Magnesium alloy is widely used in the aerospace field due to its light weight and high strength. As an anti-gravity casting process, low differential pressure casting can reduce pores, shrinkage cavities, shrinkage porosity, and pinhole defects in castings; improve surface quality of castings; feeding pressure of differential pressure casting is large, feeding effect is obvious, reducing tendency of loose defects, and at the same time reducing tendency of hot cracking during solidification of castings. Therefore, low differential pressure casting has been widely used.     and the porosity defects are reduced It can also reduce tendency of hot cracking when the casting solidifies. Therefore, low differential pressure casting has been widely used. As application of large magnesium alloy shell castings continues to expand in aerospace and aviation fields, in order to obtain high-quality castings, low (differential) pressure casting technology has been widely used and developed. Most of ZM5, ZM6 magnesium alloy and rare earth heat-resistant magnesium alloy shell castings used in aerospace are Class I castings, which require 100% X-ray flaw detection and have high internal quality requirements. ZM5 and ZM6 alloy shell cores are generally made of clay sand surface dry type, and outer shape is clay sand wet mold; rare earth heat-resistant magnesium alloy shell core is made of resin sand, and outer shape is made of clay sand wet mold. It is produced using a differential (low) pressure casting process. Casting defects such as looseness, cracks, segregation, pores, and flux slag inclusions often occur in poured castings.
This topic combines production characteristics of shell castings, summarizes casting defects often encountered in differential (low) pressure casting and proposes corresponding solutions.

Figure 1 is a three-dimensional diagram of shell parts. Material is ZM5, size is Φ420 mm×700 mm, and it needs to be 100% X-ray inspected. It is a Class I casting. Common defects in castings include porosity, segregation, cracks, flux inclusions and pore defects.

 

Defects and Countermeasures of Low-Pressure Casting of Magnesium Alloy Shells

Porosity is main defect in magnesium alloy castings. It is widely distributed and difficult to feed. Crystallization temperature range of magnesium alloy is wide, solidification shrinkage is large, and tendency of mushy solidification is great during solidification, which often causes micropores formed by liquid shrinkage and solidification shrinkage to be dispersed and not replenished by liquid of external alloy, resulting in looseness. Sometimes even shrinkage cracks occur due to excessive intergranular tensile stress during solidification shrinkage.
In magnesium alloy shell casting, porosity mainly occurs in thick and thin transition areas of casting and near gap runner, as well as at intersection between cold iron and core sand. By rating contraction that has occurred, it is generally grade 4, and some even reach severe grade 5. Castings belong to Category I castings, and the highest required level can only be level 3, and looseness level must be strictly controlled.
Technological design
Process of shell castings mostly adopts principle of sequential solidification. System is solidified sequentially from lateral runner to sprue connected to liquid riser mouth, thereby ensuring that casting feeding channel is smooth and giving full play to feeding advantages of differential (low) pressure casting. Figure 2 shows magnesium alloy shell casting gating system. Measures such as setting up cold iron and graphite sand to adjust local cooling rate, adding gap runner and transition runner to shorten feeding distance, etc.

 

Defects and Countermeasures of Low-Pressure Casting of Magnesium Alloy Shells
Figure 2: Magnesium alloy shell gating system
Figure 3 is a schematic diagram of cold iron layout of magnesium alloy shell casting. A cold iron must be set up in front of gap runner. Position and thickness of cold iron are particularly important to reduce looseness in front of gate. Gap runner must not only be aligned with cold iron and stagger cold iron gap, but also ensure that cold iron on both sides of gap runner has sufficient width. In front of gap runner, cold iron must gradually transition from thick to thin. Edge of cold iron must be transitioned with graphite sand, and graphite sand must also gradually transition from thick to thin. Edge of cold iron must be transitioned with graphite sand, and graphite sand must also gradually transition from thick to thin.

 

Defects and Countermeasures of Low-Pressure Casting of Magnesium Alloy Shells
Figure 3: Schematic diagram of cold iron layout of the magnesium alloy shell
During process design, cold iron is generally installed at the boss to enhance cooling to reduce hot spots. However, when boss is too thick, loose defects are likely to occur, which are difficult to eliminate even if thickness of cold iron is increased. Designing a feeding runner at boss area to increase feeding can effectively eliminate loose defects.
Pouring temperature
Lowering pouring temperature can reduce overheating of casting and help reduce porosity level, but the lower temperature, the better. Too low a temperature will reduce feeding effect. Generally speaking, pouring temperature can be appropriately lowered when pouring a large-diameter vertical cylinder, and pouring temperature should be appropriately increased when pouring a small-diameter vertical cylinder.
Pouring process
On the premise of ensuring that mold is full, filling speed is generally selected at the lower limit. This can not only avoid turbulence and splashing caused by gas entrainment during mold filling, but also extend baking time of molten metal on runner, thereby adjusting solidification speed of casting and enhancing sequential solidification effect. According to production experience, liquid rising speed is higher than mold filling speed.
Increasing holding pressure can strengthen feeding ability of alloy during solidification, thereby filling microscopic pores between dendrites. However, excessive pressure will cause deformation of casting, sand sticking and misfire, etc. Therefore, the higher pressure, the better. Generally, holding pressure is 30kPa~60kPa.

Segregation of magnesium alloys is generally component segregation, which often leads to direct scrapping of castings. Alloy smelting process and casting solidification conditions can cause segregation defects in castings. By adjusting refining temperature, improving chilling capacity and lowering pouring temperature, tendency of segregation defects can be effectively reduced.
Refining temperature
During smelting process of magnesium alloy, other metals with higher melting points than magnesium alloy are heavy metals. Phenomenon of segregation of heavy metal elements is due to failure of heavy metal elements to be fully integrated into alloy liquid. If refining temperature is increased, heavy metal elements can be better melted into alloy liquid, and segregation of heavy metal elements in castings is significantly reduced.
Chilling capacity and pouring temperature
Component segregation is likely to occur in gaps between thick-walled cold irons and thin-walled areas between thick walls. Placing graphite sand in these locations can enhance chilling and reduce local overheating, which can effectively prevent occurrence of segregation defects.
Reduce pouring temperature, shorten alloy solidification time, reduce alloy supercooling degree, prevent heavy metals from precipitating during solidification process, and reduce occurrence of segregation. In production of heat-resistant magnesium alloy shells, segregation defects are greatly improved by lowering pouring temperature.

Cracks are fatal casting defects in castings. They appear as straight or zigzag gaps and cracks on castings. Cross-sections are oxidized to black or dark gray, which is easy to occur at thickness junction of boss and lower end frame. Process design, raw material quality and cold iron position will all lead to crack defects.
Technological design
When a casting is designed with multiple adjacent bosses, as shown in Figure 4, crack defects are likely to occur in thin walls between the bosses. Crack defects can be effectively eliminated through properly designed anti-crack ties.

 

Defects and Countermeasures of Low-Pressure Casting of Magnesium Alloy Shells
Figure 4: Where cracks occur
Control raw material quality
It can be seen from heredity of metal that composition of raw magnesium ingot determines quality of casting. A series of defects of magnesium ingots can be inherited to cast parts after casting. When primary magnesium ingots have crack defects, batch crack defects will occur in cast castings. Therefore, strengthening control of crack defects in primary magnesium ingots can effectively eliminate occurrence of batch crack defects.
Cold iron location
Rare earth heat-resistant magnesium alloys shrink greatly and are prone to crack defects when they are hindered during solidification. Once cracks occur, they will develop and penetrate to the end, which is a fatal flaw. Therefore, gap between cold irons must be large enough during modeling, otherwise it will hinder shrinkage of alloy and cause cracks. Generally, gap between cold irons should not be less than 5 mm, and arc surface cold irons should be centripetal to prevent rear parts from touching each other.

Flux plays an important role in covering and refining during magnesium alloy smelting. Improper use will cause flux inclusion defects in lower part of casting position, near inner gate and in dead corners. It is one of common defects of magnesium alloy castings and mainly comes from refining agent and detergent of smelting tool. Standing time of alloy liquid, use of flux and pouring conditions can all lead to flux inclusions.
Magnesium alloys must have a certain standing time after refining. Extend standing time after refining alloy liquid from 20 minutes to 30 minutes. After extending standing time, it ensures that flux can have sufficient time to separate from alloy liquid, and slag inclusion degree of casting will be significantly improved.
Alloy is refined with argon gas, and upper limit of amount of refining flux is used only for covering, reducing amount of flux. Temperature of flux crucible is controlled at 800℃, so that flux on smelting tool and riser pipe can easily flow down. These measures can reduce occurrence of flux inclusion defects.
There should be sufficient distance between rising tube and bottom of crucible to prevent flux from being sucked into rising tube and poured into casting during pouring.

Air holes are a common defect in housings. Generally divided into entangled pores and intrusive pores, factors such as use of liquid riser, quality of chilled iron, air permeability of molding sand, baking quality of mold and pouring speed will all affect formation of pore defects in castings.
Lift tube usage
After repeated use, liquid riser tube will corrode due to long-term immersion in alloy liquid. If local corrosion is too rapid, pits will be formed. As use time increases, pits will deepen until air leakage occurs. Pores caused by air leakage in riser tube are larger, so riser tube must be carefully inspected before use, and any dents must be replaced in time.
Cold iron quality
If pores appear on the surface of casting in contact with cold iron, it may be caused by oil stains on the surface of cold iron or insufficient baking of cold iron surface. Cold iron must undergo sand blowing treatment before sanding, and must be baked thoroughly after sanding. Surface of cold iron that has not been fully baked will turn yellow if sand is hung on it. For cold iron with too large quenching area, air grooves can be opened on the surface to enhance exhaust capacity of cold iron during pouring.
Molding sand breathability and mold baking
Control mud content of original sand and adjust particle size of original sand from 70/140 mesh to 40/70 mesh to improve air permeability of molding sand; bake mold twice to improve baking quality of mold. These measures can effectively reduce occurrence of pores on the surface of casting.
In actual production, it was found that subcutaneous pores are prone to occur on thin wall of shell, especially when thin wall area is large, which appear as black spots during X-ray inspection and are often misjudged as flux inclusions. By opening shallow exhaust grooves in thin-walled mud core, surface pore defects are reduced.
Choose a reasonable filling pressure
Defects such as air entrainment often occur in castings during pouring process. Main reason is that filling speed is too fast during filling process. At the same time, due to complex internal structure of cabin, sudden change in wall thickness of casting will also cause instantaneous flow rate of liquid to continuously change, increasing possibility of air entrainment. Reducing filling rate can effectively prevent occurrence of entangled pores.

(1) In low differential pressure casting of magnesium alloy shells, each link such as raw materials, melting tools, molten metal quality, melting process and refinement and refining must be strictly controlled to ensure that molten metal is qualified and to produce high-quality castings. premise.
(2) Reasonable casting technology is the key to solving low differential pressure casting defects of magnesium alloy shells, which can effectively reduce production costs.