1、 Introduction to Casting

Among casting alloys, aluminum alloy casting is the most widely used and incomparable to other alloys. The types of aluminum alloy casting are as follows:

Due to the different components of aluminum alloys, their physical and chemical properties vary, and their crystallization processes are also different. Therefore, it is necessary to select casting methods reasonably based on the characteristics of aluminum alloys in order to prevent or reduce the occurrence of casting defects within the permitted range, and thus optimize castings.

1. Aluminum alloy casting process performance

The casting process performance of aluminum alloy is usually understood as the comprehensive performance of the most outstanding properties during the filling, crystallization, and cooling processes of the mold. Liquidity, shrinkage, airtightness, casting stress, and gas absorption. These characteristics of aluminum alloy depend on the composition of the alloy, but are also related to casting factors, alloy heating temperature, complexity of the mold, sprue system, sprue shape, etc.

(1) Liquidity

Liquidity refers to the ability of alloy liquid to fill the mold. The size of fluidity determines whether the alloy can cast complex castings. The flowability of eutectic alloys is the best in aluminum alloys.

There are many factors that affect fluidity, mainly including composition, temperature, and the presence of solid particles of metal oxides, metal compounds, and other pollutants in the alloy liquid. However, the fundamental external factors are the pouring temperature and pouring pressure (commonly known as pouring head).

In actual production, in the case where the alloy has been determined, in addition to strengthening the melting process (refining and slag removal), it is also necessary to improve the casting processability (sand mold permeability, metal mold exhaust and temperature), and increase the pouring temperature without affecting the quality of the casting to ensure the fluidity of the alloy.

(2) Contractility

Shrinkage is one of the main characteristics of cast aluminum alloys. Generally speaking, alloys are divided into three stages from liquid casting to solidification, and then to cooling to room temperature, namely liquid shrinkage, solidification shrinkage, and solid shrinkage. The shrinkage of alloys has a decisive impact on the quality of castings, affecting the size of shrinkage cavities, the generation of stress, the formation of cracks, and changes in size. Usually, casting shrinkage is divided into bulk shrinkage and linear shrinkage, and in actual production, linear shrinkage is generally used to measure the shrinkage of alloys.

The shrinkage size of aluminum alloy is usually expressed as a percentage and is called the shrinkage rate.

① Body contraction

Body contraction includes liquid contraction and solidification contraction.

From pouring to solidification, the casting alloy liquid will exhibit macroscopic or microscopic shrinkage at the final solidification point. This macroscopic shrinkage caused by shrinkage is visible to the naked eye and can be divided into concentrated shrinkage and dispersed shrinkage. The diameter of the concentrated shrinkage cavity is large and concentrated, and distributed at the top of the casting or at the hot nodes with thick cross-sections. The dispersed shrinkage cavity morphology is dispersed and small, mostly distributed in the axis and hot joint of the casting. Microscopic shrinkage pores are difficult to see with the naked eye, and most of them are distributed under grain boundaries or between dendrites of dendrites.

Shrinkage and porosity are one of the main defects in castings, caused by liquid shrinkage being greater than solid shrinkage. In production, it has been found that the smaller the solidification range of cast aluminum alloys, the easier it is to form concentrated shrinkage cavities, and the wider the solidification range, the easier it is to form dispersed shrinkage cavities. Therefore, in the design, it is necessary to make the cast aluminum alloy conform to the principle of sequential solidification, that is, the volume shrinkage of the casting during the liquid to solidification period should be supplemented by the alloy liquid, and the shrinkage cavities and looseness should be concentrated in the external riser of the casting. For aluminum alloy castings that are prone to dispersion and looseness, the number of risers should be more than that of concentrated shrinkage holes, and cold iron should be installed at the locations where looseness is likely to occur to increase the local cooling rate and enable simultaneous or rapid solidification.

② Line contraction

The size of wire shrinkage will directly affect the quality of the casting. The greater the linear shrinkage, the greater the tendency for cracks and stress to occur in aluminum castings; The size and shape of the castings also change significantly after cooling.

For different cast aluminum alloys, there are different casting shrinkage rates. Even for the same alloy, the shrinkage rate varies depending on the casting. On the same casting, the shrinkage rates of length, width, and height are also different. It should be determined based on specific circumstances.

(3) Thermal cracking property

The occurrence of hot cracks in aluminum castings is mainly due to the fact that the shrinkage stress of the casting exceeds the bonding force between the metal grains, and most of them occur along the grain boundaries. From the observation of the crack fracture surface, it can be seen that the metal at the crack site is often oxidized and loses its metallic luster. Cracks extend along grain boundaries in a serrated shape, with a wide surface and narrow interior, and some penetrate the entire end face of the casting.

The tendency for cracks to occur in different aluminum alloy castings is also different. This is because the larger the difference between the temperature at which a complete crystalline framework begins to form during the solidification process of cast aluminum alloys and the solidification temperature, the greater the alloy shrinkage rate and the greater the tendency for hot cracks to occur. Even for the same alloy, the tendency for hot cracks to occur varies due to factors such as casting resistance, casting structure, and pouring process. In production, measures such as using setback casting molds or improving the pouring system of cast aluminum alloys are often taken to avoid cracking in aluminum castings. The hot cracking method is usually used to detect hot cracks in aluminum castings.

(4) Air tightness

The airtightness of cast aluminum alloy refers to the degree to which the cavity shaped aluminum casting does not leak under the action of high-pressure gas or liquid. The airtightness actually characterizes the degree of density and purity of the internal structure of the casting.

The airtightness of cast aluminum alloy is related to the properties of the alloy. The smaller the solidification range of the alloy, the smaller the tendency to produce looseness, and the smaller the precipitation porosity, the higher the airtightness of the alloy. The airtightness of the same cast aluminum alloy is also related to the casting process, such as reducing the casting temperature of the cast aluminum alloy, placing cold iron to accelerate the cooling rate, and solidifying and crystallizing under pressure, all of which can improve the airtightness of aluminum castings. The infiltration method can also be used to block the leakage gap to improve the airtightness of the casting.

(5) Casting stress

Casting stress includes three types: thermal stress, phase transition stress, and shrinkage stress. The causes of various stresses are not the same.

① Thermal stress

Thermal stress is caused by uneven thickness and cooling at the intersection of different geometric shapes of castings. Forming compressive stress at the thin wall, resulting in residual stress in the casting.

② Phase transition stress

Phase transition stress is caused by the phase transformation of certain cast aluminum alloys during the cooling process after solidification, resulting in changes in volume and size. Mainly due to uneven wall thickness of aluminum castings, phase transformation occurs in different parts at different times.

③ Shrinkage stress

Aluminum castings are hindered by the mold and core during shrinkage, resulting in tensile stress. This kind of stress is temporary, and aluminum castings will automatically disappear when opened. However, improper opening time can often cause hot cracks, especially for aluminum alloys cast in metal molds, which are prone to hot cracks under such stress.

The residual stress in cast aluminum alloy parts reduces the mechanical properties of the alloy and affects the machining accuracy of the castings. Residual stress in aluminum castings can be eliminated through annealing treatment. Alloys have good thermal conductivity and no phase transformation during cooling. As long as the casting structure is designed reasonably, the residual stress of aluminum castings is generally small.

(6) Inspiratory property

Aluminum alloy is prone to absorbing gases, which is the main characteristic of casting aluminum alloy. The hydrogen gas generated by the reaction between the components of liquid aluminum and aluminum alloys and the moisture contained in furnace materials, organic combustion products, and molds is absorbed by the aluminum liquid.

The higher the temperature of the aluminum alloy melt, the more hydrogen it absorbs; At 700 ℃, the solubility of hydrogen in 100g of aluminum is 0.5-0.9. When the temperature rises to 850 ℃, the solubility of hydrogen increases by 2-3 times. When containing alkali metal impurities, the solubility of hydrogen in aluminum liquid significantly increases.

Aluminum casting alloys not only absorb gas during melting, but also generate gas during casting. As the temperature of the liquid metal entering the mold decreases, the solubility of the gas decreases, and excess gas is released. Some of the gas that cannot escape remains in the casting, forming pores, which are commonly referred to as 'pinholes'. Gas sometimes combines with shrinkage cavities, and the gas precipitated in the aluminum liquid remains inside the shrinkage cavities. If the pressure generated by the heating of bubbles is high, the surface of the pores will be smooth, and there will be a bright layer around the pores; If the pressure generated by bubbles is low, the surface inside the hole will have many wrinkles, resembling 'fly feet', and upon careful observation, it will also have the characteristic of shrinkage.

The higher the hydrogen content in the cast aluminum alloy liquid, the more pinholes are generated in the casting. Pinholes in aluminum castings not only reduce the airtightness and corrosion resistance of the castings, but also decrease the mechanical properties of the alloy. The key to obtaining aluminum castings without or with few pores is the melting conditions. If a covering agent is added for protection during melting, the gas absorption of the alloy is greatly reduced. Refining aluminum melt can effectively control the hydrogen content in the aluminum melt.

2、 Sand casting

The casting method of using sand, clay, and other auxiliary materials to make molds is called sand casting. The materials of sand molds are collectively referred to as molding materials. Sand molds for non-ferrous metal applications are made by mixing sand, clay, or other binders with water.

The process of forming aluminum castings is the interaction between the metal and the mold. After injecting aluminum alloy liquid into the mold, heat is transferred to the mold, while sand mold molds are subjected to the thermal, mechanical, and chemical effects of the liquid metal. Therefore, in order to obtain high-quality castings, in addition to strictly mastering the melting process, it is also necessary to correctly design the proportion, shape, and pouring process of the mold (core) sand.

3、 Metal mold casting

1. Introduction and Process Flow

Metal mold casting, also known as hard mold casting or permanent mold casting, is a method of pouring melted aluminum alloy into a metal mold to obtain castings. Aluminum alloy metal mold casting mostly uses metal cores, but can also use methods such as sand cores or shell cores. Compared with pressure casting, aluminum alloy metal molds have a longer service life.

2. Advantages of Casting

(1) Advantages

Metal molds have a faster cooling rate and a denser casting structure, which can be strengthened by heat treatment. Their mechanical properties are about 15% higher than those of sand casting.

Metal casting has stable casting quality, better surface roughness than sand casting, and a low scrap rate.

Good working conditions, high productivity, and easy for workers to master.

(2) Disadvantages

Metal type has a high thermal conductivity and poor filling ability.

The metal type itself has no breathability. Corresponding measures must be taken to effectively exhaust.

Metal molds have no yielding properties and are prone to cracking and deformation during solidification.

3. Common Defects and Prevention of Metal Castings

(1) Pinhole

Measures to prevent pinholes:

It is strictly prohibited to use contaminated cast aluminum alloy materials, materials contaminated with organic compounds, and materials severely oxidized and corroded.

Control the smelting process and strengthen degassing and refining.

Control the thickness of metal type coatings, as excessive thickness can easily cause pinholes.

The temperature of the mold should not be too high, and measures such as copper inlay or watering should be taken for the thick walled parts of the casting.

Strictly control the moisture content when using sand molds and try to use dry cores as much as possible.

(2) Stomata

Measures to prevent the formation of pores:

Modify the unreasonable sprue and riser system to ensure smooth liquid flow and avoid gas entrapment.

The mold and core should be preheated before coating, and must be fully baked before use.

Sufficient exhaust measures should be considered when designing molds and cores.

(3) Oxidation slag inclusion

Measures to prevent oxidation slag inclusion:

Strictly control the melting process, quickly melt, reduce oxidation, and thoroughly remove slag. Al Mg alloy must be melted under a covering agent.

The furnace and tools should be clean, free of oxides, and preheated. After coating, they should be dried and used.

The designed pouring system must have stable flow, buffering, and slag skimming capabilities.

Adopting a tilted pouring system to stabilize the liquid flow and prevent secondary oxidation.

The selected coating has strong adhesion and does not peel off during the pouring process, resulting in slag inclusion in the casting.

(4) Hot cracking

Measures to prevent thermal cracking:

During the actual pouring system, local overheating should be avoided to reduce internal stress.

The inclination of the mold and core must be ensured to be above 2 °. Once the sprue is solidified, the core can be extracted and the mold can be opened. If necessary, sand cores can be used instead of metal cores.

Control the thickness of the coating to ensure consistent cooling rates for all parts of the casting.

Choose the appropriate mold temperature based on the thickness of the casting.

Refine the alloy structure and improve its thermal cracking ability.

Improve the casting structure, eliminate sharp corners and sudden wall thickness changes, and reduce the tendency for hot cracking.

(5) Loose

Measures to prevent looseness:

Reasonably set up the riser to ensure its solidification and have the ability to compensate for shrinkage.

Adjust the working temperature of the metal mold appropriately.

Control coating thickness and reduce thickness at thick walls.

Adjust the cooling rate of various parts of the metal mold to provide greater chilling capacity at the thick wall of the casting.

Reduce the pouring temperature of the metal appropriately.