Causes of blackening of die casting and preventive measures

Time:2020-04-16 09:42:14 / Popularity: / Source:

Surface of die casting has uneven soot-like, flow-spot-like, spot-like black or yellowing that is different from color of base metal. Generally, alloy liquid is contaminated due to excessive release agent, punch oil, soot and carbides produced by combustion, then alloy liquid is formed by oxidation and discoloration. Residual water in cavity, when encountering hydrogen and oxygen decomposed by high-temperature alloy liquid, will oxidize die-cast alloy liquid, which will also cause alloy liquid to change color and appear black. Oxide blackening and rusting will also occur when castings are well die casting and exposed to moisture. Aiming at phenomenon of blackening of die castings, we analyze phenomenon and causes of blackening of die castings, and discusses influencing factors of blackening of die castings and their solutions.

1. Blackening phenomenon on the surface of die castings

Aluminum, zinc, and magnesium die casting alloys are all active metals. They will oxidize slowly in a dry environment. And they are susceptible to oxidation and erosion under high temperature or humidity conditions, which is determined by characteristics of alloy itself. After surface of die casting is oxidized, color will become dark, and even become moldy. Not only are die casting parts easily oxidized and polluted, high-temperature die-casting alloy liquid is more easily oxidized and polluted.

1.1 Surface oily blackening

Blackening of oil fume on the surface of die casting is shown in Figure 1. On the surface of die casting, there seems to be black cloud smoke, black shape is irregular, and depth is uneven. This is because in the process of filling cavity with alloy liquid, mold release agent or punch oil encounters high temperature alloy liquid and burns insufficiently, generating a large amount of oil fume to accumulate in cavity, and smoke cannot be completely and quickly excluded from cavity. Accumulated oil fume pollutes surface of alloy liquid and mold cavity, causing surface of die casting to appear black like oil fume.
die casting parts 
Figure1 Casting surface is black like soot
cloud-like blackening of die casting surface 
Figure2 A cloud-like blackening of die casting surface

1.2 Cloud-like blackening of die casting surface

Cloud-like blackening of surface of die casting is shown in Figure 2. Surface of die-casting is black, as if there is a cloud, shape of black is irregular, but color of black is more uniform. This is because surface of cavity of mold is contaminated by carbon black and oil fume of release agent, carbon black on mold surface is printed and dyed on the surface of casting, so that surface of each mold has a blackened pattern that is relatively close in shape. Generally, black color is relatively light, and color is dark without oil fume.

1.3 Dark spots on the surface

Dark spots on the surface of die-casting are black as shown in Figure 3. Sizes of spots are different, shape is round, and dark spots are darker in color. This is because black particles formed by release agent or punch oil, on the surface of mold or casting, each black particle individually causes casting surface to become black. These black particles may also be deposits or aggregates of release agents or punch oil, which are relatively viscous and are not easily decomposed into black particles by high temperature.
Die casting surface blackened like black spots 
Figure3 Die casting surface blackened like black spots

1.4 Surface oxidation black

Die casting surface blackened like black spots is shown in Figure 4. Fig. 4 (a) and Fig. 4 (b) are die-casting parts whose surface is blackened by natural oxidation. Figure 4 (a) shows trace amount of mold release agent remaining on the surface of casting. After natural oxidation of atmosphere, color becomes darker. Figure 4 (b) A is darker than Figure 4 (b) B. Figure 4 (b) A is a surface that is naturally oxidized and blackened by air after being placed in a cleaning workshop for 10 days under natural condition of no packaging protection of die casting. This is because atmosphere contains moisture, especially during rainy season in the south, a large amount of moisture in the air is deposited on casting surface, hot and humid moisture will quickly oxidize and blacken entire casting. Figure 4 (b) B shows unoxidized and blackened surface of casting in box for 35 days. It can be seen that good packaging can prevent oxidized and blackened surface of casting.
Naturally oxidized blackened surface of die casting 
(A) Naturally oxidized blackened surface (b) Naturally oxidized blackened surface
Figure4 Naturally oxidized blackened surface of die casting

1.5 Black spots appear on the surface

Surface condition of die casting parts after being left in die-casting workshop for 10 days is shown in FIG. 5. After oxidation-blackened surface of die casting parts is left for 10 days in die-casting workshop, it will be polluted and oxidized by release agent, soot, and mist in air, black spots will appear. Release agent, oil fume and water mist aggravate alloy oxidation of casting, oxidized black spots are relatively deep. Using shot peening method, black spots cannot be removed. Therefore, after die casting is taken, sprayed release agent should be prevented from splashing on casting surface. For castings that are boxed, top layer of casting should be covered with a cover plate to prevent water spray of release agent suspended in the air of die-casting workshop from falling on the surface of castings.
die casting 
Figure5 Surface condition of die castings after 10 days in the die casting shop

1.6 Carbon deposits are formed on the surface

1. Carbon deposits on the surface of casting
As shown in Figure 6. Carbon deposits are caused by carbon on the surface of mold, which is relatively rough, like traces of a sticky mold, and generally there is no sign of sticky mold being pulled. If quality of release agent or punch oil is incorrect, or release agent is not pure, wax and organic grease in release agent are liable to burn, carbon black after combustion residues will aggregate and adhere to mold surface to form carbon deposits. Carbon deposits appear on the surface of mold and are a layer of dark black dirt. Carbon deposits usually appear as a rough surface similar to a sticky mold, and color is same as color of aluminum alloy that is adhered, because a layer of aluminum alloy is adhered to surface of carbon deposit. Carbon deposit and aluminum alloy sticky mold will be generated one after another and based on each other.
die casting 
Figure 6 One of carbon impressions on the surface of casting
2. Carbon deposits on the surface of casting
As shown in Figure 7, surface of casting shows a rough surface imprint (Figure 7A), and a smooth flat surface appears on the surface of carbon deposit part of casting, which looks like peeling and it is a mark formed after carbon deposits fall off (Figure 7B).
die casting alloys 
Figure 7 Carbon deposition mark (ii) formed on the surface of casting
In addition, once carbon deposit layer on mold surface comes off, carbon deposit adheres to surface of casting, and a small dot or a small piece of black carbon black will appear on the surface of casting.
Carbon deposits on the surface of mold: in the mold where casting has a thick thermal joint; where mold has relatively relatively concentrated heat; where mold temperature is relatively high (≥ 220℃); parts where mold contacts with alloy liquid at a high temperature for a long time; parts that away from gate, the last filled part of alloy liquid. This is because it is easy to sinter and deposit organic matter of release agent at a high temperature, and carbon deposits are liable to occur at a place where oil fume is collected.
Black spots of oil stains still remaining after shot peening are shown in Figure 8. Oil stains still remaining after shot blasting are shown in Fig. 9, which is surface alloy liquid of die casting is blackened by oil smoke pollution. Oil shot black spots and cloud-like blackening still remain on the surface of casting after shot peening. Oil fume is not only attached to casting surface, but oil stain has contaminated internal alloy liquid, so such blackening cannot be removed by shot blasting.
die casting parts 
Figure 8 Shot peening after the oil residue black spots
die casting parts 
Figure 9 Shot peening after remaining oil and black smoke like

1.7 Mild blackening

Blackening of die-casting is shown in Figure 10. Yellow spot formed by punch oil on the cake at the moment of opening mold is sprayed onto casting surface as shown in Fig. 10 (a); casting is oxidized and blackened as shown in Fig. 10 (b); corrosion of casting after long-term erosion by rain, wind and sun, and severe oxidative mildew is shown in Figure 10 (c).
die casting parts 
A. Macular map caused by punch oil
die casting parts 
B. Moisture and darkening of castings
die casting parts 
(C) Casting mold oxidation map
Figure10 Die casting blackened

1.8 Oil stains

Oil on the conveyor belt of transfer casting contaminates casting, resulting in blackening of casting. Oil-contaminated casting on conveyor belt is shown in Figure 11. If storage box, working platform and tools for storing castings, trimming dies for trimming, operating gloves, etc. are stolen and oily, they will also contaminate casting and cause casting surface to become black. After casting or finishing is cleaned, moisture is not blown clean. After oxidation blackening and rusting, oxidation blackening of casting surface caused by negative residue of finishing coolant is shown in Figure 12. If packaging carton is damp and absorbs water, it will also cause mold to contact carton with moisture.
die casting parts 
Figure11 Casting was contaminated by oil on conveyor belt
die casting parts 
Figure12 Finishing coolant causes casting to oxidize and blacken

2. Causes and solutions of blackening of die-casting

2.1 Excessive spraying of punch oil

If punch oil in injection chamber is too much, it will produce a large amount of flame, oil smoke and carbon black when it encounters poured superalloy. These flames, soot and carbon black will not only promote oxidation of alloy liquid, but also pollute alloy liquid and cavity wall. Therefore, it is necessary to reduce amount of punch oil and amount of punch oil as much as possible to observe that outer circle of cake does not become black, and black is only on the end face of punch.
Because graphite oil containing punch easily blackens casting, you should choose a graphite oil with little or no graphite content. For example, water-soluble punch oil can be used to reduce amount of wax, grease and organic matter.
If punch oil produces too much and too thick oil fume, oil fume will flow into cavity from gate and stick to cavity near gate, resulting in blackening of casting surface near gate.
Spray lubricating oil of punch into upper part of punch outside shot chamber, or spray it into shot chamber from back of punch after shot in the form of mist, so accumulated punch oil will be pushed out of injection chamber by returned punch to reduce accumulation of punch oil. If injection oil is sprayed into injection chamber in the form of a mist after return of injection punch, or is dripped into injection chamber in the form of oil, not only should nozzle oil be sprayed less, but also pay attention to use of compressed air to blow up accumulated punch oil on the surface of injection chamber.
If mold core extractor leaks oil, hydraulic oil of core extractor flows into cavity, which will also make casting surface black.

2.2 Spraying amount of release agent is too much or concentration is too strong

If release agent is excessively accumulated after die-casting cavity is sprayed, or concentration of release agent is too strong, oil, grease, wax, silicone oil, other organic substances and moisture in release agent will generate a large amount of oil fume after encountering high temperature alloy liquid. Excessive oil fume not only seriously pollutes alloy liquid and surface of mold cavity, but also causes spray-filled alloy liquid to rapidly oxidize, causing black spots, blackening (Figure 9) and carbon deposits on the casting surface and inside. Therefore, a mold release agent with good effect of preventing sticking mold, high temperature resistance, not easy to burn, and not easy to produce oil smoke should be selected. A good quality release agent not only has effect of preventing oxidation and corrosion on the surface of die casting, but also does not affect quality of casting.
Use of lighter release agents can also reduce production of soot. In general, proportion of release agent is about 1: 120 to 150. For example, mold of heat sink has a thin and deep cavity of heat sink, and it is easy to stick mold. Spraying amount of release agent cannot be reduced, but a lighter concentration release agent can be used. Ratio of 1: 140 can be used. .
In order to reduce generation of oil fume, pay attention to spraying location, distance, flow rate and length of time when spraying mold release agent; spray and atomization effect of release agent is better, spray amount of release agent and punch oil is small and uniform. Film formed by release agent on mold surface should be thin and uniform. Do not let release agent flow or accumulate in cavity. After spraying, surface of mold cavity should be blown clean with compressed air, excess water in mold release agent should be evaporated before mold is closed.
It should also be noted that water used to prepare release agent should be pure water or soft water, hard water cannot be used because metal ions of hard water not only affect stability, coating properties, release properties of release agent, but also promote casting oxidation and rust.

2.3 Casting surface carbon black

Quality of release agent or punch oil is improperly selected, or release agent is not pure. Wax and organic grease in release agent are easy to burn, residual carbon black after burning will sinter and adhere to mold surface and form carbon deposits. Carbon deposit layer is relatively firm on the surface of mold and usually needs to be polished with oil stone, water sandpaper and abrasive cloth to remove it. Surface of mold can be regularly polished to prevent excessive carbon deposits and blackening defects.
Generally, when mold surface temperature is higher than 240 ℃ at the moment of mold opening, release agent will be sintered to form carbon deposits. Therefore, it is necessary to reduce gating temperature, control mold temperature within a certain range, and maintain mold thermal balance. Reduce mold temperature in overheated area and increase cooling water flow of the mold, so that mold temperature is not too high to prevent release agent from sintering and collecting carbon. In addition, the rougher molding surface of mold, the easier it is for oil fume to adhere to produce carbon deposits; mold release agent selected cannot withstand high temperatures, the higher gating temperature and mold temperature, the easier it is to produce oil fume and sintered carbon deposits.
If there is carbon deposit on the surface of mold, speed of heat transfer from alloy liquid to mold is affected, shrinkage and thermal crack defects are easily formed in thick parts of product. At this time, you must use oil stone or abrasive cloth to polish carbon deposit on the surface of mold.
In large thermal joints and thick-walled parts of casting, alloy liquid is at a high temperature for a long time, sintering of release agent is intensified, and carbon deposit defects are more likely to be formed. Therefore, cooling and heating system of mold should be well set up to control temperature of mold.

2.4 Excessive water residue on mold cavity surface mold release agent

Mold cavity's mold release agent has not been blown clean or volatilized. Excessive water remains on mold surface. Alloy liquid that is exposed to high temperatures decomposes into oxygen (O), hydrogen (H), and HO- ions. Oxygen oxidizes and contaminates alloy liquid, making surface layer of casting oxidized and blackened. At the same time, when there is too much moisture in mold release agent, mold release agent will not get sufficient oxidation and combustion when it encounters high temperature alloy liquid, and it will form smoke.
Mold surface temperature is higher than 160 ℃, moisture will evaporate faster and cleaner. Therefore, it is necessary to appropriately increase temperature of mold away from gate and temperature of mold which is too low; cooling water of small mold can be properly turned off, so that mold temperature is not too low, and too much water of mold release agent on the surface of mold can be quickly volatilized, so as not to generate excessive oil fume.
After spraying release agent, mold release agent water on mold surface should be blown clean accurately and thoroughly. Excessive release agent should be blown away with compressed air, blown off, and cleaned. Align deep cavity, deep hole, deep groove, slider groove of mold, and use air pipe to stay for 1 to 3 sec. Moisture on the parting surface of mold must also be cleaned with an air pipe, not only to prevent water on the parting surface from flowing into cavity, but also to prevent excessive water on parting surface from blocking parting surface and exhaust groove, which affects effect of exhaust.
Regularly check and prevent mold cooling water, mold release agent water, and mold release agent water in slider groove from flowing into cavity and injection chamber.
If mold release agent uses water with a high salt content, salt will adhere to surface of casting, promote oxidation and corrosion of casting. Therefore, pure or soft water can be used as mold release agent for die casting, and purified tap water can also be used.

2.5 Poor quality of release agent and punch oil

O2, CO2, CO, H2O produced by oxidative combustion and decomposition of organic substances such as paraffin, vegetable oil, mineral oil, various silicone oils and graphite in release agent react with Al to form Al2O3. At the same time, soot is generated to cause oxidation and blackening of casting. These organics are unstable and will condense into viscous, particulate oil droplets. If they are infected by bacteria, they will also condense into flocs and build up in piles. These viscous oil droplets encounter oil fume when they are filled with filled superalloy fluid. When condensed oil droplets are burned and insufficiently oxidized, oil fume pollutes oil droplet particles, turning oil droplet particles black into release agent black particles, and are washed by alloy liquid and penetrates into casting as alloy liquid flows, so that alloy liquid is partially or entirely oxidized. Therefore, after local finishing of casting, phenomenon of blackening may occur. When emulsifying and dissolving of release agent and water are insufficient, or when combustion decomposition of high temperature alloy liquid is insufficient, release agent black particles may also appear.
After die-casting is stored for a long period of time, if there are black spots, mildew spots, and white spots on the surface (frost, black when removed), it indicates that corrosion has occurred. This may be due to presence of a component in release agent that promotes corrosion of casting. Surface of casting just after casting is slightly blackened. After casting is left in nature for 10 to 30 days, surface of casting will automatically oxidize and become darker.
When punch oil accumulates more and punch oil encounters insufficient combustion decomposition of superalloy fluid, black particles of punch oil will also appear. If surface of mold is contaminated, such black spots will contaminate superficial alloy liquid of casting. After die casting, superficial layer of casting will form small black dots. There is also accumulation of contamination into a small piece of blackened, irregular shape. Blackening of casting caused by black particles is not same as blackening of oil fume pollution on casting surface. It is not outside casting surface, but oxidizes inside of casting and cannot be wiped off with a clean cloth. If internal alloy liquid is contaminated during filling process, alloy liquid will be contaminated, oxidized and discolored, small black holes or small black slags will be formed inside casting, which will look like a small black spot to naked eye. Small black dots are usually less than 2 mm in size and appear individually or scattered. Location, degree, and size of small black cavities or small black slags are related to size, location, and filling flow of black particles.
Small black cavities are mainly formed by gas of oil fume. Inner surface of black small cavities is rounder than inner surface of shrinkage pores, but surface of pores without air or hydrogen is smooth and bright. This is because pollution color of oil fume is relatively dark. Generally, small black cavity formed by punch oil has a darker color than color formed by release agent. Small black slag is oxidized slag formed after alloy liquid is oxidized and contaminated by black particles. Oxidized slag is darker than normal oxidized slag due to oil fume pollution.
Small black cavities or small black slags will not only appear on the surface of casting's finishing or dissection, but also on the fracture surface of gate in the casting, which will affect quality of casting.
Therefore, release agents and punch oils that have good emulsification effects with water, good suspendability, no precipitation, no agglomeration, no scaling, no bacterial infection, and easy dispersion should be used.

2.6 Gating system and mold structure

Although die casting process can solve some casting defects, under normal process debugging, most of casting defects still need to be improved by mold, casting system and mold structure will directly cause blackening of casting. Generally, parts that are far from gate, parts with vortex, deep cavity of mold and final filling part are prone to blackening. Firstly, because there are more fumes gathered in these parts, alloy liquid and mold surface have been in contact with fumes for a long time, amount of eddy current entrained gas in alloy liquid is large, which causes oxidation of alloy liquid on the surface of casting to be aggravated, degree of adhesion of soot and black hair on mold surface is increased. Second reason is that flow rate of alloy liquid at last filling part is slow, degree of scouring on mold surface is light. Oil fume accumulated on mold surface cannot be washed away by alloy liquid.
Especially for large flat castings or large flat parts of castings, most of alloy liquid is jet-filled, which exceeds speed of flue gas flow and discharge. The more alloy liquid is in contact with and mixed with flue gas, the more amount of soot gas is wrapped in alloy liquid, and the more easily alloy liquid is oxidized and polluted, the more serious phenomenon of blackening on casting surface. Inner gates should be provided to allow alloy liquid to be filled with full wall thickness as much as possible so that alloy liquid does not entrain gas. Filling is completed quickly and in a short time, so that oil fume is too late to produce and oil fume does not have time to oxidize alloy liquid.
Improve gating system of mold, change position, size and direction of gate and overflow slot, can change flow direction and flow of alloy liquid, change generation, flow direction and discharge of oil fume.
Figure 13 shows heat sink gating system. Figure 13 (a) is a casting with many heat sinks. Figures 13 (b) and 13 (d) are heat sinks on both sides of casting (appearance is blackened) phenomenon). Add a gating system to mold as shown on left or right of Figure 13 (c), so that alloy liquid can fill darkened parts of casting as soon as possible, directly and smoothly. Widen both sides of gate to part of heat sink on the side of casting, so that alloy liquid directly fills heat sink on the side of casting. After mold is modified in this way, blackening of surface of heat sink on both sides of casting has been eliminated, alloy liquid directly fills heat sink on the side of casting as shown in Figure 14.
 die casting process 
Figure13 Fin casting system
aluminum die casting parts 
Figure 14 Alloy fluid fills heat sink directly on the side of casting
In addition, appropriately increase cross-sectional area of overall and local exhaust grooves, overflow ports of mold, so that exhaust system can smoothly, quickly discharge liquid and exhaust, timely discharge oil fume and contaminated alloy liquid out of cavity, which will also have better results.

2.7 Cast structure

Generally, in thinner part of casting wall, mold temperature is relatively low, release agent water volatilizes slowly, residual release agent and moisture are relatively large, so more oil fume is generated. Castings have deep cavity parts and blind hole parts that cannot be filled smoothly and are difficult to evacuate, and there is more oil fume remaining in cavity. Due to shape or structure of casting, clamping force on mold is large, there will be a phenomenon of mold sticking. It is necessary to spray a thicker and more mold release agent, mold cavity will leave too much mold release agent, which will increase blackness of casting. Therefore, some blackening cannot be completely eliminated because release agent cannot be reduced.
Side wall of radiator casting which is more black is thinner, and it is far away from other heat sinks alone. Amount of alloy liquid collected in cavity is relatively small, so that temperature of mold and slider here is not high enough, mold release agent on the surface of cavity is not fully volatilized, more residual mold release agent produced relatively more oil fume when it encountered superalloy. Solution is to increase mold temperature.
Cavities that cannot be filled directly with alloy liquid and eddy currents during filling process are susceptible to eddy-like blackening on casting surface. Solution is to change position and flow direction of filling, directly fill to eliminate eddy current phenomenon.
Blackening often coexists with cold barriers and flow marks. Because temperature of mold or alloy liquid in cold insulation and flow mark parts is low, mold release agent burns and volatilizes slowly when it encounters alloy liquid. Flue gas and slow flowing alloy liquid have a long contact time, alloy liquid is polluted by oil fume for a long time. Solution is to increase mold temperature.
For large-scale castings, alloy liquid will be sprayed to distal end or advanced to surrounding parting surface, instead of being able to discharge soot gas in the cavity to outside of cavity in the same way as a complex casting; Therefore, casting surface of large plane is prone to blackening. Solution is to set more gates, so that alloy liquid can be filled quickly in a short time.

2.8 Die-cast alloy material

Most die-cast alloys are aluminum alloys that are susceptible to oxidation, especially at high temperatures. During pouring process and moment of die casting filling, it will be oxidized and defects of oxidized slag inclusion will appear.
During pouring process of alloy liquid from furnace by pouring spoon, alloy liquid will be oxidized by oxygen in air during flow and tumble, forming an aluminum oxide film on the surface of alloy liquid. The longer it takes alloy liquid, the more alloy liquid tumbles, and the more severe oxidation becomes.
When alloy liquid is poured into injection chamber, high-temperature alloy liquid causes punch oil sprayed into injection chamber to burn, ignite, and smoke, oxygen atoms decomposed by these oil fumes and combustion gases will accelerate, intensify oxidation of alloy liquid. Therefore, the more punch oil is used, the more the alloy liquid is oxidized and polluted.
When aluminum die casting parts are stored in a natural state, surface will soon be oxidized and color will become darker; if stored in a humid environment, surface will also have rust defects within a few days. After shot peening, dense oxide layer formed during high temperature demolding of die casting is sprayed off, surface oxidation speed and degree will increase.
Anti-oxidation ability of die-cast alloys with different chemical compositions is not same. Therefore, degree of blackening of surface of castings produced by die-casting alloys of different brands is different. Therefore, you can choose a die-cast alloy that is not easily oxidized and blackened. For example, ENAC-434000 alloy is more easily oxidized and blackened than ADC12 alloy.
When refining slag of aluminum alloy liquid, adding 0.1% to 0.3% of mass of alloy liquid by sodium fluoroaluminate or sodium fluorosilicate is also beneficial to prevent oxidation and discoloration of casting.

2.9 Surface becomes black and discolored after shot casting

Stainless steel shots should be regularly screened and dusted. This is because after stainless steel shots are used for a period of time, not only will steel shots be worn down, but also alumina dust will adhere to surface of steel shots. At the same time as shot blasts die casting, steel shot will also hit alumina dust on the surface of casting, causing overall color of casting surface to become dark, or to become matte.
Zinc shot material: bright appearance, high specific gravity, shot blasting force is large enough; low hardness, can remove large burrs without wearing surface of product; dust produced by zinc shot is not easy to burn and explode, it is a relatively safe shot blasting material which are widely used for deburring and cleaning of zinc and aluminum products. However, zinc shot itself is easy to oxidize, and it is easy to produce oxide dust, which causes surface of casting to become darker.
Aluminum shot material: Surface of casting after shot blasting with aluminum shot is the most beautiful, but because of light weight of aluminum shot, impact strength is relatively small, and small burrs cannot be shot out. Shot peening efficiency is not high enough. Dust is flammable and explosive, so it is rarely used.
Adding 20% to 30% aluminum shot into stainless steel shot can achieve a whiter and brighter surface effect than using stainless steel shot alone.
During shot blasting process, not only moisture and dust in air will be sprayed onto casting surface, but casting will also be heated by shot material to heat up. Temperature increase will accelerate oxidation reaction on casting surface, so casting will quickly turn black.
For shot blasted products, gloves should be worn during operation. If casting is directly touched by hand, surface of casting will leave a hand mark and cause blackening.
After die casting is in a high temperature state after die casting, an oxide film will quickly form on the surface. After shot peening, oxide film disappears, and casting surface is more prone to oxidation defects. Therefore, shot-casting parts should be dust-proof, moisture-proof, water-proof and oil-proof, sealed and packaged, and then surface-treated quickly.
If casting is polished by vibration grinding, only dust, oil and small burrs on the surface of casting can be polished, blackening of casting substrate cannot be polished. Before and after finishing, castings must be stored in a sealed and dry place to prevent substrate from being wet, oxidized and darkened. In finishing process, pay attention to cleaning powder deposited by abrasive. After finishing, casting should be cleaned, baked and dried immediately.

2.10 Blackening, bubbles, and peeling of electroplating caused by poor castings

Surface of die casting must be free from defects such as cracks, looseness, shrinkage, pores, air bubbles, pinholes, cold separation, flow marks, etc. Otherwise surface of casting is liable to blistering after plating, and plating layer is separated from substrate.
During solidification of die casting parts, a dense cold hard layer is formed on the surface due to rapid cooling, internal structure may have defects such as pores and shrinkage. If this good surface layer is removed during grinding, defects such as pits and bubbles will appear during plating. When polishing die casting parts, do not press polishing wheel too tightly, do not overheat castings to prevent abrasives from sticking to castings, which will cause products to not be blackened or peeling. In other words, grinding and polishing of die casting parts before plating should not be excessive.
Die casting parts are easy to be oxidized. If storage time is long, an oxide film will be formed on the surface, which will also affect plating performance. Before electroplating, a reducing substance should be used to remove oxide film.

2.11 Moisture in air promotes oxidation and blackening of castings

Air is wet during rainy season in the south. During blowing process after release agent is sprayed, water in compressed air is sprayed onto mold surface, causing more water on mold surface. Therefore, it is necessary to dry compressed air.
When air humidity is high, if seal is not well sealed when die casting is stored indoors or during transportation, moisture in air will condense on the surface of casting, slightly darkening and blackening surface of casting, severely oxidize and whiten surface of casting to produce mold spots.
In winter, when die castings are transferred from outdoor to indoor finishing or storage, if there is air conditioning in the room, indoor temperature is significantly higher than outdoor, surface of casting will also condense moisture and frost in the air, which will cause rapid oxidation of casting. Therefore, when casting is just inside room, do not open packaging immediately; or after finishing packaging, finish machining immediately to prevent casting from frosting and blackening due to long-term storage.
In high temperature season, temperature of product in machined air-conditioning plant is relatively low, and it cannot be directly loaded and shipped. It must enter buffer zone of warehouse for transition, otherwise water will condense on product.
Aluminum alloys are amphoteric metals. They will oxidize and discolor when they encounter acidic or alkaline substances. Therefore, neutral cutting fluids should be used during finishing and ultrasonic cleaning.
Finished casting is washed in hot water, and then water drops on the surface of casting are blown off immediately with compressed air. After being left for a period of time, it is boxed. If it is stored in a packing box while it is still hot, casting will soon be oxidized, blackened, and mildewed.

2.12 Method for removing black on the surface of die casting

100% inspection of blackening of surface of castings. After discovering individual blackened castings, you can use a polishing machine to polish them, or use manual cleaning with a clean cloth.
For oil stains and mildew on castings, the most effective method is to use a special mold remover cleaning agent for die-casting aluminum alloy to thoroughly clean oil stains and mildew at one time.
Although aluminum is easy to oxidize, a thin oxide film will form on the surface after oxidation. Density of oxide film is very high, which can block aluminum alloy inside from further oxidation. Therefore, after casting is cleaned, passivation treatment should be performed immediately to form a passivation film on the surface of aluminum alloy casting to achieve purpose of anti-corrosion and antibacterial. In humid weather or environment, it can also prevent mold from being corroded by mold.
Anti-oxidation and drying castings should be placed in a dry and ventilated place, inspected regularly. If casting is found to be in contact with water or moisture, another mold point appears, it should be cleaned and anti-oxidized in time. Most of castings can be completely renewed.

2.13 Operation, management and storage to prevent oxidation and blackening of surface of die castings

Die castings are easily oxidized and eroded under high temperature or humidity conditions. After surface of die casting is oxidized, color will become dark and even moldy. Good die castings, or cleaned die castings, if not treated with anti-oxidation in time and stored in a wet warehouse for a long time, as short as one or two months, and as long as half a year, it may oxidize and become black and moldy. Therefore, there must be appropriate anti-oxidation measures and storage management for die castings.
Due to different levels of humidity, ventilation and temperature inside storage warehouse, aluminum alloy die castings are stored in warehouse at different positions and heights, their oxidation, blackening and moldy conditions will be different. Therefore, warehouse where castings are stored should be dry, ventilated in sunny weather, closed in humid weather, and dehumidified if necessary. If warehouse is on the ground, storage floor should be elevated to support ground to prevent ground from returning to moisture.
Castings should be sealed and packed with desiccant, castings should not be wet during storage and transportation. Special protective measures should be taken during rainy season to prevent castings from getting wet and moldy. It can be added with a plastic bag packaging, and castings can be sealed in a small plastic bag. It can also be stored in a plastic bag as large as a large box, and put in a desiccant to seal packaging.
Die-casting parts should be washed and dried after die casting and machining processes. If no cleaning treatment is performed, surface of die casting parts will have stains such as corrosive substances such as mold release agents, cutting fluids, saponification solutions, and water stains, which accelerate growth of mold blackening points of aluminum alloy die-casting parts. Therefore, after die-casting parts are finished, they should be cleaned with ultrasonic waves and dried. However, if cleaning agent is not selected properly, it will also cause corrosion and oxidation of die castings.
Do not use items such as wet cardboard boxes, cardboard and turnover boxes. Plastic boxes and plastic plates can be used. Immediately after packing, top layer should be covered.
For castings that require special rust protection, stagnation time of castings during circulation should be reduced. For castings with steel and iron inserts, apply rust preventive oil to inserts and there must be no omissions.
During transportation, it should be protected from rain, covered and tied with protective rain cloth. When castings are exposed to rain, this batch of castings should be isolated immediately, washed and dried. In addition, castings must be placed for 7 days to confirm that they are qualified.
In order to prevent water droplets splashing when spraying mold release agent on casting, casting should be moved to other areas for storage in time.
Conveyor belt beside die-casting machine, trimming dies for trimming, and workbench should be wiped regularly at regular intervals. Gloves of operator should be clean, free of moisture and oil to avoid contamination of casting.
Check castings out of box before leaving warehouse, especially for the first two or three months. For castings that have been boxed for 6 months, boxing must be rechecked.

2.14 Surface coating treatment method to prevent oxidation of die castings

Common surface coating methods for die castings are as follows:
(1) Painting.
(2) Anodizing can prevent aluminum alloy from continuing to oxidize.
(3) Porcelain anodizing, beautiful surface, high hardness and good decoration.
(4) Micro-arc oxidation, high hardness, good corrosion resistance, disadvantages are rough surface and poor appearance.
(5) Hard anodizing, high hardness, wear-resistant, unsightly color, gray.
(6) Electroplating.
(7) Chemical oxidation.
(8) Organic matter coating, plastic spraying.
(9) Physical vapor deposition (PVD treatment), good effect.
(10) Ceramic coating, high cost.

3 Conclusion

There are many causes of blackening of die castings. Corresponding measures need to be taken according to each cause of blackening in order to effectively eliminate blackening phenomenon on the surface of die castings and improve quality of die castings.

Go To Top