Q&A
Basic knowledge of zinc alloy die casting
Zinc alloy die-casting parts are widely used in various decorative aspects, such as furniture fittings, building decoration, bathroom fittings, lighting parts, toys, tie clips, belt buckles, all kinds of metal decorative buckles, etc., so the casting surface quality is required to be high, and good surface treatment performance is required. The most common defect of zinc alloy die casting is surface blistering.
Defect Characteristics:
(1) Surface blisters on the surface of the die casting.
(2) Found on the surface of the die casting.
(3) Appears after polishing or machining.
(4) Appears after oil spraying or plating.
Causes
1. Hole causes: mainly air holes and poor shrinkage of these two causes cause holes, air holes are often round, and poor shrinkage is mostly irregular shape.
(1) Pneumatic hole causes:
a metal liquid in the filling, solidification process, due to gas intrusion, resulting in the casting surface or the internal production of holes.
b. Gas intrusion from the volatilization of paint.
c Alloy liquid gas content is too high, solidification precipitation.
d When the gas in the cavity, the gas volatilized by the paint, and the gas precipitated from the solidification of the alloy, in the case of poor mold venting, and eventually left in the casting to form the air holes.
(2) Shrinkage hole causes:
a metal liquid solidification process, due to the volume shrinkage or the last solidified parts do not get the metal liquid complementary shrinkage, and the production of shrinkage holes.
b uneven thickness of the casting or casting local overheating, resulting in a part of the solidification is slow, volume contraction surface to form a concave.
Due to the existence of air holes and shrinkage holes, the holes may enter into the water when the die casting is being processed on the surface. When baking after painting and plating, the gas in the holes will be heated and expanded; or the water in the holes will turn into vapor and the volume will expand, which will lead to blisters on the surface of the casting.
2. Intercrystalline corrosion:
Harmful impurities in the composition of zinc alloy: lead, cadmium, tin will be gathered at the junction of the grain to cause intergranular corrosion, the metal substrate is broken due to intergranular corrosion, and plating accelerates the damage, the parts subject to intergranular corrosion will be expanded and the top of the plating layer, resulting in blistering of the casting surface. Especially in the humid environment, the inter-crystalline corrosion will make the casting deformed, cracked, and even broken.
3. Cracks caused by: water cracks, cold cracks, hot cracks.
Water cracks, cold segregation: metal liquid in the filling process, the first into the metal liquid contact wall solidified too early, after entering the metal liquid can not be fused with the solidified metal layer into one, the casting surface at the formation of overlap, appearing stripes defects. Water lines are generally shallow in the casting surface; and cold segregation lines may penetrate into the casting.
Thermal cracks:
a When the casting thickness is not uniform, solidification process to produce stress;
b Premature ejection, metal strength is not enough;
c Uneven force during ejection
d Coarse grain size due to high mold temperature;
e Presence of harmful impurities.
All of the above factors may cause cracks.
When there are water cracks, cold segregation cracks and hot cracks in the die casting, the solution will penetrate into the cracks when plating, and then turn into vapor when baking, and the air pressure will top up the plating layer to form blisters.
Solutions to defects:
Control the production of porosity, the key is to reduce the amount of gas mixed into the casting, the ideal metal flow should be continuously accelerated from the nozzle through the manifold cone and gate into the cavity, forming a smooth and consistent direction of the metal flow, the use of conical runner design, that is, the flow should be accelerated from the nozzle to the inner gate is gradually reduced, to achieve this purpose. In the filling system, the mixed gas is due to turbulence and metal liquid phase mixing and the formation of air holes, from the metal liquid from the casting system into the cavity of the analogous die-casting process study, it is obvious that the sharp change in the runner and incremental cross-sectional area of the runner will make the metal liquid out of the present turbulence and roll gas, smooth metal liquid is conducive to the gas from the runner and the cavity into the overflow tanks and venting grooves, discharged out of the mold.
For shrinkage holes: To make all parts of the die casting solidification process as much as possible at the same time uniform heat dissipation and solidification at the same time. Shrinkage holes can be avoided through reasonable spout design, inner gate thickness and location, mold design, mold temperature control and cooling.
For intergranular corrosion: The main concern is 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 materials.
For water and cold segregation patterns, increase the mold temperature, increase the inner gate speed, or increase the overflow channel in the cold segregation area to reduce the appearance of cold segregation patterns.
For thermal cracks: Do not change the thickness of die casting parts drastically to reduce the stress; adjust the relevant die casting process parameters; reduce the mold temperature.
