Search Results (63)

The formation of hot tearing during direct chill casting of aluminum alloys, specifically AA6111, is a significant challenge in the production of ingots for industrial applications. This study investigates the role of hydrostatic pressure and tensile stress in the formation of hot tearing during direct chill casting of rectangular ingots. Combining experimental results and finite element modeling with ABAQUS/CAE 2022, the mechanical behavior of the semi-solid AA6111 alloy was analyzed under different cooling conditions. “Hot” (low water flow) and “Cold” (high water flow) conditions were the two types of cooling conditions that produced cracked and sound ingots, respectively. The outcomes indicate that high tensile stress and localized negative hydrostatic pressure in the hot condition are the main factors promoting the initiation and propagation of cracks in the mushy zone, whereas the improvement of the cooling conditions reduces these defects. Full article

This study explores the feasibility of generating a globular microstructure in situ during the thixotropic 3D printing of the EN AW-4043 alloy, starting from a conventional cold-rolled wire. Thermodynamic simulations using Thermo-Calc software were first conducted to identify the semi-solid processing window of the alloy, based on the evolution of liquid and solid fractions as a function of temperature. Guided by these results, thermal treatments were performed on cold-rolled wires to promote the formation of a globular microstructure. A laboratory-scale printing head prototype was then designed and built to test continuous heating and deposition conditions representative of a thixotropic additive manufacturing process. The results showed that a globular microstructure could be achieved in the cold-rolled EN AW-4043 wires by heating them at 590 °C for 5 min in a static muffle furnace. A similar effect was observed when continuously heating the wire while it flowed through the heated printing head. Preliminary deposition tests confirmed the viability of this approach and demonstrated that thixotropic 3D printing of EN AW-4043 alloy is achievable without the need for pre-globular feedstock. Full article

The 7075 aluminum alloy semi-solid billet is prepared using the extrusion alloy semi-solid isothermal treatment (EASSIT) process. These findings indicate that as the isothermal time increases, there is a noticeable increase in both the average grain size (AGS) and shape factor (SF). The relationship between the AGS, SF, and isothermal temperature is complex due to the influence of grain refinement mechanisms. The HV0.2 of isothermal samples decreased with the increase in isothermal temperature, which may be related to the increase in liquid-phase composition and AGS; Cu and Si show obvious segregation at grain boundaries and within intracrystalline droplets. The segregation of Cu and Si in the initially melted solid grains leads to the creation of intracrystalline droplets. The diffraction peaks of Al₇Cu₂Fe, Al₆(Cu, Fe), Al₂CuMg, and MgZn₂ gradually decrease as the isothermal temperature increases. Due to the influence of the grain refinement mechanism and melting mechanism, the coarsening behavior of grains at high isothermal temperatures is more complicated, and the coarsening rate constant shows an increment followed by a subsequent decrease as the isothermal temperature rises. The coarsening kinetics of 7075 aluminum alloy in a semi-solid state can be described using the LSW equation of n = 3. Full article

In this study, the thixotropic behavior of an Fe-rich Al-1.2Si-1.1Fe-0.8Zn aluminum alloy was thoroughly investigated. Firstly, ideal semi-solid billets were prepared through thermal deformation-induced isothermal spheroidization (TDIIS). The application of severe plastic deformation (SPD) via hot upsetting provided a strong driving force for recrystallization. As a result, the fibrous elongated grains gradually transformed into equiaxed grains following the TDIIS process. Overall, the grain size decreased with increasing deformation. However, as the temperature rose, the grain size initially decreased and then increased. The optimal conditions for the hot upsetting process were determined to be a temperature of 400 °C and a deformation of 50%. Under these conditions, the average grain size was 71.1 μm, and the shape factor was approximately 0.8, showcasing the excellent thixotropic properties of the semi-solid billets. Furthermore, the microstructure and mechanical properties of the thixotropic Al-1.2Si-1.1Fe-0.8Zn components were examined. These components, which were deep-cavity cylinders, exhibited an inconsistent wall thickness. Due to variations in the extrusion pressure, the grain morphology transitioned from dendritic at the upper part of cylinder wall to equiaxed at the bottom. This transition caused the elongation of the cylinder wall, resulting in it being lower than the cylinder bottom. During the thixoforming process, the equiaxed grains underwent deformation, and new recrystallized grains were formed. The multiscale synergy between the deformed grains, recrystallized grains and subgrains enabled the Fe-rich Al-1.2Si-1.1Fe-0.8Zn aluminum alloy to achieve well-balanced mechanical properties. Full article

In this study, the mechanical properties of SSM-ADC12 aluminum alloy specimens with a ZnAl4Cu3 zinc alloy interlayer were observed after Transient Liquid Phase Diffusion Bonding (TLPDB), a welding process conducted in a semi-solid state. The purpose of the experiment was to study how the following parameters—bonding temperature (400, 430, 460, 490, and 520 °C), bonding time (60, 90, and 120 min), and thickness of the ZnAl4Cu3 zinc alloy (0.5, 1.0, and 2.0 mm)—affect the mechanical properties and the types of defects that formed. The results show that the bonding strength varied significantly with different parameters following the TLPDB process. A maximum bonding strength of 32.21 MPa was achieved at a bonding temperature of 490 °C, with 20 min of bonding and a ZnAl4Cu3 zinc alloy layer that was 2.0 mm thick. Conversely, changing the welding parameters influenced the bonding strength. A minimum bonding strength of 2.73 MPa was achieved at a bonding temperature of 400 °C, with a bonding time of 90 min and a ZnAl4Cu3 zinc alloy interlayer that was 2.0 mm thick. The Vickers microhardness results showed that the bonded zone had a lower hardness value compared to the base materials (BMs) of the SSM-ADC12 aluminum alloy (86.60 HV) and the ZnAl4Cu3 zinc alloy (129.37 HV). The maximum hardness was 83.27 HV, which resulted from a bonding temperature of 520 °C, a bonding time of 90 min, and a ZnAl4Cu3 zinc alloy that was 2.0 mm thick. However, in the near interface, the hardness value increased because of the formation of MgZn₂ intermetallic compounds (IMCs). The fatigue results showed that the stress amplitude was 31.21 MPa in the BMs of the SSM-ADC12 aluminum alloy and 20.92 MPa in the material that results from this TLPDB process (TLPDB Material) when the limit of cyclic loading exceeded 10⁶ cycles. Microstructural examination revealed that transformation from a β-eutectic Si IMC recrystallization structure to η(Zn–Al–Cu) and β(Al₂Mg₃Zn₃) IMCs occurred. A size reduction to a width of 6–11 µm and a length of 16–44 µm was observed via SEM. Finally, voids or porosity and bucking defects were found in this experiment. Full article

The main aim of this research was to investigate the aluminum AlSi9Cu3(Fe) machining chips recycling possibility utilizing a direct hot extrusion process and thixoforming. The thixo feedstock was prepared directly from the aluminum alloy AlSi9Cu3(Fe) machining chips waste without any remelting step. The machining chips were compacted, and direct hot extruded to create the solid samples and thixo feedstock. The aluminum alloy AlSi9Cu3(Fe) machining chips had a high degree of plastic deformation and after extrusion and heating in the semisolid temperature range, the suitable globular microstructure was achieved, which is a precondition for a successful thixoforming process. This approach can be characterized as a semisolid recycling process with a lower energy consumption, a higher material yield, and reduced greenhouse gas emissions into the atmosphere compared with conventional casting and recycling. Optical metallography, scanning electron microscopy accompanied with energy dispersive spectroscopy, electrical conductivity, and mechanical properties investigation were performed on the reference casted sample with a dendritic microstructure, the extruded sample with a severely deformed microstructure, and finally the thixoformed samples with a globular microstructure produced with different parameters, according to the Taguchi L4 (2³) experimental plan. Full article

As an important piece of equipment for hydrogen energy application, the hydrogen internal combustion engine is helpful for the realization of zero carbon emissions, where the aluminum connecting rod is one of the key core components. A semi-solid forging forming process for the 7075 aluminum alloy connecting rod is proposed in this work. The influence of process parameters, such as the forging ratio, sustaining temperature, and duration time, on the microstructures of the semi-solid blank is experimentally investigated. The macroscopic morphology, metallographic structure, and physical properties of the connecting-rod parts are analyzed. Reasonable process parameters for preparing the semi-solid blank are obtained from the experimental results. Under the reasonable parameters, the average grain size is 41.48~42.57 μm, and the average shape factor is 0.80~0.81. The yield strength and tensile strength improvement ratio of the connecting rod produced by the proposed process are 47.07% and 20.89%, respectively. Full article

Advanced processing techniques are required to produce functionally graded metal matrix composites due to the metallurgical conditions required during production. In this study, we developed a novel approach for this task by using a combination of two different methods to produce functionally graded 7075 Al/SiC_p (5–20 wt.%) composites. The first process was direct semisolid stirring, which was used to prevent particle agglomeration, brittle reaction products, floating or settling of the reinforcements, and poor wettability. The second process was sequential squeeze casting, which enabled liquid diffusion between the two composite layers that were used to produce a functionally graded aluminum matrix composite. Thus, a method was developed to eliminate the problems encountered in the production of particle-reinforced metal matrix composite materials using liquid stirring methods and to produce composite materials with the desired functionally graded structure. The resulting functionally graded material was subjected to spectrometer analyses, density measurements, and metallographic examinations to determine the characteristics of its layers and interfacial zones, as well as to assess the formation of the graded structure. The results indicate the potential of using this new combined manufacturing method, which is efficient and controllable, to produce functionally graded metal matrix composites. Full article

We propose a methodology for the rheological characterization of a semisolid metal slurry using experimental squeeze-flow data. The slurry is modeled as a structural thixotropic viscoplastic material, obeying the regularized Herschel–Bulkley constitutive equation. All rheological parameters are assumed to vary with the structure parameter that is governed by first-order kinetics accounting for the material structure breakdown and build-up. The squeeze flow is simulated using finite elements in a Lagrangian framework. The evolution of the sample height has been studied for wide ranges of the Bingham and Reynolds numbers, the power-law exponent as well as the kinetics parameters of the structure parameter. Systematic comparisons have been carried out with available experimental data on a semisolid aluminum alloy (A356), where the sample is compressed from its top side under a specified strain of 80% at a temperature of 582 °C, while the bottom side remains fixed. Excellent agreement with the experimental data could be achieved provided that at the initial instances (up to 0.01 s) of the experiment, the applied load is much higher than the nominal experimental load and that the yield stress and the power-law exponent vary linearly with the structure parameter. The first assumption implies that a different model, such as an elastoviscoplastic one, needs to be employed during the initial stages of the experiment. As for the second one, the evolution of the sample height can be reproduced allowing the yield stress to vary from 0 (no structure) to a maximum nominal value (full structure) and the power-law exponent from 0.2 to 1.4, i.e., from the shear-thinning to the shear-thickening regime. These variations are consistent with the internal microstructure variation pattern known to be exhibited by semisolid slurries. Full article

A semi-solid route is expected to be a fabrication method that can fabricate aluminum alloy foams with a variety of mechanical properties, but the allowance fluctuation of the fabrication conditions of aluminum alloy foams with high reproducibility is not clear. The objective of this study was to reveal the allowance fluctuation between the setting temperature and the actual temperature of the melt to fabricate stable foams, having pores with small pores and high circularity, and the influence of the increasing volume fraction of the solid on the pore morphology. Al-Si alloy foams were fabricated five times by adding a blowing agent into a semi-solid slurry under the same setting fabrication conditions, such as the temperature and concentration of oxygen in the atmosphere. The results of small relative standard deviations of pore diameter and circularity indicated that the conducted fabrication process had high reproducibility, even if the volume fraction of the solid changed in a range of 5%. When the volume fraction of the solid exceeds the minimal fraction of primary crystals for prevention of drainage, the clogging effect works more efficiently because the ratio of clogged cell walls increases. Additionally, the preferred range of the volume fraction of the solid for the fabrication of stable foam was revealed to be around 15% to 35%. Full article

The non-dendritic microstructure plays a crucial role in determining the rheological properties of semi-solid alloys, which are of the utmost importance for the successful industrial application of the thixoforging process. To further understand the impact of the reheating process on the evolution of microstructure and thixotropic deformation behavior in the semi-solid state, a hot extruded and T6 treated 7075 aluminum alloy was reheated to the selected temperature ranges using varying heating rates. Subsequently, thixo-compression tests were performed. The study found that during reheating and isothermal holding, the elongated microstructure of the as-supplied alloy can transform into equiaxed or spherical grains. The presence of recrystallized grains was found to be closely linked to the penetration of the liquid phase into the recrystallized grain boundaries. Furthermore, it was observed that higher heating rates resulted in smaller grain sizes. The thixotropic flow behavior of the alloy with various microstructures was analyzed using the true stress–strain curves obtained by thixo-compression experiments, which exhibited three stages: a rapid increase in true stress to a peak value, followed by a decrease in true stress and a steady stress until the end of compression. The stress fluctuated with strain during the formation of the slurry at a strain rate of 10 s⁻¹, indicating the significant role of strain rate in material flow during semisolid formation. Full article

Despite the dozens of earlier research verifications, the contribution of shearing of molten metallic alloys during their solidification to grain formation is still ambiguous. Also, modeling of this phenomenon has received very little attention. Experiments were conducted in this study to investigate the effect of the shear rate on the density, size, and shape factor of the formed grains up to a solid fraction of 0.15 for the solidifying A356 aluminum alloy in the coaxial cylinder viscometer. The rheology of the formed semisolid slurry was studied as well. Results exhibited morphological evolution and grain refinement. The grain number density increased from 5 × 10⁸ m⁻³ in the absence of melt shearing to reach 4 × 10⁹ m⁻³ at the shear rate of 250 s⁻¹. Also, the shape factor was improved to reach 0.78. Based on the experimental investigations, the grain number density under shearing was correlated to the shear rate and the grain number density in the absence of shearing via an empirical formula. A shear-dependent grain multiplication factor was deduced. The alloy exhibited a shear-thinning behavior where the viscosity obeyed the power law with a constant and an exponent of 0.9264 and 0.468, respectively. Moreover, the measured data were fitted to several proposed viscosity models and the model of Hirai et al. showed the best fit; therefore, it was recommended for predicting the viscosity of semisolid slurries. Full article

In this study, an enclosed cooling slope channel (ECSC) was used to produce a semi-solid slurry of the 7075 aluminum alloy. The effects of the pouring temperature and the rate of cooling water on the microstructure of the semi-solid slurry were studied. The microstructure, solidification behavior, mechanical properties, and fracture mechanism of rheological squeeze casting (Rheo-SC) and liquid squeeze casting (LSC) samples were compared. The results indicate that lowering the pouring temperature and increasing the rate of cooling water can refine the crystals of the semi-solid slurry. The best process is a pouring temperature of 670 °C and a rate of cooling water of 200 L/h. The microstructure of the LSC samples was made up of coarse dendritic crystals, but the microstructure of the Rheo-SC samples was made up of almost spherical primary α₁-Al and refined secondary α₂-Al under this method. The ultimate tensile strength, yield strength, and elongation of the Rheo-SC samples were 238 MPa, 151 MPa, and 5.2%, respectively, which were 10%, 10.5%, and 44.4% higher than those of the LSC sample. The key factor contributing to the increased performance of the Rheo-SC samples is the combination of decreased casting flaws, strengthened grain refinement, and improved segregation. Full article

The microstructure plays a key role in the mechanical properties of hyper-eutectic Al-Si alloys. In this study, we investigate the microstructural evolution of rheo-die-casting (RDC) on the Al-15Si-4Cu-0.5Mg alloy using a mechanical rotational barrel system. Our findings demonstrate that higher rotational speed and pouring temperature reduce the size and roundness of primary Si particles in the semisolid slurry. Additionally, during RDC, the dendritic aluminum matrix and skeletal iron-containing inter metallics are sheared off, leading to a more uniform and dispersed Al₂Cu phase. Ultimately, our rheo-diecasting results indicate the formation of a near globular aluminum matrix, fine primary Si particles, and a homogeneous Al₂Cu phase, thus highlighting the efficacy of this processing method for improving the microstructure and properties of the Al-15Si-4Cu-0.5Mg alloy. We suggest that these results hold promise for enhancing the quality of aluminum-based alloys in various industrial applications. Full article

The effect of final thermomechanical treatment (FTMT) on the mechanical properties and microstructure of a T-Mg₃₂(Al Zn)₄₉ phase precipitation hardened Al-5.8Mg-4.5Zn-0.5Cu alloy was studied. The as-cold rolled aluminum alloy samples were subjected sequentially to solid solution treatment, pre-deformation, and two-stage aging treatment. Vickers hardness was measured during the aging process under various parameters. Tensile tests were conducted on the representative samples based on the hardness results. Microstructural characteristics were analyzed via transmission electron microscopy and high-resolution transmission electron microscopy. The conventional T6 process was also carried out for comparison. The hardness and tensile strength are increased evidently by the FTMT process for the Al-Mg-Zn-Cu alloy, while the ductility is adversely affected to a small extent. The precipitation at the T6 state consists of a coherent Guinier–Preston zone and T″ phase in the form of intragranular, fine, and spherical particles, while a semi-coherent T′ phase appears after the FTMT process as a new constituent. The distribution of dislocation tangles and isolated dislocations is another feature of FTMT samples. Enhanced precipitation hardening and dislocation strengthening account for the improved mechanical performance of FTMT samples. Full article