Introduction

With the development of technology, the quality of die casting more and more high requirements, not only on its mechanical properties, internal organization has strict requirements, the appearance of defects also put forward more stringent requirements.

Such as aluminum alloy die casting in the casting process, often appear sintering sticky aluminum, pulling and casting after shot blasting skin phenomenon, can not meet the customer on the casting of the appearance of the quality requirements, not only reduce the casting qualification rate and production efficiency, but also increase the production cost.

For die casting casting defects such as burnout, aluminum sticking, straining, cracking, etc., through the partial use of die casting mold coating process, can reduce the casting appearance of the defective rate and leakage rate, to provide a reference for the solution of similar problems.

 

1. Introduction of die-casting mold coating process

Mould coating process is in the mold surface is coated with a layer of high hardness alloy, the use of cold welding to the super-hard alloy in a short period of time, quickly coated in the mold surface, and penetrate into the mold tissue inside, can be coated to increase the coated mold abrasion resistance, corrosion resistance, impact resistance, mold release and heat resistance.

1.1 Microstructure of die casting molds after coating

Figure 1 is the microstructure of the overlay layer of the die casting mold. It can be seen, after being coated mold, in the substrate outside the surface layer to form a layer of alloy layer, cladding and strengthen the layer thickness of 0.02 ~ 0.03mm, after the local coated mold, the thickness of the coated parts increased. In order to increase the effect of cladding within the casting tolerance, the cladding area of the mold can be clad in multiple layers, and the thickness of the cladding layer can be controlled at 0.01～0.1mm.

Microstructure of the coating layer of the die casting mold

 

1.2 Mold Coating Materials and Physical Properties

WTC-90 is a cladding material for molds, which is based on WC with a small amount of Ti added to reduce its brittleness. In the process of mold coating, there is no need to heat the coated mold, it is coated at room temperature without thermal deformation, there is no stress concentration in the coated area, and the coated area will not be cracked due to stress, and the WC coated layer shows ultra-high hardness, which is outstanding in the aspect of anti-washout.

2. The influence of mold coating process on the quality of castings

In the actual production process, mold design by the casting structure, as well as by the casting, row system design limitations, process window is relatively narrow, casting defects exist in parallel. Process setup principle, the first to ensure the internal quality of the castings, but the appearance of the castings often do not meet customer requirements.

Such as with the gate flushing lead to casting gate parts strained, cracked; mold sharp corners of the location can not be set up cooling water, resulting in high temperature, burning adherence to the sticky aluminum; mold grooves are more, the aluminum filling process is subject to a greater resistance, the casting surface to produce flow marks, the casting of the appearance of the casting after shot blasting, peeling, and so on. In the mold pre-design and production of the introduction of the coated process, can effectively solve the above defects.

2.1 Improvement of casting strain and crack by mold coating process

Figure 2 shows the design of the oil cooler cover casting system and the position of the pulling mold.

Schematic diagram of the oil cooler cover casting system and the film drawing position

It can be seen that the pulling mold position is located at the end of the inner gate. Through repeated verification of the die casting process, it was found that lowering the pouring speed can reduce the scouring of the pulling mold area, but the pulling mold position can not be eliminated, and it will lead to poor appearance of the castings. The depth of the pulled mold exceeds 0.5 mm (see Figure 3), and cracks, especially hidden cracks, will appear.

External defects of casting film pulling

The mold to take the coated process, the number of coated coating layer for 2 layers, the thickness of the coated layer is 0.05mm. After verification, pulling the mold part defects have been solved, the adjustment of the die casting process, especially for the speed of the requirements can be relaxed. Figure 4 for the mold is coated, castings before and after shot blasting treatment comparison, fully meet the technical requirements.

Comparison of die castings before and after shot blasting after mold covering

2.2 Improvement of sintering and aluminum sticking of castings by die coating process

Aluminum liquid high-speed impact cavity or core, the impact of the mold parts of the temperature rises, and thus can not form a cold layer, the phenomenon of aluminum corrosion (erosion) of the mold (sintering) will occur. Blanks on the coarse grain layer (see Figure 5).

Partial sintering of castings

Generally in the die casting production process, for the sintering part by setting cooling water, reduce the local mold temperature, can reduce the sintering. But often affected by the mold structure and product structure, part of the position can not set cooling water. If the amount of mold release agent sprayed is increased, the production beat will be affected. It has been proven that localized sintering can be effectively solved by doing a coating treatment on the localized sintered parts (see Figure 6).

Comparison of mold coating before and after

2.3 Improvement of shot peening of castings by die casting mold coating process

The surface of the casting after blasting will appear a layer of thin skin warping, or even fall off, its size, area and location is not fixed, this phenomenon is known as skinning after blasting (see Figure 7).

The main reason for casting skinning is mostly due to the special design of some castings, more grooves, aluminum in the filling process fluctuations, high-speed filling, aluminum flow to the mainstream filling can not be places (dead ends), molten liquid splash, its temperature drop is relatively fast, in the mainstream to reach the beginning of the pressurization, before the start, it has solidified. Therefore, the formation of cold scars on the surface of the casting, flow marks or pitting, which is the main reason for shot peening (see Figure 8).

Schematic diagram of water-cooled shell gating system and casting structure

Aluminum liquid in the cavity flow process, due to the mold surface is very smooth, if in the casting easy to produce cold scars, flow marks and pitting position, the mold to do the coating treatment, the mold surface will form a convex and concave state, the formation of high and low grooves, can be dispersed in the direction of the flow of liquid aluminum in the cavity, and the formation of a layer of air between the grooves, to improve the position of the liquid liquid aluminum mobility, to avoid or reduce the cold scars, flow marks or pitting. avoid or reduce the generation of cold scars, flow marks or pockmarks. It has been proved that the mold coating treatment effectively solves the cold scars, flow marks and pitting on the casting surface, and there is no peeling phenomenon in the appearance after shot blasting (see Figure 9).

Appearance of shot blasted casting after mold coating

 

3. Conclusion

The partial coating process of the mold can effectively solve the pulling and cracking of the gate part of the mold, can effectively solve the sticking aluminum and sintering of the local hot joint part of the mold, and can effectively solve the problem of skinning of the castings due to shot blasting. Through the coating of the mold, it solves the casting appearance quality problems in the production and improves the production efficiency.