With advent of 5G communication era, structure of products in transportation and communication fields is developing towards integration and lightweighting. Therefore, requirements for material heat dissipation performance and load-bearing capacity are constantly increasing. Since most of complex heat dissipation structural parts in communication and transportation fields have uneven wall thickness, requirements for material fluidity and heat dissipation performance are getting higher and higher. Casting Al-Si alloy can not only meet its formability but also enable mass production. However, traditional Al-Si cast aluminum alloys, such as ADC12 aluminum alloy, have a thermal conductivity of 92 W/(m·K) and a yield strength of 156MPa, which can no longer meet usage requirements. Researchers have conducted a series of studies based on composition, technology, and structural properties to improve the overall performance of aluminum alloys. In recent years, Japan has developed DMS series and DX series of high thermal conductivity cast aluminum alloys. Thermal conductivity of DMS1, DMS3, DX17, and DX19 alloys is as high as 150-210W/(m·K), but yield strength is less than 120MPa. DMS5 alloy is a replacement product of ADC12 aluminum alloy. Thermal conductivity is about 1.6 times that of ADC12 aluminum alloy, and yield strength is similar to ADC12 aluminum alloy. Al-10Si-0.3Mg die-cast aluminum alloy designed by ourselves in this project has good fluidity and can be strengthened by heat treatment. Thermal conductivity is close to that of DMS5, yield strength is higher than that of DMS5 alloy, formability is equivalent to ADC12, and corrosion resistance is better than ADC12 aluminum alloy. It can meet urgent demand for higher thermal conductivity and yield strength of new generation communication devices and automotive heat dissipation parts.
Pouring temperature has a great influence on performance of castings. Liquidus line of Al-10Si-0.3Mg aluminum alloy simulated using JMatPro software is 10℃ higher than that of ADC12 aluminum alloy. Therefore, choosing appropriate pouring temperature plays an important role in improving its overall performance. When pouring at low temperatures, fluidity of aluminum alloys decreases, making die-casting mold filling more difficult. Using higher pouring temperatures will increase shrinkage of alloy, increase gas solubility in aluminum liquid, and easily cause defects such as shrinkage and pores, thereby reducing performance of alloy and even causing product to be scrapped. Taking Al-10Si-0.3Mg aluminum alloy as object, effect of pouring temperature on its structure, mechanical properties and thermal conductivity was studied, and appropriate pouring temperature of Al-10Si-0.3Mg alloy was determined, aiming to improve alloy properties and improve quality of castings.

Self-designed Al-10Si-0.3Mg aluminum alloy was used in test. Its chemical composition is shown in Table 1. Main raw materials used are 99.9% pure aluminum, 2202 crystalline silicon, pure magnesium ingots, and 75% iron additives. A DCC280 2800 kN die-casting machine is used, with a clamping force of 280kN and a material handle thickness of about 15mm. A mold temperature machine is used to control mold surface temperature and is set to 200℃. Injection force is 330kN, injection time is 3.5s, and cooling time is 2.0s. Stroke position of punch during die casting: the first fast position is set to 100mm, the second fast position is set to 250mm, and supercharged position is 280mm. Formed standard tensile test bar is shown in Figure 1. In order to study effect of pouring temperature on microstructure, mechanical properties and thermal conductivity of samples, three groups of samples were prepared at pouring temperatures of 650, 680 and 720 ℃.

Table 1 Main components of Al-10Si-0.3Mg alloy (%)

 

Figure 1 Die-casting mechanical properties test rod and thermal conductivity sample

 

Figure 2 Microstructure of Al-10Si-0.3Mg aluminum alloy at different pouring temperatures
White round and elliptical structures in structure are primary α-Al, short needles and white α solid solution between dendrites constitute (α+Si) eutectic, and there is a small amount of primary Si, and black dots are pores. Due to low Mg content in alloy, only a small amount of Mg2Si exists in the form of particles, fishbones, and dendrites. In addition, it can be seen that as die-casting temperature increases, pores of alloy gradually increase, sizes of primary Si and α-Al increase. Because aluminum has good plasticity, but its strength is low. However, as pouring temperature increases, number of pores in alloy increases, resulting in in a decrease in plasticity. Primary Si is a brittle phase, which easily causes stress concentration at tips and edges. Increase in its size will further reduce strength of alloy.

 

Figure 3 X-ray nondestructive testing of Al-10Si-0.3Mg aluminum alloy at different pouring temperatures

 

Figure 4 Mechanical properties of Al-10Si-0.3Mg aluminum alloy at different pouring temperatures
When pouring temperature is 650℃, tensile strength, yield strength and elongation of Al-10Si-0.3Mg alloy reach maximum values, which are 298MPa, 201MPa and 5.62% respectively. When pouring temperature increases, other parameters (speed, time, pressure) remains constant, solidification time of alloy increases, resulting in a decrease in solidification rate, thus causing alloy grains to become coarser, distance between primary dendrites and secondary dendrites to increase, number of pores to increase, resulting in deterioration of mechanical properties of alloy.     Properties of die-cast aluminum alloys depend on shape, size and distribution of primary α-Al phase, eutectic Si, primary Si, secondary phase intermetallic compounds and pores.

 

Figure 5 Thermal conductivity and density of Al-10Si-0.3Mg aluminum alloy at different pouring temperatures

 

Figure 6 Tensile fracture morphology of Al-10Si-0.3Mg aluminum alloy at different pouring temperatures

(1) As pouring temperature increases, Al-10Si-0.3Mg aluminum alloy absorbs air seriously, alloy density decreases, and effective thermal conductivity area decreases, resulting in a decrease in thermal conductivity of alloy.
(2) Observation of fracture morphology through scanning electron microscopy found that when pouring temperature increased, number and size of dimples in fracture surface of Al-10Si-0.3Mg aluminum alloy decreased, resulting in a decrease in plasticity of alloy.