Analysis of Common Defects of Zinc Alloy Die Castings
Zinc alloy die-casting parts are widely used in various decorative aspects, such as furniture fittings, architectural decoration, bathroom fittings, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc., so the surface quality requirements of castings are high. At the same time, good surface treatment properties are required.
Characterization of defects: There are protruding vesicles on the surface of die-casting parts, which are found after die-casting, exposed after polishing or processing, appearing after injection or plating. cause:
1. Hole causes: mainly pores and shrinkage mechanisms, the pores are often round, and the shrinkage is mostly irregular.
(1) Causes of pores: a. During the filling and solidification of the molten metal, holes are formed on the surface or inside of the casting due to gas intrusion. b The gas volatilized by the paint invades. c alloy liquid contains too much gas and precipitates during solidification.
When the gas in the cavity, the gas volatilized by the paint, and the gas which is solidified and precipitated by the alloy, when the mold is poorly exhausted, the pores formed in the casting are finally left.
(2) Causes of shrinkage cavities: a During the solidification process of the molten metal, shrinkage holes are generated due to shrinkage of the volume or the fact that the molten metal is not replenished at the final solidified portion. b The casting or casting with uneven thickness is locally overheated, causing a certain part to solidify slowly, and the surface is concave when the volume is contracted. Due to the existence of pores and shrinkage cavities, the holes may enter the water during the surface treatment of the die-casting part. When the baking is performed after painting and electroplating, the gas in the holes is thermally expanded; or the water in the holes may become vapor and expand in volume. This causes the surface of the casting to foam.
2. Intergranular corrosion caused by:
Harmful impurities in zinc alloy composition: lead, cadmium, and tin will accumulate at the grain boundary to cause intergranular corrosion, and the metal matrix will be broken by intergranular corrosion, and electroplating accelerates this scourge, and the site affected by intergranular corrosion will expand. The coating is lifted up, causing the surface of the casting to foam. Especially in the humid environment, intergranular corrosion will deform, crack and even break the casting.
3. Cracks caused by: water marks, cold lines, hot cracks.
Water pattern, cold pattern: During the filling process, the metal liquid contact wall that enters firstly solidifies prematurely, and then the metal liquid cannot be fused with the solidified metal layer, forming a fold on the surface of the casting. Strip defects appear. Water marks are generally shallow on the surface of the casting; cold welts may penetrate into the interior of the casting.
Hot crack: a when the thickness of the casting is uneven, the stress is generated during the solidification process; b is prematurely ejected, the strength of the metal is insufficient; the temperature at which the force is uneven when the c is ejected is too high, so that the grain is coarse;
All of the above factors may cause cracks.
When the die casting has water marks, cold lines and hot cracks, the solution will penetrate into the crack during electroplating, be converted into steam during baking, and the air pressure will push up the plating layer to form foam.
Solve the defect plan:
The key to controlling the generation of pores is to reduce the amount of gas mixed into the casting. The ideal metal flow should be accelerated from the nozzle through the splitter cone and runner into the cavity to form a smooth and uniform flow of metal, using a tapered flow path. The design, that is, the pouring flow should be continuously accelerated from the nozzle to the inward gate, which can be achieved. In the filling system, the gas mixed in is formed by the mixing of turbulent flow and metal liquid phase. From the study of the simulated die-casting process in which the molten metal enters the cavity from the casting system, it is obvious that the sharp transition position and the incremental pouring in the runner are obvious. The cross-sectional area of ​​the channel causes the turbulent flow of the molten metal to be ventilated, and the smooth molten metal facilitates the flow of gas from the runner and the cavity into the overflow and exhaust vents and out of the mold.
For shrinkage cavities: all parts in the die casting solidification process should be uniformly dissipated as much as possible while solidifying. The shrinkage can be avoided by reasonable nozzle design, gate thickness and position, mold design, mold temperature control and cooling. For the intergranular corrosion phenomenon: mainly to control the content of harmful impurities in the alloy raw materials, especially lead <0.003%. Pay attention to the impurity elements brought by the waste.
For water ripples and cold insulation, the mold temperature can be increased, the gate speed can be increased, or the overflow tank can be increased in the cold partition to reduce the occurrence of cold insulation.
For hot cracks: the thickness of the die-casting part should not be changed sharply to reduce the stress generation; the relevant die-casting process parameters are adjusted; the mold temperature is lowered.
Concerned about surprises
Label: zinc alloy die casting
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