introduction
With the improvement of automobile emission standards, reducing engine emissions, reducing fuel consumption, and improving combustion efficiency have all made cars tend to be lightweight,Lightweight is of great significance for the development of the automotive industry. According to statistics, currently domestic domestic passenger cars have a weight increase of about 8% to 10% compared to similar foreign cars,casting defects Domestic domestic commercial vehicles have a weight that is about 10% to 15% higher than similar foreign vehicles. According to research figures, if the weight of a car is reduced by 10%, fuel efficiency will decreaseThe rate can be increased by 6% to 8%.casting defects In terms of materials, many cast steel, cast iron structural and functional components are gradually being replaced by aluminum alloys or other alloys with lower density,casting defects includingThe proportion of aluminum alloy parts in the cylinder block and cylinder head of some gasoline engines, especially in new energy vehicles, is increasing. In recent years, domestic aluminum alloy castingsSignificant progress and breakthroughs have also been made in casting technology, casting defects providing strong support for the rapid development of new energy vehicles.There are several methods for forming aluminum castings, including high-pressure casting, low-pressure casting, and gravity casting,casting defects each of which has its own unique advantages and disadvantagesPoint. However, for some cavity and tube castings with complex shapes (especially inner cavity shapes), disappearing cores such as sand cores, salt cores, wax molds, etc. are required,Generally, gravity casting is used.
Material composition, modification control, and purification treatment of aluminum alloy castings
The composition of alloys has a significant impact on their structure and properties, and controlling the composition of alloys is also one of the important tasks in the alloy treatment process. oftenThe composition of conventional elements is generally analyzed using a direct reading spectrometer, which can quickly and accurately grasp the composition state of alloys.An appropriate amount of trace elements can be added to aluminum silicon alloys to refine grain size and enhance their fluidity, such as intermediate alloys for elements such as titanium, zirconium,casting defects and vanadium.At the same time, refining the grain size also has the effect of improving the filling and shrinking ability of the alloy, reducing the tendency of hot cracking, and improving the electrochemical properties of the alloy. Magnesium in ZL104 willThe precipitation effect of MgSi2 in the aluminum matrix has a significant strengthening effect on the alloy. casting defects By controlling the magnesium content, good plasticity and resistance can be achievedComprehensive properties such as tensile strength and yield strength.In the chemical composition of aluminum alloys, there are often some impurity elements that are harmful to the alloy’s performance, mainly including iron, tin, lead, calcium, etc.,casting defects which have an impactThe biggest one is iron. These impurity elements may be introduced from the furnace charge during the alloy melting process, or they may be introduced into the alloy from the crucible or tool.Iron in aluminum silicon alloys mainly appears in the form of Al9Fe2Si2, with a hard and brittle iron phase that often passes through in the form of coarse needles α Phase grains, weaken the matrix, and reduce aluminumSilicon alloy performance. When Fe phase is present in aluminum silicon alloy, the surface oxide film will lose continuity and precipitate at grain boundaries β Iron phase can also promote aluminum siliconElectrochemical corrosion occurs in alloys,casting defects resulting in a decrease in their corrosion resistance. Therefore, it is necessary to strictly control the iron content in the alloy, and use rotating N2 or inert gas to remove H and metal oxide inclusions from the aluminum liquid. At the same time, flux is used to minimize the slag in the aluminum liquid.The main factors affecting the spoilage process are: type and dosage of spoilage agent, spoilage temperature, and spoilage time.
Type and dosage of modifier: The appropriate type of modifier should be selected based on the type of alloy, casting process, and specific requirements for microstructure controlClass and dosage. Choosing a non-toxic, pollution-free, and long-lasting modifier is currently the development direction of aluminum alloy smelting technology. Aluminum casting aluminum alloyThe commonly used methods for gold modification include Na modification, casting defects Sr modification, rare earth modification, etc. We adopt strontium modification agents with better comprehensive performance and combine them with titaniumMetaalloys are used as refining agents.In production practice, it should be considered that the reaction of the modifier may not be complete, so the amount of modifier used should not be too small, otherwise the modification effect will be poor.casting defects But the amount of spoilage agent should not be too much, otherwise it will cause excessive spoilage. Therefore, the amount of modifier generally accounts for 0.2% to 1.0% of the weight of the furnace charge.In production, adding 0.5% is usually sufficient to ensure good spoilage effect. For castings cast in metal molds, the amount of modifier can be appropriately reduced.Metamorphic temperature: casting defects A higher temperature of aluminum liquid is beneficial for the metamorphic reaction, with a faster metamorphic speed and better effect. However, the metamorphic temperature should not be too high, otherwiseThe oxidation and suction of aluminum liquid will increase sharply,casting defects and the service life of the crucible will be reduced. Generally speaking, the metamorphic temperature should be selected slightly higher than the pouring temperature10 ℃ is better, so as to avoid high modification temperature, reduce the time for temperature adjustment after modification, and improve the modification effect and aluminum liquidMetallurgical quality.Metamorphic time: The higher the metamorphic temperature and the better the contact between the aluminum liquid and the metamorphic agent, the shorter the required metamorphic time. The time of deterioration should beDetermine based on specific circumstances and experiments. If the metamorphic time is too short, the metamorphic reaction will not proceed completely; If the deterioration time is too long, casting defects it will increase the deteriorationThe burning of the agent increases the suction and oxidation of the alloy.
Casting Quality Control of Aluminum Alloy Castings
Due to the complex structure of most casting products, which are thin-walled pipe parts with significant differences in wall thickness and multiple hot spot positions, forming difficulties during castingDifficult, ordinary pouring methods cannot fully shape it.casting defects To select the most reasonable molding method, use casting simulation analysis software such as PROCAST to analyzeAnalyze the parts. To ensure the complete molding of the product and facilitate the discharge of gas from the mold cavity and sand core, a tilting pouring method is adoptedThe tilt angle, casting defects tilt speed, and action mode of the equipment must be infinitely adjustable. Tilting pouring is beneficial for complete filling of aluminum liquid, and in tilting pouringDuring the process, the flow of the alloy liquid is relatively stable, and the flushing force on the mold cavity is small. The filling of the aluminum liquid from slow to fast and then to slow can achieve the alloy liquid in the mold cavityThe laminar flow and sequential exhaust are beneficial for the gas to be discharged from the parting surface,casting defects exhaust plug, exhaust groove, and riser, and are also beneficial for the inevitable absence of gas in the aluminum liquidCompletely clean impurities, slag, and other impurities that float up and accumulate in the sprue can result in castings with dense structure and high mechanical properties. Inclined pouringWhen pouring, adjust the tilting angle and flipping speed of the mold according to the shape and forming sequence of different castings to achieve the best effect.
Choose a reasonable casting process and parameters
The main casting process parameters that affect the molding and quality of castings include pouring temperature, pouring speed, cooling strength, casting defects solidification sequence, etc.⑴ Pouring temperatureRefers to the temperature of the liquid injected into the mold. If the pouring temperature is too low, the gas in the liquid during crystallization cannot float up and escape from the liquid surface in time, causing pores and drainageLoose, may also cause quality defects in castings such as slag inclusion and cold shuts; If the pouring temperature is too high,casting defects it will lead to the formation of oversized grains during crystallization, due to excessive volume shrinkageDue to insufficient filling and shrinkage of the sprue, significant shrinkage porosity and shrinkage holes are formed, resulting in excessive casting stress and cracking. casting defects It is also possible to produce structural defects in feather products.Therefore, the scientifically standardized pouring temperature is generally 70-100 ℃ higher than the actual crystallization temperature of the alloy.At the same time, the pouring temperature should be adjusted according to the differences in product structure and shape, as well as the differences in wall thickness. For example, due to the complex structure of a certain intake pipe, the wall thickness differenceThere is a significant variation in wall thickness (from 3mm to 20mm within a length range of 10mm), with many protrusions and difficulty in filling the mold. The pouring temperature needs to reachIt takes about 760 ℃ to form completely.⑵ Pouring speedThe pouring speed is not a fixed speed, and the start and process of casting are not at the same pouring speed: the speed of pouring affects the aluminum liquidFilling, exhaust,casting defects casting molding, crystallization, microstructure, and properties have a significant impact. On the premise of ensuring the quality of castings, the highest pouring speed should be adopted.⑶ Cooling intensityCooling intensity, also known as cooling rate, not only affects the internal stress and cracks of castings, but also affects the crystallization rate of castingsThe direction of grain growth and other organizational factors have a greater impact. As the cooling rate increases, the crystallization rate of the casting increases, and the intragranular structure becomes more refined; along withThe cooling intensity increases,casting defects the casting liquid cavity becomes shallower, and the size of the transition zone decreases, which improves the metal shrinkage conditions and reduces or eliminates porosity in the castingDefects such as pores; The density of the casting can be increased, casting defects and the size of the initial compound can be refined to reduce the degree of regional segregation.
In order to achieve sufficient cooling speed during gradual crystallization and obtain a better microstructure, circulating water cooling is generally used to form a largeTemperature gradient enables rapid cooling of castings. The requirement of cooling intensity on the temperature of cooling water cannot be ignored. Generally, casting defects the cooling water temperature is set atAround 20-30 ℃, the temperature of our circulating cooling water is generally stable at around 25 ℃, and the cooling intensity can be set as a curve according to the needs of the casting process,The adjustment of cooling time and interval can be set and controlled by the equipment PC to eliminate internal stress in the casting and reduce the possibility of defects such as cracksSex means obtaining high-quality castings.(4) Solidification sequenceSome products are affected by the structure, resulting in significant differences in wall thickness or local wall thickness that is too thick. Due to the limitations of the mold structure, it is not possible to arrange cooling water pipes, casting defects orBecause the distance between the cooling water pipes is too far to achieve rapid cooling, it is necessary to place cold iron on the mold or sand core for forced coolingCooling improves the metal shrinkage conditions and reduces or eliminates defects such as porosity and porosity in castings.Cold iron materials commonly used in practical production include: copper, cast iron, steel, aluminum alloy, etc. Among them, copper has the highest thermal conductivity,The best cooling effect. But red copper also has its limitations in production, due to its higher cost compared to other materials, and its high melting point, such as cold ironThe shape cannot be obtained by casting and must be machined. The forming cycle is long, the process is complex, and it is not suitable for large-scale production. Cast iron,The thermal conductivity of steel and aluminum alloys is lower than that of copper, and the cooling effect is slightly worse. Therefore, in production applications, the effective surface area and volume of cold iron must be determinedSlightly larger. But they can all be obtained by casting, with a simple molding process and suitable for large-scale production.The installation arrangement of cold iron can generally be divided into two types: one is directly installed on the mold, usually using purple copper material; casting defects Another method is to place it in sandOn the core, other materials can be used. Both installation methods have their own advantages and disadvantages. Arrange and install copper cold iron on the mold, with the shape, position, and size of the cold ironDue to the limitations of the mold structure, casting structure, mold heating pipeline, and mold cooling water circuit, the cold iron contacts the mold material and conducts heat quickly,The cooling effect is very good. The cold iron installed on the sand core has poor thermal conductivity due to the poor thermal conductivity of the sand core, mainly relying on the thermal capacity of the cold iron itselfHeat absorption, casting defects limited cooling effect.
Process Control of Aluminum Casting Casting
⑴ Guarantee of mold temperatureThe temperature of the mold affects the solidification sequence of the casting, which can transfer some foreseeable casting defects to the gating system and machining allowance of the castingMedium. To ensure that the casting gradually cools and solidifies according to the design scheme of the mold sprue, the temperature of the upper mold is generally set to be 20-30 ℃ higher than the temperature of the lower mold.⑵ Paint controlIn gravity casting of aluminum castings, the most difficult to master and document or verbally inherit is the spraying of mold coatings. However, it has a significant impact on castingsThe impact of quality is also crucial. In the thin-walled part of the casting, in order to ensure the fluidity of the aluminum liquid and the integrity of the casting, the coating must be sprayedApply thicker; In the thicker parts of the casting, the coating is sprayed thicker and gradually thinner near the aluminum liquid filling end;casting defects And at the roots of some smaller arcsTransition areas that are prone to cracking should be coated as thinly as possible, or even partially unpainted. If necessary, heat dissipation ribs can be engraved on the corresponding parts of the mold.In production, areas with small draft angles on the mold are prone to paint peeling off. It is important to observe and promptly spray the paint.⑶ Production rhythmIn the production process, it is necessary to ensure the continuity of production and the stability of production rhythm, or the stability of several consecutive production cycles.The continuity of production and the stability of rhythm are relatively easy to understand, mainly to control the stability of mold temperature field and casting quality. evenThe stability of the continuous production cycle for several beats mainly refers to the inability of the casting quality to adjust process parameters, coatings, casting defects and mold temperature after several beats of continuous productionWhen adjusting the mold temperature based on temperature setting, it is necessary to raise or lower the temperature of the mold, resulting in a brief pause in production.
Epilogue
In gravity casting of aluminum castings, there are many factors that affect quality. Choose a reasonable casting method, control the quality of aluminum liquid, casting defects optimize the casting process, andParameters are the main way to improve the quality of castings.