Advent of 5G communication era is driving electronic communication equipment and products towards a highly integrated direction. However, requirements for thermal conductivity of equipment materials are also constantly increasing to ensure service life of equipment and products. As an important component of 5G base stations, communication filters have high power and high integration. There are many irregular thin-walled heat sinks designed on shell structure to improve heat dissipation capacity. High-pressure casting has become main forming method for large-scale manufacturing of heat dissipation shells due to its high production efficiency and cost advantages. Metal aluminum has characteristics of low density, high specific strength and excellent corrosion resistance, and is main material for manufacturing communication filters. Room temperature thermal conductivity of pure aluminum is about 237W/(m·K), but its strength is low. Alloying can improve mechanical properties of pure aluminum, but it will have a certain negative impact on its thermal conductivity. Alloy elements generally exist in aluminum in the form of solid solution atoms, precipitated phases, and second phases, which will bring crystal defects such as vacancies, dislocations, and phase interfaces, thereby increasing probability of electron scattering and reducing mean free path of electron scattering, which ultimately leads to a decrease in electrical (thermal) performance. There is an exclusive relationship between strength and electrical (thermal) performance. At present, materials of high thermal conductivity aluminum heat dissipation shells are mainly improved on basis of Al-8Si alloy. Considering service life of mold, die-cast high thermal conductivity aluminum alloy material generally contains 0.8%~1.0% Fe to improve anti-sticking property of mold. On the other hand, considering electrical and thermal conductivity of alloy, it is necessary to further strictly control content of impurity elements in alloy to reduce influence of solid solution elements on electrical conductivity. Studies have shown that transition metal elements such as Cr, Mn, V, Ti have the most significant effect on reducing electrical and thermal conductivity. Therefore, high thermal conductivity aluminum alloy used for die casting is generally an Al-Si-Fe alloy system, with Si content between 6% and 9%, Fe content between 0.6% and 1.0%, content of other impurity elements is generally controlled below 0.01% to meet dual requirements of formability and performance.
However, relative content of Si and Fe will have a direct impact on volume fraction of eutectic Si phase, crystal structure type and volume fraction of Fe phase, solidification temperature range of alloy, solid solubility of Fe and Si elements in aluminum matrix, thereby affecting strength, plasticity and electrical (thermal) properties of alloy. High thermal conductivity Al-Si-Fe alloys generally require aging treatment at 300~350℃ to further improve electrical conductivity. However, aging precipitation structure evolution and aging precipitation kinetics of high thermal conductivity alloys are still unclear. In addition, complex interaction between Fe and Si elements makes development of high thermal conductivity die-casting materials often based on trial and error experiments, which reduces R&D efficiency and increases R&D costs. In recent years, field of materials science has begun to widely use phase diagram calculation software such as Thermo-Calc, FactSage, PANDAT and JMATPro to guide design of aluminum alloys, overcoming limitations and singleness of relying solely on experimental exploration, improving efficiency of product research and development, saving resources and energy.
In this study, phase composition, phase volume fraction, solidification temperature range and solid solubility changes of Fe and Si elements in Al-Si-Fe system were calculated by PANDAT thermodynamic calculation software to reveal interaction mechanism between Fe and Si elements. Calculated alloy composition was used to trial-produce die-cast products, and cast structure, aging structure, strengthening and conductive mechanism of alloy were studied and theoretically analyzed, in order to provide a reference for design of high thermal conductivity materials.

Thermodynamic calculations were performed using PANDAT software, and database used was PANAl2016, which fully described thermodynamics of Al-Si-Fe ternary system. It is intended to calculate solid solubility of Fe and Si elements in primary α-Al phase by point calculation method. Line calculation was used to obtain structure, phase composition and phase molar fraction of cast alloy at room temperature; cross-sectional calculation was used to obtain pseudo-binary phase diagram of Al-7.5Si-xFe with different Fe contents and specific Si content (7.5%), in order to clarify changes in microstructure caused by changes in Fe content. Finally, alloy composition of experimental study was given using obtained thermodynamic calculation results. Figure 1 is the overall technical route for design of high thermal conductivity die-casting aluminum alloys guided by thermodynamic calculations.

 

Figure 1 Technical route for design of high thermal conductivity die-casting aluminum alloys guided by thermodynamic calculations

 

Figure 2 Schematic diagram of size of cast test bar
By designing several groups of Al-7.5Si-xFe alloys with different Fe contents (designed components are shown in Table 1), effects of Fe content changes on solidification behavior, structure, phase composition, phase molar fraction, solid solubility of Fe and Si elements in primary α-Al were calculated, and calculation results are shown in Figure 3. It can be seen that increasing Fe content will intensify reaction between Fe and Si, increase content of β-AlFeSi phase, and reduce content of eutectic Si. At the same time, increasing Fe content has a more dramatic effect on solidification behavior of alloy, and proportion of primary α-Al decreases rapidly, especially when Fe content reaches 0.8%. Reduction of primary α-Al will increase discontinuity of alloy structure and probability of electron scattering, which is not conducive to improvement of electrical and thermal conductivity. Taking into account strength, plasticity and electrical conductivity of material, combined with results of thermodynamic calculations, Al-7.5Si-0.8Fe was selected as research object, and test rods were prepared by die casting to further study relationship between structure and performance of alloy.

 

Table 1 Compositions of several alloys calculated by thermodynamics

 

Figure 3 Results of thermodynamic calculation

 

Table 2 Designed composition and actual composition of T1 alloy after thermodynamic optimization (%)

 

(a) Metallographic structure (b) SEM backscattering photo (c) Al surface scan (d) Si surface scan (e) Fe surface scan (f) Sr surface scan
Figure 4 Microstructure and element surface distribution of die-cast alloy

 

Figure 5 XRD spectrum of cast and aging alloy samples
Figure 6 is a SEM secondary electron morphology photo after deep corrosion treatment. It can be seen that eutectic Si presents a dendritic metamorphic morphology, and eutectic divides aluminum matrix into islands. No obvious precipitation phase was found in aluminum matrix in cast state. As can be seen from Figure 6c, compared with cast state, some eutectic network chains are partially disconnected, and island-like primary α-Al becomes gradually continuous. At the same time, granular and short rod-shaped precipitation phases were precipitated in primary α-Al, as shown in Figure 6d. To further explore type of precipitation phase, solute element distribution of precipitation phase was analyzed using EDS element surface scanning, as shown in Figure 7. It can be seen that there are mainly two types of precipitation phases, one is rich in Si, and the other is rich in Fe, Mn, and Si. Size of precipitation phase is between tens and hundreds of nanometers, showing a dispersed distribution. Combined with XRD spectrum (see Figure 5), no extra diffraction peaks appeared after aging, and organizational composition was still α-Al, Si and β-AlFeSi phases, indicating that no new phases were generated after aging.

 

(a) As-cast (b) As-cast, local magnification (c) Aged (300℃*1h) (d) Aged, local magnification (300℃*1h)
Figure 6 SEM microstructure of die-cast and aged alloy samples

 

(a) SEM, low magnification (b) SEM, high magnification (c) Al surface scan (d) Si surface scan (e) Fe surface scan (f) Mn surface scan
Figure 7 EDS element surface distribution of precipitated phase

 

(a) TEM bright field image (b) Al element surface distribution (d) Si surface distribution (d) Fe surface distribution
Figure 8 TEM microstructure of 300℃*1h aged alloy sample

 

Figure 9 Calculated relationship between equilibrium solid solubility of Fe, Si elements and temperature
Figure 10 Mechanical properties of as-cast and heat-treated alloys. It can be seen that tensile strength, yield strength and elongation of alloy in cast state are 245MPa, 114MPa and 5.97% respectively. After aging heat treatment at 300℃*1h, tensile strength, yield strength and elongation of material are 198.8 MPa, 96.6 MPa and 6.96% respectively. Compared with die-cast state, tensile strength and yield strength have decreased significantly, and elongation has increased slightly. After aging treatment at 300℃*2h, mechanical properties such as tensile strength and yield strength remain basically unchanged, and elongation decreases. Figure 11 shows a disc heat dissipation shell product prepared by die casting. Since difference in wall thickness will lead to differences in cooling rate, which will also affect microstructure and conductivity, conductivity at different wall thicknesses was measured. It can be seen that conductivity of die-cast state is between 23.5~24.2MS/m, and after aging treatment at 300℃*1h, conductivity rises to 25.5~26.5MS/m. However, after aging treatment at 300℃*2h, conductivity showed a slight decrease compared with state after aging treatment for 1h. Combined with mechanical properties data, it can be seen that reasonable heat treatment process should be aging at 300℃*1h. Prolonging aging time will not only not improve conductivity, but also worsen strength-conductivity matching.

 

(a) Engineering stress-engineering strain curve (b) Mechanical properties
Figure 10 Engineering stress-engineering strain curve and mechanical properties of die-cast alloy in cast and aging state

 

Figure 11 Die-cast products and conductivity in different states

(1) Through thermodynamic calculation, effect of different Fe contents on microstructure, phase composition and solid solubility of Fe and Si elements of Al-7.5Si alloy was obtained. It was found that with increase of Fe content, molar fraction of primary α-Al decreased, and solid solubility of Fe and Si was the smallest when Fe addition was 0.8%.
(2) Tensile strength of die-cast Al-7.5Si-0.8Fe alloy is 245MPa, elongation is 5.97%, and electrical conductivity is between 23.5~24.2MS/m; tensile strength after aging at 300℃×1h is 198MPa. Electrical conductivity is between 25.5~26.5MS/m, and aging treatment can significantly improve electrical conductivity of alloy.
(3) Morphology of eutectic Si phase and solid solubility of Fe and Si are two main microstructural factors affecting conductivity. Among them, spheroidization of eutectic Si during aging treatment enhances continuity of aluminum matrix and increases mean free path of conduction electrons, which is main reason for improvement of aging conductivity.