Analysis of the Common Defects in Zinc Alloy Die Castings
Zinc alloy die castings are widely used in various decorative fields, such as furniture accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, bags, various metal decorative buckles, etc. Therefore, the surface quality of the castings is required to be high, and good surface treatment performance is required. The most common defect of zinc alloy die castings is surface blistering.
Defect characterization: There are protuberant bubbles on the surface of die castings, which are found after die casting, exposed after polishing or processing, and appearing after oil spraying or electroplating.
Causes:
1. Cavitation: mainly stomata and contraction mechanism, stomata are often round, and contraction is mostly irregular.
(1) Causes of pore formation: In the process of filling and solidification of liquid metal a, the gas intrusion leads to the formation of holes on the surface or inside of castings. B. Invasion of vaporized gas from paint. C alloy liquid contains too much gas and precipitates during solidification.
When the gas in the cavity, the volatile gas from the coating and the gas from the alloy solidification are not exhausted properly in the die, the air holes formed in the castings will be left at last.
(2) Causes of shrinkage: during the solidification of liquid metal, shrinkage occurs because of shrinkage of volume or lack of liquid metal feeding at the final solidification part. B. Local overheating of uneven thickness castings or castings results in slow solidification at a certain part and concave surface formation during volume shrinkage. Because of the existence of air holes and shrinkage holes, the holes may enter into water during surface treatment of die castings. When spraying paint and baking after electroplating, the gas in the holes will expand by heat; or the water in the holes will change into vapor and volume expansion, which will cause foaming on the surface of castings.
2. Intergranular corrosion causes:
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 by intergranular corrosion. Electroplating accelerates this disadvantage. The parts affected by intergranular corrosion will expand and the coating will be jacked up, resulting in foaming on the surface of castings. Especially in wet environment, intergranular corrosion will cause casting deformation, cracking, and even breakage.
3. Cracks: water marks, cold isolation marks and hot cracks.
Watermark and cold barrier: In the filling process of liquid metal, the contact wall of liquid metal first solidified too early, and then the liquid metal can not be fused with the solidified metal layer, forming overlapping lines at the butt of the casting surface, resulting in strip defects. Water marks are usually on the shallow surface of castings, while cold barrier marks may penetrate into the inner part of castings.
Hot crack: a when the thickness of the casting is uneven, the solidification process produces stress; B is premature ejection, metal strength is not enough; C is uneven ejection, D is too high die temperature to make the grain coarser; e is harmful impurities.
All the above factors may cause cracks.
When there are water marks, cold barrier lines and hot cracks in die castings, the solution will infiltrate into the cracks during electroplating, and turn into steam when baking, and the plating layer will be bubbled by air pressure.
Solutions to defects:
The key to control the production of gas holes is to reduce the amount of gas mixed into the casting. The ideal metal flow should be accelerated from the nozzle through the diversion cone and runner into the cavity to form a smooth and uniform metal flow. The conical runner design means that the flow should be gradually reduced from the nozzle to the inner runner, which can achieve this goal. In the filling system, the gas mixing is due to the mixing of turbulence and liquid metal to form pore. From the study of the simulation die casting process of liquid metal entering the cavity from the casting system, it is obvious that the sharp transition position and increasing cross-section area of the runner will cause turbulence and entrainment of the liquid metal flow. Only a smooth liquid metal can facilitate the gas entering the overflow slot from the runner and the cavity. And exhaust groove, discharge out of the die.
For shrinkage: to make all parts of the die casting solidification process as uniform as possible heat dissipation, and solidification at the same time. Shrinkage can be avoided by reasonable nozzle design, thickness and location of inner nozzle, die design, temperature control and cooling. For the phenomenon of intergranular corrosion, it is mainly to control the content of harmful impurities in alloy raw materials, especially lead < 0.003%. Pay attention to the impurities brought by waste.
For water marks and cold partition marks, the appearance of cold partition marks can be reduced by increasing the temperature of the die, increasing the speed of the inner gate, or increasing the overflow groove in the cold partition area.
For hot cracks: the thickness of die castings should not change sharply to reduce stress generation; the relevant die casting process parameters should be adjusted; and the die temperature should be lowered.
