Study on Mechanical and Thermal Conductivity of Die Casting Aluminum Alloys

Al-xZn-8Si-2Cu (x=12, 14, 16, 18) alloy profiles were prepared by die-casting process. OM and SEM were used to analyze the effect of different Zn content on the microstructure and morphology of the alloy, and the effect of Zn content on the mechanical properties and thermal conductivity was further analyzed. The results show that when the Zn content in the alloy increases from 12% to 18%, the content of Zn dissolved in the α-Al matrix increases, which has a solid solution strengthening effect on the alloy. At the same time, the size of the eutectic Si increases to form sharp corners. stress concentration; the tensile strength and yield strength of the alloy increased from 345.5 MPa and 263.5 MPa to 395.4 MPa and 343.8 MPa, respectively; the elongation decreased slightly; the thermal conductivity decreased from 109.4 W/(m K) to 85.7 W/ (m·K).

Aluminum alloy has the advantages of low density, high strength and good comprehensive performance, so it is widely used in radiators, electronic communications and other fields. With the increasing development and progress of science and technology, electronic products are constantly becoming lighter and miniaturized, and people's requirements for the operating performance of electronic products such as computers, mobile phones, and tablets are also increasing day by day. The improvement of the operating performance of electronic products needs to be supported by the good thermal conductivity and mechanical properties of materials. According to relevant statistics, about 55% of electronic parts fail due to equipment overheating or other heat-related problems, and a 10 ℃ increase in the temperature of semiconductor components will also reduce their service reliability by about 50%, and when components When operating at higher temperatures, its failure rate also increases with increasing temperature. Equipment damage caused by impact, drop, bending, etc. faced by electronic products in daily use needs to be improved by improving the mechanical properties of materials. At present, the comprehensive properties of existing materials are difficult to meet the requirements of heat dissipation and mechanical properties during equipment operation at the same time. Therefore, it is particularly important to develop materials with high strength and high thermal conductivity for electronic products.

There are many factors that affect the thermal conductivity and mechanical properties of aluminum alloys, mainly including the following three aspects: ①Solubility of alloying elements and their forming phases in the aluminum matrix. The thermal conductivity of the alloy is affected by affecting the degree of lattice distortion of the aluminum matrix; the strengthening phase is formed between other elements and Al or between other elements to affect the strength; ② impurities and defects. The higher the number, the less dense the alloy, the lower the thermal conductivity and the lower the strength. ③ heat treatment. By affecting the shape, size and distribution of the phases in the alloy, the scattering effect on the moving electrons is changed, thereby affecting the thermal conductivity of the alloy, and the precipitation of the strengthening phase has a certain effect on the thermal conductivity and mechanical properties of the aluminum alloy. Among them, alloying elements are one of the main factors affecting the thermal conductivity and mechanical properties of alloys, which mainly depend on the existence form and content of alloying elements. Based on the research status at home and abroad, Deng Banghui and others have shown that the thermal conductivity of Al10SiMnMg after die-casting is 125 W/(m·K), and the tensile strength and yield strength are 155 MPa and 80 MPa, respectively. The thermal conductivity of traditional ZL101 after low pressure casting is 125 W/(m·K), and the tensile strength and yield strength are 155 MPa and 95 MPa, respectively. However, the thermal conductivity and mechanical properties of traditional Al-Si alloys have certain limitations, which cannot fully meet the needs of actual industrial production and applications. Due to the low solid solubility of Zn in α-Al, the negative effect on thermal conductivity is small. Through the addition of Zn element, on the premise of ensuring the good thermal conductivity of Al-Zn alloy, it is expected to further improve the thermal conductivity of aluminum alloy. Mechanical properties of as-cast room temperature. Therefore, Zn was used as a strengthening element in this experiment to study the changing trends of microstructure, thermal conductivity and mechanical properties in Al-Zn alloys with Zn content of 12%-18%. At the same time, by means of SEM, OM, XRD and other detection methods, the existence of Zn in the alloy was analyzed to reveal the influence of Zn on the mechanical properties and thermal conductivity of Al-Zn alloy.

The chemical compositions of the test alloys are shown in Table 1. The raw materials are prepared with pure aluminum, pure Zn, pure Cu, Al-30Si, Al-10Mn and Al-10Fe master alloy.

The pure aluminum is added to the resistance furnace for heating and melting, and the melting temperature is 730 ℃. Subsequently, Al-10Mn master alloy, Al-10Fe master alloy, Zn, Cu and Al-30Si master alloy were added, and molybdenum rods were used to stir after all the alloys were melted to make the alloy composition evenly distributed. Then let it stand, and after slag removal, die-cast tensile specimens. The dimensions of the die-casting specimens are shown in Figure 1. A small amount of Fe is added to facilitate the demoulding of the casting. However, since Fe will reduce the mechanical properties of the alloy, adding a small amount of Mn reduces the negative effect of Fe. The tensile test was carried out with HD-B615-A-S microcomputer-controlled electronic universal material testing machine. The surface of the sample was ground and polished to a smooth surface, and the conductivity was tested with a FIRST FD-102 eddy current conductivity meter. After mechanical polishing, the microstructure and fracture morphology of the samples were observed under a ZEISSM10A scanning electron microscope.

2.1 The effect of Zn content on the mechanical properties of the alloy

With the increase of Zn content from 12% to 18%, the tensile strength and yield strength of the alloy gradually increased. 30.5%; elongation decreased gradually with the increase of Zn content, from 1.72% to 1.15%. It can be seen that the increase of Zn content in the alloy can greatly improve the strength of the alloy, but it will reduce the plasticity of the alloy.

2.2 Analysis of fracture morphology

Tensile fracture morphologies of Al-Zn alloys with different Zn contents. It can be seen that the fracture structure of the alloy is mainly composed of a small number of cleavage platforms, a large number of dimples and tearing edges. With the increase of Zn content in the alloy, the number of dimples and tear edges in the fracture structure of the alloy gradually decreased, and the area of ​​the cleavage platform increased relatively. When the Zn content increased to 18%, the number of dimples and tearing edges in the fracture tissue reached the minimum, and the overall brittle fracture characteristics were exhibited.

The variation law of thermal conductivity with Zn content. It can be seen that with the increase of Zn content, the thermal conductivity of the alloy tends to decrease. It is reduced from 109.4 W/(m·K) to 85.6 W/(m·K), a decrease of 21.8%. The essence of the heat conduction process of metal materials is the heat conduction of free electrons and the heat generated by lattice vibration. According to the crystal characteristics of metal materials, the heat conduction mode of metal materials is mainly the heat conduction of free electrons, so there is a certain relationship between the thermal conductivity and electrical conductivity of metals, that is, the Wiedemann-Franz law.

According to the classical theory of electrical conductivity and thermal conductivity, the less defects, grain boundaries and lattice distortions in a metal material, the more complete the lattice of the metal material, the less the scattering effect on electrons, and the better the thermal conductivity.

The addition of Zn has an important effect on the thermal conductivity of the alloy. A part of Zn dissolves into the α-Al matrix, which changes the original lattice lattice inside the alloy and increases the lattice distortion. A part of it fails to dissolve into the matrix, and a Zn-rich phase is formed at the grain boundary, which reduces the mean free path of electron movement and increases the difficulty of electron passage, thereby reducing the thermal conductivity.

2.4 The effect of Zn on the microstructure of Al-Zn alloy

Microstructures of Al-Zn alloys with different Zn contents. It can be seen that the α-Al dendrites in the Al-12Zn-8Si-2Cu alloy are relatively coarse, the Si morphology in the eutectic structure is relatively small, and the distribution is relatively uniform, so the elongation is relatively high. When the Zn content in the alloy was increased to 14%, the size of α-Al dendrites became significantly smaller, and the content of Zn dissolved in the α-Al matrix increased, which played a solid solution strengthening effect on the alloy. As the Zn content continued to increase to 18%, the size of α-Al dendrites became smaller, and the solid solution strengthening effect was further enhanced, so the yield strength was improved; on the other hand, the addition of Zn increased the size of eutectic Si, and these sizes The enlarged eutectic Si has some sharp corners, which will cause stress concentration and reduce the elongation of the alloy; at the same time, the increase of the Zn content in the α-Al matrix caused by the increase of the Zn content increases the lattice defects. thereby reducing thermal conductivity.

Fig.6 Metallographic structure of Al-xZn-8Si-2Cu alloy

2.5 XRD and EDS phase analysis

FIG. 7 is the XRD patterns of Al-Zn alloys with different Zn contents. It can be seen that the Al-xZn-8Si-2Cu alloy is mainly composed of five phases including Al, Al2Cu, Zn, Si, and Al6(FeMn). With the increase of Zn content, the change of Zn content had no effect on the phase composition of the alloy, and the peak and position of the phase without Zn did not change, while the peak of the Zn-containing phase increased and the peak intensity increased.

Fig.7 XRD patterns of aluminum alloys with different Zn contents

Figure 8 shows the SEM micrographs of Al-Zn alloys with different Zn contents, in which the white dotted phase at A is the Zn-rich phase that is not dissolved at the grain boundary. It can be seen that since the limit solubility of Zn in Al can reach 32.8%, more Zn dissolves into the matrix during the solidification process of the alloy, and since this position is near the Al matrix, the main element content in the EDS results is Al, Zn content lower. The gray block at B is the primary Si phase, and the Chinese character at C is the Al6(FeMn) phase. Due to the low content of Fe and Mn, and the formation of less Chinese-shaped Al6 (FeMn) phase, it will not produce splitting effect on the structure, so it has little effect on the mechanical properties of the alloy. The gray-white short rod at D is the Al2Cu phase. With the increase of Zn content, the morphology, number and size of the second phase particles did not change significantly. It can be seen that the increase of Zn content has little effect on the second phase particles in the alloy, and this result is also consistent with the XRD result.

Table 2 Al-xZn-8Si-2Cu(x=12, 14, 16, 18) EDS results %

3in conclusion

(1) With the increase of Zn content, the content of Zn dissolved in the α-Al matrix continues to increase, which has a solid solution strengthening effect on the alloy, and also increases the size of eutectic Si, and the eutectic Si with larger size increases. There are some sharp corners, which will cause stress concentration and reduce the elongation of the alloy; the increase of the Zn content in the α-Al phase will increase the lattice defects and reduce the thermal conductivity.

(2) The increase of Zn content can significantly improve the strength of the alloy, but it will reduce the elongation and thermal conductivity of the alloy. When the Zn content of the alloy is increased from 12% to 18%, the tensile strength and yield strength of the alloy are respectively In order to increase from 345.5 MPa, 263.5 MPa to 395.4 MPa, 343.8 MPa, increased by 14%, 30.5%, respectively; thermal conductivity decreased from 109.4 W/(m K) to 85.7 W/(m K).

(3) The increase of Zn content has little effect on the phase composition in the structure. The number of dimples and tear edges in the fracture structure increases with the increase of Zn content, and the relative area of the cleavage platform decreases with the increase of Zn content, showing brittle fracture characteristics as a whole. .