Zinc alloy has a large specific gravity of 2: good castability, and can die-cast precision parts with complex shapes and thin walls, and the casting surface is smooth.
3. Surface treatment can be carried out: electroplating, spraying and painting.
4: No iron absorption, no corrosion, no die sticking in melt die casting.
5: It has good mechanical properties and wear resistance at room temperature.
6: Low melting point, melting at 385℃, easy to die-cast.
Problems needing attention in use: 1: poor corrosion resistance. When the impurity elements such as lead, cadmium and tin in the alloy composition exceed the standard, the casting will undergo aging deformation, which shows volume expansion, mechanical properties, especially plasticity, decrease significantly, and even break after a long time.
Lead, tin and cadmium have little solubility in zinc alloys, so they are enriched on grain boundaries to become cathodes, and aluminum-rich solid solutions become anodes, which promote intergranular electrochemical corrosion in the presence of water vapor (electrolyte).
Die castings are aged due to intergranular corrosion.
Zinc alloy 2: The microstructure of aging zinc alloy is mainly composed of zinc-rich solid solution containing Al and Cu and Al-rich solid solution containing Zn, and their solubility decreases with the decrease of temperature.
However, due to the extremely fast solidification speed of die casting, the solubility of solid solution is greatly saturated at room temperature.
After a certain period of time, this supersaturated state will gradually ease, and the shape and size of the casting will change slightly.
3. The temperature affects the zinc alloy die casting, and it is not suitable to be used in the working environment of high temperature and low temperature (below 0℃).
Zinc alloy has good mechanical properties at room temperature.
However, the high-temperature tensile strength and low-temperature impact properties decreased significantly.
Material type zinc alloy Zamak3: good fluidity and mechanical properties.
Suitable for castings with low requirements on mechanical strength, such as toys, lamps, decorations and some electrical equipment.
Zamak5: Good fluidity and mechanical properties.
Suitable for castings with certain requirements on mechanical strength, such as automobile parts, electromechanical parts, mechanical parts, electrical components, etc.
Zamak2: Used for mechanical parts with special requirements on mechanical properties, high hardness and general requirements on dimensional accuracy.
ZA8: Good fluidity and dimensional stability, but poor fluidity.
Suitable for die casting small-sized, high-precision and high-mechanical strength workpieces, such as electrical appliances.
Superloy: It has the best fluidity and is suitable for die-casting thin-walled, large-sized, high-precision and complex-shaped workpieces, such as electrical components and their boxes.
Different zinc alloys have different physical and mechanical properties, which provides a choice space for die casting.
Superplastic zinc alloy
Superplastic zinc alloy.
Its typical representative is Zn-22Al's * * * precipitation alloy, which has high elongation (1000%), low flow stress, high strain rate sensitivity index (m=0.5) and good microstructure machinability. The room temperature strength can be improved by strengthening treatment and adjusting microstructure.
The superplastic deformation temperature of the alloy is not high (250℃), which is convenient for production.
But its comprehensive performance at room temperature is not ideal, such as poor creep resistance and corrosion resistance. In order to improve its comprehensive properties, a small amount of alloying elements such as Cu and Mg can be added.
Commonly used zinc-based superplastic alloys are Zn-22Al-0.2Cu, Zn-4Al and Zn-5Al*** crystal alloys.
They can be used to produce handicrafts, plastic molds, knobs and rubber products molds.
The material selection of zinc alloy is mainly considered from three aspects: (1) the use of die casting itself and the performance requirements to be met.
1: Mechanical properties and tensile strength are the maximum resistance when the material breaks; Elongation is a measure of brittleness and plasticity of materials; Hardness is the ability of material surface to resist plastic deformation caused by hard object extrusion or friction.
2. Working environment status: working temperature, humidity, medium contacted by workpiece and air tightness requirements.
3. Accuracy requirements: achievable accuracy and dimensional stability.
(2) Good process performance: 1: casting process.
2. Mechanical manufacturing capacity.
3. The technicality of surface treatment.
(three) the cost of raw materials with good economy and the requirements for production equipment (including smelting equipment, die casting machines, molds, etc.). ), and the production cost.
The role of elements Among the components of zinc alloy, the effective alloying elements are aluminum, copper and magnesium; Harmful impurity elements: lead, cadmium, tin and iron.
(1) The function of aluminum ① improves the castability of the alloy, increases the fluidity of the alloy, refines the grain, causes solid solution strengthening, and improves the mechanical properties.
② Reduce the reaction ability of zinc to iron, and reduce the corrosion of iron materials, such as gooseneck, mold, crucible, etc.
The aluminum content is controlled at 3.8-4.3%.
Mainly considering the required strength and fluidity, good fluidity is a necessary condition to obtain a complete, accurate size and smooth surface casting.
The influence of aluminum on fluidity and mechanical properties is shown in Figure 3.
When the aluminum content is 5%, the fluidity reaches the maximum; Drop to the minimum of 3%.
The influence of aluminum on impact strength is shown by the dotted line in Figure 3.
When the aluminum content is 3.5%, the impact strength reaches the maximum; Minimum 6%.
When the aluminum content exceeds 4.3%, the alloy becomes brittle.
The aluminum content is lower than the specified range, which leads to the difficulty of filling thin-walled parts and the possibility of cooling and cracking after casting.
The adverse effect of aluminum in zinc alloy is the formation of Fe2Al3 scum, which leads to the decrease of its content.
(2) copper action 1: increase the hardness and strength of the alloy; 2. Improve the wear resistance of the alloy; 3. Reduce intergranular corrosion.
Disadvantages 1: When the copper content exceeds 1.25%, the size and mechanical strength of die castings will change due to aging; 2. Reduce the ductility of the alloy.
(3) Magnesium can reduce intergranular corrosion, refine the alloy structure, thus improving the strength and wear resistance of the alloy; ① When the magnesium content is 0.08%, the thermal brittleness, toughness and fluidity decrease.
② In the molten state of the alloy, it is easy to be oxidized and lost.
(4) Impurity elements: lead, cadmium and tin make the intergranular corrosion of zinc alloy very sensitive, accelerate the intergranular corrosion of zinc alloy in warm and humid environment, reduce the mechanical properties, and cause the size change of castings.
When the content of lead and cadmium in zinc alloy is too high, the surface quality of the workpiece is normal at the time of die casting, but after being stored at room temperature for a period of time (eight weeks to several months), bubbles appear on the surface.
(5) Impurity elements: Iron ① Iron reacts with aluminum to generate Al5Fe2 intermetallic compound, which causes the loss of aluminum and forms scum.
② Hard spots formed in die casting will affect the later polishing.
③ Increase the brittleness of the alloy.
The solubility of iron in zinc solution increases with the increase of temperature. Every time the temperature of zinc liquid in the furnace changes, iron will be supersaturated (when the temperature drops) or unsaturated (when the temperature rises).
When the iron element is supersaturated, the supersaturated iron will react with aluminum in the alloy, resulting in an increase in the amount of scum.
When iron element is unsaturated, the corrosion of alloy to zinc pot and gooseneck will be enhanced and return to saturation.
The result of these two temperature changes is the final consumption of aluminum and the formation of more scum.
Matters needing attention in the production of zinc alloy 1: the control of alloy composition begins with the purchase of alloy ingots, which must be based on ultra-high purity zinc and added with ultra-high purity aluminum, magnesium and copper. Suppliers have strict composition standards.
High-quality zinc alloy is the guarantee to produce high-quality castings.
2. The purchased alloy ingots should be stored in a clean and dry stacking area to avoid white rust due to long-term moisture, or increase slag generation and metal loss due to factory dirt pollution.
A clean factory environment is very useful for effective control of alloy composition.
3. The ratio of new materials and recycled materials such as nozzle shall not exceed 50%, and general new materials: old materials = 70: 30.
Aluminum and magnesium in continuous remelting gold gradually decrease.
4. When the nozzle material is remelted, the remelting temperature must be strictly controlled not to exceed 430℃ to avoid the loss of aluminum and magnesium.
5. Conditional die casting plants had better use centralized smelting furnace to smelt zinc alloy, so that the ratio of alloy ingot and return charge is uniform, the flux is used more effectively, and the alloy composition and temperature are uniform and stable.
Electroplating waste and fines should be melted separately.
The material is zinc alloy luxury zinc alloy automobile standard 1. This standard adopts zinc alloy chrome-plated material, which will not rust.
2, olive bottom, heavy and steady.
Fine workmanship and first-class quality.
3. Use 3M glue at the bottom.
Usage: 1 Thoroughly clean the place where the logo needs to be pasted and dry it.
2. Remove the 3M glue from the logo base, stick it in a proper position, and press it with a little force for about one minute.
The product characteristics of new zinc alloy security window traditional stainless steel have begun to expose its disadvantages, and its quality commitment has also begun to be questioned, and its channel price system has been in a relatively transparent state. Jinsheng zinc alloy color steel protective window absorbs the advantages of existing doors and windows and becomes a new product with unique functions and its own characteristics, which is the third generation protective window product. It integrates safety and aesthetics, adopts unique welding-free assembly line operation and combined shuttle structure, accepts the baptism of the market and conforms to the needs of the market and consumers.
Security window, zinc alloy color steel, is made of the same material as outdoor facilities such as highway guardrail and high-voltage electric tower.
High strength, beautiful appearance, bright color, high quality and low price are the best substitutes for stainless steel protective nets.
classify
Zinc alloy, also known as zinc-based alloy, is generally divided into binary alloy, ternary alloy and multicomponent alloy.
Binary zinc-based alloys generally refer to zinc-aluminum alloys; Ternary zinc-based alloys generally refer to zinc-aluminum-copper alloys; Multicomponent alloys generally refer to trace metals such as zinc, aluminum and copper.
Zinc-based alloy, zinc alloy and zinc-aluminum alloy are all broad concepts, which does not mean that alloys can meet certain functions.
For example, zinc-aluminum alloys are divided into low aluminum-zinc-based alloys, aluminum-zinc-based alloys and high aluminum-zinc-based alloys according to aluminum content.
Although they are all zinc-aluminum alloys, their properties are quite different.
Low aluminum zinc-based alloys are generally binary alloys, which are mainly used for corrosion protection. Now spraying galvanized aluminum alloy has basically replaced the galvanizing process (new technology).
Al-Zn-based alloys are generally ternary alloys, which are mainly used for fastening functions and are often used in fasteners such as rivets. The reason is that it has certain strength and elongation, and the most important thing is that it has good construction convenience.
High aluminum zinc-based alloys are generally ternary or multicomponent alloys, which have the characteristics of different melting parameters and casting processes, and the properties of the materials produced are very different. Some are suitable for fasteners with good elongation, some are suitable for high-strength shells with high strength, and only a few are suitable for sliding bearings with low friction reduction coefficient; Therefore, high aluminum zinc-based alloys are called "magic alloys" abroad.
Generally speaking, zinc-based bearing alloys are all high aluminum zinc-based alloys, but not all high aluminum zinc-based alloys are sliding bearing alloys.
The "zinc-based alloy" in the new sliding bearing alloy classification is essentially different from the zinc-based alloy in the above broad concept, so it is not a material strictly speaking.
The alloy of sliding bearing requires certain strength, elongation and hardness, and the most important thing is to have very good antifriction performance.
Good antifriction is definitely not produced naturally by the mixing and melting of several related metal components, but requires a complete process to ensure its performance; For example, diamond and graphite have the same chemical composition. Diamond or graphite can be produced by different processes. The molecular structure of diamond is triangular, which is characterized by its extreme hardness and can be used as a tool. The molecular structure of graphite is parallel, which is very soft and can be used as a lubricant. Diamond and graphite have the same composition, but their properties are quite different.
The process of multicomponent alloy is much more complicated than ternary alloy. Ternary alloy can be produced by primary melting or secondary melting process.
Because the cost of secondary smelting is higher than that of primary smelting, many enterprises are willing to use primary smelting process to produce ternary alloys.
Multicomponent alloys are based on ternary alloys with one or more alloy components, so the melting technology is naturally much more complicated. At the level of general smelting technology, it is actually very difficult to add one or more elements at will.
With the birth of nanotechnology in the world, micro-nano application technology derived from nanotechnology has brought new development ideas to the basic materials industry and completely changed people's thinking.
Micro-nano application technology has been applied to the field of bearing alloys, and advanced "combined casting process" technology has been born, thus realizing zinc-based microcrystalline alloys synchronized with the world on the basis of various bearing alloys.
Microcrystalline alloy is a kind of zinc-based alloy material whose alloy grains are refined to micron level. Zinc-based alloys with such ultra-fine grains show excellent comprehensive mechanical properties, over-dimensional stability and wear resistance in a special aspect.
Note: 1. Poor corrosion resistance.
When the impurity elements lead, cadmium and tin in the alloy composition exceed the standard, the casting will undergo aging deformation, which shows volume expansion, mechanical properties, especially plasticity, decrease significantly, and even break after a long time.
Lead, tin and cadmium have little solubility in zinc alloys, so they are enriched on grain boundaries to become cathodes, and aluminum-rich solid solutions become anodes, which promote intergranular electrochemical corrosion in the presence of water vapor (electrolyte).
Die castings are aged due to intergranular corrosion.
2. The microstructure of aging zinc alloy is mainly composed of zinc-rich solid solution containing Al and Cu and Al-rich solid solution containing Zn, and their solubility decreases with the decrease of temperature.
However, due to the extremely fast solidification speed of die casting, the solubility of solid solution is greatly saturated at room temperature.
After a certain period of time, this supersaturated state will gradually ease, and the shape and size of the casting will change slightly.
3. Zinc alloy die castings should not be used in high temperature and low temperature (below 0℃) working environment.
Zinc alloy has good mechanical properties at room temperature.
However, the high-temperature tensile strength and low-temperature impact properties decreased significantly.
4. The surface of zinc alloy die casting will have burrs due to the problems of insufficient clamping force, poor clamping, insufficient mold strength and high melting temperature. This phenomenon is called product capping, which is often a post-processing process that enterprises must face. At present, according to the nature of the product, it is mainly solved by manual grinding, hydrogen and oxygen and Zhaoling frozen shot blasting machine. Before defect analysis, such as furniture accessories, architectural decoration, bathroom accessories, lighting accessories, toys, tie clips, belt buckles, various metal buckles, etc. , widely used in zinc alloy die casting, which requires high surface quality and good surface treatment performance.
The most common defect of zinc alloy die casting is surface blistering.
Defect characterization: there are protruding bubbles on the surface of die casting.
Mainly manifested in: die casting, polishing or post exposure, oil spraying or electroplating.
Reason: 1. Cavity causes: mainly stomata and contraction mechanism, stomata are often round, while contraction is mostly irregular.
(1) Causes of blowholes: A During the solidification process of liquid metal mold filling, blowholes are generated on the surface or inside of the casting due to gas invasion.
B the gas volatilized from the paint invades.
The gas content of C alloy liquid is too high, and it precipitates during solidification.
When the gas in the mold cavity, the gas volatilized from the paint and the gas precipitated from the solidification of the alloy are not discharged smoothly in the mold, they will eventually remain in the pores formed in the casting.
(2) Causes of shrinkage cavity: A In the solidification process of molten metal, shrinkage cavity occurs due to volume reduction or the final solidification position cannot be supplemented by molten metal.
B castings with uneven thickness or local overheating will cause a certain part to solidify slowly, and when the volume shrinks, a concave position will be formed on the surface.
Due to the existence of pores and shrinkage cavities, water may enter the holes of die casting during surface treatment, and the gas in the holes will expand when heated during painting and baking after electroplating. Or the water in the hole will turn into steam and expand in volume, thus foaming the surface of the casting.
2. Intergranular corrosion: harmful impurities in zinc alloy components, such as lead, cadmium, tin, etc., will gather on grain boundaries to cause intergranular corrosion, and the metal matrix will be broken due to intergranular corrosion. Electroplating will accelerate this scourge, and the parts affected by intergranular corrosion will expand and lift up the coating, resulting in blistering on the casting surface.
Especially in humid environment, intergranular corrosion will make castings deform, crack and even break.
3. Causes of cracking: water ripple, cold cracking and hot cracking.
Water ripple and cold parting: In the mold filling process, the molten metal first enters the mold wall and solidifies prematurely, and then enters the mold wall and cannot be integrated with the solidified metal layer, forming overlapping lines at the butt joint of the casting surface, resulting in strip defects.
Water ripple is generally in the shallow layer of casting surface; However, cold ribs may penetrate into the casting.
Thermal crack: when the thickness of casting A is uneven, stress will be generated during solidification; B premature pop-up, metal strength is not enough; C is forced unevenly when it is ejected, and the high die temperature of D makes the grain coarse; There are harmful impurities.
All the above factors may cause cracks.
When there are waterlines, cold lines and hot cracks in die casting, the solution will penetrate into the cracks during electroplating, and will be converted into steam during baking, and the electroplated layer will be pushed up by air pressure to form bubbles.
Solution to the defect: The key to control the porosity is to reduce the amount of gas mixed into the casting. The ideal metal flow should enter the cavity from the nozzle through the shunt cone and the runner at an accelerated speed, forming a stable and consistent metal flow. This goal can be achieved by adopting the tapered runner design, that is, the pouring flow rate should be gradually reduced from the nozzle to the gate at an accelerated speed.
In the mold filling system, the mixed gas forms pores due to turbulent mixing with liquid metal. It can be clearly seen from the study of the simulated die casting process of liquid metal entering the cavity from the gating system that the sharp transition position and the increasing cross-sectional area of the runner will make the liquid metal flow turbulent and entrain gas, and the stable liquid metal will be beneficial for the gas to enter the overflow groove and exhaust groove from the runner and cavity and be discharged from the mold.
For shrinkage cavity: all parts during die casting and solidification should be uniformly radiated and solidified at the same time.
Shrinkage can be avoided by reasonable nozzle design, gate thickness and location, mold design, mold temperature control and cooling.
For intergranular corrosion, it is mainly to control the content of harmful impurities in alloy raw materials, especially lead 0.003%.
Pay attention to the impurity elements brought by waste.
For water ripple and cold parting, the appearance of cold parting can be reduced by increasing the mold temperature, increasing the speed of inner gate or increasing the overflow groove in the cold parting area.
For hot cracks: the thickness of die casting should not change sharply to reduce stress; Adjust related die casting process parameters; The aluminum die casting with die temperature reduction, electroplating and zinc removal is a kind of die casting with zinc as the main component.
This part of the surface has a dense surface layer, and the interior is porous structure and active amphoteric metal.
Therefore, only by adopting appropriate pretreatment method and electroplating process can the electroplating layer on zinc alloy have good adhesion and meet the requirements of qualified products.
The zinc alloy material commonly used in electroplating is ZA4- 1, and its main components are: 3.5% ~ 4.5% of aluminum, 0.75% ~ 1.25% of copper, 0.03% ~ 0.08% of magnesium, the balance being zinc, and the sum of impurities is ≤0.2%.
925 brand zinc alloy has high copper content and is easy to electroplate.
Generally, the density of zinc alloy is 6.4 ~ 6.5g/cm. If the density is 6.4g/cm, it is easy to blister and pock after electroplating.
In short, the selection of materials must be strictly controlled.
In addition, when die casting, the mold must be designed reasonably to avoid insurmountable defects (such as pitting) in electroplating. The zinc alloy sacrificial anode is produced according to the national standard GB/T4950-2002 "Sacrificial Anode of Zinc-Aluminum-Cadmium Alloy", and the anode for pipeline also conforms to SY/T00 19-97 "Code for Cathodic Protection Design of Sacrificial Anode for Buried Steel Pipeline".
Implementation standard: GB/T 4950-2002 SY/T0019-97.
Zinc alloy sacrificial anode is suitable for anticorrosion cathodic protection of ships, mechanical equipment, marine engineering and seaport facilities in seawater and fresh water, and pipelines, cables and other facilities in low resistivity soil.
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