Analysis of Air Bubble in Zinc Alloy Die Castings

A major and frequent defect (blistering) of zinc alloy die castings has plagued many die casting practitioners and die casting processing enterprises. At the same time, wrangling between raw material suppliers and die-casting companies, polishing and electroplating companies, and small company departments sometimes occurs.
Zinc alloy die castings are currently widely used in various decorations, such as furniture accessories, architectural decorations, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc., so the surface quality of die castings is relatively high. , At the same time requires good surface treatment performance. The most common defect of zinc alloy die castings is surface blistering.

1. Defect characterization

There are small protruding bubbles on the surface of die-casting parts, which are found after die-casting, are exposed after polishing or processing, and appear after oil spraying or electroplating.

2. Reasons

1. Caused by pores: mainly caused by pores and contraction mechanism. The pores are often round, while 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. Intrusion of gas from paint.

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 from the coating, and the gas precipitated by the solidification of the alloy, when the mold row is defective, the pores formed in the casting are finally left.
(2) Causes of shrinkage:

A. In the process of molten metal solidification, shrinkage cavities occur due to the shrinkage of the product 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 too slowly, and the surface will form a recess when the volume shrinks. Due to the existence of pores and shrinkage holes, the castings may be exposed to water during surface treatment. When baking is suitable after painting and electroplating, the gas in the pores will expand by heat or the water in the pores will become steam and volume expansion. Cause blistering on the surface of the casting.

2. Caused by intergranular corrosion:

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, and the parts affected by intergranular corrosion will expand. The coating is lifted up, 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.

3. Cracks cause: water pattern, cold barrier pattern, hot crack.
(1) Water pattern and cold partition pattern:

During the filling process of the molten metal, the molten metal that enters first contacts the mold wall and solidifies prematurely, and then the molten metal cannot be melted into 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.
(2) Thermal cracking:
A. When the thickness of the casting is uneven, stress will be generated during the solidification process.

B. The strength of the metal ejected prematurely is not enough.

C. Uneven ejection force.
D. Excessive mold temperature makes the crystal intergranular coarse.

E. The presence of harmful impurities.

The above factors may cause cracks.
When there are water marks, cold barrier marks, and hot cracks in the casting, the solution will penetrate into the cracks during electroplating, and it will be converted into steam during grilling, and the air pressure will lift the electroplating layer to bubble.
4. Improper mold and product design:

Pouring speed, pouring direction, cooling system, profile wall changes, etc.!

5. Caused by improper post-processing technology:
(1) Polishing:

For example, the intensity of polishing, some are roller burnish (the stone material and specification used, “granite ceramic”), vibration light (the specification of the abrasive material used, the amplitude of vibration, the frequency of vibration).
(2) Polishing:

The specification and model of the polishing wheel, and the strength during polishing (too large force will destroy the hard layer, and too light will not achieve the polishing effect).
(3) Degreasing and cleaning:

Because some auxiliary materials (wax, vibrating liquid, etc.) will remain on the surface of the die-casting part when polishing or polishing, so to clean, generally use lye, acid and electrolytic degreasing. Zinc alloy die castings, whose main components are zinc and aluminum, will produce segregation during die casting. Some parts of the product will be rich in zinc or aluminum. Alkali can make aluminum and acid can make zinc preferentially dissolve. Pinholes will be formed on the surface of the product and the solution will remain in between, which will easily produce bubbles after electroplating. Therefore, the cleaning and degreasing of zinc alloy die castings is very important!

(4) Process and storage:

A. After die-casting, it could not be preserved! And caused by water absorption and oxidation.

B. The surface of the product is oxidized if there is no electroplating treatment in time after polishing.

C. After the cleaning and degreasing are completed, there is no timely electroplating, which causes the surface of the product to corrode.

D. After the electroplating is completed, it is not saved as required!

3. Solutions to defects

1. For stomata:

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 through the split cone and runner into the cavity, forming a smooth and consistent metal flow, using a tapered runner The 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 forms pores due to the turbulent flow and the liquid phase of the metal. From the study of the simulation process of the molten metal entering the cavity from the casting system, it is obvious that there are sharp transitions and increasing pouring in the runner. The cross-sectional area of ​​the channel will cause the molten metal flow to appear turbulent and entrain gas, and the stable molten metal is conducive to the gas entering the overflow groove and exhaust chute from the runner and cavity, and exiting the mold.

2. For shrinkage:

It is necessary to make all parts of the die-casting solidification process uniformly dissipate heat at the same time 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 must be less than 0.003%. Pay attention to the impurity elements brought by waste (added by classification ratio).
3. For water pattern and cold partition pattern:

The mold temperature can be increased, the internal gate speed can be increased, or the overflow groove (slag bag) can be increased in the cold compartment area to reduce the appearance of cold barrier lines.
4. For thermal cracks:

The thickness of the die-casting parts should not be changed sharply to reduce the stress; the relevant die-casting process parameters should be adjusted; the mold temperature and the material temperature should be reduced.

5. Carefully check the mold and product, review the optimization of the mold and product plan!

6. The adhesive particle size used for polishing is about 0.045-0.069.

The roller burnishing should be granite ceramics, and it should be matched with roller burnishing liquid, cleaning agent and wax removal water. Choose different specifications of granite ceramics according to product requirements, and the ratio of rolling stone to product; 2.5:1.0. And the roller speed should not be too high, the time is 1.5-2.0H.

Polishing is selected according to product requirements. Generally, cloth wheel grease is used for rough polishing, and then white grease is used for fine polishing. The circumferential speed of the cloth wheel should not be too fast, and the smaller product has a relatively slow speed, and the hard surface layer of the product should not be damaged! After finishing polishing, add white powder and place it well and separate it with soft objects! It must be electroplated in time!

7. It can often be cleaned before degreasing, generally with organic solvents, surfactants, or degreasing liquid for preliminary cleaning. The degreasing of zinc alloy die castings is generally carried out with a combination of alkaline chemical solvents and electric degreasing oil! Etching and activation are necessary after completion (in order to improve the adhesion of electroplating in the subsequent process), but including electroplating! Every process must ensure the purity and agricultural degree of the potion and solution, and the cleanliness of the tank! From time to time, thoroughly clean the cleaning tank and the etching activation tank. This is the top priority! It is also the key to determining the effect of electroplating!

8. The product should be electroplated within 12H after polishing, degreasing and cleaning. The electroplated products should be packaged and placed in a dry, ventilated and cool place, and should not be exposed to the sun.