Effect of heat treatment on mechanical and thermal conductivity properties of die-cast aluminum alloy ZL102 was studied using microstructure characterization, mechanical and thermal conductivity testing. Results show that physical phases in room temperature structure of die-cast aluminum alloy ZL102 include primary α (Al), aluminum-silicon eutectic structure, primary crystalline silicon and a small amount of intermetallic compounds. After solid solution treatment, silicon phase in die-cast aluminum alloy ZL102 fuses and spheroidizes; after aging treatment, fine point-like second phase precipitates on α (Al) matrix, and sphericity of silicon phase at grain boundary is further improved. Among three heat treatment states, aluminum alloy ZL102 has the highest mechanical properties after solution treatment, but the lowest thermal conductivity. In summary, aging treatment takes into account mechanical and thermal conductivity properties of alloy. At this time, tensile strength of alloy is 212MPa, elongation is 3.9%, and room temperature thermal conductivity is 142.7W/(m·K).
With advent of 5G communication era, integration of electronic communication equipment and products is gradually increasing, and amount of heat generated per unit volume is also increasing. At this time, relevant materials and structures are required to have good thermal conductivity properties to ensure normal operation of equipment and products and extend their service life. Take 5G communication filter as an example. It has high power and high integration. In order to improve heat dissipation capacity, filter housing structure is usually designed with many irregular thin-walled heat sinks. For mass forming and manufacturing of this type of structural shells, die-casting process has significant efficiency and cost advantages. Density of metallic aluminum is only 1/3 of steel and iron, and it has huge potential for lightweighting. In recent years, it has been widely used in automobiles, communications, aerospace and other fields. Room temperature thermal conductivity of pure aluminum is approximately 237 W/(m·K), and it has excellent thermal conductivity. However, strength of pure aluminum is low. In actual production, some alloying elements are often added to improve its mechanical properties, and addition of alloying elements will have a certain impact on its thermal conductivity properties. Usually, alloying elements strengthen aluminum alloys in the form of solid solution atoms, intermediate phases or precipitation strengthening phases. However, whether it exists in the form of solid solution atoms or intermediate phases, it will bring a large number of vacancies, dislocations and other crystal defects to alloy, and precipitated phase will also cause lattice distortion in alloy. Existence of these defects increases probability of free electron scattering in alloy and reduces number of electrons for effective heat conduction, resulting in a reduction in thermal conductivity of alloy.
In order to take into account mechanical and thermal conductivity properties of aluminum alloys, researchers have conducted in-depth research. Wen Cheng studied effects of 22 alloy elements on electrical and thermal conductivity of industrial pure aluminum and found that different elements have different effects. Addition of transition elements such as Mn, Cr, etc. will cause electrical and thermal conductivity of pure Al to decline rapidly, while Zn, Sr and rare earth metamorphic elements have less impact. Li Linjun found that different magnesium to silicon ratios have different effects on thermal conductivity of aluminum alloy 6063. When magnesium to silicon ratio is 1.5, alloy has the best thermal conductivity. Lumley et al. studied effects of alloy composition and heat treatment on thermal conductivity of Al-Si-Cu aluminum alloy die castings. Study showed that thermal conductivity of alloys with certain compositions can be increased by more than 60% through use of heat treatment. Kim et al. tested thermal diffusivity of Al-1Si and Al-9Si alloys under different heat treatment conditions, studied relationship between thermal diffusivity and silicon phase solid solution and precipitation, concluded that re-precipitation of dissolved silicon in solution-treated samples will increase thermal diffusivity of alloy. Choi et al. studied effect of mold temperature on thermal and mechanical properties of aluminum alloys and concluded that the higher mold temperature, the slower solidification rate of alloy. At this time, the larger silicon particles are, the better thermal properties of alloy are. After aging treatment, mechanical strength of alloys at different mold temperatures becomes similar.
This article takes a 5G communication filter housing as an actual die-cast aluminum alloy ZL102 as research object, uses flash method to test thermal conductivity of alloy at different temperatures, changes microstructure of alloy through heat treatment to explore impact of alloy microstructure changes on mechanical and thermal conductivity properties, with a view to providing a reference for improving mechanical and thermal conductivity properties of aluminum alloy die castings in actual production.

An actual die-cast product of a 5G communication filter housing was used as test piece. As shown in Figure 1a, die-cast piece weighs about 5.4kg and has an overall size of about 539mm * 410mm * 45mm. Die-casting part has a complex structure and shape. Wall thickness of main body is only about 2mm, while wall thickness at mounting lugs and other locations reaches 6mm. Sampling position for microstructure characterization, mechanical and thermal conductivity testing in test is shown in Figure 1b. Wall thickness at this position is wall thickness of main body of die casting, which is well representative and convenient for sampling. Chemical composition of aluminum alloy ZL102 measured by X-ray fluorescence spectrometer (XRF) is shown in Table 1. Mechanical properties tensile specimens were prepared by wire cutting, and dimensions were determined in accordance with national standard GB/T228.1-2010. At the same time, Φ4mm*1mm disc samples were wire-cut for testing thermal conductivity of alloy. Heat treatment of samples is divided into three control groups. One group is in die-cast state without heat treatment; second group undergoes solution treatment at 500℃*4h; and third group undergoes aging treatment at 200℃*3h based on second group.

 

Figure 1 Actual die-cast product of a 5G communication filter housing

Metallographic samples were mechanically ground with 240#, 600#, 1200#, 1500#, and 2000# sandpaper in sequence, polished with diamond polishing agent, then used 95%H2O+2.5%HNO3+1.5%HCl+1%HF Keller's reagent to corrode for 10~20s. Structural morphology was observed under a Leica DM2700M optical microscope (OM) and a JEOL 6301F scanning electron microscope (SEM). Instron 5967 electronic universal testing machine was used to conduct room temperature tensile test. Tensile speed was 2mm/min. Mechanical property test data was average of 5 effective specimens.
According to physical laws of heat conduction, temperature field inside object will change with time during heat conduction, which makes it difficult to directly measure thermal conductivity. In this paper, based on measuring density, specific heat capacity and thermal diffusion coefficient of alloy, thermal conductivity of alloy is calculated using equation (1)
λ=αρCρ (1)
In formula: λ is thermal conductivity of sample to be measured, unit is W/(m·K); α is thermal diffusion coefficient of sample to be measured, unit is mm2/s; ρ is density of sample to be measured, unit is g /cm3; Cρ is specific heat capacity of sample being tested, in J/(g·K). LFA457 laser thermal conductivity meter was used to measure thermal diffusion coefficient of sample, specific heat capacity was measured using TA-DSC2500 differential scanning calorimetry, and Archimedean drainage method was used to measure density of sample.

Filter housing is die-cast from aluminum alloy ZL102. According to Table 1, silicon content in alloy is 12.8% (mass fraction, same below), which is close to eutectic composition of aluminum-silicon binary alloy (Si content of eutectic composition point in Al-Si binary equilibrium phase diagram is 12.6%). When alloy solidifies, eutectic reaction L→α(Al) + β(Si) mainly occurs, forming a large amount of α(Al) + Si eutectic structure. However, due to high cooling rate of alloy during die-casting process and solidification process away from equilibrium, phases present in die-casting structure of aluminum alloy ZL102 at room temperature include primary α (Al), aluminum-silicon eutectic structure, primary crystal silicon and a small amount of intermetallic compounds.. Figure 2 shows surface and core microstructure of die-casting obtained by OM. Gray-white matrix is primary α (Al), and there are a large number of gray-black alternating aluminum-silicon eutectic structures distributed between matrix, as well as needle-shaped / lath-shaped primary crystal silicon. Comparing Figure 2a and Figure 2b, surface structure is finer and more uniform. This is because surface layer of die casting is in direct contact with mold wall during forming. Alloy has a high cooling rate and a high degree of supercooling, making grains in surface structure finer and more uniform. In addition, a small amount of large gray polygonal structures were found in core structure of die casting, as shown in Figure 2b. Since iron content in alloy composition is slightly higher, it is initially judged to be an iron-containing phase. Alloy phase composition was tested using X-ray diffraction (XRD), with a scanning range of 10°~90° and a scanning speed of 4°/min. Test results were imported into MDI Jade 6 for analysis. Results are shown in Figure 3. It can be seen from XRD test results that there are only α (Al) and Si phases in alloy, and no iron-containing intermetallic compounds were detected. Reason for above situation may be that content of intermetallic compounds in alloy is low, resulting in XRD not detecting detected.

Table 1 Chemical composition of aluminum alloy ZL102 wB/%

 

Figure 2 Microstructure of aluminum alloy ZL102 die casting

 

Figure 3 XRD pattern of die-cast aluminum alloy ZL102
SEM was used to further analyze the above polygonal phases, as shown in Figure 4. Based on energy spectrometer (EDS) results (Table 2), it is speculated that it is a complex AiSiFeMn quaternary intermetallic compound. Based on some literature such as Yuan, Wang, etc., it was determined that phase is α-Al 15 (Mn, Fe)3Si2. This phase is further evolved by adding Mn element to β-Al5FeSi phase. When β-Al5FeSi phase is formed, some Mn atoms take away positions occupied by Fe atoms in β phase, which is equivalent to Mn atoms partially replacing Fe atoms, thus forming AlSiFeMn quaternary composite phase.

 

Figure 4 SEM image of polygonal phase

Table 2 EDS analysis results of point P in Figure 4 %

Figure 5 compares and analyzes aluminum alloy ZL102 die-casting structure before and after heat treatment. It can be seen that primary α (Al) changes little before and after heat treatment, and dendrite orientation has no obvious rules, while morphology and distribution of silicon phase have changed significantly. Silicon phase in die-cast structure shows a needle-like/lath-like distribution, which seriously splits α(Al) matrix; after solid solution treatment, silicon element originally dissolved into α(Al) matrix in solid solution stage precipitates, showing a fine point-like shape on α-Al matrix, while silicon phase at grain boundary is more rounded and sphericity is further improved. For polygonal AiSiFeMn quaternary intermetallic compound that appears in die-casting structure, its morphology does not change significantly during solution and aging treatment processes. It is speculated that reason is that solution treatment temperature in this article is not sufficient to dissolve it into α(Al) matrix. It can also be seen from Figure 5 that there are a certain number of pores distributed in die-casting structure. Pores in structure have a tendency to increase after solution treatment, but pores do not expand further with continued aging treatment.

 

Figure 5 Comparative analysis of die-cast, solid solution and aging structures

Mechanical properties of die-cast aluminum alloy ZL102 under different heat treatment processes are shown in Figure 6. It can be seen that in terms of strength and elongation, solution-treated sample is better than die-cast and aging-treated sample. Its tensile strength and elongation are 222.8MPa and 6.1% respectively. Compared with die-cast state, tensile strength has increased by 9.2%, and elongation has been significantly improved, increasing by 205 %, while mechanical properties of aged specimens are in the middle. Analyzing reasons, although there have been studies that ordinary die castings are not suitable for heat treatment due to existence of pores, and aluminum alloy ZL102 is a non-heat treatment strengthened alloy. However, fusing and spheroidization of silicon phase during solid solution treatment process significantly improves its splitting effect on α (Al) matrix, reduces stress concentration generated around alloy when it is loaded, is conducive to improvement of alloy strength and elongation. After further aging treatment, mechanical properties of alloy have declined to a certain extent compared to solid solution state. Reason is that silicon phase precipitated in α (Al) grain boundaries and solid solution further aggregates and grows, causing coarsening, which damages mechanical properties of alloy, no aging strengthening phase precipitates during aging process of alloy.

 

Figure 6 Effect of heat treatment on mechanical properties of die-cast aluminum alloy ZL102
Figure 7 shows influence of different temperatures and heat treatment processes on thermal conductivity of die-cast aluminum alloy ZL102 measured in experiment. First of all, it can be seen that thermal conductivity of alloy in three states of die-casting, solid solution and aging increases with increase of temperature. From room temperature to 300℃, it shows a trend of rapid growth first and then slow growth. This changing trend is mainly related to thermal conductivity mechanism of alloys. For general metals, thermal conductivity consists of electronic thermal conductivity and phonon thermal conductivity. Correspondingly, thermal resistance is also divided into electronic thermal resistance and phonon thermal resistance. Electronic thermal resistance is caused by electrons being scattered by various media and consists of electron-phonon scattering and electron-defect scattering. Phonon thermal resistance is also determined by two processes: one is collision between phonons caused by nonlinear vibration of crystal lattice; the other is collision between phonons and defects in solid. When temperature is low, vibration amplitude of atoms on crystal lattice is very small, and contribution of phonons to thermal conductivity is small. Thermal conductivity is dominated by electrons. At this time, thermal conductivity of alloy is mainly determined by interaction between electrons and defects. Number of internal defects in alloy basically does not change with temperature changes in low-temperature region, and movement rate of electrons basically remains unchanged whether in the high-temperature or low-temperature region. At this time, mean free path of electrons can be approximately considered to be a constant. At this time, thermal conductivity of alloy is mainly determined by specific heat, and specific heat has a linear relationship with temperature. Therefore, thermal conductivity of alloy in three heat treatment states increases rapidly with increase of temperature at the beginning. As temperature continues to rise, vibration of atoms in alloy under three heat treatment states slowly intensifies. At this time, scattering effect of phonons on electrons begins to increase, probability of electrons being scattered increases, and mean free path of electrons begins to decrease, which causes thermal conductivity of alloy to slowly increase as temperature continues to increase.

 

Figure 7 Effect of temperature and heat treatment process on die-cast aluminum alloy ZL102
Effect of thermal conductivity
Comparing thermal conductivity of alloys in three heat treatment states in Figure 7, it can be seen that die-cast alloy has the highest thermal conductivity at different temperatures. That is, solution and aging treatment processes in this article cannot improve thermal conductivity of die-cast aluminum alloy ZL102, but instead reduce thermal conductivity of alloy. Analysis of reasons shows that there are a large number of solid solution atoms in the sample after solution treatment, which causes lattice distortion, resulting in an increase in crystal defects in alloy, an increase in probability of electrons being scattered, a decrease in mean free path of electrons, and a significant reduction in thermal conductivity of alloy. After aging treatment, thermal conductivity of alloy recovers somewhat compared to solid solution state. This is because second phase precipitates during aging treatment, which reduces solid solubility of alloy and reduces degree of lattice distortion. However, it can be seen that thermal conductivity of aged alloy is still lower than that of die-cast state. This is because second phase precipitated during aging treatment will increase phase interface of alloy, which will in turn increase probability of electrons being scattered, reducing thermal conductivity of alloy. Generally speaking, among negative effects of alloying elements on thermal conductivity of aluminum alloy ZL102, solid solution form is greater than precipitated phase form. In addition to solid solution and precipitation of alloy elements, growth of grains and pores in structure of die-cast aluminum alloy ZL102 during heat treatment process will also have a certain impact on thermal conductivity of alloy. Relevant studies have shown that grain growth reduces grain interface of alloy and can improve thermal conductivity of alloy to a certain extent, while existence of holes will obviously weaken thermal conductivity of alloy.

(1) Phases in die-casting structure of aluminum alloy ZL102 at room temperature include primary α (Al), aluminum-silicon eutectic structure, primary crystal silicon and a small amount of intermetallic compounds. Polygonal phase is AlSiFeMn quaternary composite phase.
(2) After solution treatment, silicon phase in die-casting structure of aluminum alloy ZL102 has fused and spheroidized; after aging treatment, fine point-like second phase precipitates on α(Al) matrix, and silicon phase balls at grain boundaries degree further increased.
(3) Among the three heat treatment states, aluminum alloy ZL102 has the highest mechanical properties after solution treatment. Tensile strength and elongation are 222.8MPa and 6.1% respectively, which are respectively increased by 9.2% and 205% compared with die-cast state. However, At this time, thermal conductivity of alloy is the lowest, and thermal conductivity at room temperature drops from 155.8 W/(m·K) in die-cast state to 127.8 W/(m·K). In summary, aging treatment takes into account mechanical and thermal conductivity properties of alloy. At this time, tensile strength of alloy is 212MPa, elongation is 3.9%, and room temperature thermal conductivity is 142.7 W/(m·K).