1. Introduction
1.1 Overview of Ductile Iron
Ductile iron, emerging in the 1950s, is a cast iron material formed by spheroidizing and inoculating molten iron. Its graphite structure is spherical. The mechanical properties of ductile iron depend on its microstructure, which consists of graphite, ferrite, and pearlite in the as-cast state. The spheroidization rate, size, and distribution uniformity of the graphite structure are crucial factors affecting its mechanical properties. Ductile iron can be classified into ferrite ductile iron, pearlite ductile iron, and ferrite – pearlite ductile iron based on the matrix structure. It has many excellent properties such as good wear resistance, high anti-vibration ability, low notch sensitivity, and excellent machinability. Additionally, it has a high carbon content, close to the eutectic composition, a low melting point (about 1200°C), good metal fluidity, and a small shrinkage rate, making its casting performance superior to steel. It is widely used in various fields such as automobile manufacturing, wind power generation, and rail transit. In recent years, the production of ductile iron in China has been increasing steadily, and its proportion in the total casting output has also been rising. However, there are still some problems, such as poor comprehensive quality and stability of castings, a low degree of specialized production scale, and the mechanical properties of ductile iron that cannot meet the needs of actual industrial technology development.
1.2 Application of External Force Field in the Casting Process
In the 21st century, casting technology has entered a new stage of development. With the development of science and technology, researchers hope to improve the performance of castings while meeting the requirements of environmentally friendly casting technology and achieving large-scale and rapid industrial production. Various special casting technologies have emerged, and lost foam casting is one of the relatively well-developed ones. However, under the current technical conditions, lost foam casting has some shortcomings. Since it uses dry sand to fill the sand box, the heat dissipation rate is relatively slow, resulting in a large dendrite size and low mechanical properties of the cast iron alloy. To solve this problem, external force fields are often applied during the lost foam casting process. There are mainly the following methods:
- Vacuum Low-Pressure Lost Foam Casting Technology: This technology combines vacuum lost foam casting and low-pressure casting. It can improve the filling ability of the alloy, with high casting precision, low surface roughness, high productivity, and good performance. The straight runner can act as a feeding riser, reducing temperature loss during pouring. The liquid alloy is fed under controllable pressure, resulting in a high yield and a dense structure.
- Pressure Lost Foam Casting Technology: This technology combines lost foam casting and pressure solidification crystallization. During pouring, the sand box is placed in a pressure tank, and pressure gas is introduced to solidify the metal liquid under pressure. This method can reduce defects such as shrinkage holes and porosity, thereby improving mechanical properties. During the pressure solidification process, the external force can squeeze the liquid metal between dendrites and cause micro-deformation of the first-solidified dendrites, which can greatly improve the feeding ability of the riser and reduce shrinkage holes and porosity.
- Vibration Lost Foam Casting Technology: This technology combines mechanical vibration with lost foam casting, integrating the advantages of vibration solidification theory and lost foam casting. The casting solidifies in a vibration field, and the mechanical vibration causes relative motion between the liquid phase and the solid phase, resulting in dendrite fracture, an increase in nucleation cores in the liquid phase, a refined solidification structure, improved feeding, and enhanced mechanical properties of the casting. Compared with the other two strengthening methods, vibration lost foam casting technology can directly use the vibration table during sand compaction, does not require additional equipment, can achieve the same effect, and is simple to operate and cost-effective, meeting the requirements of green and environmentally friendly casting.
1.3 Research on the Influence of Vibration Field on the Metal Solidification Process
1.3.1 Influence of Vibration Field on Metal Solidification Crystallization
Applying mechanical vibration during the solidification of liquid metal has the main effect of increasing the number of nucleated grains in the liquid metal, resulting in a refined grain structure and improved mechanical properties of the metal casting. Periodic mechanical vibration has an important impact on the gas exhaust process of the metal liquid. It accelerates the formation of bubbles in the metal liquid, facilitating the faster removal of gas, especially hydrogen, which is beneficial for reducing the porosity of lost foam casting. Additionally, vibration during lost foam casting has a positive impact on the crystallization and nucleation rate of the liquid metal, resulting in refined grains, the fragmentation and dispersed distribution of non-metallic inclusions, a reduced viscosity of the metal liquid, and enhanced feeding ability. In summary, it can refine the grains during the crystallization process of the liquid metal, improve the comprehensive performance of lost foam casting; make the microstructure and chemical composition of lost foam casting more uniform during the metal crystallization process, reducing segregation phenomena within and between grain boundaries; eliminate or significantly reduce defects such as casting pores; accelerate the flow of the metal liquid, enhancing the feeding ability and reducing or eliminating the shrinkage of lost foam casting; improve the fluidity of the metal liquid, increasing the filling ability of lost foam casting; and improve the crystallization process of the metal liquid, accelerating the solidification crystallization process, homogenizing the residual stress of lost foam casting, and improving the deformation resistance of the material.
1.3.2 Research Progress of Vibration Field in lost foam Casting Process at Home and Abroad
Foreign research on vibration solidification casting began relatively early. In 1868, a Russian scientist applied simple vibration to the solidification process of liquid metal. In 1950, Soviet researchers used alloy steel as the research object and found that when the vibration frequency was 1 – 5 Hz and the amplitude was 0 – 1 mm, the influence of mechanical vibration on the crystallization of lost foam casting was weak. When the vibration frequency was 10 – 20 Hz, the microstructure of lost foam casting was improved, the grains were refined, and lost foam casting quality was significantly improved. However, when the vibration frequency was increased to 40 – 60 Hz again, obvious segregation phenomena occurred in the microstructure of lost foam casting. In recent years, many scholars in China have also entered this field and achieved certain results. For example, some scholars have studied the solidification crystallization process of Al – Si alloys under mechanical vibration conditions and found that the application of the vibration field can refine the grains of lost foam casting and improve its mechanical properties.
1.4 Research Significance and Research Contents
1.4.1 Research Significance
Lost foam casting technology has been well developed and widely used in industrial production such as steel casting, iron casting, and aluminum casting. By applying mechanical vibration during the lost foam casting process, the solidification process of the casting is in a vibration field. In the initial solidification stage, the vibration causes relative motion between the solid phase and the liquid phase, resulting in the fracture of the first-solidified dendrites, an increase in crystallization cores, and a refined microstructure, thereby improving the mechanical properties of lost foam casting and its filling ability. This is a green and environmentally friendly casting method that is simple to operate and has a low cost.
1.4.2 Research Contents
This experiment uses the vibration lost foam casting method to prepare ductile iron, combining the advantages of lost foam casting and vibration solidification, and applying mechanical vibration to the lost foam casting of ductile iron. The research contents mainly include: (1) determining the optimal vibration frequency and amplitude by comparing the microstructure and tensile properties of ductile iron prepared with different vibration frequencies and amplitudes; (2) conducting low-temperature impact experiments on the ductile iron prepared with the optimal vibration frequency and amplitude and the ductile iron prepared without vibration to explore the influence of mechanical vibration on the strength and toughness of ductile iron; (3) conducting friction and wear experiments on the ductile iron prepared with the optimal vibration frequency and amplitude and the ductile iron prepared without vibration to explore the influence of mechanical vibration on the wear resistance of ductile iron.
2. Experimental Materials, Equipment and Methods
2.1 Experimental Materials
The QT400 – 18 ductile iron used in this experiment was prepared from raw materials such as pig iron, scrap steel, and ferrosilicon. The chemical element contents are shown in Table 1. The spheroidizing agent used was 1.6 wt.% FeSiCaMgRE, and the inoculant was 0.5 wt.% CaBa – FeSi. The chemical element contents of the spheroidizing agent and the inoculant are shown in Table 2.
| Material | C | Si | Mn | S | P | Fe |
|---|---|---|---|---|---|---|
| Pig iron | 4.07 | 0.72 | 0.06 | 0.012 | 0.026 | Remainder |
| Scrap steel | 0.05 | 0.01 | 0.26 | 0.005 | 0.014 | Remainder |
| Ferrosilicon | 0.035 | 72.15 | 0.003 | 0.008 | Remainder |
| Material | Si | Al | Mg | Ca | RE | Ba | Fe |
|---|---|---|---|---|---|---|---|
| Spheroidizing agent | 45.96 | 0.72 | 5.69 | 1.09 | 0.89 | Remainder | |
| Inoculant | 72.99 | 1.15 | 1.73 | 2.25 | Remainder |
2.2 Experimental Equipment
The experiment adopted the form of dual-motor one-dimensional vibration. According to relevant literature, vertical vibration has the most significant impact on casting materials. The vibration frequency was controlled within 0 – 50 Hz, and the amplitude was controlled within 0 – 2 mm. The mechanical vibration table was used, and the vibration frequency was adjusted by a frequency converter, and the amplitude was adjusted by the angle of the eccentric block of the motor.
2.3 Vibration Field Ductile Iron Preparation Process
First, the required foam was cut and bonded into a model. The surface of the foam model was brushed with refractory paint and dried three times. Then, the foam model was buried in a sand box filled with dry quartz sand (20 – 40 mesh). The sand box was fixed on the vibration platform, and the vibration table was started to make the quartz sand dense. A plastic film was covered on the quartz sand, and a negative pressure system was used to make the sand box in a negative pressure state (the negative pressure value was controlled at 0.06 MPa). A layer of dry sand was covered on the plastic film. The metal liquid was prepared in a medium-frequency induction melting furnace. When the temperature of the metal liquid reached 1450°C, ferrosilicon was added, and when the temperature reached 1500°C, the metal liquid was poured out. Before pouring, the carbon and silicon contents in the metal liquid were measured. If they did not meet the expected values, appropriate carbon-increasing agents or ferrosilicon were added for composition adjustment. The spheroidizing agent and the inoculant were placed in a ladle and covered with iron filings. The metal liquid was poured into the ladle for spheroidizing and inoculating treatment. During pouring, the vibration table was started according to the preset vibration frequency and amplitude. After pouring, the negative pressure was maintained for 10 minutes and then the negative pressure system was closed.
2.4 Microstructure Analysis and Mechanical Property Testing
2.4.1 Microstructure Analysis
- Microscopic Structure Analysis: The morphology of the graphite structure in the ductile iron specimen was observed using an Olympus optical microscope. After observing the graphite structure, the surface was corroded with a 4% nitric alcohol solution to observe the morphology of the matrix structure.
- Fracture Morphology Analysis: The fracture morphology of the ductile iron was observed using a TM3030 scanning electron microscope. The three-dimensional reconstruction of the fracture morphology was carried out using an Olympus OLS4100 laser confocal microscope to measure the height difference of the fracture and the surface roughness of the fracture.
- Friction and Wear Morphology Analysis: The surface wear marks of the ductile iron prepared under different process conditions after the friction and wear experiment were observed using a TM3030 scanning electron microscope to analyze the influence and mechanism of mechanical vibration on the friction and wear properties of the ductile iron.
2.4.2 Mechanical Property Testing
- Tensile Property Testing: The tensile experiment was carried out using an E45 – 305 microcomputer-controlled electronic universal tensile testing machine with a tensile speed of 1 mm/min. The actual thickness and width of the tensile specimen were measured before the experiment. Each process was repeated three times, and the average value was taken.
- Low-Temperature Impact Property Testing: The low-temperature impact experiment was carried out using an MTS impact testing machine. The impact specimen was a Charpy V-notch specimen with a size of 55 mm × 10 mm × 10 mm, a V-notch depth of 2 mm, an included angle of 45°, and a curvature radius of the notch bottom of 0.25 mm. Each process was repeated three times, and the average value was taken.
- Hardness Testing: The surface hardness of the ductile iron specimen before and after friction and wear was detected using a UH250 fully automatic universal hardness tester with a holding time of 30 s. Three regions were selected for detection, and the average value was taken.
- Friction and Wear Property Testing: The friction and wear experiment was carried out using an IPC – 7010 friction and wear testing machine with a loading force of 50 N, a frequency of 2 Hz, a sliding distance of 10 mm, and a wear time of 30 min. The friction pair was a tungsten carbide with a diameter of 6 mm. The specimen surface was polished with sandpaper before the experiment. The weight loss of each process was measured, and the average value was taken.
3. Influence of Mechanical Vibration on the Microstructure and Tensile Properties of QT400 – 18 Ductile Iron
3.1 Influence of Vibration Frequency on the Microstructure and Tensile Properties of QT400 – 18 Ductile Iron
3.1.1 Influence of Vibration Frequency on the Graphite Structure
When the amplitude was constant at 1.5 mm and the vibration frequencies were 0 Hz, 30 Hz, 40 Hz, and 50 Hz, the graphite structure of the ductile iron prepared with different vibration frequencies had different degrees of change compared with that of the ductile iron prepared without vibration. As the vibration frequency increased from 0 Hz to 40 Hz, the content of the graphite structure increased, the spheroidization rate increased, the average size decreased, and the graphite was more evenly distributed in the ductile iron. When the vibration frequency was 50 Hz, although the average size of the graphite continued to decrease, the spheroidization rate decreased due to the strong mechanical vibration destroying the growth environment of the graphite. The measurement results of the graphite structure of the ductile iron prepared with different vibration frequencies are shown in Table 3.
| Specimen No. | Vibration Frequency (Hz) | Spheroidization Rate (%) | Spheroidization Grade | Average Graphite Ball Diameter (μm) | Graphite Content (%) |
|---|---|---|---|---|---|
| a | 0 | 78.8 | 4 | 33.86 | 13.58 |
| b | 30 | 83.1 | 3 | 29.61 | 15.21 |
| c | 40 | 85.9 | 3 | 27.09 | 16.26 |
| d | 50 | 82.3 | 3 | 26.44 | 16.41 |
3.1.2 Influence of Vibration Frequency on the Ferrite Structure
When the amplitude was constant at 1.5 mm and the vibration frequencies were 0 Hz, 30 Hz, 40 Hz, and 50 Hz, as the vibration frequency increased, the content of the graphite structure in the ductile iron solidification structure increased, the diffusion distance of carbon atoms in the metal solution decreased, the concentration difference at the phase interface increased, and the nucleation rate of ferrite increased, resulting in an increase in the ferrite structure content in the ductile iron. At the same time, the ferrite grain size showed a decreasing trend. The ferrite content in the matrix was calculated using image – pro plus 6.0, and the ferrite grain size was measured using the three-circle intercept method. The measurement results are shown in Table 4.
| Specimen No. | Vibration Frequency (Hz) | Ferrite Content in Matrix (%) | Ferrite Grain Size (μm) |
|---|---|---|---|
| a | 0 | 68.28 | 33.76 |
| b | 30 | 70.57 | 31.85 |
| c | 40 | 71.42 | 29.93 |
| d | 50 | 72.01 | 28.62 |
3.1.3 Influence of Vibration Frequency on the Pearlite Structure
When the amplitude was constant at 1.5 mm and the vibration frequencies were 0 Hz, 30 Hz, 40 Hz, and 50 Hz, compared with the ductile iron prepared without vibration, the pearlite structure of the ductile iron prepared with 30 Hz, 40 Hz, and 50 Hz vibration frequencies was refined to different degrees. As the vibration frequency increased, the lamellar pearlite in the ductile iron gradually decreased, and the short rod-like and granular pearlite gradually increased. The layer spacing of the pearlite structure was measured using Nano Measure software, and the measurement results .
3.1.4 Influence of Vibration Frequency on the Tensile Properties
The tensile experiments were carried out on the ductile iron prepared with vibration frequencies of 0 Hz, 30 Hz, 40 Hz, and 50 Hz. The tensile strength and elongation of the ductile iron prepared with different processes were measured three times, and the average value was taken as the result. the tensile strength and elongation of the ductile iron prepared with different processes increased with the increase of the vibration frequency. When the vibration frequency was 40 Hz, the tensile strength and elongation of the ductile iron reached the peak. When the vibration frequency continued to increase, the tensile strength and elongation decreased slightly. The tensile strength of the ductile iron prepared without vibration was 419.5 MPa, and the elongation was 18.3%. The tensile strength of the ductile iron prepared with a vibration frequency of 30 Hz was 446.5 MPa, and the elongation was 21.1%. Compared with the ductile iron prepared without vibration, the tensile strength increased by 27 MPa, with an increase rate of 6.4%, and the elongation increased by 2.8%, with an increase rate of 15.3%. The tensile strength of the ductile iron prepared with a vibration frequency of 40 Hz was 457.1 MPa, and the elongation was 22.2%. Compared with the ductile iron prepared without vibration, the tensile strength and elongation increased by 37.6 MPa and 3.9% respectively, with an increase rate of 9.0% and 21.3%. When the vibration frequency was 50 Hz, the tensile strength of the ductile iron reached 452.3 MPa. Compared with the ductile iron prepared without vibration, the tensile strength increased by 32.8 MPa, with an increase rate of 7.8%. Compared with the ductile iron prepared with a vibration frequency of 40 Hz, the change was not significant. The elongation was 20.6%, and compared with the ductile iron prepared without vibration, the elongation increased by 2.3%, with an increase rate of 12.6%. It can be seen from the above analysis and comparison that the tensile strength and elongation of the ductile iron prepared with different vibration frequencies are greater than those of the ductile iron prepared without vibration, which further verifies that mechanical vibration has a strengthening effect on ductile iron.
The tensile strength of ductile iron is determined by the combined action of ferrite, pearlite and graphite structures. After the introduction of mechanical vibration, the spheroidization rate of the graphite structure of ductile iron increased, the number of spherical graphite increased, the size decreased, the content of irregular graphite structure decreased significantly, the ferrite grains in ductile iron were refined, and the layer spacing of pearlite decreased, thus increasing the tensile strength of ductile iron. However, when the vibration frequency is too high, the violent vibration destroys the conditions for the growth of graphite in a spherical shape, resulting in a decrease in the spheroidization rate. The irregular graphite will cause a splitting effect on the matrix during the plastic deformation process, preventing the matrix from fully exerting its performance and reducing the deformation resistance of ductile iron. The increase in the elongation of ductile iron is determined by its microstructure. The introduction of mechanical vibration significantly increased the spheroidization rate of the graphite structure of ductile iron and decreased the average size. The increase in the number of spherical graphite in ductile iron can cause plastic deformation during the tensile process to a certain extent. On the other hand, the introduction of mechanical vibration increased the non-uniform nucleation substrate of ductile iron, refined the ferrite grains, and increased the number of favorable grains during plastic deformation, delaying the appearance of microcracks in ductile iron during the plastic deformation process and effectively reducing the local stress concentration of ductile iron. The interface of short rod-like and granular pearlite is smaller than that of lamellar pearlite, so the resistance generated during dislocation movement is smaller, which is more conducive to dislocation slip and cross-slip during deformation. In summary, ductile iron has good plastic deformation ability during the tensile experiment process.
The tensile fracture morphology of ductile iron was observed, the tensile fracture morphology of the ductile iron prepared without vibration. The size of the graphite balls exposed on the fracture surface was large, there were a small number of dimples, the number was relatively small, and the size was large. There were cleavage planes with different orientations on the fracture surface, and there were microcracks in some areas of the cleavage planes, showing the characteristics of river patterns. After the application of mechanical vibration, the tensile fracture morphology of the ductile iron prepared with a vibration frequency of 30 Hz. Compared with the ductile iron prepared without vibration, the size of the graphite balls exposed on the surface decreased and the number increased, and the number of dimples centered on the graphite balls increased, indicating that the matrix had undergone plastic deformation, and the river patterns and cleavage fractures gradually decreased. the tensile fracture morphology of the ductile iron prepared with a vibration frequency of 40 Hz. The number of dimples centered on the graphite balls or the pits after the graphite balls were detached on the fracture surface further increased, and the tearing ridges were more obvious. The tearing ridges were the phenomenon generated when the dislocations and slip bands were blocked and gathered during the plastic deformation process of the matrix. There was no cleavage plane on the fracture surface, indicating that the ductile iron had undergone plastic deformation. the tensile fracture morphology of the ductile iron prepared with a vibration frequency of 50 Hz. Compared with the ductile iron prepared with a vibration frequency of 40 Hz, the distorted graphite structure on the fracture surface increased, the number of dimples decreased, the characteristics of the tearing ridges weakened, and the river patterns appeared in a small part of the fracture surface. The main body of the ductile iron still underwent plastic deformation.
To better characterize the tensile fracture morphology of the ductile iron prepared with different vibration frequencies, a three-dimensional laser confocal microscope was used to reconstruct the tensile fracture morphology of the ductile iron prepared with different vibration frequencies with a magnification of 100 times. The three-dimensional imaging technology of the three-dimensional laser confocal microscope was used to analyze the tensile fracture. After the three-dimensional reconstruction of the tensile fracture morphology, the different heights can be represented by different colors. The colors from high to low are: red, yellow, green, blue and purple. The plastic deformation degree of the tensile fracture can be expressed by the difference in colors. The height difference of the ductile iron prepared without vibration was 711.9 μm, and the height differences of the ductile iron prepared with mechanical vibration were 838.7 μm, 934.2 μm, and 825.3 μm respectively. The height difference reflects the plastic deformation degree of the ductile iron. The larger the height difference of the ductile iron specimen, the greater the plastic deformation degree. Therefore, it can be concluded that the ductile iron specimens prepared with different vibration frequencies all have different degrees of plastic deformation, and the ductile iron prepared with different vibration frequencies has a greater plastic deformation degree than that prepared without vibration.
To more accurately express the deformation degree of the ductile iron prepared with different vibration frequencies, the roughness detection software was used to measure the roughness of the three-dimensional surface morphology after reconstruction. The roughness index Sa can reflect the plastic deformation of ductile iron under tensile load. with the increase of the vibration frequency, the trend of the roughness index Sa was first increased and then decreased. Compared with the change in elongation in the tensile stress – strain curve, it can be found that the change trends of the surface roughness Sa of the fracture and the elongation of ductile iron were basically the same, both first increased and then decreased.