Metal semi-solid forming technology has many advantages, preparation of semi-solid slurry or billet is its basis and key, requiring organization of billet to be a uniform and fine spherical non-dendritic organization. A lot of research has been done at home and abroad around preparation of semi-solid slurry. However, due to influence of container size, it is difficult for a large volume of melt to enter supercooled state at the same time, and there is a significant temperature difference between various parts of liquid alloy, so that cooling rate of different parts is different, resulting in inconsistent organizational morphology. In view of this, this project has developed a set of processing devices (device includes a stainless steel hollow heat-absorbing circular tube) and adopts a new type of thermal balance method slurry making process. Process is low in cost, short in process, simple to operate, and can quickly absorb superheat of inner melt of barrel, improving chilling effect of core melt. In addition, heat balance method processing device was connected to an ultrasonic vibrator, ultrasonic vibration and heat balance method were combined for the first time to act on aluminum alloy melt, further improving structure of slurry. In previous experiment, A356 aluminum alloy semi-solid slurry was prepared by composite ultrasonic vibration heat balance method and direct heat balance method, organization was compared and analyzed. It was found that organization of sample obtained by the former was better than that of the latter. Taking A356 aluminum alloy as research object, aluminum alloy semi-solid slurry was prepared by ultrasonic vibration heat balance method, direct heat balance method and traditional casting process, and same pouring temperature, mold temperature, injection pressure and injection speed were used for forming. By analyzing influence of different slurrying process conditions on metallographic structure and mechanical properties of die castings, optimization effect of composite action of ultrasonic vibration and heat balance on alloy structure was studied.
Three slurry preparation processes were used in experiment: (1) Ultrasonic vibration heat balance method: heat balance method processing device was combined with an ultrasonic vibrator with a power of 1.8kW and a vibration frequency of 20kHz, and ultrasonic vibration was turned on to prepare semi-solid slurry; (2) Direct heat balance method: ultrasonic vibration was not turned on, and semi-solid slurry was prepared using heat balance method processing device; (3) Traditional die casting: heat balance method processing device was not used, and direct die casting was performed.
Heat balance method slurry preparation parameters are that melt treatment temperature is 630℃, outer diameter of heat absorbing round tube is φ34mm, and wall thickness is 1.1mm. Die casting forming process parameters: mold preheating temperature is above 250℃, melt pouring temperature is 620℃, die casting machine injection pressure is 400kN, and injection speed is 0.3m/s. After slurry die casting, metallographic structure and mechanical properties of die castings obtained under different process conditions were compared, and some samples were subjected to T6 heat treatment (530℃×6h solid solution + 80℃ water quenching + 150℃×6h aging and air cooling).

 Table 1 Chemical composition of A356 aluminum alloy (%)

 

Figure 1 Schematic diagram of ultrasonic vibration heat balance method treatment device
1. Control cabinet 2. Ultrasonic transducer 3. Ultrasonic amplitude transformer 4. Hollow heat absorbing tube 5. Barrel 6. Refractory brick 7. Support frame 8. Lifting motor

 

Figure 2 Schematic diagram of die casting test device
1. Injection chamber 2. Trolley 3. Injection rod 4. Mold 5. Cylinder 6. Column 7. Guide rail

Table 2 Main technical parameters of J15-300 vertical die casting machine

 

Figure 3 Three-dimensional model of mold and die casting
After preheating crucible resistance furnace and other tools, A356 aluminum alloy ingot is placed in furnace and heated to 720℃, kept warm. Melt is deslaged and degassed. Alloy slurry is prepared according to 3 different processes and die-casting is performed: (1) Ultrasonic vibration heat balance method, Transfer pre-cast melt to a small crucible preheated with furnace, measure temperature of aluminum liquid with a thermocouple, and pour melt into barrel when temperature drops to 630℃; turn on lifting motor with a lifting speed of 20 mm/s, and turn on ultrasonic vibration at the same time. Driven by motor, heat-absorbing round tube enters barrel, and after heat-absorbing round tube reaches bottom of barrel, it is pulled out after standing for 5 seconds; measure temperature of aluminum liquid in barrel, and when temperature drops to 620℃, semi-solid slurry is die-cast; (2) Direct heat balance method, do not turn on ultrasonic vibration, heat-absorbing round tube directly enters barrel, after reaching bottom of barrel, it is pulled out after standing for 5 seconds. Die-casting is performed when aluminum liquid temperature drops to 620℃; (3) Traditional die casting, transfer pre-cast melt to a small crucible preheated with furnace, and directly die-cast when temperature drops to 620℃.

 

Figure 4 Actual picture of die casting

 

Figure 5 Core structure of A356 aluminum alloy die casting under different process conditions before T6 heat treatment

 

Figure 6 Edge structure of A356 aluminum alloy die casting under different process conditions before T6 heat treatment

 

Figure 7 Core structure of A356 aluminum alloy die casting under different process conditions after T6 heat treatment

 

Figure 8 Edge structure of A356 aluminum alloy die casting under different process conditions after T6 heat treatment

 

Figure 9 Average equivalent diameter and average shape factor of metallographic structure under different process conditions
Heat balance method slurrying process uses a heat absorbing component as an internal cooler to absorb heat and control temperature, and to prepare slurry. Compared with traditional method of directly preparing slurry based on idea of external cooling, this process provides a means to improve controllability and efficiency of cooling process. When semi-solid slurry is prepared by ultrasonic vibration heat balance method, heat-absorbing circular tube absorbs heat from melt while applying ultrasonic vibration. When ultrasonic wave propagates in melt, it can produce mechanical effects, cause microscopic turbulence, promote convection heat transfer between barrel wall, heat-absorbing circular tube wall and melt, accelerate uniformization of melt temperature, thereby increasing nucleation rate and providing conditions for uniform nucleation, so that grains grow uniformly in all directions, and metallographic structure becomes uniform, fine and round.

 

Figure 10 Schematic diagram of tensile test bar

Table 3 Mechanical properties of A356 aluminum alloy die castings under different process conditions
Metallographic structure of semi-solid die castings obtained under ultrasonic vibration heat balance method process is the best, average equivalent diameter is the smallest, reaching 41.3μm, and average shape factor is the largest, reaching 0.95. Metallographic structure of die castings obtained by traditional process is the worst. Comprehensive mechanical properties of semi-solid die castings obtained under ultrasonic vibration heat balance method process are the best, and comprehensive mechanical properties of die castings obtained by traditional process are the worst. T6 heat treatment can effectively improve comprehensive mechanical properties of die castings.