6061 aluminum alloy has characteristics of medium strength, good machining performance, can be heat treated, strengthened and anodized. It is widely used in transportation, architectural decoration, aerospace and other fields. From perspective of actual production, when manufacturing many complex thin-walled parts using 6061 aluminum alloy bar blanks or slabs, a large amount of machining is often required, resulting in low material utilization and high production costs. Using traditional casting technology to cast 6061 aluminum alloy complex thin-walled parts blanks, then performing appropriate machining can significantly reduce production costs and improve material utilization. However, when casting 6061 aluminum alloy complex thin-walled parts blanks using traditional casting processes, there is a serious tendency for hot cracking, and pass rate of castings is very low.
Semi-solid metal forming technology combines advantages of solid metal forming and liquid metal forming. It has low forming pressure and small solidification shrinkage, which can reduce tendency of hot cracking of castings, improve structure and performance of castings, and achieve near-net shape of parts. In addition, increasing Si content in 6061 aluminum alloy (called modified 6061 aluminum alloy) may improve fluidity and mold filling ability of alloy and reduce tendency of hot cracking of castings. Therefore, semi-solid metal forming technology combined with increasing Si content can be used to reduce hot cracking tendency of 6061 aluminum alloy castings. Preparing semi-solid slurries with uniformly fine and nearly spherical solid phase grains is the key to semi-solid metal forming technology. Over years, researchers at home and abroad have successively developed a variety of preparation methods for semi-solid slurries and conducted rheological die-casting research on semi-solid slurries. Some researchers used ultrasonic vibration method to prepare a hypereutectic Al-Si alloy slurry with a mass fraction of 2% Fe and performed rheological die casting. They found that δ-Al4 (Fe, Mn) Si2 phase in semi-solid Al-Si alloy slurry prepared by ultrasonic vibration method was significantly refined. A forced convection stirring process for preparation of aluminum and magnesium alloy semi-solid slurries was proposed, and a new rheological high-pressure die-casting process was developed in combination with high-pressure die-casting equipment. A variety of Al-Si alloy slurries were prepared using inclined plate vibration cooling method, and good grain refining effects were achieved. Self-inoculation method was used to prepare 6061 aluminum alloy semi-solid slurry and rheological die-casting research was conducted to prepare rheological die-casting parts with spherical primary α-Al grains.
Some researchers have carried out research on preparation of 6061 aluminum alloy slurry by serpentine channel casting process, analyzed influence of casting process parameters on preparation of 6061 aluminum alloy slurry, which provided strong technical support for this research work. However, there are currently no reports on preparation of serpentine channels and rheological die-casting of semi-solid modified 6061 aluminum alloy slurry. Therefore, in this study, semi-solid modified 6061 aluminum alloy slurry prepared from graphite serpentine channels was rheologically die-casted. Structures of traditional liquid die-casting and rheological die-casting 6061 aluminum alloy samples were comparatively analyzed to study influence of Si content on structure and mechanical properties of semi-solid modified 6061 aluminum alloy rheological die-casting samples, aiming to provide a reference for its application.
Graphic and text results
Commercial 6061 aluminum alloy and Al-28Si alloy were selected as raw materials. Chemical compositions of two alloys are shown in Table 1. NETZSCHSTA409C/CD differential scanning calorimeter was used to conduct thermal analysis (Differential Scanning Calorimetry, DSC) of 6061 aluminum alloy. Solidus and liquidus temperatures of 6061 aluminum alloy were measured to be 597℃ and 646℃ respectively, solid- liquid temperature interval was 49℃, as shown in Figure 1 .

Table 1 Chemical composition of raw materials (%)

 

Figure 1 DSC curve of 6061 aluminum alloy

 

(a) Schematic diagram of device (b) Alloy liquid (c) Snake channel
Figure 2 Schematic diagram of preparation of semi-solid slurry in serpentine channel
1. Well type resistance furnace 2. Graphite crucible 3. Alloy melt 4. Ni-Cr/Ni-Si type thermocouple 5. Graphite serpentine channel 6. Semi-solid slurry 7. RFM-composite ceramic thermal insulation pouring spoon
Specific semi-solid rheological die-casting process is: first, ingredients are prepared according to alloy composition set in test, then aluminum alloy ingot is put into a well-type resistance furnace for melting. Melting temperature is 800℃; after it is completely melted, use a graphite tube to pass argon gas into melt for refining for 10 minutes; then take out graphite crucible and let it cool, while removing slag and impurities from surface of alloy melt; use a Ni-Cr/Ni-Si type thermocouple to measure alloy melt. When temperature of alloy melt cools to 690℃, immediately pour alloy melt into graphite serpentine channel, and alloy melt forms a semi-solid slurry after flowing through serpentine channel. Use an RFM-composite ceramic insulated pouring spoon preheated to 350℃ to collect slurry at exit of serpentine channel, then immediately pour semi-solid slurry into pressure chamber of YYC180B die-casting machine, and quickly complete die-casting filling process. Among them, semi-solid rheological die-casting process parameters: injection specific pressure is 90MPa, injection speed is 0.5m/s, and preheating temperature of pressure chamber and mold is 200℃. After pressure holding is completed, die casting with four standard tensile specimens can be obtained, as shown in Figure 2.

 

Figure 3 Physical picture and dimensions of semi-solid rheological die-cast tensile specimen

 

(a) Side tissue (b) Heart tissue
Figure 4 Structure of traditional liquid die-casting 6061 aluminum alloy sample

 

(a) Side tissue (b) Heart tissue
Figure 5 Microstructure of rheological die-casting sample of semi-solid modified 6061 aluminum alloy
It can be seen that primary α-Al grains in edge structure of sample are mainly fine equiaxed crystals, while primary α-Al grains in the center structure are mainly coarse dendrites, and secondary primary α2-Al grains are mainly fine equiaxed crystals, with a network of eutectic structures distributed at grain boundaries. Edge structure of rheological die-casting sample is mainly composed of a large number of small spherical secondary α2-Al grains, while primary α-Al grains in core structure are mainly spherical crystals or rosette crystals. Average grain diameter and shape factor of primary α-Al grains are 58 μm and 0.68 respectively. Secondary α2-Al grains and eutectic structures at grain boundaries increase. Compared with core structure of traditional liquid die-cast 6061 aluminum alloy sample, primary α-Al grains in core structure of semi-solid modified 6061 aluminum alloy rheological die-cast sample evolved from coarse dendrites to globular crystals or rosette crystals with a smaller average grain diameter. Secondary primary α2-Al grains and non-equilibrium eutectic structure at grain boundaries increase. This shows that use of serpentine channel casting composite rheological die-casting process can significantly improve microstructure and morphology of 6061 aluminum alloy, making primary α-Al grains finer and rounder.

 

(a)0.6%Si (b)1.0%Si (c)1.4%Si (d)1.8%Si (e)2.2%Si (f)2.6%Si
Figure 6 Edge structure of rheological die-casting samples of semi-solid modified 6061 aluminum alloy with different Si contents

 

(a)0.6%Si (b)1.0%Si (c)1.4%Si (d)1.8%Si (e)2.2%Si (f)2.6%Si
Figure 7 Microstructure of core of semi-solid die-cast 6061 aluminum alloy rheological die-cast samples with different Si contents

Table 2 Mechanical properties of semi-solid modified 6061 aluminum alloy with different Si contents

 

(a)0.6%Si (b)1.8%Si (c)2.6%Si
Figure 8 Stress-strain curves of semi-solid modified 6061 aluminum alloy rheological die-casting specimens with different Si contents
In conclusion
(1) Composite rheological die-casting process of serpentine channel casting can effectively improve as-cast structure of 6061 aluminum alloy. Primary α-Al grains evolve from coarse dendrites to small, rounded globular crystals or nearly globular crystals.
(2) Increasing Si content can significantly refine structure of semi-solid modified 6061 aluminum alloy rheological die-casting sample. As Si content increases, average grain diameter of primary α-Al in rheological die-casting sample structure gradually decreases. When Si content increases from 0.6% to 2.6%, diameter of primary α-Al grains in edge structure increases. Average grain diameter gradually decreased from 52 μm to 32 μm; average grain diameter of primary α-Al grains in core structure gradually decreased from 64 μm to 34 μm.
(3) Under same Si content, semi-solid modified 6061 aluminum alloy rheological die-casting samples show better mechanical properties than traditional liquid die-casting samples. When Si content increases from 0.6% to 2.6%, as-cast tensile strength of semi-solid modified 6061 aluminum alloy rheological die-casting sample gradually increases from (107±4) MPa to (209±14) MPa. Length rate gradually increased from 2.2%±0.3% to 5.5%±0.5%.