Effects of melt temperature and casting speed on the structure and defect formation during direct-chill casting of an Al-Cu alloy

A thorough experimental investigation of the effects of melt temperature and casting speed on the structure and defect formation during the steady and nonsteady stages of direct-chill (DC) casting of an Al-2.8 pct Cu alloy is performed. In addition, the temperature and melt-flow distributions in the sump of billets cast at different melt temperatures are numerically simulated and used in the discussion on the experimental results. Apart from already known phenomena such as the coarsening of the structure, deepening of the sump, and increased probability of bleed-outs during DC casting with increased casting temperature, a few new observations are made. The increased melt temperature is shown to increase the severity of subsurface segregation, whereas the macrosegregation in the rest of the billet remains virtually unaffected. Hot-tearing susceptibility is strongly diminished by an increased melt superheat. The amount and distribution of “floating” grains is demonstrated to depend on both the melt temperature and the casting speed. The porosity was found to only slightly depend on the melt temperature. The amount of nonequilibrium eutectic in the center of the billet increases with increasing melt temperature. The effects of melt temperature on the dimensions of the sump, transition region, and mushy zone and on the melt-flow pattern in the sump are discussed and used in the interpretation of experimentally observed phenomena. An erratum to this article is available at .     

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.     

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.     

TearSim: A two-phase model addressing hot tearing formation during aluminum direct chill casting

A two-phase mathematical model for the study of hot tearing formation is presented. The model accounts for the main phenomena associated with the formation of hot tears, i.e., the lack of feeding at the late stages of solidification and the localization of viscoplastic deformation. The model incorporates an advanced viscoplastic constitutive model for the coherent part of the mushy zone, allowing for the possibility of dilatation/densification of the semisolid skeleton under applied deformation. Based on quantities computed by the model, a hot tearing criterion is proposed where liquid feeding difficulties and viscoplastic deformation at the late stages of solidification are taken into account. The model is applied to study hot tearing formation during the start-up phase for direct-chill (DC) casting of extrusion ingots, and to discuss the effect of different phenomena and process parameters. The modeling results are also compared to experimentally measured hot tearing susceptibilities, and the model is able to reproduce known experimental trends such as the effect of the casting speed and the importance of the design of the starting block.     

Two-phase modeling directed toward hot tearing formation in aluminum direct chill casting

The two-phase mass and momentum conservation equations governing shrinkage-driven melt flow and thermally induced deformation are formulated for the aluminum direct chill (DC) casting process. Two main mechanisms associated with hot tearing formation during solidification and subsequent cooling are thus addressed simultaneously in the same mathematical model. The approach unifies the two-phase mushy zone model outlined by Farup and Mo, the constitutive relations that treat the mushy zone as a viscoplastic porous medium saturated with liquid outlined by Martin et al., and the “classical” mechanics approach to thermally induced deformations in solid (one-phase) materials using the linear kinematics approximation. A temperature field and a unique solidification path are considered as input to the model. The governing equations are solved for a one-dimensional (1-D) situation with some relevance to the DC casting process. The importance of taking into account the transfer of momentum from the liquid phase to the solid phase is then demonstrated through modeling examples. Furthermore, the modeling results indicate that the constitutive law governing the viscoplastic behavior of the solid skeleton of the mushy zone should take into account that the solid skeleton can be compressed/dilated as well as stress space anisotropy. Calculated peak values for liquid pressure and solid stress turn out to correlate to the hot tearing susceptibility measured in casting trials in the sense that trials having the largest cracks are those for which the highest pressures and stresses are computed.     

Effects on composite composition and structure from compound loading during hot pressing

Translated from Poroshkovaya Metallurgiya, No. 5(341), pp. 1–4, May, 1991.     

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.     

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.     

Finite element method simulation of mushy zone behavior during direct-chill casting of an Al-4.5 pct Cu alloy

In this article, the stresses, strains, sump depth, mushy zone length, and temperature fields are calculated through the simulation of the direct-chill (DC) casting process for a round billet by using a finite-element method (FEM). Focus is put on the mushy zone and solid region close to it. In the center of the billet, circumferential stresses and strains (which play a main role in hot cracking) are tensile close to the solidus temperature, whereas they are compressive near the surface of the billet. The stresses, strains, depth of sump, and length of mushy zone increase with increasing casting speed. They are maximum in the start-up phase and are reduced by applying a ramping procedure in the start-up phase. Stresses, strains, depth of sump, and length of mushy zone are highest in the center of the billet for all casting conditions considered.     

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.     

Influence of surface morphology, water flow rate, and sample thermal history on the boiling-water heat transfer during direct-chill casting of commercial aluminum alloys

An experimental investigation has been conducted on as-cast samples from three commercially significant aluminum alloys (AA1050, AA3004, and AA5182) to quantify the influence of surface morphology, water flow rate, and sample thermal history on the boiling-water heat transfer under conditions similar to those experienced in the direct-chill (DC) casting process. The study involved characterization of the as-cast surface morphology using a laser profilometer and quantification of the sample surface temperature and heat extraction to the cooling water using a DC casting simulator in combination with an inverse heat-conduction (IHC) analysis. The results from the study indicate that alloy’s thermal conductivity, surface morphology, and sample initial temperature all dramatically influence the calculated “boiling curve.” The intensity of the heat extraction was found to be enhanced at high heat fluxes in the nucleate boiling regime as the thermal conductivity was increased and was also found to increase as the surface of the sample became rougher, presumably through promotion of nucleation, growth, and/or detachment of bubbles. The heat transfer was also found to increase with increasing sample starting temperature, resulting in a series of boiling curves dependent on initial sample temperature. Finally, the effect of the water flow rate on heat transfer was found to be comparatively moderate and was limited to the sample with the smooth (machined) surface.     

Effect of low-frequency electromagnetic casting on the castability,microstructure, and tensile properties of direct-chill cast Al-Zn-Mg-Cu alloy

Microstructure and grain boundary segregation of the direct-chill (DC) cast Al-Zn-Mg-Cu (7A60) alloy, with and without low-frequency electromagnetic field, were investigated. The surface quality, hot-tearing tendency, and tensile properties of the ingots manufactured by DC and low-frequency electromagnetic field casting (LFEC) were compared. The results show that LFEC significantly improves the surface quality and reduces the hot-tearing tendency of DC ingots. It is possible to generate a fine, uniform, and equiaxed microstructure with LFEC. Under optimum conditions, the average grain size varies from 30 μm near the surface to 45 μm at the center of the LFEC ingots. Decreasing electromagnetic frequency or increasing intensity significantly refines microstructure, suppresses grain boundary segregation, and increases as-cast fracture strength and elongation. In the range of frequency and ampere turns employed in the experiments, the optimum frequency is found to be 15 to 25 Hz and the number of ampere turns to be 18,000 to 20,000 A_t.     

Transient memory effects during plastic strain cycling of Al-Cu alloy

Results of cyclic tests on a commercial Al-Cu alloy show the transient memory effects mainly observed in the alloy aged at 200°C. Asymmetric tests give experimental proof that the constricted loops observed during the first cycles of this material are a form of strain memory. This transient effect is associated with the back stresses caused by the misfit of θ′ lamellae in the sheared matrix lattice; it is explained by means of a simple accommodation model. The isotropic hardening component of strain-hardening can be determined from cyclic tests by a method of which an outline is presented. According to the model of dislocation shuttling used to describe the cyclic hardening, the reverse straining should repeat the forward hardening: we present experimental proof that this is indeed the case for ε_α > 0.008, except for a difference in stress level caused by isotropic hardening during forward straining. During saturation cycling reduction of the strain amplitude gives rise to a second memory effect: the small loops are formed by curves which closely repeat the former large-amplitude curves. As a corollary the small-amplitude peak stresses are shifted towards the first of these values, whether these are in tension or in compression.     

Microstructural evolution during hot working of ti aluminide alloys: Influence of phase constitution and initial casting texture

The hot-working behavior of γ(TiAl)-based alloys was investigated in order to understand fundamental aspects of the evolution of the microstructure and to establish guidelines for advanced alloy design and processing. The investigations involved a wide range of Al compositions and are based on metallographic investigations of the deformed samples. Particular emphasis was placed on the effects of phase composition and casting texture. It was found that the behavior of dynamic recrystallization was significantly influenced by the Al content of the alloys. Under the same deformation conditions. dynamic recrystallization was fastest for alloys with nearly stoichiometric composition, whereas the recrystallization kinetics decreased for lower or higher Al contents. This result can be attributed to the effect of the Al concentration on the micromechanisms of deformation and diffusion as well as on the initial cast microstructure, which changed from fully lamellar to equiaxed near-γ microstructures by raising the Al content from 45 to 50 at. pct. Further, it was observed that the casting texture, i.e., the orientation of lamellae with respect to the deformation axis, significantly influenced the recrystallization behavior. In this respect, the development of shear bands due to kinking and bending of lamellae is concluded to play an important role in the recrystallization behavior and seems in general, to be a particular feature of the microstructural evolution of lamellar alloys on hot working. R.M. IMAYEV and V.M. IMAYEV, Senior Scientists, formerly with the GKSS Research Centre, Institute for Materials Research     

Microstructural evolution during hot working of Ti aluminide alloys: Influence of phase constitution and initial casting texture

The hot-working behavior of γ(TiAl)-based alloys was investigated in order to understand fundamental aspects of the evolution of the microstructure and to establish guidelines for advanced alloy design and processing. The investigations involved a wide range of Al compositions and are based on metallographic investigations of the deformed samples. Particular emphasis was placed on the effects of phase composition and casting texture. It was found that the behavior of dynamic recrystallization was significantly influenced by the Al content of the alloys. Under the same deformation conditions, dynamic recrystallization was fastest for alloys with nearly stoichiometric composition, whereas the recrystallization kinetics decreased for lower or higher Al contents. This result can be attributed to the effect of the Al concentration on the micromechanisms of deformation and diffusion as well as on the initial cast microstructure, which changed from fully lamellar to equiaxed near-γ microstructures by raising the Al content from 45 to 50 at. pct. Further, it was observed that the casting texture, i.e., the orientation of lamellae with respect to the deformation axis, significantly influenced the recrystallization behavior. In this respect, the development of shear bands due to kinking and bending of lamellae is concluded to play an important role in the recrystallization behavior and seems, in general, to be a particular feature of the microstructural evolution of lamellar alloys on hot working.