Development of rheoforming technology in metal casting

The preparation of metal semi-solid slurry is the key link of its processing technology, which is directly related to the quality and performance of semi-solid parts. Semi solid slurry requires stable spherulites or near spherulites with uniform and fine non dendritic structure. At present, the main semi-solid slurry preparation methods include electromagnetic stirring, liquidus casting, mechanical stirring, strain induced melt activation, chemical grain refinement and spray forming. Electromagnetic stirring (EMS) is a kind of non dendritic semi-solid structure, which can be prepared by changing the heat transfer and flow mode of alloy melt with the help of electromagnetic force. Because of its non-contact characteristics, it can reduce the pollution and entrainment of slurry. This technology has been applied in industry, but because of its “skin effect”, it can reduce the pollution and entrainment of slurry Therefore, electromagnetic stirring is not suitable.

Liquidus casting method means that the liquid alloy is cooled to liquidus, and then poured according to the process time after holding temperature to obtain uniform distribution of fine, equiaxed semi-solid structure. Because of its simple process and high preparation efficiency, it has been widely used in aluminum, magnesium alloys, especially wrought aluminum alloys. The earliest invention method of semi-solid pulping is mechanical stirring, which means that the alloy melt is sheared by the external force generated by mechanical stirring, and the uniform semi-solid structure of spherulite or near spherulite can be obtained by controlling the stirring speed, slurry temperature and cooling speed. Strain induced melt activation is a cold deformation process of breaking the dendrite structure of the common structure billet, then heating to the solid-liquid two-phase region, recovery and recrystallization occur. The sharp part with large interfacial energy in the two-phase region is melted, while the concave part is solidified due to the diffusion effect, and the non dendrite semi-solid slurry is obtained. This process is complicated and costly High efficiency, low efficiency.

Chemical grain refinement method is to increase the number of crystal nucleus in alloy melt by modifier or refiner, so as to improve the crystal structure and obtain semi-solid slurry with spherulite or near spherulite. The process is simple and easy to operate. The key is to control the composition, dosage and time of refiner. The spray forming method is that the atomized metal liquid is sprayed onto the cooling target by the gas atomizer. The atomized alloy droplets are broken under the shearing pressure of the spray pressure, and then the semisolid metal melts with spherulites or near spherulites can be obtained when heated again. The process flow is complicated and the cost is relatively high, which is only used in some special products.

Rheoforming technology is based on the preparation of semi-solid slurry. The advantage of rheoforming lies in the on-line preparation of slurry, which can save energy consumption and recycle waste in time. The first solidification is in the slurry preparation stage, and to ensure the on-line preparation of semi-solid slurry, the quality, cost and efficiency of slurry must be ensured in this solidification stage. The second solidification is the whole process of the subsequent movement, pouring, injection molding and solidification under pressure of the semi-solid slurry. In these two solidification processes, the main problem is how to quickly prepare high-quality semi-solid alloy melt online; the other is the perfect connection between slurry forming process and slurry without pollution.