This research aims to evaluate horizontal and vertical constrained rod casting (CRC) molds on hot tearing susceptibility (HTS) of Al-xCu casting alloys with 2.2, 3.6, 7.5, and 12.5 percent Cu. The hot tears on the casting product were observed using a macroscopic approach. In addition, the hot tearing susceptibility of each casting product prepared using these molds was evaluated using the HTS formula. The results show that the vertical CRC mold has a higher HTS value than the horizontal CRC mold. The rod length is a significant factor in causing hot tearing. Longer rods are more susceptible to hot tearing. The horizontal CRC mold provides a clearer effect of rod length and Cu composition on the average HTS value. In the vertical CRC mold, the effect of Cu composition on the average HTS value is less clear. Therefore, it is highly recommended to use horizontal CRC mold for HTS testing of aluminum casting alloys.@en

The alloy design and homogenization processes are intimately associated with the microstructure, phase composition and performance for Al-Zn-Mg-Cu alloys. The microstructures and phase composition of a series of Al-Zn-Mg-Cu alloys before and after the homogenization treatments were investigated along with thermodynamic calculation to understand the underlying relationship. The eutectic microstructures (α-Al + M (Mg(ZnAlCu)2)) are dominating with Cu-enriched [AlCuMgZn] particles, both depending on the Zn:Mg ratio and (Cu + Mg) content, in addition to minor constituent θ (Al2Cu) and Al7Cu2Fe phases in the as-cast alloys. The optimal homogenization process was suggested based on the analysis of the residual phases (i.e., the S (Al2CuMg) phase) since all (for low/mediate-(Cu + Mg) alloys) or partially (for high-(Cu + Mg) alloys (∼>4.24 wt%)) S (Al2CuMg) particles were dissolved during the homogenization. This residual S phase may be transformed from the primary M and/or Cu-enriched [AlCuMgZn] phases. The homogenization kinetics calculation results agreed well with above experimental results. A critical (Cu + Mg) level and a linear correlation between Cu and Mg concentrations were revealed based on the thermodynamically modelling, which can be conductive to determine the optimal homogenization process. Furthermore, the solubility limit and stoichiometric balance principles based on controlling the homogenized microstructures can guide the composition design for advanced high-strength aluminum alloys.@en

The retrogression and re-aging (RRA) processes, aimed mainly at tailoring intergranular precipitates, could significantly improve the corrosion resistance (i.e., stress corrosion cracking resistance) without considerably decreasing the strength, which signifies that an efficient control of the size, distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the (mid-)thick high-strength Al alloy plates. Compared to RRA process with retrogression at 200 °C (T77), this study investigated the impact of a modified RRA process (MT77) with lower retrogression temperatures (155-175 °C) and first-stage under-aging on the properties of a high-strength AA7050 Al alloy, in combination with detailed precipitate characterization. The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time, along with the increased electrical conductivity. The rapid responsiveness of microstructure/property typical of retrogression at 200 °C was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the (mid-)thick plates. On the other hand, higher retrogression temperature facilitated more intragranular η precipitates, coarse intergranular precipitates and wide precipitate free zones, which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys. With the preferred MT77 process, the high strength approaching T6 level as well as good corrosion resistance was achieved. However, though a relatively homogeneous through-thickness strength was obtained, some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed, possibly related to the quenching sensitivity. The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.@en

The paper aims to evaluate the effect of chemical composition on the Hot Cracking Susceptibility (HCS) using mechanical and non-mechanical hot cracking criteria during solidification. The criteria were SKK as a mechanical criterion. Feurer, Clyne Davis, and Katgerman as non-mechanical criteria. The criteria were implemented at various parameters to evaluate their abilities in the hot cracking susceptibility (HCS) prediction at varied chemical composition. In this study, The Mg content was varied in Al9Zn (1, 1.5, 2, 2.5 %wt.) Mg2Cu alloys and Cu content in Al9Zn2Mg (1, 1.5, 2, 2.5 %wt.) Cu alloys. The validation of the result is also conducted by comparing with the experimental data. Based on Feurer criterion, The hot cracking initiates at lower temperature and at higher critical rate of feeding and shrinkage with Cu content, and the hot cracking initiates at higher temperature with Mg content, and it initiates at higher critical rate of feeding and shrinkage from 1 up to 1.5 of Mg, and the critical rate of feeding and shrinkage remains constant from 1.5 up to 2.5 of Mg. Based on Clyne & Davies, the HCS decreases with Cu content from 1 up to 2 of Cu, and it increases from 2 up to 2.5 of Cu. The HCS decreases with Mg content from 1 up to 2 of Mg, and it remains constant from 2 up to 2.5 of Mg. Based on Katgerman criterion, the HCS decreases with Cu content from 1 up to 1.5 of Cu, it increases from 1.5 up to 2 of Cu, and it decreases from 2 up to 2.5 of Cu. The HCS decreases sequentially with Mg content. Based on SKK criterion, the HCS curves shift to the right with Cu content which means that the hot cracking initiates at lower temperature, and the HCS curves shift to the left with Mg content which means that the hot cracking initiates at higher temperature with Mg content. The Feurer, Clyne & Davies, and some specific range for SKK criteria are in agreement for the effect of Cu content on HCS of alloys, and Katgerman and some specific range for Clyne&Davies criteria are in agreement for the effect of Mg content on HCS of alloys.

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Over the last 4 decades, remarkable progress has been made in the modelling of casting processes. The development of casting models is well reflected in the proceedings of the 15 Modelling of Casting, Welding and Advanced Solidification Processes (MCWASP) conferences that have been held since 1980. Computer simulations have enabled a better understanding of the physical phenomena involved during solidification. Modelling gives the opportunity to uncouple the physical processes. Furthermore, quantities that are difficult or impossible to measure experimentally can be calculated using computer simulations e.g. flow patterns and recalescence. However, when it comes to accurately predicting casting performance and in particular, the occurrence of defects like cracks, segregation and porosity there is certainly some way to go. In this paper, the current understanding of the main mechanisms of defect formation during shape and DC casting processes will be reviewed and requirements will be discussed to give a direction to making casting models more predictive and quantitative.

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Hot tearing is one of the most severe and irreversible casting defects for many metallic materials. In 2004, Eskin et al. published a review paper in which the development of hot tearing of aluminium alloys was evaluated (Eskin and Suyitno, 2004). Sixteen years have passed and this domain has undergone considerable development. Nevertheless, an updated systematic description of this field has not been presented. Therefore, this article presents the latest research status of the hot tearing during the casting of aluminium alloys. The first part explains the hot tearing phenomenon and its occurrence mechanism. The second part presents a detailed description and analysis of the characterisation methods of the mushy zone mechanical properties and hot tearing susceptibility. The third part presents considerable data pertaining to the mushy zone behaviour, including those of the linear contraction and load behaviour during solidification, semi-solid strength and ductility, and characteristic points related to hot tearing. The fourth part examines the effect of the composition and casting process parameters on the hot tearing susceptibility of aluminium alloys. The fifth part describes the hot tearing simulations and the associated criteria and mechanisms. Finally, recommendations for the further development of hot tearing research are presented.

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issn 1757-8981@en

NEO@en

NEO@en

NEO@en

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.

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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.

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Cold cracking is a severe challenge during the direct-chill casting of high-strength 7××× series aluminum alloys. A finite element method (FEM) simulation combined with a cold cracking criterion has been demonstrated to possess obvious technical advantages in cold cracking prediction. However, the current absence of mechanical properties and effective criteria for 7××× series aluminum alloys inhibits the progress of the technique. In this study, the corresponding mechanical properties of four typical 7××× series aluminum alloys are investigated. The cold cracking tendencies of the alloys are evaluated by a new cold cracking index (CCI) developed by the authors. It is shown that AA7055 has the highest cold cracking propensity among the four alloys, followed by AA7050, AA7085, and AA7022, respectively. The cold cracking tendency is basically consistent with the amount of nonequilibrium eutectics of the alloys under the same casting process. It is also shown that the application of water wiper can effectively decrease the occurrence of cold cracking.@en

The knowledge on constitutive mechanical behavior at the temperatures close to the solidus is essential for predicting high-temperature deformation and fracture, e.g. cold and hot cracking of aluminum alloys. In this work we studied the tensile mechanical properties of an as-cast AA7050 alloy in a near-solidus temperature regime. Tensile tests were carried out using Gleeble-3800™ system at temperatures from 400 to 465 °C and at strain rates from 0.0005 to 0.05 s⁻¹. The results show that the strength decreases with increasing temperature and decreasing strain rate. Meanwhile, ductility decreases with the increase of temperature and strain rate. The constitutive parameters were extracted by fitting the test data to the extended-Ludwik and creep-law equations. The parameters for the extended-Ludwik equation are continuous with the values from a lower temperature regime obtained earlier, while the parameters for the creep-law equation are comparable with those obtained on other 7XXX aluminum alloys. We observed a change in fracture mode at 450 °C; from ductile transgranular to intergranular. This temperature coincides with the discontinuity point of the temperature-ductility slope. On the fracture surface of a sample that was deformed at 465 °C with a strain rate of 0.0005 s⁻¹, we observed features characteristic of micro-superplasticity. Considering the test conditions, viscous flows of incipient melt or liquid-like substances are suggested to be responsible for the formation of this feature.

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The knowledge on constitutive mechanical behavior at the temperatures close to the solidus is essential for predicting high-temperature deformation and fracture, e.g. cold and hot cracking of aluminum alloys. In this work we studied the tensile mechanical properties of an as-cast AA7050 alloy in a near-solidus temperature regime. Tensile tests were carried out using Gleeble-3800™ system at temperatures from 400 to 465 °C and at strain rates from 0.0005 to 0.05 s⁻¹. The results show that the strength decreases with increasing temperature and decreasing strain rate. Meanwhile, ductility decreases with the increase of temperature and strain rate. The constitutive parameters were extracted by fitting the test data to the extended-Ludwik and creep-law equations. The parameters for the extended-Ludwik equation are continuous with the values from a lower temperature regime obtained earlier, while the parameters for the creep-law equation are comparable with those obtained on other 7XXX aluminum alloys. We observed a change in fracture mode at 450 °C; from ductile transgranular to intergranular. This temperature coincides with the discontinuity point of the temperature-ductility slope. On the fracture surface of a sample that was deformed at 465 °C with a strain rate of 0.0005 s⁻¹, we observed features characteristic of micro-superplasticity. Considering the test conditions, viscous flows of incipient melt or liquid-like substances are suggested to be responsible for the formation of this feature.

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Network models allow to study fluid flow around numerous grains during solidification. It appears that the predicted behavior of a network model depends heavily on the rate of solidification estimated for each individual channel. Therefore, a suitable 1D solidification model is investigated with direct methods. The model includes heat, solute and mass conservation as well as the Wilson¿Frenkel formula. An accurate but CPU-time consuming method is presented. Keywords: Network modeling; Simulation; Solidification@en

AA7050 is an aluminum alloy with superior mechanical properties; however, it is prone to hot tearing (HT) during its production via direct-chill casting. This study focuses on extracting constitutive parameters of the alloy thermomechanical behavior in semi-solid state as well as gaining insight in its failure behavior. Tensile tests were performed using an Instron 5944 at solid fractions between 0.85 (550 °C) and 1.0 (465 °C), at deformation rates of 0.2 and 2 mm/min. The results showed that there are three mechanical behavior regimes in this solid fraction range: ductile at 1.0 (T = 465 °C) ≤ f_s < 0.97 (T = 473 °C), brittle at 0.97 (T = 473 °C) ≤ f_s ≤ 0.9 (T = 485 °C) and then ductile again (at 0.9 (T = 485 °C) < f_s ≤ 0.85 (T = 550 °C)). Fracture surface analysis revealed that the fracture mode was mostly intergranular with fracture propagating through solid bridges as well. Semi-solid constitutive parameters were obtained by making a simple thermal model and numerical tensile tests in ALSIM software package and comparing the simulation results with experimental mechanical tests. The extracted constitutive parameters and available information from the literature support the fact that AA7050 is more susceptible to HT than AA5182 and Al-2 wt pct Cu alloys. The obtained parameters can further enhance the predictive capability of computer simulations of direct-chill casting.

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New 7xxx aluminum alloys with high alloying contents are being designed, which could induce serious hot tearing defects during direct-chill (DC) casting. Among all factors affecting hot tearing of 7xxx alloys, undoubtedly alloying elements play a significant role. In this study, the effect of main alloying elements (Zn, Mg, and Cu) on hot tearing of grain-refined Al-xZn-yMg-zCu alloys was investigated by a dedicated hot tearing rating apparatus simulating the DC-casting process. It was found that the minimum and maximum hot tearing susceptibilities occur for 4 to 6 and 9 wt pct Zn, respectively, indicating the complicated effect of Zn content. The hot tearing resistance of grain-refined Al-9Zn-yMg-zCu alloys is enhanced with increasing Mg content but is deteriorated with increasing Cu content. This can be attributed to the interaction of the thermal stresses, melt feeding, and final eutectics. The observed tendencies of the main alloying elements on hot tearing were also confirmed for four commercial 7xxx alloys. In addition, both the load value at non-equilibrium solidus and the SKK criterion proposed by Suyitno et al. using measured load developments were found to be good indicators in predicting hot tearing susceptibility. This study can provide a beneficial guide in designing 7xxx alloys considering the potential occurrence of hot cracks beforehand.@en

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.

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