Causes and solutions of blister defects in zinc alloy die castings
Causes and solutions of blister defects in zinc alloy die castings:
Zinc alloy die castings are widely used in various decorative aspects, such as furniture accessories, building decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc. Therefore, the casting surface quality is required to be high, and at the same time requires good surface treatment performance. The most common defect of zinc alloy die castings is surface blistering. Defect characterization: there are protruding bubbles on the surface of die casting, which are found after die casting, polished or processed, and appear after oil injection or plating.
Causes:
1. Cavity: mainly caused by stomata and shrinkage mechanism, stomata are often round, and most of the shrinkage is irregular shape. (1) Causes of porosity: a metal liquid in the mold filling and solidification process, due to gas invasion, resulting in the surface or internal casting holes. B paint volatile gas intrusion. C alloy has high gas content and precipitates during solidification. When the gas in the mold cavity, the vaporized gas from the coating and the solidified gas from the alloy are not well exhausted, the air holes formed in the casting are finally left. (2) Causes of shrinkage: shrinkage occurs in the solidification process of a molten metal due to shrinkage of volume or the last solidification part can not be filled by molten metal. B. Local overheating of castings or castings with uneven thickness results in slow solidification of a certain part and concave surface formation during volume shrinkage. Because of the existence of pores and shrinkage holes, the holes may enter water during surface treatment of die castings. When spraying and baking after electroplating, the gas in the holes will be heated and expanded; or the water in the holes will become vapor and the volume will expand, thus causing foaming on the surface of the castings.
2. Intergranular corrosion: harmful impurities in zinc alloy composition: lead, cadmium and tin will gather at the grain boundary and lead to intergranular corrosion. Metal matrix will be broken due to intergranular corrosion, and electroplating accelerates this disaster. Intergranular corrosion will swell the coating and cause foaming on the surface of the castings. Especially in wet environment, intergranular corrosion will cause deformation, cracking and even breakage of castings.
3. Cracks are caused by water streaks, cold streaks and hot cracks. 1) Watermark and cold partition: In the filling process, the liquid metal contact mold wall solidifies prematurely, and then the liquid metal can not fuse with the solidified metal layer, forming overlapping lines on the surface of the casting, resulting in stripe defects. Water marks are usually on the surface of the casting surface, while the cold streak may penetrate into the inside of the casting. 2) Hot crack: a when the casting thickness is uneven, solidification process produces stress; B premature ejection, metal strength is not enough; C ejection stress uneven D too high mold temperature makes grain coarse; e harmful impurities exist. All these factors may cause cracks. When the die castings have water marks, cold diaphragm lines, hot cracks, electroplating solution will penetrate into the cracks, in the baking time into vapor, gas pressure jack up the electroplating layer to form bubbles.
The way to solve the problem is:
1. Controlling the formation of porosity: the key is to reduce the amount of gas mixed into the castings. The ideal metal flow should be accelerated continuously from the nozzle through the diversion cone and the runner into the cavity, forming a smooth and uniform metal flow. Conical runner design is adopted, that is, the flow should be accelerated gradually from the nozzle to the inner runner to achieve. To this end. In the filling system, the mixed gas is formed by the mixture of turbulence and liquid metal. From the study of the simulated die casting process in which liquid metal enters the mold cavity from the casting system, it is obvious that the sharp transition position in the runner and the increasing cross-section area of the runner will make the liquid metal flow turbulent and entraining gas, and the stable liquid metal will appear. This will help the gas enter the overflow slot and the exhaust slot from the runner and the mold cavity, and discharge the mold outside.
2, for shrinkage cavity: to make all parts of the die casting process uniform heat dissipation at the same time, at the same time solidification. The shrinkage can be avoided by reasonable nozzle design, gate thickness and position, die design, die temperature control and cooling. For intergranular corrosion, it is mainly to control harmful impurities in raw materials, especially lead <0.003%. Pay attention to impurity elements from waste.
3. For water mark and cold partition mark: it can increase mold temperature, increase inner gate speed, or increase overflow groove in cold partition area to reduce the appearance of cold partition mark.
4. For hot crack: do not change the thickness of die casting sharply to reduce stress; adjust the relevant die casting process parameters; reduce the mold temperature.
