With help of Image-Pro Plus assisted analysis, optical microscope (OM), scanning electron microscope (SEM), electron backscattered (EBSD) and other methods were used to study effect of cryogenic treatment on microstructure and mechanical properties of die-cast aluminum-silicon alloy. Results show that cryogenic treatment can effectively improve mechanical properties of die-cast aluminum-silicon alloy. As cryogenic treatment time increases, effect of cryogenic treatment on improving mechanical properties of alloy first increases and then decreases. Compared with as-cast alloy, mechanical properties of alloy were greatly improved after 12 h cryogenic treatment. Tensile strength was 213.2 MPa, an increase of 15.4%, elongation was 8.65%, an increase of 36.2%, and hardness HV was 106.6, an increase of 18.1%. Continuing to extend cryogenic time, α-Al phase, eutectic Si phase, etc. begin to grow slowly, resulting in a decrease in mechanical properties. Cryogenic treatment can effectively refine α-Al phase of alloy, improve size and morphology of eutectic Si phase and iron-containing phase, play a role in refining structure and strengthening dislocations.
Foreword: Cars are major energy consumers, accounting for more than 13% of carbon emissions. According to statistics, for every 100 kg reduction in vehicle weight, carbon dioxide emissions can be reduced by about 5g/km; at the same time, for every 10% reduction in vehicle weight, power consumption can be reduced by 5.5%, cruising range can be increased by 5.5%, daily wear and tear costs can be saved by 20%. Therefore, weight reduction is of great significance to development of automobile industry. Aluminum alloy is currently the most popular lightweight material, and die-casting parts have advantages of high quality, good mechanical properties, high efficiency, and are suitable for mass production. Therefore, aluminum alloy die-casting parts are widely used in automobile bodies, wheels, chassis, anti-collision beams, battery casings, etc. However, due to fast speed and short time of die-casting process, it is difficult for gas in die-casting cavity to escape in a short period of time and is thus trapped in casting. As a result, it is easy to produce pore defects during conventional heat treatment, which greatly reduces mechanical properties of castings and limits large-scale application of die-cast aluminum alloys.
Cryogenic treatment is a method of treating materials below -130℃ using liquid nitrogen as a medium, which can change microstructure of material and thereby improve mechanical properties of material. At present, cryogenic treatment, as an important strengthening method, has been widely used in steel materials. In recent years, affected by global emphasis on energy conservation and environmental protection, research on cryogenic treatment processes for aluminum alloy materials has become a hot and focused issue. Li Guirong and others conducted cryogenic treatment on aluminum-based composite materials and aluminum-silicon alloys respectively, found that cryogenic treatment can promote precipitation of second phase and improve mechanical properties of material by hindering dislocation movement. Wu Zhisheng et al. conducted cryogenic treatment on 5A06 aluminum alloy welded joints and found that after cryogenic treatment, grain structure of welded joint was refined, pore defects were reduced, and matrix density was increased. Chen Ding et al. performed cryogenic treatment on aluminum alloys of different compositions and proposed that volume shrinkage effect induced by cryogenic treatment plays an important role in improving mechanical properties. Yang Ye et al. performed cryogenic treatment on aluminum-silicon alloy and found that cryogenic treatment can passivate iron-rich phase in the structure, greatly reducing splitting effect on matrix, and effectively improving mechanical properties of aluminum alloy. Zhou Jianzhong et al. conducted cryogenic treatment on laser shot-peened aluminum alloy and found that cryogenic treatment can inhibit dynamic recovery of dislocations, generate high-density dislocations in sample, and produce a good structural strengthening effect. In short, cryogenic treatment has different strengthening effects on aluminum alloys with different compositions, and there is no consensus on strengthening mechanism. There are also studies on combined use of traditional heat treatment methods such as solid solution and aging with cryogenic treatment, and found that synergy of the two can further improve mechanical properties of aluminum alloys, but treatment process is more complex and treatment time is longer. At present, there are few studies on cryogenic treatment of die-cast aluminum-silicon alloys, effect and influence of cryogenic time on die-cast aluminum-silicon alloys are still unclear.
This article takes die-cast aluminum-silicon alloy as research object. By comparatively analyzing mechanical properties and microstructure change patterns of cryogenically treated samples at different times, strengthening mechanism of cryogenic treatment on die-cast aluminum-silicon alloys was analyzed in depth, and the best cryogenic treatment process for improving performance of die-cast aluminum-silicon alloys was revealed, providing a theoretical basis for further improving application of cryogenic treatment in the field of cast aluminum alloys.

Test material is die-cast aluminum-silicon alloy, and its composition is tested with a German Bruker S1-TITAN handheld spectrometer as shown in Table 1. Test samples are divided into 5 groups. Specific processing technology and sample numbers are shown in Table 2. Ingot was processed into a 10 mm * 10 mm * 5 mm cube and a tensile specimen as shown in Figure 1 using wire cutting. Cryogenic treatment is carried out in liquid nitrogen cryogenic tanks. MTS-DDL100 electronic universal testing machine was used to test mechanical properties of each group of samples. Test conditions were: room temperature, tensile rate was 1 mm/min, each group of samples was tested three times and average value was taken. Use FM-ARS900 microhardness tester to measure microhardness. Each group of samples is tested at 5 points and average value is taken.

Table 1 Chemical composition of die-cast aluminum-silicon alloy wB/%

Table 2 Cryogenic treatment process scheme of die-cast aluminum-silicon alloy

 

Figure 1 Shape and size of tensile specimen
After metallographic samples were ground and polished, surface was etched with 0.5% volume fraction of HF reagent, microstructure of samples was observed using Observer.Z1m Zeiss optical microscope (OM), HitachiSU-1510 scanning electron microscope (SEM) and electron backscattering (EBSD). Image-Pro Plus image analysis software was used to perform statistical analysis on composition phases of die-cast aluminum-silicon alloy. Among them, calculation method of average particle size of α-Al phase is shown in Figure 2 and formula (1).
Calculation method is as shown in formula (1):

 

In the formula: Li is diameter size passing through center of mass. D is average grain diameter: in any grain, average length of 90 line segments passing through grain center of mass at intervals of 2°.

 

Figure 2 Schematic diagram of calculation method of average particle size

Figure 3 shows mechanical properties of die-cast aluminum-silicon alloys under different cryogenic treatment processes. When cast, alloy has a tensile strength of 184.8 MPa, an elongation of 6.35%, and a hardness of HV 90.3. Cryogenic treatment can effectively improve mechanical properties of die-cast aluminum-silicon alloys. After cryogenic treatment for different times, mechanical properties of alloy first increase and then decrease. When cryogenic treatment time is 12 h, mechanical properties of alloy are maximized, with a tensile strength of 213.2 MPa, an elongation of 8.65%, and a hardness of HV106.6. Compared with properties of as-cast alloy, tensile strength increased by 15.4%, elongation increased by 36.2%, and hardness increased by 18.1%. Continuing to extend cryogenic treatment time, although improvement in mechanical properties of alloy gradually decreases, it is still higher than mechanical properties of cast aluminum-silicon alloy. When cryogenic treatment time is 48 h, tensile strength of alloy is 200.6 MPa, elongation is 7.5%, and hardness is HV97.4, which is already lower than mechanical properties of alloy after cryogenic treatment for 6 h. From perspective of energy conservation, environmental protection and production efficiency, it is of little significance to continue to increase cryogenic treatment time to improve mechanical properties of die-cast aluminum-silicon alloys.

 

Figure 3 Mechanical properties of die-cast aluminum-silicon alloys under different cryogenic treatment processes

Figure 4 shows morphology of α-Al phase of die-cast aluminum-silicon alloy under different cryogenic treatment processes. For as-cast die-cast aluminum-silicon alloy, as shown in Figure 4a, α-Al phase is relatively coarse and has more columnar crystals. Average particle size measured using Image-Pro Plus analysis was 27.22 μm. Figure 4b shows structure after cryogenic treatment for 6 h. α-Al phase has been refined and coarse columnar crystals have been reduced, but there is still some agglomeration, and average particle size is 24.42 μm. As cryogenic time increases, α-Al phase is further refined. When cryogenic time is 12 h, α-Al phase is the smallest, with an average particle size of 22.41 μm. Continuing to increase cryogenic time, compared with 12 h cryogenic sample, average particle size of α-Al phase increases slightly, but its size is still lower than that of as-cast alloy. In short, cryogenic treatment can refine grains, reduce agglomeration of columnar crystals and α-Al phases, and promote a more uniform distribution of organizational structure.

 

Figure 4 Microstructure morphology of α-Al phase of die-cast aluminum-silicon alloy under different cryogenic treatment processes

Figure 5 shows morphology of eutectic Si phase of die-cast aluminum-silicon alloy under different cryogenic treatment processes. Eutectic Si phase of cast alloy is mostly in the form of flakes or strips, as shown in Figure 5a. After cryogenic treatment, eutectic Si phase begins to become shorter and finer, transforming into granular form. When cryogenic time is 12 h, eutectic Si phase is significantly refined and tends to be round, as shown by arrow in Figure 5c. Continuing to increase cryogenic time, eutectic Si phase begins to grow slowly again, as shown in Figure 5d and e.

 

Picture 5 Microstructure of eutectic Si phase of die-cast aluminum-silicon alloy under different cryogenic treatment processes

Figure 6 shows morphology of iron-containing phase of die-cast aluminum-silicon alloy in as-cast and cryogenically cooled 12 h state. As-cast alloy contains polygonal block AlFeMnSi phase and long needle-shaped Al5FeSi phase, as shown in Figure 6a. After cryogenic treatment, Al5FeSi phase shrinks and becomes passivated, and its outline becomes smooth. Sharp-edged AlFeMnSi phase also tends to be rounded, as shown by arrow in Figure 6b. After cryogenic treatment, size of iron-containing phase in die-cast aluminum-silicon alloy becomes smaller and its morphology changes, which greatly reduces splitting effect on aluminum matrix.

Volume of aluminum-silicon alloy shrinks under cryogenic treatment, which causes microscopic stress inside material and promotes plastic deformation of material. Figure 7 is a schematic diagram of tissue refinement process produced by cryogenic process. Before cryogenic treatment, dislocations in alloy are scattered and have a low density, as shown in Figure 7a; at the beginning of cryogenic treatment, as shown in Figure 7b, microscopic stress is small, dislocations inside grains will gather locally and form dislocations. As cryogenic treatment proceeds, stress accumulation increases, and dislocations gather into small-angle grain boundaries, which refines grains, as shown in Figure 7c; solubility of Si, Fe and other solute atoms in alloy in aluminum matrix decreases, and solute atoms will segregate to dense dislocations near grain boundaries. As cryogenic time increases, Si phase and iron-rich phase precipitate at grain boundaries, as shown in Figure 7d. These precipitated phases are smaller, more uniform and round in shape, pinning grain boundaries and hindering dislocation movement, which play a significant role in improving the mechanical properties of aluminum alloys. Figure 8 is a schematic diagram of EBSD grain size distribution of the two samples as cast and in cryogenic 12 h state. Under cryogenic treatment, aluminum-silicon alloy does produce many fine-grained structures, as shown by arrows in figure. According to Hall-Petch formula:

 

In the formula: σs is yield strength of polycrystal; d is average diameter of grains; σ0 is resistance to deformation within grain, which is equivalent to yield strength of single crystal; K is coefficient of influence of grain boundaries on deformation, which is related to grain boundary structure. It can be seen that the smaller grain size, the greater yield strength, and it is difficult for dislocations to move. This is main reason why tensile strength and plasticity of alloy are improved after 12 h of cryogenic cooling.

 

Figure 6 Microstructure morphology of iron-containing phase of die-cast aluminum-silicon alloy as cast and cryogenically cooled for 12 h

 

Figure 7 Schematic diagram of grain changes during cryogenic cooling

 

Figure 8 Schematic diagram of EBSD grain size as cast and after cryogenic cooling for 12 h.

(1) Cryogenic treatment can significantly improve mechanical properties of cast aluminum-silicon alloys. As cryogenic time increases, lifting effect first increases and then decreases. When cryogenic time is 12 h, its lifting effect reaches its best.
(2) Compared with as-cast alloy, after 12 hours of cryogenic cooling, tensile strength of die-cast aluminum-silicon alloy changed from 184.8 MPa to 213.2 MPa, an increase of 15.4%, and elongation increased from 6.35% to 8.65%, an increase of 36.2%. %, hardness changed from HV90.3 to HV106.6, an increase of 18.1%.
(3) Cryogenic treatment will cause volume shrinkage of die-cast aluminum-silicon alloys to produce plastic deformation, reduce solubility of elements in aluminum, resulting in refinement of α-Al phase and eutectic Si phase structures, rounding morphology of iron-containing phase, playing a role in grain refinement and dislocation strengthening.