Semi-solid Forming & Die Casting: Method, Structure, Process and Advantage

What is the principle of aluminum alloy semi-solid forming? The unique rheological and stirring properties of non dendritic semi-solid metals (SSM) are used to control the quality of die castings. In this article, we detail everything you should know about semi-solid forming method, structure, process, and advantage.

Semi solid Forming Method

Rheoforming:

In the process of metal solidification, vigorous stirring is applied to fully break the dendritic primary solid phase to obtain a solid-liquid mixed slurry with a certain spherical primary solid phase uniformly suspended in the liquid metal mother liquor. The solid component is generally about 50%, that is, rheological slurry. The method of direct processing with this rheological slurry is called rheological molding of semi-solid metal.

Thixoforming:

If the slurry solidifies into ingot, cut the metal ingot into a certain size as required, and then reheat (i.e. secondary heating of the blank) to the semi-solid temperature zone of the metal (the metal ingot is called a semi-solid metal blank). Thixotropic forming is the processing method of forming with a semi-solid blank of metal.

Metal materials show obviously different physical characteristics in liquid, solid, and semi-solid stages. Using these characteristics, many metal hot working forming methods such as solidification processing, plastic processing, and semi-solid processing have been produced.

The internal characteristics of semi-solid metals (alloys) are the mixed coexistence of solid and liquid phases. There are metal liquids at the grain boundary. Their states are different according to the different solid fractions. When the solid fraction is high, the liquid composition is limited to some grain boundaries; When the solid fraction is low, the solid particles are free in the liquid phase.

The internal structure of semi-solid metal:

1. High solid fraction

2. Low solid fraction

Advantages of semi-solid metal processing over conventional processing methods:

1. The viscosity is higher than that of liquid metal, which is easy to control: the gas entrained in the die is less, which reduces oxidation, improves processability, reduces die bonding, can carry out higher speed part forming, improve surface finish, and is easy to realize automation and form new processing technology;

2. The flow stress is lower than that of solid metal: semi-solid slurry has rheology and thixotropy, and the deformation resistance is very small. It can form parts at a higher speed, and can form complex parts, shorten the processing cycle, improve the material utilization rate, which is conducive to energy saving and material saving, and can carry out high-speed forming of continuous shape (such as extrusion), with low processing cost;

3. Wide application range: all alloys with solid-liquid two-phase zone can realize semi-solid processing, can be applied to a variety of processing processes, such as die-casting, rolling, extrusion, and forging, and can carry out material compounding and forming.

The basic technology of semi-solid processing

Microstructure characteristics, formation mechanism, and mechanical behavior of semi-solid metals

1. Formation and evolution of nondendrites

The liquid metal is stirred and cooled in the solidification process, and its crystallization causes the initial shape of solid particles to be dendritic. Then, under the action of shear force, the planting crystals will break and form small spherical crystals. Compared with the microstructure of conventional die casting and semi-solid die casting, the semi-solid metal produced by rheological die casting has a unique nondendritic and nearly spherical microstructure.

2. Mechanical behavior of semi-solid metals

In order to obtain the mechanical properties of A356 aluminum alloy by different processing methods. Advantages of semi-solid metal processing technology. For example, thermoforming and T6 properties have better mechanical properties than those obtained by metal mold die casting, and the properties of forgings are similar.

Advantages of semi-solid die casting

The range of complex formed parts is expanded, near-final forming (such as thin-wall parts, undercut parts, hole parts and edge radiation parts) can be realized, and the process links are significantly reduced, the processing cost is low, the cutting amount is small, and the material utilization rate is high.

Rheological forming of semi-solid metal

Compared with thermoforming, rheological forming eliminates secondary heating and is formed directly by semi-solid slurry, but its process control is more difficult. In recent years, due to the short production process and low relative cost, rheological forming has attracted the attention of many researchers at home and abroad, such as the “new rheological die casting process” of LKR company in Austria, the double helix rheological technology of Z. fan in Britain, the inclined plate of T. HAGA in Japan to prepare semi-solid slurry and directly cast and roll thin strip and rheological extrusion, The continuous emergence of semi-solid direct rolling and cone barrel semi-solid rheological forming technology by Kang Yonglin in China has laid a solid foundation for the industrial application of this technology.