Al-Zn-Si-Cu alloy was prepared by die-casting forming process, refinement and modification effect of ceramic nano-reinforced particles (TiC/TiB2) and Al-10Sr modifier on mechanical properties, thermal conductivity/electrical properties and microstructure of heat-free die-cast Al-Zn-Si-Cu alloy was studied. Results show that after adding 0.5% ceramic nanoparticles alone, compared with alloy without adding refiner, α-Al phase is significantly refined and nearly rose-shaped, fibrous eutectic Si phase is more dense, short rod-shaped Al2Cu phase is finer, and content of Zn, Si, and Cu elements in matrix increases. At this time, mechanical properties are improved, and thermal conductivity/electrical properties are slightly reduced. This is mainly due to increase in solute elements, which leads to a greater degree of lattice distortion, increases electron scattering ability, and reduces mean free path of electrons, which reduces thermal and electrical conductivity of metal; after combined action of 0.5% ceramic nanoparticles and 0.1% Al-10Sr master alloy, edges and corners of eutectic Si phase are blunted, and content of solute elements in matrix is further increased. At this time, mechanical properties of alloy reach the best, its tensile strength, yield strength and elongation are 361 MPa, 213 MPa and 6.8% respectively, and thermal conductivity is 118.73 W/(m·K).
Energy conservation and emission reduction and improving mileage of new energy vehicles are mainstream of current automotive industry, reducing weight of car itself is the most direct and effective means. Realization of goal of lightweighting automobiles generally starts from three aspects: materials, processes and design. Now they have been integrated to promote development of large-scale components and integrated die-casting. Integration of large-scale component die-casting can greatly simplify production process of body manufacturing and significantly improve production efficiency of body. Die-casting parts need to be heat treated before they can meet performance requirements, but there are many uncontrollable factors in heat treatment process, which will eventually lead to quality defects such as deformation and blistering of die-casting parts after heat treatment. Therefore, automotive market urgently needs heat-free materials to avoid this phenomenon, so that materials do not need to be heat treated and can meet mechanical performance requirements under cast conditions.
Since heat-treatment-free aluminum alloy cannot be subjected to subsequent heat treatment processes for performance enhancement, melt treatment process in the early stage of test is very important. Traditional melt treatment process usually uses Al-Ti-B alloy for refinement and Al-Sr alloy for modification to achieve purpose of refining alloy grains. However, there are also disadvantages such as large size of effective nucleation particles such as TiAl3 and TiB2, which are easy to aggregate and sink, are easily "poisoned" by elements such as Zr, Cr, and Mn, resulting in refinement decay. In addition, metallurgical quality of Al-Ti-B master alloy is poor, component stability is not good, there are also difficult to solve problems such as aggregation of TiB2 and poisoning effect of Si and TiAl3, which have a great impact on quality stability of product after refinement. Compared with traditional refiners, nanoparticles are more effective in refining alloys. On the one hand, they can serve as nucleation particles of alloys, and on the other hand, they can be enriched and pinned at solid-liquid interface to promote nucleation and reduce average grain size.
In this paper, aluminum alloy composition is designed independently, and heat-free die-cast aluminum alloy castings are prepared by liquid ultrasonic vibration method combined with die-casting forming process. Main research is influence of modifier and ceramic nanoparticle refiner on microstructure and properties of heat-free strengthened die-cast aluminum silicon alloy, in order to improve performance of aluminum alloy and meet performance requirements of modern automobile industry for high-strength aluminum alloy materials.

Experimental material is Al-Zn-Si-Cu alloy, which is prepared by melting pure aluminum ingot (99.7%, mass fraction, same below), pure zinc ingot (99.8%), single crystal Si and Al-50Cu alloy, Al-10Sr master alloy as modifier, TiC/TiB2 as ceramic nanoparticle refiner, and balance is Al. In order to study effect of refinement and modification on microstructure and mechanical properties of alloy, three groups of alloy compositions were designed in this experiment, with nominal compositions of Al-15Zn-8Si-2Cu, Al-15Zn-8Si-2Cu-0.5TiC/TiB2 and Al-15Zn-8Si-2Cu-0.5TiC / TiB2-0.1Sr. Actual alloy composition was measured by SPECTROMAX photoelectric direct reading spectrometer, as shown in Table 1.

 

Table 1 Experimental alloy composition wB/%
In smelting process, pure aluminum ingots and aluminum-silicon master alloys were first placed in a smelting furnace, and smelting furnace was heated to 760 ℃. After alloy in furnace was completely melted, Al-50Cu master alloy and pure Zn ingots were added. After complete melting, 0.1% Al-10Sr master alloy was added, then 0.5% TiC/TiB2 ceramic nanoparticles were added. FB900 ultrasonic vibrator was used, and ultrasonic vibration process parameters were set to 3 kW power, 20 kHz frequency, and ultrasonic treatment for 5 min. After treatment was completed, it was left to stand for die casting. Die casting test used a 280 t vertical LK IMPRESS-III die casting machine, aluminum liquid pouring temperature was 680~700 ℃, injection speed was 0.10~0.15 m/s at low speed, and 3.5~5.5 m/s at high speed. Specific pressure selected for test was 200 MPa, holding time was 30 s, and mold temperature was 200 ℃.
After test was completed, HD-B615A-S computer servo double-column tensile material testing machine was used for tensile testing. Size of tensile test bar is shown in Figure 1. Metallographic sample was cut into a 20 mm section from the end of tensile test bar. After grinding and polishing, it was corroded with Keller solution. Sample structure was observed with an OLYMPUS GX-51 optical microscope. Second phase morphology and solute element distribution of die casting structure were analyzed with a Verios G4 UC scanning electron microscope. Solute element analysis was performed at the center of sample. Five points were taken for each sample and average value was taken. Phase composition of alloy was analyzed with help of XRD. Mechanical properties of alloy were tested with a HD-B615A-S computer servo double-column tensile material testing machine. Casting system is shown in Figure 1. Surface of sample was polished to a smooth surface, and conductivity was tested with a FIRST FD-102 eddy current conductivity meter.

 

Figure 1 Casting system and tensile sample size

Figure 2 shows effect of nanoparticle refiner and modifier on mechanical properties of Al-15Zn-8Si-2Cu casting alloy. Test results show that after adding TiC/TiB2 nanoparticles alone, strength and elongation of alloy are significantly improved. After adding nanoparticle refiners and modifiers together, mechanical properties of alloy are further improved compared with alloy samples with only refiners and without them. Modifiers can improve morphology of eutectic Si phase in alloy. In this experimental alloy composition, mechanical properties of alloy depend on size, morphology and distribution of α-Al and eutectic Si, solid solubility of solute elements in the matrix and the size and distribution of the second phase. From mechanical properties results, it can be seen that alloy without grain refiner and modifier has a yield strength of 158 MPa, a tensile strength of 330 MPa, and an elongation of 6.1%. Under combined action of TiC/TiB2 nanoparticle refiner and Al-10Sr modifier, yield strength, tensile strength and elongation of alloy reach 213 MPa, 361 MPa and 6.8%, respectively, which are 34.9%, 9.3% and 11.0% higher than those of alloy without refiner and modifier.
Figure 3 shows fracture morphology of Al-15Zn-8Si-2Cu alloy before and after addition of nanoparticle refiner and modifier. By comparing fracture morphology of alloy under three alloy compositions, it can be observed that in alloy without refiner and modifier, since Si phase in alloy structure has not been modified, morphology is still mainly lath-shaped and relatively coarse, so when loading, it is easy to generate stress concentration in coarse block Si phase. When stress intensity reaches limit, Si phase will break, and a large number of micropores will connect to each other to produce microcracks, which will then connect to each other and continue to expand, finally sample will break. Therefore, alloy fracture morphology shows a large number of large-sized cleavage platforms and a small number of dimples, as shown in Figures 3a and d. After adding TiC/TiB2 nanoparticles, alloy grains are refined to a certain extent, and size of cleavage platform is significantly reduced, as shown in Figures 3b and e. After adding nanoparticle refiners and modifiers, relatively obvious dimples exist in tensile fracture of alloy, and size of cleavage platform is significantly reduced. Due to modification of Sr element, eutectic Si phase is passivated, splitting effect is reduced, and cracking caused by stress concentration is reduced, so it has higher strength and plasticity, as shown in Figures 3c and f.

 

Figure 2 Mechanical properties of Al-Zn-Si-Cu alloy before and after adding refiner and modifier

 

Figure 3 Fracture morphology of Al-Zn-Si-Cu alloy before and after adding refiner and modifier

Figure 4 shows thermal conductivity of Al-15Zn-8Si-2Cu alloy before and after adding nanoparticle refiner and modifier. According to test results, before adding nanoparticle refiner and modifier, thermal conductivity of alloy can reach 111.82 W·(m·K)-¹. After adding nanoparticle refiner and modifier, thermal conductivity of alloy decreases slightly. After adding Al-10Sr modifier, thermal conductivity of alloy increases to 118.73 W·(m·K)-¹.

 

Figure 4 Thermal conductivity of Al-Zn-Si-Cu alloy before and after adding refiners and modifiers
Good electrical and thermal conductivity of metals is determined by electrons in them. Positive ions in metal are arranged in a lattice in a certain way, and outer electrons separated from atoms become free electrons. Properties of free electrons are similar to those of molecules in ideal gases, forming free electron gas. When a large number of free electrons move in a directional manner, an electric current is formed. Wiedemann-Franz law can well reflect relationship between thermal conductivity and electrical conductivity of metal materials. This law believes that ratio of thermal conductivity to electrical conductivity of a metal is not affected by material itself, but only related to temperature of material. Ratio of thermal conductivity (λ) to electrical conductivity (σ) of a metal is proportional to temperature (T), that is:
λ =LσT+C（1）
Where: λ is thermal conductivity, L is Lorentz coefficient, σ is electrical conductivity, T is Kelvin temperature, and C is lattice thermal conductivity. Olasson et al. listed commonly used values of L and C when using formula (1) to theoretically calculate thermal conductivity of aluminum alloys, that is, . Because Wiedemann-Franz law can be applied at different temperatures and is very consistent with actual situation, it is more reliable to convert electrical conductivity measured at room temperature into thermal conductivity.
In metal materials, free electrons are main carriers. When elements are added in the form of solid solution, lattice will be distorted. When alloying elements exist in the form of a second phase, new interfaces will be introduced. In both cases, electron scattering is increased and mean free path of electrons is reduced, resulting in a decrease in thermal and electrical conductivity of metal. In experimental alloy material designed in this experiment, due to addition of nanoparticles in alloy, particle content in composite material is high and grain size is fine, which will increase scattering area of alloy and reduce thermal conductivity of alloy. Before adding Al-10Sr, eutectic Si phase is in the form of coarse laths and aggregated distribution. It is difficult for electrons to pass through or bypass eutectic Si grains to complete energy transmission. After refinement and modification, eutectic Si phase is refined, alloy density is improved, atomic distance is shortened, matrix connectivity is enhanced, and free electron free path is increased, thus improving thermal conductivity of alloy.

Figure 5 shows XRD spectrum of Al-15Zn-8Si-2Cu alloy before and after adding nanoparticle refiner and modifier. According to PDF card analysis, alloy composition of this test mainly consists of four phases, namely Al, Si, Zn-rich phase and Al2Cu phase. It can be seen from figure that with addition of nanoparticle TiC/TiB2, diffraction peaks corresponding to diffraction peaks of Si (111), Al (111), Al2Cu (311) and Zn (002) in alloy gold sample shifted to the right. According to basic theory of X-ray diffraction, Bragg formula can be used to explain relationship between diffraction angle corresponding to peak and corresponding crystal plane spacing. Formula is:
2dsinθ=nλ（2）

 

Figure 5 XRD spectrum of Al-Zn-Si-Cu alloy before and after adding refiner and modifier
Where: d is crystal plane spacing; θ is diffraction angle; λ is X-ray wavelength; n is a constant. α-Al has a face-centered cubic structure, and lattice constant a at room temperature is 0.404 96 nm. Relationship between interplanar spacing and lattice constant satisfies following formula:
 
Where: d is interplanar spacing; a is lattice constant; h, k, l are crystal plane indices. Diffraction peaks of alloy sample all move to the right, and 2θ angle becomes larger, while interplanar spacing d is inversely proportional to θ. From formula (2), it can be seen that d is proportional to lattice constant a. When d decreases, lattice constant a of Al also decreases, indicating that after adding nanoparticles, grains are refined, which promotes solid solution of Si atoms into Al matrix. Al undergoes lattice distortion during forming, and atomic radius of aluminum rAl=0.143 nm is larger than atomic radius of silicon rSi=0.117 nm, so lattice constant a of Al decreases. At the same time, it can be observed that diffraction peaks corresponding to Zn-rich phase and Al2Cu phase in XRD curves of Al-15Zn-8Si-2Cu-TiC/TiB2 and Al-15Zn-8Si-2Cu-TiC/TiB2-Sr samples are significantly more intense than diffraction peaks corresponding to Al-15Zn-8Si-2Cu sample, which further indicates that addition of nanoparticles will cause lattice distortion in Al matrix, further promoting solid solution of solute atoms into matrix, and playing a role in solid solution strengthening.
Figure 6 shows microstructure of Al-15Zn-8Si-2Cu alloy before and after adding nanoparticle refiner and modifier. Figures 6a, c and e are optical microstructures, Figures 6b, d and f are scanning electron microscope microstructures. It can be observed from optical microstructure that alloy is mainly composed of α-Al dendrites and eutectic Si phases. Without adding refiner and modifier, there are relatively coarse α-Al dendrites and lath-shaped eutectic Si phases in alloy structure. In scanning electron microscope structure, it can be observed that second phase in the form of dots and short rods is distributed at grain boundaries, as shown in Figures 6a and b. After adding TiC/TiB2 nanoparticles, size of α-Al dendrites in matrix structure is significantly reduced, and alloy structure is denser, as shown in Figure 6c and d. After modification treatment of Al-10Sr master alloy, eutectic Si phase undergoes obvious passivation, transforming from lath shape to short rod shape, and splitting effect on matrix is reduced. At this time, α-Al becomes nearly equiaxed, as shown in Figure 6e , f, optimal mechanical properties are reached at this time. In order to further explore refinement and modification effect of alloy, grain size of alloy was analyzed using Image Pro software. Analysis and test results are shown in Table 2. It can be seen from table that after alloy was refined, average grain size was significantly reduced and roundness was greatly improved. At this time, average area of α-Al grains is 31.5 μm, roundness reaches 0.79, equivalent diameter is 26.8 μm, and secondary dendrite spacing is 26.8 μm. After further modification treatment, grain size of alloy changed slightly, with an equivalent diameter of 22.4 μm, and roundness was slightly improved to 0.84.

 

 

Figure 6 Microstructure of Al-Zn-Si-Cu alloy before and after adding refiner and modifier

 

Table 2 Quantitative statistics of alloy α-Al grain size
Upper right part of Figure 6b, d, and f is partial enlarged SEM view of corresponding alloy. Table 3 shows EDS energy spectrum analysis of alloy composition at different positions. According to energy spectrum analysis combined with XRD results, it can be concluded that block phase corresponding to spectrum 1 is Si phase, white short rod second phase in spectrum 2 is Al2Cu phase, and gray long strip second phase in spectrum 3 is Zn-rich phase. Matrix under three alloy compositions was further scanned. Scanning results are shown in Figure 7. It was found that there were certain differences in content of Si, Zn, and Cu elements in matrix under three alloy compositions. After composite addition of TiC/TiB2 nanoparticles and Al-10Sr master alloy , content of solute elements in matrix is further increased. According to XRD pattern, it can be concluded that addition of nanoparticles causes lattice distortion of α-Al, promoting solid solution of solute atoms into matrix, thereby increasing solid solution strengthening effect of alloy.

 

Table 3 Corresponding EDS energy spectrum

 

Figure 7 Solute element content in alloy matrix

(1) Yield strength, tensile strength and elongation of heat-free die-cast aluminum alloy without adding ceramic particles and modifiers are 158 MPa, 330 MPa and 6.1%. Under combined action of TiC/TiB2 nanoparticle refiner and Al-10Sr modifier, yield strength, tensile strength and elongation of alloy reach 213 MPa, 361 MPa and 6.8%, respectively, which are 34.8%, 9.3% and 11.0% higher than those of alloy without adding refiner and modifier.
(2) Before adding nanoparticle refiners and modifiers, thermal conductivity of alloy can reach 111.82 W·(m·K)-¹. When TiC/TiB2 nanoparticles and Al-10Sr modifiers are added, thermal conductivity of alloy increases to 118.73 W·(m·K)-¹, which is mainly due to refinement of eutectic Si phase, increase in alloy density, shortening of interatomic distance, enhancement of matrix connectivity, and increase in free path of free electrons.
(3) After composite addition of TiC/TiB2 nanoparticles and Al-10Sr master alloy, α-Al dendrites transformed into rosettes, eutectic Si phase was passivated, Al2Cu phase was refined, and solute element content in matrix was further increased. Addition of nanoparticles caused α-Al lattice distortion, promoted solute atoms to dissolve into matrix, thus increased solid solution strengthening effect of alloy.