Reasons and Solutions for Unqualified Zinc Alloy Die Castings
Causes of blistering in zinc alloy die castings:
1. Caused by holes: mainly due to the stomata and shrinkage mechanism, the stomata are often circular, while the shrinkage is mostly irregular. (1) Causes of pores: During the filling and solidification process of a molten metal, holes are generated on the surface or inside of the casting due to the intrusion of gas. B attack of the gases volatile from the paint. c The gas content of the alloy liquid is too high, and the coagulation precipitates. The gas in the cavity, the volatile gas of the paint, the alloy solidifies and precipitates, and when the mold exhaust is not good, it will eventually remain in the formation of the casting pores. (2) Reasons for shrinkage: During the solidification process of liquid metal, due to volume reduction or no liquid metal material at the final solidified part, and shrinkage holes. B castings with uneven thickness or local overheating of the castings, resulting in slow solidification sites, and a depression on the surface when the volume shrinks. Due to the existence of pores and shrinkage holes, the holes can enter water when the die-casting parts are surface-treated. When baking after painting and electroplating, the gas in the holes expands due to heat; or the water in the holes turns into water vapor and expands in volume, thereby Causes blistering on the casting surface.
2. Harmful impurities in the intergranular corrosion of zinc alloy composition: lead, cadmium, tin will gather at the grain boundary to cause intergranular corrosion, intergranular corrosion and broken metal matrix and coating, accelerate the disaster, the intergranular corrosion in the area will expand Covering the top, causing blistering on the surface of the casting. Especially in a humid environment, intergranular corrosion can deform, crack, and even shatter castings.
3. Reasons for cracks: water, cold separation, hot cracks. 1) Water, cold grain separation: During the filling process, the molten metal enters the first contact wall and solidifies prematurely, and then the solidification of the liquid metal and the metal can be integrated into one, and the moiré in the form of joints on the surface of the casting , a stripe defect occurs. The water is generally shallow on the surface of the casting; and the cold barrier may penetrate into the interior of the casting. 2) Hot cracks: When the wall thickness of the casting is uneven, the stress produces a solidification process; B is pushed out prematurely, and the metal strength is not enough; C, the mold temperature with uneven pushing force is high, and the grains are coarse; E The existence of harmful impurities. These parameters may be cracked. When there are water lines, cold separation lines, and hot cracks in the die casting, the plating solution will penetrate into the cracks, which will be transformed into steam baking, and the top-deposited metal will form blisters.
The solution to the defects of zinc alloy die castings:
1. Control of porosity generation: The key is to reduce the amount of mixed gas in the casting. The ideal metal flow should continuously accelerate the nozzle and enter the cavity through the shunt cone and runner to form a smooth metal flow direction. The conical runner design is adopted. That is, the pouring flow should continue to accelerate the nozzle to the gate and gradually reduce, which can achieve this purpose. In the filling system, the gas mixture is formed by turbulent flow and the liquid metal phase to form pores, and the gating system enters the cavity to simulate the liquid metal during the die casting process. Obviously, greatly increasing the transformation of the runner gate can make the metal liquid flow turbulent. While entraining gas, the liquid metal is smooth, which is beneficial for the gas to enter the overflow groove and the exhaust groove from the gate and cavity, and be discharged out of the mold.
2. Shrinkage: Make all parts of the zinc alloy die casting casting and solidification process as far as possible while dissipating heat evenly and solidifying at the same time. Through reasonable nozzle design, mold design, gate thickness and location, mold temperature control and cooling to avoid shrinkage. For intergranular corrosion phenomenon: the main impurity content controls harmful alloy materials, especially lead <0.003%. Pay attention to waste and impurity elements.
3. Water, cold partition: It can increase the mold temperature, increase the gate speed, or increase the overflow groove in the cold partition area to reduce the cold appearance and close the grain.
4. Hot cracks: The casting thickness of zinc alloy die castings does not change rapidly, reducing the generation of stress; die casting process parameters and adjustment; reduce the mold temperature.
