Basic Knowledge of Zinc Alloy Die Casting Huizhou Hardware Die Casting Factory

At present, zinc alloy die castings are widely used in various decoration fields, such as furniture accessories, architectural decoration, bathroom accessories, lighting accessories, toys, tie clips, belt buckles, various metal decorative buckles, etc. Therefore, high requirements are placed on the surface quality of castings and good surface treatment performance is required. The most common defect of zinc alloy die castings is surface blistering

Defect characterization: there are protrusions and small bubbles on the surface of the die casting

It was found after die casting

Exposed after polishing or machining

Appears after oil spraying or electroplating

Causes:

1. Caused by pores: mainly due to pores and shrinkage mechanism. The pores are usually round, while most shrinkage is irregular

(1) Cause of porosity: a. During the filling and solidification process of molten metal, holes are formed on the surface or inside of the casting due to gas intrusion

b The gas volatilized from the paint invades

The gas content of c alloy liquid is too high and precipitates during solidification

When the gas in the mold cavity, the gas volatilized from the coating, and the gas separated from the alloy solidification are poorly vented, they will eventually remain in the casting to form pores

(2) Causes of shrinkage cavity: a Shrinkage cavity occurs due to the reduction of the volume or the lack of liquid metal feeding at the final solidification position during the solidification of liquid metal

b The casting with uneven thickness or local overheating of the casting results in slow solidification of a certain part, and concave position is formed on the surface when the volume shrinks

Due to the existence of pores and shrinkage cavities, the holes may enter water during the surface treatment of die castings. When the holes are baked after painting and electroplating, the gas in the holes will expand due to heat; Or the water in the hole will become vapor and expand in volume, which will lead to blistering on the surface of the casting

2. Caused by intergranular corrosion:

Harmful impurities in the composition of zinc alloy: lead, cadmium and tin will gather at the grain boundary to cause intergranular corrosion, and the metal matrix will be broken due to intergranular corrosion. Electroplating accelerates this disaster, and the parts subject to intergranular corrosion will expand to jack up the coating, causing blistering on the surface of the casting. Especially in the humid environment, the intergranular corrosion will make the castings deformed, cracked, or even broken. (See Figure 1 on the right)

3. Caused by cracks: water marks, cold striations and hot cracks

Water lines and cold separation lines: during the filling process, the metal liquid that enters first contacts the mold wall and solidifies prematurely, and the metal liquid that enters later cannot be fused with the solidified metal layer as a whole. The overlapping lines are formed at the butt joint of the casting surface and strip defects appear, as shown in Figure 2. The water mark is generally in the shallow layer of the casting surface; However, the cold barrier may penetrate into the casting

Hot cracks:

a When the casting thickness is uneven, the solidification process produces stress

b Premature ejection and insufficient metal strength

c Uneven force during ejection

d Excessive mold temperature makes grains coarse

e Harmful impurities exist

The above factors may cause cracks

When there are water lines, cold lines and hot cracks in the die casting, the solution will infiltrate into the cracks during electroplating, and will be converted into steam during baking, and the air pressure will lift the electroplated coating to form bubbles

There are two levels of forced feeding: extrusion feeding and forging feeding: there are two levels of forced feeding for castings. One is basically to eliminate the shrinkage cavity and porosity of the casting, and the other is to make the inside of the blank reach the level of broken grain or forged structure. If we want to use different words to express the two different degrees, we can use” for the former; Extrusion feeding” To express the latter, we can use” Forging feeding” To express. We should pay full attention to the understanding that replenishment is a direct means and cannot be completed indirectly. In terms of process, we can have a process parameter to express it, which is” Feed pressure”. In physical principle, the concept of pressure can occur in two ways: one is in the liquid situation, namely” Pascal's Law” For clarity, we define it as” Liquid pressure”, The other one appears in the solid state. We define it as” Solid pressure”. It should be noted that the applicable conditions of the pressure concept in these two different states will cause major problems if we mix them up