Defect analysis and precautions of zinc alloy
Surface treatment of zinc alloy
Precautions of zinc alloy
1. Poor corrosion resistance.
When the impurity elements of lead, cadmium, and tin in the alloy composition exceed the standard. It leads to aging and deformation of the casting, which is manifested as volume expansion. The mechanical properties, especially the plasticity, decrease significantly, and even break down after a long time.
Lead, tin, and cadmium have very little solubility in zinc alloys. Therefore, it concentrates on the crystal grain boundary and becomes the cathode. The aluminum-rich solid solution becomes the anode, which promotes intergranular electrochemical corrosion in the presence of water vapor (electrolyte). Die castings are aged due to intergranular corrosion.
2. Aging effect
The structure of zinc alloy is mainly composed of a zinc-rich solid solution containing Al and Cu and an Al-rich solid solution containing Zn, and their solubility decreases with the decrease of temperature.
However, the solidification rate of die castings is extremely fast. Therefore, at room temperature, the solubility of the solid solution is greatly saturated. After a certain period of time, this kind of oversaturation will gradually resolve. The shape and size of the casting are slightly changed.
3. Temperature
Zinc alloy die castings should not be used in high and low temperature (below 0℃) working environment. Zinc alloy has better mechanical properties at room temperature. However, the tensile strength at high temperature and the impact performance at low temperature are significantly reduced.
4. Die casting parts of zinc alloy
Zinc alloy die castings due to insufficient clamping force. Problems such as poor mold clamping, insufficient mold strength, and too high melting temperature will cause burrs on the surface.
This phenomenon is called product blasting, which is often a post-processing process that enterprises must face.
Zinc alloy defect analysis
Various decorative aspects. Such as furniture accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc., are widely used in zinc alloy die castings.
This also requires the surface quality of its castings to be high, and it needs to have good surface treatment performance. The most common defect of zinc alloy die castings is surface blistering.
Defect characterization: There are small bumps on the surface of die casting. The main manifestations are: found out after die-casting, exposed after polishing or processing, and after oil spraying or electroplating.
Causes of defects:
Pores are caused by: mainly the pores and contraction mechanism. The pores are often round, and the contraction is mostly irregular.
(1) Causes of stomata:
a. In the process of filling and solidification of molten metal, due to gas intrusion, pores are formed on the surface or inside of the casting.
b. Gas intrusion from paint volatilization.
c. The gas content of the alloy liquid is too high and will precipitate during solidification. When the gas in the cavity, the gas volatilized by the paint, the gas precipitated by the solidification of the alloy,
When the mold is poorly vented, it will eventually remain in the pores formed in the casting.
(2) Causes of shrinkage:
a. In the process of molten metal solidification, shrinkage cavities occur due to the reduction in volume or the failure of the final solidified part to be fed by the molten metal.
b. Castings with uneven thickness or local overheating of castings cause a certain part to solidify slowly, and the surface will form a recess when the volume shrinks.
Due to the existence of pores and shrinkage holes, the holes may enter water during the surface treatment of the die-casting parts, and when baking is performed after painting and electroplating, the gas in the holes is heated and expanded.
Or the water in the hole will become steam and expand in volume, which will cause blistering on the surface of the casting.
Intergranular corrosion
Intergranular corrosion causes:
Harmful impurities in the composition of zinc alloys: lead, cadmium, and tin will accumulate at the grain boundaries and cause intergranular corrosion.
The metal matrix is broken due to intergranular corrosion, and electroplating accelerates this scourge. The parts subject to intergranular corrosion will expand and lift up the coating, causing blistering on the surface of the casting.
Especially in a humid environment, intergranular corrosion will cause the casting to deform, crack, or even break.
Crack caused: water pattern, cold barrier pattern, hot crack
Water pattern, cold barrier pattern: During the filling process of the molten metal, the molten metal that enters first contacts the molding wall and solidifies prematurely. After entering the molten metal, it cannot be fused with the solidified metal layer.
Moirés are formed at the butt joints on the surface of the casting, and strip defects appear. The water mark is generally in the shallow layer on the surface of the casting; while the cold barrier may penetrate into the inside of the casting.
Thermal cracking:
a. When the thickness of the casting is uneven, stress will be generated during the solidification process;
b. Premature ejection, insufficient metal strength;
c. Uneven force during ejection
d. Too high mold temperature makes the crystal grains coarse;
e. The presence of harmful impurities. The above factors may cause cracks. When the die casting has water marks, cold barrier marks, and hot cracks, it will melt during electroplating.
The liquid will penetrate into the cracks and be converted into vapor during baking, and the pressure will lift the electroplated layer to form blisters.
Solution to defects
The key to controlling the generation of pores is to reduce the amount of gas mixed into the casting. The ideal metal flow should continue to accelerate from the nozzle into the cavity through the split cone and runner to form a smooth and consistent metal flow.
It adopts a tapered runner design. That is, the pouring flow should be gradually reduced from the nozzle to the gate at an accelerating rate, which can achieve this goal. In the filling system, the mixed gas is due to the turbulence mixing 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 transition point and the increasing cross-sectional area of the runner are seen. Will cause turbulence and entrainment in the molten metal flow.
The stable molten metal is conducive to the gas entering the overflow groove and exhaust groove from the runner and cavity, and then exiting the mold.
For shrinkage cavity
make all parts of the die-casting solidification process at the same time as possible to uniformly dissipate heat and solidify at the same time. Reasonable nozzle design, inner gate thickness and position, mold design, mold temperature control and cooling can be used to avoid shrinkage.
For the phenomenon of intergranular corrosion
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 lines and cold partition lines
increase the mold temperature, increase the speed of the inner gate, or increase the overflow groove in the cold partition area to reduce the appearance of cold partition lines.
For hot cracks
do not change the thickness of the die-casting parts sharply to reduce stress; adjust the relevant die-casting process parameters; reduce the mold temperature.
Electroplating and stripping of zinc alloy
Zinc-aluminum die-casting is a kind of die-casting part with zinc as the main component. The surface of this kind of part has a very dense surface layer, and the inside is an open porous structure and a lively amphoteric metal.
Therefore, only by adopting appropriate pretreatment methods and electroplating processes can the electroplated layer on the zinc alloy have good adhesion and meet the requirements of qualified products.
The zinc alloy materials commonly used for electroplating are mainly composed of aluminum 3.5% to 4.5%, copper 0.75% to 1.25%, magnesium 0.03% to 0.08%, the balance is zinc, and the sum of impurities is ≤0.2%. The zinc alloy of 925 grade contains high copper content and is easy to be electroplated.
Generally, the density of zinc alloy is 6.4～6.5g/cm3. If the density is less than 6.4g/cm3, blistering and pitting are likely to occur after electroplating. In short, we must strictly control the selection of materials. In addition, the mold must be designed reasonably during die-casting to avoid insurmountable defects (such as pitting) caused by electroplating.
