Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys

Casting temperature on die casting aluminum alloy properties which influence?

With the arrival of the 5G communication era, the structure of products in the field of transportation and communication is developing in the direction of integration and lightweight, so the requirements for the thermal performance and load capacity of materials are increasing. For the complex heat dissipation structural parts in the field of communication and and transportation, due to its uneven wall thickness, the requirements for material fluidity and heat dissipation performance are getting higher and higher, and the use of cast aluminum-silicon alloys can meet both its formability and mass production. Traditional Die-Cast Aluminum Alloys, such as ADC12 thermal conductivity of 92 W / (m – K), yield strength of 156 MP, has been unable to meet its performance requirements, domestic researchers according to the composition, process, organization and performance to carry out a series of studies in order to improve the overall performance of aluminum alloys. In recent years, Japan has developed DMS series and DX series of high thermal conductivity casting aluminum alloy, of which DMS1, DMS3, DX17, DX19 alloy thermal conductivity as high as 150~210W/(m-K), however, the yield strength is less than 120 MPa, DMS5 is a replacement of ADC12, the thermal conductivity of ADC12 is about 1.6 times. DMS5 is a replacement for ADC12, with a thermal conductivity about 1.6 times that of ADC12 and a yield strength similar to ADC12. The Al-10Si-0.3Mg die-casting aluminum alloy designed by this project has good fluidity, can be strengthened by heat treatment, the thermal conductivity is close to that of DMS5, the yield strength is higher than that of DMS5, the formability is comparable to that of ADC12, and the corrosion resistance is better than that of ADC12, so it can satisfy the urgent needs of new-generation communication devices and automobile heat dissipation parts for higher thermal conductivity and yield strength., such as ADC12 thermal conductivity of 92 W / (m – K), yield strength of 156 MP, has been unable to meet its performance requirements, domestic researchers according to the composition, process, organization and performance to carry out a series of studies in order to improve the overall performance of aluminum alloys. In recent years, Japan has developed DMS series and DX series of high thermal conductivity casting aluminum alloy, of which DMS1, DMS3, DX17, DX19 alloy thermal conductivity as high as 150~210W/(m-K), however, the yield strength is less than 120 MPa, DMS5 is a replacement of ADC12, the thermal conductivity of ADC12 is about 1.6 times. DMS5 is a replacement for ADC12, with a thermal conductivity about 1.6 times that of ADC12 and a yield strength similar to ADC12. The Al-10Si-0.3Mg die-casting aluminum alloy designed by this project has good fluidity, can be strengthened by heat treatment, the thermal conductivity is close to that of DMS5, the yield strength is higher than that of DMS5, the formability is comparable to that of ADC12, and the corrosion resistance is better than that of ADC12, so it can satisfy the urgent needs of new-generation communication devices and automobile heat dissipation parts for higher thermal conductivity and yield strength.

The casting temperature has an important influence on the casting performance and quality. Fang Yuanming et al. found that the casting temperature has an important influence on the casting defects by simulating the connecting rod die casting of aluminum alloy, and Wang Shaozhi et al. produced aluminum alloy automobile bracket, and the use of near-liquid-phase line pouring temperature can obtain uniform and fine spherical organization. Using JMatPro software to simulate the liquid phase line of Al-10Si-0.3Mg aluminum alloy is 10 oC higher than that of ADC12, and the selection of the appropriate pouring temperature plays an important role in improving its comprehensive performance. When the low temperature casting, the alloy mobility is reduced, resulting in die casting filling more difficult, using a higher pouring temperature will make the alloy shrinkage increases, the gas solubility in the metal liquid increases, the defects such as loosening and porosity are easy to produce, thus reducing the performance of the alloy or even cause product scrap therefore. This project takes Al-10Si-0.3Mg as the object, studies the effect of casting temperature on its organization, mechanical properties and thermal conductivity, and determines the appropriate casting temperature for Al-10Si-0.3Mg aluminum alloy, aiming to achieve the purpose of improving the alloy properties and enhancing the quality of castings.

1. Test program

1.1 Alloy composition

The Al-10Si-0.3Mg alloy of this test is a self-designed composition, taking into account the mobility, strength and thermal conductivity, and its chemical composition is shown in Table 1. The main raw materials used are 99.9% pure aluminum, 2202 crystalline silicon, pure magnesium ingot, and 75% iron additives.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys1

Table 1. Main components of the alloy wb/%.

1.2 Preparation process

Prepare 200 kg of alloy, melting in a 300 kg resistance furnace, the first baking furnace, when the furnace temperature reaches 700 ℃ to add 99.9% pure aluminum ingot, melting for 3 h, to be completely melted ingot and the aluminum temperature reaches 800 ℃ to add 2202 crystalline silica, heating to 850 ~ 900 oC molten silicon, during the period of continuous stirring, until completely melted. Subsequently add iron additives, stirring, to be completely melted add Mg, magnesium block completely immersed in the liquid aluminum, eliminate open flame, stirring for 5 min after the melt refining, refining temperature of 710-730 oC, refining agent using sodium-free calcium-free refining agent, the amount of use according to the 2 kg / t, rotational speed of 250~350 r/min, the gas flow rate of 5~10 L / min. refining time of 20min. After the completion of refining slagging, composition to reach a slightly higher design value, take samples, in the Spectrum M12 (LAB) direct-reading spectrometer to analyze the composition of qualified for die casting, the test process is completed in the same furnace, and the order of low temperature to high temperature.

DCC280 type 2800 kN die-casting machine is used, its clamping force is 280 k N, the thickness of the material shank is about 15 mm, and the mold surface temperature is controlled by the mold temperature controller, set at 200 ℃. The injection force is 330 k N, injection time is 3.5 s, cooling time is 2.0 s, the stroke position of the charging head in die casting: a fast position is set to 100 mm, the second fast position is set to 250 mm, the position of the selection of the pressurization is 280 mm, the standard tensile specimen bar shaped in Fig. 1. In order to study the effect of casting temperature on the microstructure, mechanical properties and thermal conductivity of the specimen, a total of 650. 680, 720 ℃ three preparations, In order to study the effect of casting temperature on the microstructure and mechanical properties and thermal conductivity of the specimens, a total of 650, 680, 720 ℃ 3 groups of specimens were prepared.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys2

Figure 1:Die-casting mechanical properties of test bars and thermal conductivity specimens

1.3 Organization performance analysis

Shenzhen Sansi electronic universal testing machine is used to test the mechanical properties of the tensile test bar, the tensile rate of 1 mm/mm; die-casting Ф12.7 mm × 2 mm standard thermal conductivity specimen (see Figure 1), using the German NeiChi LFA467 laser flash instrument test; from the tensile test bar in the middle of the spacing section of the sample, through the standard sampling procedures for the preparation of metallurgical specimens and corrosion, after the use of Olympus GX53 microscope and Phenom XL desktop scanning electron microscope for microstructure observation and analysis of the specimen.

2. test results and discussion

2.1 Microstructure observation

Figure 2 shows the metallographic organization of the alloy at different casting temperatures. It can be seen that the white circular and elliptical organizations are primary α-Al, and the white α solid solution between short needles and dendrites constitutes (α+Si) eutectic, and a small amount of primary silicon exists. The black dots are pores, and due to the low Mg content of the alloy, only a small amount of Mg2Si is present in a granular state, fishbone, and dendrites. In addition, it can be seen that as the die-casting temperature increases, the pores of the alloy gradually increase, the size of the primary crystal Si and α-Al increases, because the metal aluminum has very good plasticity, but its strength is low, however, with the casting temperature increases, the number of pores in the alloy increases, resulting in the plasticity advantage of the metal aluminum is difficult to reflect, and the primary crystal Si is a brittle should be the phase, the tip and the edge is easy to cause stress concentration, the size of its increase leads to the strength of the alloy further reduced. The strength of the alloy is further reduced by the increase of its size.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys3

Figure 2:Microstructure of Al-10Si-0.3Mg aluminum alloy at different casting temperature

2.2 X-ray non-destructive examination of the alloy at different casting temperatures

Figure 3 shows the X-ray nondestructive testing of the alloy at different die-casting temperatures. It can be clearly seen that as the die-casting temperature increases, the pores in the casting gradually increase, from the point-like distribution of pores at 650 oC die-casting to the net-like distribution of pores at 720 oC. When the die-casting temperature increases, the cooling gradient of the metal liquid becomes larger, the volume contraction during solidification is large, and air holes are formed inside the casting, which is consistent with the results observed in the metallographic organization of the casting.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys4

Figure 3:X-ray NDT of Al-10Si-0.3Mg aluminum alloy at different casting temperatures

2.3 Mechanical properties of Al-10Si-0.3Mg aluminum alloy with different casting temperature

Figure 4 shows the mechanical properties of the castings with the change of die casting temperature. It can be seen that the average mechanical properties of the die casting specimens decrease with the increase of die casting temperature. When the casting temperature is 650 oC, the alloy tensile strength, yield strength and elongation to obtain the maximum value, respectively, 298MPa, 201MPa, 5.62%. When the die casting temperature rises, other parameters (speed, time, pressure) remain unchanged, filling the end of the die casting temperature will rise accordingly, the solidification time of the alloy increases, resulting in a reduction in its solidification speed, thus causing the alloy grain becomes coarse, primary dendrite and secondary dendrite spacing increases, the number of porosity increases, which leads to the mechanical properties of the alloy deterioration. The performance of die-casting aluminum alloy depends on the morphology, size and distribution of primary α-Al phase, eutectic Si, primary Si, secondary intermetallic compounds and pores.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys5

Fig. 4:Mechanical properties of Al-10Si-0.3Mg aluminum alloy at different casting temperatures

2.4 Thermal conductivity and density of Al-10Si-0.3Mg aluminum alloy at different casting temperatures

Figure 5 shows the thermal conductivity and density of Al-10Si-0.3Mg alloy. It can be seen that the thermal conductivity and density of the alloy decreases as the casting temperature increases. This is because, as the casting temperature increases, the number of pores in the alloy increases, and the effective thermal conductive area decreases, which results in a decrease in thermal conductivity of the alloy. At the same time the second phase appearance is changed, resulting in increased lattice distortion. Electrons, lattice vibration waves and electromagnetic radiation is the carrier of heat conduction in the metal, the total heat conduction is the superposition of each carrier conduction, there are a large number of free electrons in the metal, can quickly realize the heat transfer, electronic heat transfer is its main heat transfer mode. Therefore any factor that produces inhomogeneity in the internal organization of the metal increases the scattering of electron waves, which leads to a decrease in the thermal conductivity of the alloy.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys6

Fig. 5:Thermal conductivity and density of Al-10Si-0.3Mg aluminum alloy at different casting temperature

2.5 Fracture morphology of Al-10Si-0.3Mg aluminum alloy at different pouring temperatures

Figure 6 shows the fracture morphology of castings after stretching at different pouring temperatures. For Al-Si alloys, the fracture mode of the alloy changes from through-crystal fracture to along-crystal fracture as the size of the dendrites becomes smaller. The second phase in the organization of the alloy in this test is mainly intermetallic compounds, primary crystal Si, eutectic Si, and these second phases have a great influence on the tensile fracture of the alloy. When the die casting temperature is 650 oC, the fracture surface has a large number of tough nest, tough nest is relatively shallow and small, in the fracture surface a large number of distribution of quasi-disintegration surface, at the same time can be seen to distribution of river-like pattern, so the alloy plasticity is relatively good, see figure 6 a. When the die casting temperature is increased to 680 oC, the fracture morphology has the existence of the tearing prongs, the number of tough nests and the size of the reduction, see figure 6 b. From the figure 6 c can be seen. As can be seen from Fig. 6c, the fracture profile at 720 oC has flat surfaces and the number of tough nests is very small, which further deteriorates the plasticity.

Influence of Casting Temperature on the Properties of Die-Cast Aluminum Alloys8

Fig. 6: Tensile fracture morphology of Al-10Si-0.3Mg aluminum alloy at different casting temperatures.

3. Conclusion

(1) When the casting temperature is 650 oC, the tensile strength, yield strength and elongation of the alloy obtain the maximum value, respectively, 298 MPa, 201 MPa, with the increase of the casting temperature, the cooling rate is small, the size of the primary crystalline Si and α-Al in the organization increases, and the spacing of the primary dendritic crystal and secondary dendritic crystals increases, and the mechanical properties of the alloy decreases.

(2) With the increase of casting temperature, the alloy absorbs gas seriously, the density of the alloy decreases, the effective thermal conductivity area decreases, which makes the thermal conductivity of the alloy decrease.

(3) The fracture morphology was observed by scanning electron microscopy, when the casting temperature increased, the number and size of tough nests in the fracture of the alloy decreased, which led to a decrease in the plasticity of the alloy.

ABOUT RUIYU

Ningbo Ruiyu Metal Products Co., Ltd. is located in the southern part of Yinzhou District, Ningbo City, Zhejiang Province, China. We are situated 30 kilometers away from Ningbo Port, and our factory covers an area of 3,000 square meters. With a workforce of over 30 employees, our team possesses more than 20 years of experience in the die-casting industry.

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