Alloy Button Surface Defects Symptoms: Protuberant vesicles on the surface of alloy buttons. The appearance of blisters may have occurred just after the blank has been cast, or it may have occurred after the polishing process, and it may be found after injection or electroplating. The reason we can analyze from the following aspects:
1. Caused by holes: Mainly pores and contraction mechanisms, pores are often round, and most shrinkage is irregular.
(1) Causes of stomata:
a During the filling and solidification process of the molten metal, due to gas intrusion, holes are created on the surface or inside of the casting.
b The gas invaded by the coating invades.
c The alloy contains too much gas and precipitates when solidified.
When the gas in the cavity, the gas that the coating volatilizes, and the gas that is solidified by the alloy, when the mold is exhausted poorly, the pores that remain in the casting are eventually formed.
(2) Causes of shrinkage:
a During the solidification of the molten metal, shrinkage occurs due to the shrinkage of the volume or the lack of liquid metal in the final solidified part.
b. Partial overheating of castings or castings with uneven thickness results in slower solidification in one part and recesses in the surface when volume shrinks.
Due to the existence of pores and shrinkage holes, the holes may enter water when the die castings are subjected to surface treatment. When baking is performed after painting and plating, the gas in the pores expands by heat; or the water in the pores may become vapor and expand in volume. As a result, the casting surface blisters.
2. Intergranular corrosion causes:
Hazardous impurities in the zinc alloy composition: lead, cadmium, tin will accumulate at the junction of the crystal grains to cause intergranular corrosion, and the metal matrix will be broken due to intergranular corrosion. Electroplating accelerates this scourge, and the parts subject to intergranular corrosion will expand. The plating is lifted up, causing the surface of the casting to foam. In particular, intergranular corrosion in a humid environment can cause the casting to deform, crack, and even break.
3. Cracks: water lines, cold cuts, hot cracks.
Water pattern and cold offset pattern: In the filling process, the metal liquid firstly enters the molten metal contact wall to solidify prematurely, and then enters the molten metal and can not be fused with the solidified metal layer, forming a moiré pattern at the butt joint surface of the casting. The appearance of stripe-like defects is generally shallow on the surface of the casting; cold embossing may penetrate into the interior of the casting.
Hot cracks:
a When the casting thickness is uneven, the solidification process generates stress;
b premature ejection, the metal strength is not enough;
c Forced unevenness when ejecting
d Excessive mold temperature makes crystal grains coarse;
e The presence of harmful impurities.
The above factors may have cracks.
When the die castings have water lines, cold cut lines, and hot cracks, the solution will infiltrate into the cracks during electroplating, convert to steam during baking, and press the electroplated layer to form bubbles.
Of course, we have to solve the problem. What can I do to prevent bubbles from forming after the alloy button is finished?
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 the runner into the cavity to form a smooth and consistent metal flow, using a tapered flow channel. The design, that is, the gushing should be gradually accelerated from the nozzle to the inward gate gradually reduced, can achieve this purpose. In the filling system, the mixed gas is formed by mixing the turbulent flow with the metal liquid phase to form pores. 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 transitional points and the incremental pouring in the runner are evident. The cross-sectional area will cause turbulence and gas flow in the metal liquid flow. The smooth metal liquid will help the gas to enter the overflow trough and exhaust trough from the runner and cavity, and exit the die.
For shrinkage cavity: It is necessary to make uniform heat dissipation at the same time in the die casting solidification process, and at the same time solidify. Through the reasonable nozzle design, thickness and position of the gate, mold design, mold temperature control and cooling, to avoid shrinkage.
For the phenomenon of intergranular corrosion: The main content is to control the content of harmful impurities in the alloy raw materials, especially lead <0.003%. Pay attention to the impurity elements brought about by the waste.
For water patterns and cold streaks, the mold temperature can be increased, the speed of the inner gate can be increased, or the overflow tank can be enlarged in the cold partition to reduce the occurrence of cold streaks.
For hot cracks: Do not change the thickness of the die casting abruptly to reduce stress generation; adjust the relevant die casting process parameters; lower the die temperature.
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