Causes of Blistering Defects in Zinc Alloy Die Castings
Zinc alloy die castings are currently widely used in various decorations, such as furniture accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc. Therefore, the surface quality of the castings is relatively high. At the same time, good surface treatment performance is required. The most common defect of zinc alloy die castings is surface blistering. Defect characterization: There are blisters on the surface of die-casting parts, which are found after die-casting, exposed after polishing or processing, and after oil spraying or electroplating.
cause:
1. Caused by holes: It is mainly due to the stomata and shrinkage mechanism. The stomata are often circular, while the shrinkage is mostly irregular. (1) Causes of pores: a. During the filling and solidification process of molten metal, holes are formed on the surface or inside of the casting due to the intrusion of gas. b The gas volatilized from the paint invades. c The gas content of the alloy liquid is too high, and it precipitates during solidification. When the gas in the cavity, the gas volatilized from the paint, and the gas precipitated by the solidification of the alloy, when the mold exhaust is not good, it will eventually remain in the pores formed in the casting. (2) Causes of shrinkage cavities: a) During the solidification process of molten metal, shrinkage cavities are generated due to the shrinkage of the volume or the fact that the molten metal cannot be fed at the final solidified part. b. The castings or castings with uneven thickness are overheated locally, resulting in slow solidification of a certain part, and a concave surface is formed on the surface when the volume shrinks. Due to the existence of pores and shrinkage cavities, when the die-casting parts are surface-treated, the holes may enter water. When baking after painting and electroplating, the gas in the holes is heated and expanded; or the water in the holes will turn into steam and expand in volume, This results in blistering on the surface of the casting.
2. Caused by intergranular corrosion: Harmful impurities in zinc alloy composition: lead, cadmium and tin will gather at the grain junction to cause intergranular corrosion, and the metal matrix will be broken due to intergranular corrosion, and electroplating accelerates this scourge. Parts of intercorrosion will expand and push up the coating, causing blistering on the surface of the casting. Especially in wet environments, intergranular corrosion can cause castings to deform, crack, and even shatter.
3. Cracks caused: water lines, cold separation lines, hot cracks. 1) Water marks and cold separation marks: During the filling process of the molten metal, the contacting wall of the molten metal that enters first solidifies prematurely, and the molten metal that enters later cannot be fused with the solidified metal layer, forming a stack at the butt joint of the casting surface. striae, stripe defects. The water marks are generally shallow on the surface of the casting; while the cold barrier may penetrate into the inside of the casting. 2) Hot cracks: a When the thickness of the casting is uneven, stress is generated during the solidification process; b is ejected too early, and the metal strength is not enough; c, the force is uneven during ejection; . All of the above factors may cause cracks. When there are water lines, cold separation lines and hot cracks in the die casting, the solution will penetrate into the cracks during electroplating, and will be converted into steam during baking, and the air pressure will lift the electroplating layer to form blisters.
