introduction

With the improvement of automobile emission standards, reducing automobile engine emissions, reducing fuel consumption, and improving combustion efficiency have made automobiles tend to be lightweight, and lightweight is of great significance to the development of the automobile industry. According to statistics, at present, the self-weight of domestic autonomous passenger cars is about 8%~10% higher than that of similar foreign cars. The weight of domestic independent commercial vehicles is about 10%~15% higher than that of similar foreign vehicles. Research figures show that if the weight of the car is reduced by 10%, fuel efficiency can be increased by 6%~8%.

In terms of materials, many cast steel, cast iron structural parts, functional parts are gradually being replaced by aluminum alloys or other alloys with less density, including some gasoline engine cylinder blocks and cylinder heads, especially in new energy vehicles, aluminum alloy parts account for an increasing proportion. In recent years, domestic aluminum alloy castings
Significant progress and breakthroughs have also been made in casting technology, providing strong support for the rapid development of new energy vehicles.

Aluminum casting forming, mainly high-pressure casting, low-pressure casting and gravity casting several ways as shown in Table 1, they all have their own different advantages and disadvantages. Some cavities and pipe castings with more complex shapes (especially inner cavity shapes) need to use sand cores, salt cores, wax molds and other disappearing cores.
Generally, gravity casting is used.

Table 1 Comparison of several molding methods

	craft	Mold cost	Mold life	Porosity	heat treatment	Processing allowance	Smooth surface	Suitable products	Production efficiency
High pressure casting	complex	high	short	difference	cannot	small	high	Thin-walled pieces	high
Low pressure casting	Simple	low	long	Good	OK	big	high	middle	middle
Gravity casting	Simple	low	long	Good	OK	big	low	Wall thickness parts	low

1 Material composition, deterioration control and purification treatment of aluminum alloy castings

Alloy composition has a great influence on the microstructure and properties of the alloy, and alloy composition control is also one of the important tasks in the alloy treatment process. Conventional element composition is generally analyzed by direct reading spectrometer, which can quickly and accurately grasp the composition state of the alloy.

An appropriate amount of trace elements can be added to the aluminum-silicon alloy to refine the grain and enhance its fluidity, such as titanium, zirconium, vanadium and other elements of the intermediate alloy. At the same time, the refinement of grains also improves the shrinkage ability of the alloy, reduces the tendency of hot cracking, and improves the electrochemical characteristics of the alloy. Magnesium in ZL104 will Through the precipitation of MgSi2 in the aluminum matrix, the alloy has a significant strengthening effect, and the magnesium content is controlled, and the comprehensive properties such as plasticity, tensile strength and yield strength can be obtained.

In the chemical composition of aluminum alloys, there are often some impurity elements harmful to the performance of the alloy, mainly including iron, tin, lead, calcium, etc., among which the most affected is iron. These impurity elements may be introduced from the charge during the alloy melting process, or they may be brought into the alloy from crucibles or tools. The iron in the aluminum-silicon alloy mainly appears in the form of Al9Fe2Si2, and the iron phase is hard and brittle, often passing through the α phase grain in a thick needle, weakening the matrix and reducing the aluminum
Silicon alloy properties. When there is Fe phase in the aluminum-silicon alloy, its surface oxide film will lose continuity, and the β ferrous phase precipitated at the grain boundary will also promote the electrochemical corrosion of the aluminum-silicon alloy, so that the corrosion resistance of the alloy is reduced, so the iron content in the alloy must be strictly controlled, and the H and metal oxidation inclusions in the aluminum liquid are removed by rotating into N2 or inert gas, and the flux is supplemented to minimize the residue in the aluminum liquid.

The main factors affecting the deterioration process are: type and dosage of metamorphic agent, deterioration temperature, and deterioration time.

Types and dosage of metamorphic agents: The appropriate types and dosages of metamorphic agents should be selected according to the type of alloy, casting process and specific requirements for tissue control. The selection of non-toxic, non-polluting and long-term deterioration effect is the development direction of the current aluminum alloy melting process. Aluminum casting aluminum bonding
Gold metamorphic treatment commonly used Na deterioration, Sr deterioration, rare earth deterioration, etc., we adopt strontium metamorphic agent with better comprehensive performance, and use titanium intermediate alloy as a refining agent.

In production practice, it should be taken into account that the reaction of the spoiler may be incomplete, so the amount of spoiler should not be too small, otherwise the deterioration effect is not good. However, the amount of detergent should not be too much, otherwise it will cause excessive deterioration. Therefore, the amount of metamorphic agent generally accounts for 0.2%~1.0% of the weight of the charge. In production, the addition of 0.5% is usually enough to ensure good deterioration results. For castings of metal mold casting, the amount of metamorphic agent can be appropriately reduced.

Deterioration temperature: the temperature of the molten aluminum is higher, which is beneficial to the deterioration reaction, the deterioration speed is fast, and the effect is good, but the deterioration temperature cannot be too high, otherwise it will sharply increase the oxidation and suction of the molten aluminum, and reduce the service life of the crucible. In general, the deterioration temperature should be selected at a slightly higher than the pouring temperature10 °C is appropriate, so as to avoid the deterioration temperature is too high, can reduce the time to adjust the temperature after deterioration, and is conducive to improving the deterioration effect and the metallurgical quality of the molten aluminum.

Deterioration time: The higher the deterioration temperature and the better the contact between the molten aluminum and the metamorphic agent, the shorter the deterioration time required. The deterioration time should be determined on the basis of experiments according to the specific situation. If the deterioration time is too short, the deterioration reaction is incomplete; If the deterioration time is too long, it will increase the burn loss of the metamorphic agent and increase the absorption and oxidation of the alloy.

2 Casting quality control of aluminum alloy castings

2.1 Choose a reasonable casting method

Due to the complex structure of most casting products, the company is a thin-walled pipe part, the wall thickness difference is too large, the hot joint position is more, the casting is difficult to form, and the ordinary pouring method can not be completely formed. In order to select the most reasonable molding method, casting simulation analysis software such as PROCAST is used to analyze the part. In order to ensure the integrity of the molding of the product, which is conducive to the discharge of cavity gas and the gas generated by the sand core, tilting pouring is adopted, and the tilting angle, tilting speed and action mode of the equipment must be steplessly adjustable. Oblique pouring is conducive to the complete filling of the molten aluminum, in the process of inclined pouring, the flow of the alloy liquid is relatively stable, the flushing force of the cavity is small, the filling of the molten aluminum from slow to fast to slow, can achieve laminar flow and sequential exhaust of the alloy liquid in the cavity, which is conducive to the discharge of gas from the parting surface, exhaust plug, exhaust groove, riser, and is conducive to the inevitable impurities and slag floating in the aluminum liquid that are not completely treated, and collect in the riser, so as to obtain castings with dense structure and high mechanical properties. When tilting pouring, adjust the tilting angle and turning speed of the mold according to the shape and molding sequence of different castings to achieve the best effect.

2.2 Select reasonable casting process and parameters

The main casting process parameters that affect the casting forming and casting quality are pouring temperature, pouring speed, cooling strength, solidification order, etc.

(1) Pouring temperature
Refers to the temperature of the liquid injected into the mold, if the pouring temperature is too low, the gas of the liquid at the time of crystallization does not have time to float up the liquid surface, resulting in porosity, looseness, and may also produce casting quality defects such as slag inclusion and cold partition; If the pouring temperature is too high, it will lead to excessive grain size during crystallization, due to excessive shrinkage Large, the riser is insufficient to compensate, the formation of large shrinkage and shrinkage holes, the casting stress is too large, cracks are produced, and feather tissue defects may also occur. Therefore, the pouring temperature of scientific specifications is generally 70~100 °C higher than the actual crystallization temperature of the alloy. At the same time, the pouring temperature should be adjusted according to the different structure and shape of the product, and the difference in wall thickness, such as a certain inlet pipe due to the complex structure, the wall thickness difference varies greatly (the wall thickness changes from 3 mm to 20 mm in the length range of 10 mm), there are many protrusions, it is difficult to fill, and the pouring temperature needs to reach about 760 °C to complete the forming.
(2) Pouring speed
The pouring speed is not a fixed speed, and the casting start is not the same as the casting process: the speed and slowness of the pouring speed have a great impact on the molten aluminum filling, exhaust, casting molding, crystallization, structure and performance. Under the premise of ensuring the quality of castings, the highest pouring speed should be used.
(3) Cooling intensity
Cooling strength is also known as cooling speed, cooling strength not only has an impact on the internal stress and cracks of the casting, but also has a greater impact on the crystallization rate of the casting, grain growth direction and other organizational factors. With the increase of cooling rate, the crystallization rate of castings increases, and the intracrystalline structure is more refined. With
The cooling strength increases, the casting cavity becomes shallow, and the size of the transition zone is reduced, so that the metal shrinkage conditions are improved, and the defects such as looseness and porosity in the casting are reduced or eliminated; The density of the casting is increased, and the size of the initial compound can also be refined, reducing the degree of regional segregation.

In order to make the gradual crystallization have enough cooling rate and obtain a better structural structure, circulating water cooling is generally used to make the casting form a large temperature gradient and make the casting cool quickly. The cooling intensity requirements for the cooling water temperature cannot be ignored, and under normal circumstances, the cooling water temperature is set at
About 20~30 °C, our circulating cooling water temperature is generally stable at about 25 °C, and the cooling intensity can be set as a curve according to the needs of the casting process, that is, the adjustment of cooling time and interval can be controlled by the equipment PC setting, eliminating the internal stress of the casting part, reducing the possibility of cracks and other defects, that is, high-quality castings can be obtained.

(4) Solidification order
Some products are affected by the structure, the wall thickness difference is too large or the local wall thickness is too thick, the cooling water pipe cannot be arranged by the mold structure limitation, or because the distance after the arrangement of the cooling water pipe is too far, the effect of rapid cooling cannot be achieved, and the cold iron must be arranged on the mold or sand core to be forced
Cooling, so that the metal shrinkage conditions are improved, reducing or eliminating defects such as looseness and porosity in castings. Cold iron materials are often used in actual production: copper, cast iron, steel, aluminum alloy, etc. Among them, copper has the highest thermal conductivity and the best cooling effect. But copper in the production also has its limitations, because the cost is higher than several other materials, and the melting point of copper is very high, the shape of cold iron can not be obtained by casting method, must be machined, long molding cycle, complex process, not suitable for large-scale production. Cast iron, steel and aluminum alloys have a lower thermal conductivity than copper and a slightly worse cooling effect, so the effective surface area and volume of cold iron must be done in production applications Slightly larger. However, it can be obtained by casting, and the molding process is simple and suitable for large-scale production.The installation arrangement of cold iron can generally be divided into two types: a direct installation mold, generally using copper material; The other is placed on the sand core, which can be used with other materials. Both installation methods have their own advantages and disadvantages. Copper cold iron is arranged and installed on the mold, and the shape, position and size of the cold iron It is limited by the mold structure, casting structure, mold heating pipeline and mold cooling water channel, but the cold iron is in contact with the mold material, the heat conduction is fast, and the cooling effect is very good. The cold iron installed on the sand core, because the sand core is a poor conductor of heat, the thermal conductivity effect is poor, mainly relying on the heat capacity of the cold iron itself to absorb heat, and the cooling effect is limited.

2.3 Process control of aluminum casting casting

(1) Guarantee of mold temperature
The mold temperature affects the solidification sequence of the casting, and some foreseeable casting defects can be transferred to the casting’s riser system and machining allowance. In order to ensure that the casting is gradually cooled and solidified according to the mold riser design scheme, the temperature of the upper mold is generally set 20~30 °C higher than the temperature of the lower mold.
(2) Coating control
In aluminum casting gravity casting, the most difficult to master and the most difficult to document or pass on orally is the spraying of mold coating, but its impact on the quality of the casting is also crucial. In the thin-walled part of the casting, in order to ensure the fluidity of the molten aluminum and ensure the integrity of the casting molding, the paint must be sprayed
thicker coating; In the thick part of the casting, the paint spraying near the shrinkage end of the molten aluminum is thicker and gradually thinned; In some small arc roots, easy to produce cracks in the transition place, etc. should be made as thin as possible, or even partially not painted, if necessary, the corresponding part of the mold can be scratched heat ribs. In production, where the draft slope on the mold is small, the paint is easy to fall off, so more observation should be carried out and the paint should be replenished in time.
(3) Production beat
In the production process, it is necessary to ensure the continuity of production and the stability of the production cycle, or the cycle stability of several consecutive production cycles. The continuity of production and the stability of the beat are relatively easy to understand, mainly to control the stability of the mold temperature field and the stability of the casting quality. join
The cycle stability of the next few beats mainly refers to the fact that the quality of the casting cannot be adjusted by adjusting the process parameters, coatings, and mold temperature settings after several consecutive beats, and the mold temperature needs to be heated or cooled down, and the production has a short pause.

3 Conclusion

In aluminum casting gravity casting, there are many factors that affect quality. Choosing a reasonable casting method, controlling the quality of molten aluminum, and optimizing the casting process and parameters are the main ways to improve the quality of castings.