied that the constitutive equation should include strain-rate dependence to simulate defects that develop from thermal stress at high temperatures, such as hot tearing and hot cracking. Recovery should be incorporated into the alloy constitutive equation to predict the casting residual stress and deformation caused by the thermal stress developed mainly in the low temperature range.

     

A new investigated method on hot tearing behavior in aluminum alloys

A new investigated method based on the applied forces for assessment on hot tearing behavior in aluminum alloys is introduced in the paper. In this method, molten metal is cast in the rod-shaped mold cavity. One side of the casting specimen is hooked by a steel bolt which restrains its free contraction and transfers the tensile forces during solidification. A steel threaded rod connected to a load cell which records the realtime measurement of the tensile forces during every experiment. Thermal history is monitored by k-type thermocouple. The data of the temperature and tensile forces are acquired by a data acquisition system. Through the use of this method, it is possible to estimate the initiation of hot tearing, its propagation and cracking during solidification. It is also obtained the critical tensile stress for hot tearing initiated and fractured. Experiment is conducted with A356 alloys to investigate the accuracy of the apparatus and modify its operating parameter. Accordingly, the tensile forces curves, the temperature curves and the microstructure of the test specimen are obtained. This data provide useful information about hot tearing formation and solidification characteristics, from which their quantitative relations are derived. In this manner, the hot tearing behavior in aluminum alloys can be studied.     

A Benchmark Study on Casting Residual Stress

A benchmark study was undertaken for casting residual stress measurements through neutron diffraction, which was subsequently used to validate the accuracy of simulation prediction. The “stress lattice” specimen geometry was designed such that subsequent castings would generate adequate residual stresses during solidification and cooling of ductile cast iron, without any cracks. The residual stresses in the cast specimen were measured using neutron diffraction. Considering the difficulty in accessing the neutron diffraction facility, these measurements can be considered as a benchmark for casting simulation validations. Simulations were performed using the identical specimen geometry and casting conditions for predictions of residual stresses. The simulation predictions were found to agree well with the experimentally measured residual stresses. The experimentally validated model can be subsequently used to predict residual stresses in different cast components. This enables incorporation of the residual stresses at the design phase along with external loads for accurate predictions of fatigue and fracture performance of the cast components.     

Modeling the Stress-Strain Behavior and Hot Tearing during Direct Chill Casting of an AZ31 Magnesium Billet

Quantitative knowledge of the thermal mechanical history experienced during direct chill (DC) casting aids the scientific understanding of the process especially in terms of defect formation such as hot tearing. In this work, a thermomechanical finite element (FE) model has been developed to simulate the DC casting of magnesium alloy AZ31 billets. The mathematical model simulates the evolution of temperature, stress, and strain within the billet during an industrial DC casting process. These quantities were then used to calculate the evolution in pressure, and hence hot tearing tendency, within the semisolid regime via the Rappaz–Drezet–Gremaud (RDG) criterion. The temperature predictions were validated against experimental thermocouple data measured during a plant trial at an industrial magnesium DC casting facility. In addition, the residual elastic strains predicted by the model were compared to residual strain measurements made at the Canadian Neutron Beam Centre (CNBC) using a magnesium billet produced during the industrial casting trial. The validated model was then used to quantitatively assess the impact of casting speed on the hot tearing tendency in AZ31 billets.     

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.     

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