Why aluminum die casting defects will oxidize and slag

Update:31 Mar 2021

Preventive measures for aluminum alloy die casting defects-oxidation slag inclusion:
Strictly control the smelting process of aluminum alloy castings, quickly smelt, reduce oxidation, and remove slag thoroughly. The Al-Mg alloy must be smelted under the covering agent. The furnace and tools should be clean, free of oxides, and should be preheated, and the paint should be dried for use after coating. The designed pouring system must have stable flow, buffering and slag skimming capabilities. The inclined pouring system is adopted, the liquid flow is stable, and no secondary oxidation occurs. The selected paint should have strong adhesion and will not peel off during the pouring process and enter the aluminum alloy castings to form slag inclusions.

Aluminum alloy die castings generally use ADC11 or ADC12. The chemical composition of ADC11 is as follows:

Chemical composition of ADC11
Slag inclusion form and formation mechanism
Through the analysis of the scrap parts of aluminum alloy die-casting, there are three main forms of slag inclusion in the gate: shrinkage cavity, oily dirt and chilled layer. Among them, oily slag inclusions account for the majority, accounting for more than 60% of the total slag inclusions; the chilled layer accounts for about 35% of the total slag inclusions; and the shrinkage cavity type accounts for 5% of the total slag inclusions.
Aluminum alloy die casting defects-reasons for oxidation and slag inclusion:
The charge is not clean, and the amount of back charge is too much
Poor gating system design
The slag in the alloy liquid is not cleaned
Improper pouring operation brings in slag
Insufficient standing time after refining and deterioration treatment
Through the analysis of the scrap parts of aluminum alloy die-casting, there are three main forms of slag inclusion in the gate: shrinkage cavity, oily dirt and chilled layer. Among them, oily slag inclusions account for the majority, accounting for more than 60% of the total slag inclusions; the chilled layer accounts for about 35% of the total slag inclusions; and the shrinkage cavity type accounts for 5% of the total slag inclusions.
1. Similar to shrinkage defects
Slag inclusion in the gate of die casting similar to shrinkage cavity
After knocking off the runner cake of aluminum alloy die-casting parts, the irregular shape, relatively clean inside, and rough surface similar to shrinkage-like holes appeared at the gate. The gate surface of aluminum alloy die castings with shrinkage cavity and slag inclusion is sunken, and the appearance is still not smooth after polishing and coating. The reason is that the inner gate temperature is relatively high (the measured maximum mold temperature is 380°C), which is the last solidified part, which causes the Al liquid to be too late to feed and shrinkage defects.
2. Oily defects

Every casting cycle needs to add lubricating oil to the punch. The punch oil is not completely burned after the Al liquid is poured, and is wrapped in the Al liquid to form impurities and fill the cavity together. Since the temperature of the Al liquid wrapped with punch oil is lower, it will solidify earlier and flow at a slower speed, so it stays at the end of the product at the end of filling. In addition, the cross-sectional area of the inner gate is small and the flow resistance is large. This part of the Al liquid that is covered with punch oil and solidified in advance is not easy to pass through and stagnates near the inner gate. After knocking off the cake, a black hole appears at the inner gate, the inner wall is rough, and the surface has obvious oil stains. Check the sprue and the slag in the cake, and also find such impurities. After knocking off the runner cake of aluminum alloy die-casting parts, the irregular shape, relatively clean inside, and rough surface similar to shrinkage-like holes appeared at the gate. The gate surface of aluminum alloy die castings with shrinkage cavity and slag inclusion is sunken, and the appearance is still not smooth after polishing and coating. The reason is that the inner gate temperature is relatively high (the measured maximum mold temperature is 380°C), which is the last solidified part, which causes the Al liquid to be too late to feed and shrinkage defects.
3. Chilled layer defects
Schematic diagram of the formation position of the chill layer
Chilled layer slag
The chilled layer refers to the shell formed by the rapid condensation of the Al liquid injected into the barrel and the low-temperature surface of the barrel (the spectrometer checks the chemical composition of the chilled layer and no abnormalities are found, all within the standard allowable range), which is filled at high speed It is filled into the cavity along with Al liquid. The density of the solid chilled layer (2.7g/cm²) is greater than that of the liquid (2.45g/cm²) Al liquid. During the low-speed operation of the punch, the chilled layer slowly gathers at the end of the Al liquid. When the punch is running at high speed, part of the chilled layer will be drawn into the cavity and form defects. The final form of the chilled layer remaining in the gate. The chilled layer is mostly thin, shiny and hard, and there are many gaps with the surrounding base material, which not only affects the appearance, but also reduces the strength of aluminum alloy die castings.
Summarize the shortcomings of the existing production conditions and the relevance to slag inclusion, so as to formulate corresponding improvement measures to ensure the appropriate mold temperature, correct the wrong punch lubricant supply method, realize the standardization of aluminum alloy smelting, and verify the effect, so that The slag inclusion problem is effectively controlled.

Methods to prevent oxidation and slag inclusion:

1. Re-pouring molten metal from the riser to improve the design of the riser
2. The charge should be clean and non-corrosive
3. A riser is set at the shrinkage porosity of the casting, and the cold iron or cold iron is placed in combination with the riser
4. Control the moisture of the moulding sand and dry the sand core
5. Take measures to refine the product
6. Improve the position of the casting in the mold and reduce the pouring temperature and pouring speed

Measures to eliminate slag inclusion by oxidation:
1. Solution measures for shrinkage cavity type slag inclusion
Shrinkage defects are mainly caused by the excessively high temperature of the die-casting mold. Therefore, the actual temperature of the die-casting mold must be determined first; secondly, the inspection found that the point-type cooling water flow of the gate attachment is small or even blocked, which is the scale of the copper pipe during use The rear cross section is reduced. The length of part of the cooling water pipe is much shorter than the design. This is because the cooling water copper pipes and cooling water holes of the die-casting mold are not numbered. When the die-casting mold is disassembled, a part of the long copper pipe will be cut off and installed into the short water hole. In this way, some of the cooling water copper pipes are shorter, and the cooling effect cannot be achieved.
Two measures can be taken:
.Add cooling water inspection items to the "Mold Preservation Inspection Item List" and specify the cooling water flow detection method and frequency;
.Check the length of the cooling water copper pipe in accordance with the requirements of the drawing, replace it if it does not meet the requirements, and number the mold cooling water hole and the cooling water copper pipe for one-to-one correspondence. After the rectification, the measured mold temperature was greatly reduced, and the shrinkage cavity-like slag hardly appeared.
2. Measures to solve oily slag inclusions

Die-casting lubricant uses Hanye 275 punch lubricant, which is black and viscous. After the release agent is sprayed, the punch is returned to clean the barrel, and then the punch lubricant is dripped. The existing dripping position is the bottom of the barrel in front of the punch, and the general die-casting lubricant consumption is 5mL/mold.
3. Three solutions for oxidation slag inclusion:

Adjusting the amount of lubricating oil for the punches. The lubricating oil for the punches was reduced from 5mL to 4mL and there was no breakage. When it was reduced to 3.5mL, breakage would occasionally occur. Analysis found that most of the punch cooling water copper pipe is shorter than the design value, and the cooling effect is poor. Therefore, restore the length of the punch cooling water pipe (15mm from the inner wall of the punch) and include it in the daily inspection items, and then reduce the amount of lubricating oil to 3.5mL.
Adjust the lubricating oil filling position and supply mode, and change the lubricating oil filling at the bottom of the original barrel to filling at the top of the punch. The lubricating oil flows downstream to the entire punch surface by means of inertia, and the lubrication area is enlarged to facilitate combustion. See the improved drip position.
Adjusting the lubricating oil supply method of the punch Although the drip lubrication method on the top of the punch has a good effect, the diffusion is not good, the lubrication area is not enough, and the combustion is incomplete. The spray can form an atomized oil film on the surface of the punch, with a large diffusion area, which is more conducive to combustion and has a better lubrication effect. After the improvement, the gate section is much cleaner than the original one, the slag inclusion is reduced by nearly 90%, and there is no breakage of the material cake.
4. Measures to solve hard layer slag inclusion:
Adjust injection delay
After Al liquid is injected into the barrel, a chilled layer will be formed on the contact surface. The longer the contact time, the more chilled layer will be produced. Therefore, shortening the injection delay, that is, shortening the contact time between the Al liquid and the low-temperature barrel is an effective method to reduce the chilled layer. The specific operation is to adjust the injection delay from the original 2.5s to 0.5s.
Adjust the standby time of the spoon return
The on-site investigation found that there are differences in casting CT due to the uneven proficiency of the operators. If completely unified process parameters are used, details will be missed. For example, the standby time of the soup ladle is set to 20s. For unskilled workers, due to the slow action, the soup ladle has already been scooped up at the completion of the mold clamping and waited for more than 6 seconds above the barrel. According to the proficiency of the operator, flexibly adjust the return time of the soup spoon, and the waiting time of the soup spoon above the barrel is controlled within 1s. In addition, returning the soup spoon to the standby time is included in the leader's daily inspection project to ensure on-site execution.
Application of filter soup spoon
The traditional soup ladle inevitably scoops some oxide layer when scooping the soup. The filter spoon style can effectively prevent the oxides from being scooped in when the Al liquid is scooped, and the purity of the Al liquid is greatly improved.

There are three problems with the existing lubricating oil supply method:
.Lubricating oil dripping at the bottom of the barrel is easily wrapped by the poured Al liquid to form scum;
.Lubricating oil gathers together and cannot be burnt completely in a short time, and it is easy to be wrapped in Al liquid
.The lubrication effect of the top of the punch is not good, and it is easy to cause uneven wear. In addition, the amount of punch oil is also too much

Standardize Al liquid deslagging and degassing operation requirements
The old and new charge, slag remover, degassing agent and melting tools must be dried; the melting temperature should be controlled below 750℃ to prevent the Al liquid from dissolving too much hydrogen; the Al liquid should be rotated and degassed and twice in the transfer bag before the Al liquid is transferred Deslagging treatment, and then standing for more than 2 hours before the Al liquid should be subjected to a second degassing treatment. This improves the purity of the Al liquid and reduces the hard layer of Al liquid cooling.
Preventive measures for oxidized slag inclusion:
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1. The charge should be sand-blown, and the amount of charge used should be appropriately reduced;
2. Improve the design of the gating system to increase its slag retaining ability.
3. Use proper flux to remove slag.
4. The pouring should be stable and pay attention to slag blocking.
5. After refining, the alloy liquid should stand for a time before pouring.
6. Control the proportion of reheating.
7. Refining and degassing the alloy liquid.
8. Clean the mold cavity, reduce the heat preservation temperature and reduce the heat preservation time.
9. The furnace should be cleaned up regularly to remove the "silt" deposited at the bottom, that is, oxide impurities with a heavier proportion
10. There is also the control of material temperature. The temperature of smelting and casting materials is too low, and it is easy to precipitate primary silicon and form hard spots.
11. Adopt the inclined pouring system to make the liquid flow stable without secondary oxidation
12. The designed pouring system must have stable flow, buffering and slag skimming capabilities.

During the smelting process, the formation of "primary oxidation slag inclusion" in the non-ferrous alloy liquid is effectively controlled. In the process design of the casting, the casting system is optimized to prevent the "secondary oxidation clamp" when the alloy liquid is poured into the mold cavity. The formation of slag.