Evaluation of Holes and Defects in Metal Die Castings

Jul. 15, 2020

For auto casting parts, die casting is the most productive of all casting methods. It is the casting of liquid or semi-liquid metal at high speed under high pressure to fill the mold and solidify into a casting under pressure. method. The pressure used is 4~500 MPa, and the metal filling speed is 0.5~120 m/s. Because the die castings are difficult to avoid when filling the cavity, which affects the strength of the casting and other indicators, it will cause failure in severe cases. This article mainly analyzes the reasons for the porosity of die castings.

1 Classification of holes

The hole-like defects of die-casting parts are generally divided into air holes, loose and shrinkage holes, and the air holes include subcutaneous air holes and single dispersed air holes. The location of the subcutaneous stomata is in the range of 1~3 mm under the epidermis, which is pear-shaped and elliptical, and the inner surface of the pore is smooth; the distribution of scattered pores is irregular, most of which are pear-shaped or elliptical, and the size is larger, generally silver-gray or Bright oxidation color. There are dark, uneven, irregularly shaped holes inside the die-casting parts. Large and concentrated holes are called shrink holes, and small and scattered holes are called loose. In actual production, shrinkage and pores are difficult to distinguish, and looseness can also be considered as a collection of tiny spores.

2 Effect of holes and defects on strength

Special attention should be paid to the influence of pores on the strength of engineering parts. The same porosity will produce different effects when the parts are subjected to static loads and dynamic loads. Because the fracture of the specimen has a considerable relationship with the radius of curvature, thickness, and width of the notch tip, when the plastic stress concentration increases σs to σf, cleavage proceeds along with the elastoplastic interface, and the fracture start at σyy=Kpσs=σf. Where: σyy is the maximum stress before the notch, σf is the cleavage fracture strength of the material at the root of the notch, and Kp is the plastic stress concentration factor.

Oem Auto Parts

Oem Auto Parts

In general, σyy increases as the radius of curvature of the notch decreases. At a given temperature, the fracture stress σf decreases significantly as the radius of curvature of the notch decreases. When subjected to static load, whether it is bending or torsional load, the stress increases due to the reduction of the cross-section, resulting in a notch effect. At the same time, when the yield strength σs of the material is determined, the load Ps it bears is proportional to the cross-sectional area F0, that is, σs=Ps/F0, so the presence of pores in the part causes the load it bears to decrease proportionally.

When subjected to dynamic loads, in addition to the factors of the material itself, the notch factor is also its main factor. The notch effect includes many factors such as the geometry of the part, scale, inclusions, uneven structure, pores, cracks, etc. The resulting notch effect depends on its shape, location, and pore distribution. For example, the failure of the aluminum alloy cylinder head of the automobile engine during the endurance test caused the early failure due to the notch effect caused by the looseness of the subsurface of the material.

3. Measurement results of die castings of different materials

The porosity of the die-casting part has little to do with the composition of the material but is related to the casting conditions during the die-casting and the flow of the molten metal during the die-casting.

3.1 Effect of die casting conditions on porosity

The injection pressure and speed determine the filling of the die casting, which affects its pores; the type and amount of paint is the main source of gas out of the die casting cavity, which has a greater impact on the pores.

3.2 Paint influence

The gas content of OEM auto parts is greater than the gas content of dissolved gas elements in the holding furnace. Hydrogen occupies 70% and other gases account for 30% in the die-casting parts; 1/3 of the hydrogen comes from the metal liquid, 2/3 comes from the paint and lubricant, and the other gas except hydrogen in the air involved in the injection. Therefore, when designing the die-casting process, choosing a suitable coating has a greater impact on the compactness of the die-casting parts.

3.3 The influence of the liquid metal flow on the die castings

The flow of molten metal during die casting is mainly related to the position, shape, and size of the pouring system, the setting of overflow channels and exhaust channels, injection pressure, and injection speed. Among them, the position, shape, and size of the inner runner are greatly affected. The cavity filling conditions caused by different in-runners are different. If the in-runner is small, the molten metal has a high velocity at the in-runner, causing jet-like filling, and the gas is enclosed in the casting to form pores; the in-runner The area is large, the speed of the molten metal runner is relatively small, and the order of filling is formed, and the probability of forming air holes is small. If the overflow groove and the exhaust channel can be opened opposite the inner runner, the air holes in the cavity can be better eliminated.

4 New die casting process to prevent defects such as holes

In order to prevent die casting porosity and improve casting performance, new processes such as vacuum die casting, oxygenated die casting, airflow die casting, and double injection punch die casting method have emerged and have been applied in production.

5 Conclusion

(1) By analyzing and studying the causes of pores, the influence of the coating on the pores in the casting conditions, and the influence of the molten metal flow on the pores in the casting are mastered. Among them, the size of the inner runner and the influence of the exhaust channel on it Larger.

(2) Through research, sample preparation is the basis for accurate measurement of die casting porosity. The deformation layer and pits produced during the sample preparation have a great influence on the measurement results.

(3) Porosity measurement refers to selecting the largest square in the densest pore area of the sample to be measured as the field of view and measuring the area percentage of the pores in the area.