Aluminum Alloy Castings! Why Does High Pressure Die Casting Need no Heat Treatment But Low Pressure Die Casting Need?

1. Review of heat treatment

Common heat treatment for common metal parts involves three process: annealing, normalizing and quenching. Here are some related knowledge points of the die cast aluminum alloy.

Annealing:

Heat the product to a certain temperature and keep it for a certain time, then cool it to room temperature at a certain speed. The purpose is to eliminate the internal stress by diffusion and migration of metal particles to make the crystal structure more uniform and stable.

Solid Solution:

The parts are heated to a high-temperature single-phase region and kept at a constant temperature, so that the surplus phase is fully dissolved in the solid solution and then quickly cooled to obtain supersaturated solid solution. Heat the heat-treated aluminum alloy parts to a higher temperature and keep the temperature for a certain time, so that the soluble components in the parts are fully dissolved in the aluminum matrix, thus forming supersaturated solid solution to the greatest extent, and then rapidly cool the supersaturated solid solution and keep it to room temperature to obtain proper grain size.

Aging:

It is divided into natural aging (placing parts at room temperature or natural conditions for a long time) and artificial aging (heating parts to a relatively high temperature and returning to normal temperature in a short time). Artificial aging is usually chosen to save time. The main principle is that the parts and components are subjected to solution treatment with cold plastic deformation or die casting, and then placed at a higher temperature or room temperature. The performance, shape and size change with time.

2. When die casting aluminium alloy, why does high pressure die casting need no heat treatment but low pressure die casting need?

Through the above knowledge, we can find the key to the answer: Internal Porosity.

From the point of view of process forming, there are certain porosity in the casting body in the process of high-pressure and high-speed filling, which is why X-ray and CT are used to measure the porosity of parts in the quality control of aluminium die casting, so as to ensure that the mechanical properties of parts can reach the standard (for common ADC12, the average tensile strength is about 228Mpa). Due to the existence of porosity, heat treatment of high pressure die casting parts will cause deformation and warpage, which determines that most of the products corresponding to this process are structural parts, protective shells, pedestal supports and so on.

In the process of low pressure die casting, the cavity sealed above the crucible is slowly filled, which makes the structure of low pressure die casting parts have good compactness and few pores. Heat treatment of solution aging can further make the crystal structure more compact and uniform, thus ensuring mechanical properties such as tensile strength. The tensile strength of excellent enterprises can reach 290 ~ 330 MPa or even higher. It is not difficult to understand why some moving parts need to adopt low pressure die casting technology.

For some parts with higher strength and better ductility, such as control arm, forging process is one of the choices. However, excellent control of low pressure die casting process can sometimes achieve certain substitution effect.