Al-Si alloy is a common die-casting alloy. With its high specific strength and good casting processing performance, it is widely used in rail transit, automobile manufacturing, aerospace and other fields. Si is an important strengthening element in this series of alloys. It generally exists in alloy in the form of eutectic phase, β-Al5FeSi phase and primary Si. At the same time, as Si content increases, strength and wear resistance of alloy also increase. However, presence of needle-like eutectic Si and coarse amorphous primary Si seriously deteriorates plasticity of alloy, making it difficult to meet usage requirements. Generally, modifiers containing Se, Na, Ba and P are used to improve existence form of Si phase, thereby improving performance of alloy. This not only increases production cost of Al-Si alloy, but also causes alloy element pollution of product.
In recent years, application of electromagnetic fields in alloy solidification has developed rapidly, providing a feasible solution for refinement and structural homogenization of aluminum alloy solidification. Researchers drew on fact that electromagnetic fields significantly affect solidification process of alloy melts and proposed electromagnetic solidification processing technology, which has positive significance for electromagnetic fields to control solidification process and improve alloy forming processing performance. Semi-solid die-casting Al-Si alloy can avoid internal pore defects of product caused by traditional die-casting technology, but it cannot improve segregation and coarse eutectic phase caused by rapid solidification of melt. At present, most methods such as traditional mechanical stirring, metamorphism, shear low-temperature casting, gas-induced semi-solid processing (GISS) and rapid slurrying (RSF) are used to improve alloy quality. This study proposes to introduce pulse current into Al-12Si alloy semi-solid slurry, control alloy to a certain solid phase ratio through electromagnetic stirring, and alloy slurry is die-cast into a slab. Morphology and structure of semi-solid slurry under influence of electromagnetic influence are comparatively studied, changes in mechanical properties of die-cast slab are analyzed and discussed to provide reference for semi-solid die-casting production.
Graphical results
Die-casting material is Al-12Si aluminum alloy, and its composition is shown in Table 1. Calculate proportion of aluminum alloy raw materials for semi-solid die casting according to Al-Si phase diagram. Industrial pure aluminum with a purity of 99.83% (mass fraction, same below) and Al-20Si master alloy are smelted through an RJ2-20-5kW medium frequency resistance furnace. Maintain furnace temperature at 635℃ for 2 hours, then take 0.3kg of alloy liquid and place it in graphite crucible, and introduce YS9000 pulse power supply at the same time. Input three-phase AC380V, 20Hz ~ 2000kHz, stepless fine adjustment, pulse duty cycle is 10% ~ 90%, stepless adjustable. Pulse voltage is 12V, pulse width is 0.01s, and pulse current is introduced into melt for about 300s. At the same time, when temperature of semi-solid alloy slurry is maintained at about 570℃, semi-solid slurry is then introduced into pressure chamber of die-casting machine and die-casting begins. Clamping force of die-casting machine is 1600kN and injection force is 200kN. Semi-solid die-casting test is divided into 3 groups: the first group is a pulse-free processing process (traditional semi-solid die-casting); the second group is an electromagnetic treatment process (electric pulse semi-solid die-casting) applying fixed pulse parameters (frequency of 20Hz, output voltage of 12V, duty cycle of 20%); in the third group, a Sr modifier with a mass fraction of 0.04% was added to melt for 10 minutes, then slurry was taken out and pulse current was applied. Schematic diagram of die casting dimensions is shown in Figure 1.

Table 1 Chemical composition of Al-12Si alloy (%)

 

Figure 1 Dimensional diagram of die casting parts

 

Figure 2 Microstructure of center of aluminum alloy die-casting slab after annealing under different semi-solid solidification treatment conditions
Pulse current used in this study has an obvious refining effect on coagulated tissue. α-Al phase nucleation core is affected by Lorentz force and increases nonlinearly at frontier of solidification interface. Electromagnetic pressure promotes heat transfer and mass transfer processes of alloy elements between dendrites. Alloy concentration around growing dendrites is significantly reduced undercooling, and alloy composition is more evenly distributed. Figure 3 shows eutectic structure morphology of die-cast Al-12Si alloy sheet. As can be seen from Figure 3a, size of eutectic structure of traditional process plate varies greatly, mostly in the form of needle-like and lamellar bifurcations, and size of eutectic structure is relatively coarse. Under pulse current semi-solid solidification treatment conditions, there is no obvious sharp structure at the edge of eutectic phase, and α-Al phase structure inside eutectic structure is granular, as shown in Figure 3b. Eutectic structure is significantly refined, tissue distribution is uniform, and there are tiny eutectic Si particles; after pulse current solidification treatment under Sr metamorphic conditions, boundaries of α-Al phase structure are clear and rounded. Distribution of eutectic structure is relatively uniform, growth direction is not obvious, it is mostly in the shape of short rods and granules, as shown in Figure 3c. Al-Si eutectic phase is the most important phase composition in Al-Si alloy system. Needle-like and complex coarse eutectic structure seriously reduces performance of alloy. Therefore, modification treatment is often used to change its size and shape, thereby improving mechanical properties of alloy. In this study, after different semi-solid solidification treatments, semi-solid die-cast Al-Si eutectic structure was analyzed by energy spectrum, as shown in Table 2.

 

Figure 3 Microstructure of Al-12Si die-cast plate under different semi-solid treatments

Table 2 Chemical composition analysis of eutectic structure (%)
After pulse treatment, a pseudo-eutectic tissue layer was formed on the surface of slab. ESi phase in this area was dense, so surface hardness of sample was improved. After modification by adding Sr, density of α-Al dendrites on the surface of slab increases significantly and content of pseudoeutectic phase decreases, resulting in a relatively low hardness value in the surface layer. In addition, pulse current effectively disperses metamorphic nucleation core, thereby refining morphology and uniform distribution of eutectic Si inside slab, improving tensile strength and elongation of slab. It can be seen that hardness, strength and plasticity of semi-solid die-cast slab are improved to varying degrees after current treatment. However, tensile properties of product under joint treatment of electric pulse and Sr modification are the most obvious and toughness is ideal.

Table 3 Room temperature mechanical properties of aluminum alloy die-casting products

 

Figure 4 Tensile fracture micromorphology of Al-12Si alloy slabs after annealing with different semi-solid treatments
Analysis conclusion
(1) Compared with traditional semi-solid die-casting process, Al-12Si aluminum alloy treated with pulse current can obtain a fine and uniform microstructure. α-Al dendrites are mostly granular; while under conditions of Sr modification and current treatment, dendrite size of microstructure is further refined and more uniform, and α-Al dendrites are mostly rounded, feather-shaped and rose-shaped.
(2) There are obvious differences in morphology and composition of Al-Si eutectic. Compared with traditional semi-solid die-cast products, pulsed current also refines eutectic phase size. Pulse current treatment reduces Si content of eutectic; under condition of current combined with Sr metamorphism, eutectic structure is further refined and mostly appears in rod-shaped and granular shapes. Its Si content is further reduced and evenly distributed.
(3) Hardness and tensile strength of semi-solid die-cast slab were tested, it was found that semi-solid treatment of current treatment combined with Sr modification can effectively improve mechanical properties of product, and at the same time, toughness is further improved. Fracture observation found that die-cast Al-12Si aluminum alloy showed obvious ductile fracture characteristics, both current treatment and Sr modification treatment can effectively improve toughness of product.