This article focuses on the research of the rheological squeeze casting process of the AI-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Sc casting aluminum alloy. Through a combination of numerical simulation and experimental verification, the effects of process parameters such as melt temperature, mold temperature, and extrusion pressure on the casting performance and microstructure were investigated. The optimal process parameters were determined to obtain castings with excellent mechanical properties and a fine and uniform microstructure.
1. Introduction
1.1 AI-Zn-Mg-Cu Alloy Research and Development Status
Aluminum alloys, as lightweight materials, have found extensive applications in various industrial fields. Among them, the AI-Zn-Mg-Cu alloy (7xxx series aluminum alloy) has attracted significant attention due to its outstanding mechanical properties. As a high-strength aluminum alloy, it demonstrates unique advantages in aerospace, automotive, and defense industries.
In the aerospace sector, 7xxx series aluminum alloys are primarily used in the manufacturing of aircraft structures, wings, fuselages, and engine components. With the continuous upgrading of aircraft and the rapid development of the aviation industry, the demand for these alloys is increasing year by year. In the automotive manufacturing field, driven by the need for vehicle lightweighting, AI-Zn-Mg-Cu alloys are widely employed in automotive parts such as engine casings and wheels. In the defense industry, they are crucial for the fabrication of structural components in rockets, missiles, and other military equipment. As the demand for higher material performance and characteristics like lightweight, high strength, and corrosion resistance grows in various industries, the application scale of 7xxx series aluminum alloys is expanding rapidly, and their usage is gradually shifting towards large and complex components.
However, the current manufacturing process of 7xxx series aluminum alloy structural parts involves multiple steps such as casting into ingots, forging, plastic deformation, heat treatment, and machining. This process results in low material utilization, long processing time, and high costs, thereby limiting the application of AI-Zn-Mg-Cu alloys. For instance, in the automotive industry, the use of aluminum alloys currently accounts for approximately 20% and is on the rise. It is projected that by 2030, the weight reduction of automobiles could reach 35%. Therefore, there is a huge potential for the growth in the demand for large and complex 7xxx series aluminum alloy parts.
1.2 Research Progress of Ultra-high Strength Cast Aluminum Alloys
Ultra-high strength aluminum alloys mainly refer to the AI-Zn-Mg-Cu alloys with better mechanical properties in the 7xxx series. Since the development of 7075 high-strength aluminum alloy by the United States in 1943, research on ultra-high strength aluminum alloys has entered a rapid development stage. Researchers have continuously studied and improved the alloy composition and corresponding heat treatment processes, resulting in significant enhancements in strength, plasticity, creep properties, and corrosion resistance.
In recent years, near-net shape casting technology has become an important development direction for 7xxx series aluminum alloys. However, the casting performance of existing 7xxx series wrought aluminum alloys cannot meet the production requirements of near-net shape casting technology. To address this issue, efforts have been made in two main aspects: optimizing the casting process and designing the alloy composition.
In terms of process optimization, the squeeze casting method has been adopted to solidify the aluminum alloy melt under high pressure. This process effectively reduces the formation of internal shrinkage porosity and other defects in the casting, resulting in a more compact microstructure. During squeeze casting, the grain size of the alloy melt is finer and the structure is more uniform. Moreover, due to the continuous application of extrusion pressure on the alloy melt, the contact between the melt surface and the casting surface is improved, leading to a clear contour and low roughness of the casting surface. This reduces the dimensional difference between the casting and the final formed part, decreases the machining allowance, and提高 the material utilization rate and降低 the production cost. Based on the squeeze casting process, the rheological squeeze casting method has been further developed. By homogenizing the alloy melt before squeeze casting, the uniformity of the temperature and composition fields of the metal melt can be improved, and the grain size can be further refined.
In terms of alloy composition optimization, efforts have been made to develop casting aluminum alloys with high casting performance to overcome the poor casting performance of existing wrought aluminum alloys. Extensive research has been conducted on the role of alloying elements in AI-Zn-Mg-Cu alloys. Zn and Mg elements play a major strengthening role, while Cu element mainly contributes to improving the corrosion resistance. The addition of Sc and Zr has a significant effect on抑制 recrystallization and refining the grain size. When Sc and Zr are added in combination, the formation of Al(Sc,Zr) phase during solidification further抑制 the growth of grains.
The content and ratio of Zn and Mg elements have an important impact on the formation and volume fraction of the main strengthening phase (MgZn₂). The Zn/Mg ratio determines the type of precipitated phase in the AI-Zn-Mg-Cu alloy. When the molar fraction ratio of Zn/Mg is greater than 2 (mass fraction ratio greater than 3.8), the η phase is the main strengthening phase; otherwise, it is the T phase. The solubility of the η phase and T phase in aluminum is temperature-dependent, and the solubility decreases sharply with decreasing temperature. The MgZn₂ phase has a high solubility at the eutectic temperature, which can reach 28%, and decreases to 4%-5% at room temperature, resulting in significant solid solution strengthening. However, increasing the mass percentage of Zn and Mg elements beyond the solubility limit may improve the tensile and yield strength of the alloy, but at the expense of reduced toughness, plasticity, and corrosion resistance.
Grain refiners are widely used in the production of aluminum alloy parts to promote grain refinement, improve mechanical properties, reduce segregation and micro-inhomogeneity,抑制热裂, and enhance the fluidity of the alloy. In recent years, rare metal elements have been increasingly added to AI-Zn-Mg-Cu alloy structural components as refiners. Zr is one of the commonly used rare metal elements, while Sc is the most effective grain refiner but is usually only applied in aerospace and military structural components due to its high cost. However, excessive grain refiners may cause segregation and lead to quality problems in the alloy casting.
Through research and development, the AI-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Sc alloy has been developed. This alloy exhibits good resistance to hot cracking. After melt treatment, the rheological squeeze casting parts have a fine and uniform grain structure, with a reduced number and size of coarse primary Al₃(Sc,Zr) phases and low-melting eutectic phases at the grain boundaries. After T6 heat treatment, the low-melting eutectic phase almost completely dissolves into the matrix, significantly improving the mechanical properties of the casting. The average tensile strength reaches 565 MPa, and the average elongation is 7.5%. At the same time, the consistency and stability of the mechanical properties are also improved.
1.3 Research Status of AI-Zn-Mg-Cu Alloy Squeeze Casting
1.3.1 Squeeze Casting Technology
Squeeze casting technology, also known as liquid forging, is a near-net shape casting process that combines the advantages of casting and hot forging. It has a series of outstanding advantages such as being environmentally friendly, producing high-quality products, having high production efficiency, and a short process flow. The process principle involves pouring the alloy melt directly into the lower mold cavity, and then the upper mold descends at a fixed pressing speed. The alloy melt fills the mold cavity under pressure, and after filling, the same pressure is continuously applied to complete the solidification and feeding process, resulting in a casting with fewer defects.
During the filling process of squeeze casting, the alloy melt is slowly filled into the lower mold cavity under the uniform extrusion of the upper mold, ensuring a stable flow velocity and effectively avoiding defects such as gas entrapment. Additionally, the continuous application of pressure during solidification increases the cooling rate of the alloy melt, reduces the critical nucleus size for nucleation, and increases the number of nuclei, resulting in a finer grain size in the casting. Squeeze casting technology combines the characteristics of forging and casting, improving the performance of the alloy. Compared with traditional casting techniques, it offers higher material utilization, more stable casting performance, and simpler operation. It can eliminate casting defects such as shrinkage porosity and is suitable for the production of castings such as wheels, valve bodies, and pistons. It is an alternative casting process for the low-cost production of large and high-strength complex components.
Squeeze casting technology was first introduced in the former Soviet Union in 1937 and has since been widely applied in China and other countries. In recent years, extensive research has been conducted on the squeeze casting of AI-Zn-Mg-Cu series ultra-high strength aluminum alloys both domestically and internationally, and castings with good strength and elongation have been produced. However, there is still a significant gap between the mechanical properties of squeeze-cast AI-Zn-Mg-Cu alloy castings and those of plastic deformation parts. Achieving the replacement of forging with casting for AI-Zn-Mg-Cu series high-strength aluminum alloys still faces great technical challenges.
Moreover, due to the solidification characteristics of AI-Zn-Mg-Cu series high-strength aluminum alloys, cracking problems near the corners and hot spots of the casting are relatively severe. These hot cracks are usually in the form of shrinkage cracks, and increasing the extrusion pressure can reduce this defect but cannot completely solve the problem. Changing other process parameters such as mold temperature and melt temperature also难以 completely eliminate the problem. In addition, the dendrite problem inside the casting is also serious. Due to the high alloying degree, segregation is more severe, and it is difficult to消除 the segregation problem even after heat treatment. These issues lead to a large差距 in strength between the casting and the forging, especially for large-sized castings where the segregation is more明显, resulting in significant differences in performance at different parts of the casting.
1.3.2 Rheological Squeeze Casting Technology
Although AI-Zn-Mg-Cu alloy squeeze casting technology can effectively reduce defects such as macro-shrinkage porosity and糊状 shrinkage porosity caused by the contraction of the alloy melt during solidification, the high alloying degree of the alloy and the non-uniform distribution of temperature and composition in the as-cast aluminum alloy melt obtained from melting can lead to the formation of defects such as hot cracking and segregation in the casting, especially in the case of large-sized castings.
To address these issues, researchers have attempted to homogenize the melt before squeeze casting, giving rise to the rheological squeeze casting technology. The key step in rheological squeeze casting is to improve the uniformity of the temperature and composition fields of the alloy melt before casting. Common melt treatment methods include mechanical stirring, ultrasonic vibration, and electromagnetic stirring.
Mechanical stirring methods, such as the mechanical stirring double-helix process developed by Fan Zhongyun and the semi-solid rheological stirring device developed by Kang Yonglin, have simple process principles and easy-to-control equipment processes. However, the stirring rod in contact with the melt容易 corrode, leading to contamination of the alloy melt, which限制了 their wide application in industrial production.
Ultrasonic vibration methods involve inserting an ultrasonic vibration amplitude transformer into the alloy melt to stir the melt using ultrasonic energy. However, the direct contact between the amplitude transformer and the alloy melt results in severe corrosion, short service life, high cost, and同样会污染 the alloy melt.
Electromagnetic stirring methods利用 electromagnetic forces to promote the flow of the metal melt, thereby improving the uniformity of the temperature and composition fields of the alloy melt. This method has the advantages of non-contact, controllability, and no pollution. AEMP公司 was the first to apply electromagnetic stirring treatment technology in the melt treatment process in industrial production and adjusted the process methods and experimental parameters based on practical experience. Subsequently, companies such as Bubler in Switzerland and EFU in Germany improved and applied this technology.
In recent years, Chinese researchers have conducted in-depth studies on the preparation of high-strength aluminum alloy melts using electromagnetic stirring. Zhang Qin et al. systematically studied the effect of electromagnetic stirring treatment on the casting results of aluminum alloy semi-continuous casting using numerical simulation analysis and experimental verification methods. The results showed that the depth of the liquid pool decreased明显 after electromagnetic stirring, and the solidification structure of the casting was finer and more uniform. The General Research Institute for Nonferrous Metals in Beijing designed a compound electromagnetic stirrer device to solve the problem of poor melt fluidity caused by low superheat of the alloy melt. After electromagnetic stirring, the alloy melt cooled rapidly, generating a large number of pre-nucleation nuclei and refining the solidification structure of the alloy casting.
When using electromagnetic stirring to treat high-strength aluminum alloy melts, the skin effect and edge effect caused by electromagnetic induction can affect the stirring effect on the uniformity of the alloy melt. To address this issue, the General Research Institute for Nonferrous Metals in Beijing developed the annular slot electromagnetic stirring (AEMS) technology, which enables uniform stirring of the alloy melt in the annular slot, resulting in a more refined and uniform solidification structure.
In summary, the rheological squeeze casting technology with melt treatment can improve the uniformity of the alloy melt’s composition and temperature fields, resulting in better组织性能 of the castings. Conducting research on the rheological squeeze casting process of ultra-high strength AI-Zn-Mg-Cu alloy complex castings is of great significance for the development and engineering application of this technology.
1.4 Research Purpose, Content, and Route of This Article
1.4.1 Research Purpose
The existing AI-Zn-Mg-Cu alloy is a wrought aluminum alloy with poor casting performance, including a large solidification temperature range, severe shrinkage, and hot cracking, making it difficult to directly cast into shape. It usually requires multiple complex processes such as casting into ingots, forging, plastic deformation, heat treatment, and machining, resulting in a long processing flow, low material utilization, and high processing costs, which limits its application. Merely improving the microstructure of the alloy through melt treatment to reduce the cracking tendency cannot fundamentally solve the problem. The research group has developed the AI-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Sc casting aluminum alloy with better casting performance than existing wrought aluminum alloys. However, the research on its optimal casting process parameters is still . This experiment aims to explore the influence of casting process parameters on the performance and microstructure of the castings of this alloy for the casting of large and complex castings, based on the previous work of the research group, and to provide theoretical and technical support for the engineering application of this alloy.
1.4.2 Research Content
The main research contents of this thesis are as follows:
(1) Establish a squeeze casting model of the target wheel-shaped part, and use the ProCAST casting simulation software to simulate the rheological squeeze casting process of the wheel-shaped part to explore the effects of melt temperature, mold temperature, and extrusion pressure on the microstructure, performance, and defect situation of the casting.
(2) Analyze and experimentally verify the simulation results, study the defect situation, microstructure, and performance of the rheological squeeze casting part, and explore the influence of the above process parameters on the casting performance.
1.4.3 Research Route
This article mainly studies the influence of different process parameters such as melt temperature, mold temperature, and extrusion pressure on the microstructure, performance, and defect situation of the casting. A combination of numerical simulation and experimental verification methods is adopted, and test methods such as microstructure observation, chemical composition analysis, and Brinell hardness testing are used to test and analyze the microstructure, performance, and defect situation of the casting under different process conditions. The research route is shown in Figure 1.6.
2. Research Methods
2.1 Numerical Simulation
2.1.1 ProCAST Software and Simulation Methods
The ProCAST simulation software developed by ESI Group is mainly used to simulate the filling and solidification process of the alloy melt during casting. By using the integrated numerical analysis method based on the finite element mathematical model, ProCAST can accurately simulate the temperature field, stress field, and flow field during the casting process. It can also predict the deformation and residual stress that may occur during casting.
The key to casting process numerical simulation technology is to analyze the changes in the temperature field, stress field, etc. during the casting process through numerical calculation methods and combine these changes with the formation mechanism of casting defects to optimize the casting quality. At present, casting process numerical simulation mainly includes several key parts such as numerical simulation of filling, numerical simulation of shrinkage porosity and shrinkage cavity formation, and numerical simulation of stress field.
2.1.1.1 Numerical Simulation of Filling Process
The simulation calculation of the casting filling process is an interdisciplinary research. The current research mainly uses the turbulence model for simulation to more accurately simulate the actual filling situation. At high temperatures, the slurry is regarded as a viscous incompressible fluid, and its flow should conform to the laws of mass conservation, momentum conservation, and energy conservation. For the filling flow of the slurry in the Euler coordinate system, the simulation calculation is mainly based on the following basic equations。
2.1.1.2 Numerical Simulation of Shrinkage Porosity and Shrinkage Cavity Formation
One of the main functions of the ProCAST simulation software is to simulate and predict the tendency of shrinkage porosity and shrinkage cavity formation in castings. The viewer module of the software provides multiple criteria for predicting shrinkage porosity and shrinkage cavity defects. In casting numerical simulation, the Total Shrinkage Porosity is the most common criterion used in various casting processes such as low-pressure casting, vacuum suction casting, die casting, and squeeze casting. The Niyama Criterion is a criterion focused on interdendritic shrinkage porosity. It evaluates the possibility of micro-shrinkage porosity formation in castings through numerical analysis, but it is not suitable for the analysis of the tendency of macro-shrinkage porosity and shrinkage cavity formation. For the defect prediction of large castings, the Hot Spots (hot spots) criterion is the most commonly used defect prediction criterion. Hot spots are usually located in the thickest part of the casting where the metal melt solidifies last. The location and existence time of hot spots can be used to predict the occurrence of macro-shrinkage porosity and shrinkage cavity. According to the numerical simulation results, by changing the shape of the upper and lower molds and the squeeze casting process parameters, the location of hot spots can be changed or the existence time of hot spots can be reduced, thereby improving the quality of the casting.
2.1.1.3 Numerical Simulation of Stress Field
The ProCAST software has a stress analysis function and has established multiple mathematical models for different materials, including pure elastic model, elastic-plastic model, elastic-plastic-creep model, and elastic-viscoplastic model. However, thermal stress numerical simulation requires a large amount of computational resources. To shorten the calculation time and simplify the simulation process, the calculation of the stress field is usually not considered in the numerical simulation of the casting process.
2.1.1.4 Simulation Method
In this study, the ProCAST numerical simulation software was used to simulate the casting process of the wheel-shaped casting. The dynamic changes of the melt temperature field during the forming process were analyzed. These studies provide key process guidance for subsequent experimental verification.
The process of numerical simulation using ProCAST is shown in Figure 2.1. First, the upper and lower molds are designed in UG according to the shape characteristics of the casting and assembled. The assembled file is imported into the Mesh module of ProCAST for meshing. After meshing, the parameters required for the casting process are set in the Cast module. After setting the process parameters and running parameters, the loading operation is performed. After the calculation is completed, the simulation results are observed and analyzed in the Viewer module. According to the simulation results, the calculation process parameters and running parameters are adjusted and the simulation calculation is repeated. By 综合 analyzing the calculation results of different simulation parameters, the optimal process parameters are selected. The simulation calculation results are verified by squeeze casting experiments. According to the experimental results, the process parameters are further adjusted and optimized. Finally, the optimal rheological squeeze casting process parameters for this alloy are selected.
2.1.2 Model Establishment and Parameter Setting
2.1.2.1 Model Establishment and Meshing
The cross-section of the target wheel-shaped part is shown in Figure 2.2. Its outer diameter is 200 mm, inner diameter is 170 mm, height is 100 mm, and the thickness in the middle is 15 mm. According to the shape characteristics of this wheel-shaped part, a mold was designed and manufactured as shown in Figure 2.3. Its three-dimensional model was drawn by UG three-dimensional modeling software and imported into ProCAST for calculation.
After assembling the upper and lower molds in UG software, the assembled prt file was directly imported into the Mesh module of ProCAST. The surface and volume meshes of the model were divided as shown in Figure 2.4.
In the numerical simulation calculation, to facilitate mesh structure division and simulation calculation, the simulation calculation model was appropriately simplified. First, the influence of the air inside the alloy melt and in the lower mold cavity on the simulation results was ignored. Second, the upper and lower molds were regarded as rigid bodies, and the influence of the elastic deformation of the upper and lower molds caused by the extrusion pressure on the filling performance and experimental results during the forming process was not considered. Finally, the alloy melt after melt treatment was regarded as having a uniform temperature field and composition field.
2.1.2.2 Alloy Melt Parameter Setting
For the alloy melt material AI-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Sc alloy, the mass fraction of each component of the alloy was defined in the database of the ProCAST software to set the alloy composition. Since the filling and solidification speed is fast and the time is short during the squeeze casting process, the melt material diffusion model can be set as the Scheil model. The calculated results of the thermal physical parameters of the material obtained by the software are shown in Figure 2.5, where the solidus and liquidus temperatures are 435 °C and 626 °C, respectively.
2.1.2.3 Interface Heat Transfer Coefficient Setting
In the numerical simulation calculation process of this experiment, the boundary conditions of the temperature field include the contact surfaces between the upper and lower molds and the air, the contact surfaces between the upper and lower molds and the alloy melt, and the contact surface between the upper and lower molds. During the meshing process, since the alloy melt and the interface nodes may overlap, the interface contact type is set as COINC. Since the upper and lower molds of the mold use the same material, the contact interface between them is set as EQUIV. The interface between the mold and the air is set as NCOINC. The heat condition needs to be set on the boundary between the mold and the air. In the experiment, the mold is air-cooled, that is, set as Air-cooling condition. According to the general standard, the ambient air temperature is set to 20 °C. The heat transfer coefficient of the interface between the mold and the air is set to . For the COINC interface, according to previous research, there is a specific relationship between the heat transfer coefficient and the specific pressure within a certain range during the squeeze casting process: . According to this empirical formula, the heat transfer coefficient corresponding to different extrusion pressures can be calculated. Since the extrusion pressure of the upper mold in this experiment is a fixed value and continuously applied, the heat transfer coefficient can be approximately set as a fixed value according to the extrusion pressure.
2.1.2.4 Other Parameter Settings
In the Cast module, other process parameters are set. The gravity condition is selected as the default setting, , and the direction is along the -Z axis. According to the extrusion pressure under different experimental conditions, the surface load is set on the contact surface between the upper mold and the alloy melt in the Pressure module. The pouring amount of the alloy melt is 3.236 kg, and the downward extrusion speed of the upper mold is 20 mm/s. The calculation termination condition is set as the alloy melt temperature dropping to 400 °C.
2.1.2.5 Running Parameters
According to different process conditions, parameters such as melt temperature and interface heat transfer coefficient are set. In the simulation parameter setting and selection module, the high-pressure casting module is selected to approximately simulate the rheological squeeze casting process. By analyzing the influence of various process parameters on the solidification behavior and defect formation during the casting process, and viewing and analyzing the simulation results in the Viewer module.
2.2 Rheological Squeeze Casting Experimental Verification
2.2.2 Melt Treatment Device
Before casting the alloy melt, stirring treatment is a commonly used physical method to refine the grains and improve the uniformity of the temperature and composition fields of the melt. Electromagnetic stirring can make the metal melt flow spontaneously compared with mechanical stirring, avoid the introduction of impurities and gas by mechanical stirring, and make the temperature and composition fields of the melt more uniform while obtaining a purer melt. In this experiment, an alternating electromagnetic stirring method was used to treat the melt. As shown in Figure 2.7, the electromagnetic stirring device consists of a frequency conversion control system and an electromagnetic generation system. A thermocouple was used to connect a temperature collector to measure the real-time temperature of the alloy melt in the electromagnetic stirrer. The temperature measuring range of the temperature collector is 49 – 1999 °C.
During electromagnetic stirring, an alternating current and an alternating magnetic field are generated in the electromagnetic stirrer. Under the action of electromagnetic induction, an induced current in the opposite direction to the alternating current is generated, and a Lorentz force is produced to drive the melt to flow, making the temperature and composition fields of the melt more uniform.
2.2.3 Squeeze Casting and Solution and Aging Treatment Experimental Process
2.2.3.1 Squeeze Casting Experimental Process
High-purity Al and Al-50Cu master alloys were added to a well-type resistance melting furnace and heated to 750 °C to be fully melted. The temperature was then降低 to 700 °C, Zn and Mg were added and stirred until completely melted, and the melt was held for 30 min. The aluminum liquid temperature was increased to 720 °C, and the prepared Al-10Zr master alloy and Al-2Sc master alloy were added and stirred until completely melted, and the melt was held for 10 min. Rotating injection of Ar was used for refining, degassing, and slag removal. The heating temperature was adjusted to ensure that the aluminum liquid temperature was around 720 °C, and the refining was carried out for 30 min. The slag on the surface of the alloy melt was removed once every 5 min.
After refining, the rheological squeeze casting experiment was carried out. The casting test was divided into two groups for comparison. In the first group, the aluminum alloy melt was poured into a stainless steel crucible preheated to 500 °C, and then the crucible was placed in the electromagnetic stirring device for electromagnetic stirring melt treatment. The current was 10 A and the frequency was 15 Hz during stirring. Then, a thermocouple was inserted into the melt to measure the temperature. When the melt temperature in the crucible reached the pouring temperature, it was poured into the extrusion machine for squeeze casting. In the second group, the operation was similar to the first group. The aluminum alloy melt was also poured into a stainless steel crucible preheated to 500 °C. However, in this group, after the melt temperature reached the same temperature as measured by the thermocouple, it was directly poured into the extrusion machine for the experiment.
2.2.3.2 Solution and Aging Treatment
The castings were solution treated using a box-type resistance furnace (as shown in Figure 2.8(a)). The power of the furnace is 7.5 KW, the maximum working temperature can reach 800 °C, and the temperature control accuracy is within ±1 °C. The solution treatment was carried out at 450 °C/3 h + 460 °C/3 h + 470 °C/3 h, and then immediately quenched in room temperature water. The transfer time during the quenching process was controlled within 5 s. The aging treatment was carried out in a drying oven. The power of the drying oven is 800 W, the maximum working temperature is 250 °C, and the temperature control accuracy is 0.2 °C. The aging treatment was carried out at 120 °C for 24 h in the drying oven.
2.3 Microstructure Analysis and Performance Testing
2.3.1 Microstructure Observation
The wheel-shaped part obtained by squeeze casting was cut垂直 along the central axis position into a cross-section as shown in Figure 2.9. Metallographic samples were taken from the positions shown in the figure for testing. The size of the sample block was , and they were numbered 1 – 3 from top to bottom (hereinafter referred to as the upper part, middle part, and lower part). The obtained experimental sample blocks were polished and ground with different grits of sandpaper until the observation surface was polished to a mirror finish and there were no明显 scratches under a low-power microscope. The sample surface was anodically coated with a 2.5% HBF₄ solution at a voltage of 30 V and a current within 1 A for 50 – 60 s. A metallographic microscope was used to observe the optical metallographic structure of the samples. The grain size, morphology, and distribution were observed using a polarizing microscope, and the average grain size of the samples was calculated using the intercept method. The arithmetic average of the results of three groups was taken.
2.3.2 Chemical Composition Analysis
The uniformity of the composition of the rheological squeeze casting casting was tested and analyzed. A floor-standing spark direct reading spectrometer was used to analyze the chemical composition of the alloy sample blocks. The process was as follows: sample blocks were taken from positions 1, 2, and 3 shown in Figure 2.9. The test surface and the corresponding ground surface were machined flat by a lathe, and the test surface was simply polished with sandpaper until it was basically flat. The sample block was placed on the sample stage of the direct reading spectrometer and clamped. The composition test was completed, and the results were read on the display. The above process was repeated three times, and the arithmetic average of the experimental results was taken.
2.3.3 Scanning Electron Microscopy Analysis
After the observation surface was polished, a JSM-7900F field emission scanning electron microscope equipped with X-ray energy dispersive spectroscopy (EDS) was used to obtain high-resolution images of the interface and surrounding组织形貌 and the morphology of intermetallic compounds through secondary electron imaging and backscattered electron imaging. The elemental composition of the scanned area was analyzed by EDS, and the elemental distribution in the scanned area was observed.
2.3.4 Brinell Hardness Testing
A HBE-3000A electronic Brinell hardness tester was used. The test load was 250 N, the holding time was 15 s, and the diameter of the indenter was 5 mm. Before the test, the surface and bottom of the sample block to be tested were ground flat to ensure that the bottom and top surfaces were水平. After the test surface was polished, the sample block to be tested was placed on the sample stage of the Brinell hardness tester, and pressure was applied to the sample. The diameter of the indentation was measured using a 3D optical image measuring instrument, and the hardness value was recorded according to the hardness standard. The experiment was repeated three times, and the arithmetic average was taken.
2.3.5 Mechanical Property Testing
Tensile test specimens were processed according to the GB/T228 – 002 standard, as shown in Figure 2.11. The sampling position is shown in Figure 2.12. A WDW – 100 microcomputer-controlled electronic universal testing machine was used. The tensile rate in the tensile test was 2 mm/min, and the tensile strength , yield strength , and elongation were measured.
To ensure the accuracy of the measurement results, three groups of tensile specimens were taken at the same position and tested, and the arithmetic average was taken.
3. Numerical Simulation of Rheological Squeeze Casting of Ultra-high Strength Cast Aluminum Alloy
The ProCAST software was used to simulate the squeeze casting process of the aforementioned wheel-shaped part to predict the influence of casting process parameters such as melt temperature, extrusion pressure, and mold temperature on the defect situation of the casting. The macro-shrinkage porosity and micro-segregation of the casting were analyzed through criteria such as hot spots, and appropriate process parameters were selected based on the simulation results to guide the setting of process parameters for subsequent squeeze casting experimental verification.
3.1 Filling Process
3.1.1 Temperature Field Distribution during Filling Process
Figure 3.1 shows the temperature field and solidification time distribution at different stages during the forming process under the conditions of a melt temperature of 660 °C, a mold temperature of 200 °C, and an extrusion pressure of 75 MPa. After the alloy melt was poured into the lower mold cavity, the upper mold descended at a speed of 20 mm/s to complete the squeeze casting filling process. During the solidification process after filling, the upper and lower edges of the wheel-shaped part solidified first, and the thickest part in the middle of the wall solidified last. Therefore, shrinkage porosity is more likely to occur in the thickest part in the middle of the wheel-shaped part, which is the last to solidify.
3.1.2 Velocity Field Distribution during Filling Process
During the squeeze casting process, the alloy melt fills the mold under the downward extrusion of the upper mold. Therefore, the extrusion speed of the upper mold affects the filling speed and velocity field distribution of the alloy melt. In this experiment, the extrusion speed was set to 20 mm/s based on the research experience of the research group. Figure 3.2 shows the cross-sectional view of the velocity field distribution of the aluminum alloy melt when it is filled to different states at an extrusion speed of 20 mm/s. Figures 3.3 and 3.4 show the velocity components in the axial and radial directions, respectively, and Figure 3.5 shows the distribution of the filling time at each position of the casting during the squeeze casting filling process.
At the beginning of the filling process, the contact area between the upper mold and the alloy melt is small. In this state, the velocity of the melt in contact with the upper mold increases, and the velocity at the rising position of the alloy melt surface on the side of the upper mold also changes明显, and the velocity near the edge of the upper mold is the largest. As the upper mold continues to descend, the melt directly below the upper mold moves downward along the axial direction, and the melt on the side wall fills upward. At the end of the filling stage, the velocity of the blank directly below the upper mold increases, and the velocity direction changes from axial to radial, and the wheel-shaped part is gradually formed. It can be seen from the figure that the closer to the upper mold, the greater the deformation velocity of the alloy melt. And as the upper mold gradually descends, the deformation velocity of the melt away from the upper mold gradually increases, and the degree of deformation also increases. According to Figure 3.4, during the rheological squeeze casting process, as the upper mold descends to different positions, the alloy melt fills the lower mold cavity smoothly, and the flow state of the melt remains stable, and no defects such as gas entrapment and slag inclusion occur in this process.
3.2 Influence of Melt Temperature on Casting Filling and Solidification
Based on the solid-liquid interval range and alloy composition characteristics of the alloy, the previous research work of the research group determined that the suitable melt temperature range is 660 °C – 700 °C. Using the single-factor variable method, after the pre-processing in the ProCAST software, the mold temperature and extrusion pressure were set as fixed values, and the melt temperature was set to 660 °C, 680 °C, and 700 °C, respectively. The simulation parameter settings are shown in Table 3.1.
| Scheme | Melt Temperature/°C | Mold Temperature/°C | Extrusion Pressure/MPa | Extrusion Speed/mm s⁻¹ |
|---|---|---|---|---|
| 1 | 660 | 200 | 75 | 20 |
| 2 | 680 | 200 | 75 | 20 |
| 3 | 700 | 200 | 75 | 20 |
According to the simulation results, post-processing was carried out. The existence region and time of hot spots were judged based on the calculation results of the Hot spot module. In squeeze casting, the shorter the existence time of hot spots, the faster the solidification speed of the isolated liquid phase region of the melt, which is more conducive to grain refinement and reducing micro-segregation. The simulation results were observed bythe Y-Z direction section. The calculation results are shown in Figure 3.6. Under the conditions of a mold temperature of 200 °C and an extrusion pressure of 75 MPa, when the melt temperature is 660 °C, 680 °C, and 700 °C, the existence time of hot spots is 13.22 s, 13.34 s, and 14.45 s, respectively.