Performance of investment casting aluminum alloy casting process

Performance of investment casting aluminum alloy casting process

Due to the different components of aluminum alloys, they exhibit varying physical and chemical properties, as well as varying crystallization processes. Therefore, it is necessary to choose a reasonable casting method based on the characteristics of aluminum alloy in order to prevent or reduce the occurrence of casting defects within the allowable range, and thus optimize the casting. Today, precision casting manufacturer Tianyao Metal will discuss in detail the casting process performance of aluminum alloys. It is usually understood as a combination of the most outstanding performance during the process of filling the mold, crystallization, and cooling. Liquidity, shrinkage, airtightness, casting stress, and air absorption. These characteristics of aluminum alloys depend on the composition of the alloy, but are also related to casting factors, alloy heating temperature, complexity of the casting mold, gating system, gate shape, etc.

1、 Liquidity refers to the ability of alloy liquid to fill molds. The size of fluidity determines whether an alloy can cast complex castings. The flowability of eutectic alloys in aluminum alloys is the best. There are many factors that affect fluidity, mainly 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 the pouring pressure head).

2、 Shrinkage is one of the main characteristics of cast aluminum alloys. Generally speaking, alloys can be divided into three stages from liquid pouring to solidification, and finally 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 body shrinkage and linear shrinkage. In actual production, linear shrinkage is generally used to measure the shrinkage of alloys. The size of aluminum alloy shrinkage, usually expressed as a percentage, is called the shrinkage rate.

2.1. Body contraction includes both liquid contraction and solidification contraction.

From pouring to solidification, the casting alloy liquid will experience 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 pore size of concentrated shrinkage is large and concentrated, and is distributed at the top of the casting or at hot spots with thicker cross-sections. The dispersed shrinkage morphology is dispersed and fine, mostly distributed in the axis and hot spot of the casting. Microscopic shrinkage pores are difficult to see with the naked eye, and most of them are distributed below grain boundaries or between 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 pores. The wider the solidification range, the easier it is to form dispersed shrinkage pores. Therefore, in the design, it is necessary to ensure that cast aluminum alloys comply with the principle of sequential solidification, that is, the body shrinkage of castings during the liquid to solidification period should be supplemented by alloy liquid, and shrinkage and porosity should be concentrated in the external riser of castings. For aluminum alloy castings that are prone to dispersed porosity, the number of risers should be greater than that of concentrated shrinkage holes, and cold iron should be installed in areas prone to porosity to increase local cooling speed and enable simultaneous or rapid solidification.

2.2. Linear shrinkage, the magnitude of which will directly affect the quality of the casting. The greater the linear shrinkage, the greater the tendency for cracks and stress in aluminum castings; The size and shape of the casting also change more after cooling. For different cast aluminum alloys, there are different casting shrinkage rates. Even for the same alloy, the shrinkage rates are different for different castings. On the same casting, the shrinkage rates for length, width, and height are also different. It should be determined based on specific circumstances.

3、 The occurrence of hot cracking in aluminum castings is mainly due to the shrinkage stress of the casting exceeding the bonding force between metal grains, which mostly occurs along grain boundaries. From the observation of the crack fracture surface, it can be seen that the metal at the crack is often oxidized and loses its metallic luster. Cracks extend along grain boundaries in a serrated shape, with a wider surface and narrower interior, some penetrating 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 greater the difference between the temperature at which a complete crystalline framework begins to form during the solidification process of aluminum alloy casting and the solidification temperature, the greater the alloy shrinkage rate and the tendency to generate hot cracks. Even for the same alloy, the tendency to generate hot cracks varies due to factors such as the resistance of the casting mold, the structure of the casting, and the pouring process. In production, measures such as using yielding molds or improving the pouring system of aluminum alloys are often used to prevent cracks in aluminum castings. The hot cracking ring method is usually used to detect hot cracks in aluminum castings.

4、 Air tightness refers to the degree to which a cavity type aluminum casting does not leak under the action of high-pressure gas or liquid. Air tightness 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 for porosity, 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 pouring 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 includes three types: thermal stress, phase transformation stress, and shrinkage stress. The causes of various stresses vary.

1. Thermal stress is caused by uneven thickness and inconsistent cooling at the intersection of different geometric shapes of castings. Forming compressive stress at the thin-walled area leads to residual stress in the casting.

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

3. Shrinkage stress is caused by the obstruction of the mold and core during the shrinkage of aluminum castings, resulting in tensile stress. This kind of stress is temporary, and the aluminum casting will automatically disappear when opened. However, improper unboxing time can often cause hot cracks, especially in aluminum alloys cast in metal molds, which are prone to hot cracks under this stress.

The residual stress in aluminum alloy castings reduces the mechanical properties of the alloy and affects the machining accuracy of the castings. The residual stress in aluminum castings can be eliminated through annealing treatment. Alloy has 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、 Aspirability is the main characteristic of cast aluminum alloys, as aluminum alloys are prone to absorbing gases. The components of liquid aluminum and aluminum alloys react with the moisture content of furnace materials, organic combustion products, and molds, resulting in the absorption of hydrogen gas 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 every 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. In addition to air intake during melting, aluminum alloy casting also generates air intake when poured into the mold. As the temperature decreases, the solubility of the liquid metal entering the mold decreases, resulting in the precipitation of excess gas. Some of the gas that cannot escape remains in the casting to form pores, which is commonly referred to as "pinholes". Gas sometimes combines with the shrinkage cavity, and the gas precipitated from the aluminum liquid remains inside the shrinkage cavity. If the pressure generated by heating the bubble is high, the surface of the pore is smooth, and there is a bright layer around the pore; If the pressure generated by the bubbles is small, the surface inside the hole will have wrinkles and look like "fly feet". Upon careful observation, it will also have the characteristic of shrinkage.

The higher the hydrogen content in the casting aluminum alloy liquid, the more pinholes will be generated in the casting. Pinholes in aluminum castings not only reduce the airtightness and corrosion resistance of the castings, but also lower the mechanical properties of the alloy. The key to obtaining aluminum castings with no or few pores is the melting conditions. If a covering agent is added for protection during melting, the gas absorption of the alloy will be greatly reduced. Refining the aluminum melt can effectively control the hydrogen content in the aluminum melt.

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