What is the melting point of aluminum? And its impact on CNC machining and casting processes

In the processing and application of metal materials, the melting point is a core physical parameter. It not only determines the thermal stability of the material, but also directly affects the selection and control strategy of the processing technology. As the second largest metal in the world (after steel), aluminum is widely used in aerospace, automobile manufacturing, electronic equipment, construction engineering and other fields due to its low density, high specific strength, good conductivity and easy forming.

From complex casting processes to precision CNC machining, the melting point of aluminum (660°C) is like an invisible "baton", silently dominating the temperature field distribution, material deformation behavior, defect generation mechanism and even tool life during the processing process. Understanding the melting point characteristics of aluminum and its impact on the manufacturing process is a key entry point for optimizing the production process and improving product quality.

1. What is the melting point of aluminum?

(1) Melting point of pure aluminum

The melting point of pure aluminum (purity ≥ 99.0%) is 660°C, which is significantly lower than common metals such as steel (1370-1538°C) and copper (1085°C). The low melting point means that aluminum can be melted at relatively mild temperatures, making it a material that is easy to smelt and form. In industry, pure aluminum is mainly used for conductive materials (such as cables) and corrosion-resistant containers, but more often it is formed into aluminum alloys by adding alloying elements (such as silicon, magnesium, copper, zinc, etc.) to meet the performance requirements of different scenarios.

(2) Melting point range of aluminum alloys

The melting point of aluminum alloys is not a fixed value, but varies within a certain range depending on the alloy composition, usually between 570°C and 750°C:

1) Casting aluminum alloys:

ADC12 (commonly used die-casting aluminum alloy): contains 10.0-13.0% silicon and 1.5-3.5% copper, with a melting point of about 580-640°C. Due to its low melting point and good fluidity, it is suitable for manufacturing complex thin-walled parts (such as automobile engine cylinder blocks).

A356 (silicon aluminum alloy for casting): contains 6.5-7.5% silicon and 0.2-0.4% magnesium, with a melting point of about 615-630°C, and is commonly used for precision castings in the aerospace field (such as aircraft wheels).

2) Deformed aluminum alloys:

6061 (heat-treated strengthened alloy): contains 1.0-1.5% magnesium, 0.4-0.8% silicon, melting point of about 652-658°C, suitable for CNC processing and manufacturing structural parts (such as aluminum alloy profiles).

7075 (high-strength aviation alloy): contains 5.6-6.1% zinc, 2.1-2.3% magnesium, melting point of about 475-635°C, due to the wide melting range, the processing temperature control requirements are higher.

Compared with pure aluminum, aluminum alloys generally have lower melting points (due to the "eutectic effect" of alloying elements) and wider melting ranges, which poses different challenges to temperature control during casting and thermal deformation management during CNC processing.

2. The effect of the melting point of aluminum on CNC machining

Although the contact temperature between the tool and the workpiece during CNC machining (usually 300-500°C) does not reach the melting point of aluminum, the melting point still indirectly affects the machining quality and efficiency through the thermophysical properties of the material (such as thermal conductivity and softening temperature).

(1) The dual effect of high thermal conductivity and low melting point

The thermal conductivity of aluminum is as high as 237 W/m·K (about 4 times that of steel and 15 times that of stainless steel). Combined with the relatively low melting point, it forms a unique thermal behavior:

1) Advantage: Cutting heat is easy to diffuse

The cutting heat generated during the machining process is quickly dissipated through the workpiece and chips, and it is not easy to form a high temperature zone locally, reducing the risk of coating failure or matrix softening caused by overheating of the tool. For example, when machining 6061 aluminum alloy, even if the cutting speed is as high as 500m/min, the tool tip temperature can still be controlled below 400°C, which is significantly better than stainless steel (the temperature can reach more than 800°C under the same parameters).

2) Disadvantages: High-speed cutting is prone to "tool sticking"

Aluminum will show obvious plastic softening above 300°C. The low melting point makes it easy to have viscous flow in the high-pressure contact area between the cutting edge and the workpiece (pressure can reach 2000MPa), causing the chips to adhere to the tool tip or the front face of the tool, forming "built-up edge". Built-up edge will destroy the surface roughness of the machined surface (Ra deteriorates from 0.4μm to 1.6μm) and accelerate tool wear (life is shortened by more than 30%).

(2)  Tool selection and coating optimization

To deal with the adhesion problem in aluminum processing, the tool must meet two core requirements:

1) Ultra-smooth surface:

Use carbide tools with mirror polishing (surface roughness Ra<0.05μm), or deposit low friction coefficient coatings (such as ZrN, TiB2, diamond coatings) to reduce material adsorption. For example, the friction coefficient of TiB2 coating is only 0.15 (common TiN coating is 0.4), which can reduce the incidence of built-up edge by 70%.

2) Sharp cutting edge:

The rake angle is designed to be 15°-25° (significantly larger than 5°-10° in steel processing) to reduce cutting force and material deformation. Combined with a cutting edge blunt radius of less than 0.02mm, "shearing" cutting rather than "extrusion" cutting is achieved to avoid material adhesion due to plastic deformation.

(3) Workpiece thermal deformation control

The linear expansion coefficient of aluminum is 23.6×10⁻⁶/°C (about twice that of steel). During long-term high-speed processing, the temperature rise of the workpiece (such as 30°C) may cause a thermal expansion of 0.07mm/m, affecting the dimensional accuracy of precision parts (such as the tolerance requirement of aerospace structural parts ≤0.05mm). Control strategies include:

1) Forced cooling:

Using the micro-quantity lubrication (MQL) technology, 50-100ml/h of vegetable oil is sprayed into the cutting area, which can not only take away the heat (reduce the temperature rise by 15-20°C), but also avoid the corrosion of aluminum by water-based coolant (chloride ions can easily cause pitting corrosion).

2) Segmented processing:

The interval between roughing and finishing is more than 30 minutes, allowing the workpiece to cool naturally; or a water cooling channel is integrated in the fixture to stabilize the workpiece temperature at 25±2°C.

3. The effect of the melting point of aluminum on the casting process

Casting is a molding process that directly utilizes the melting-solidification characteristics of the material. The melting point and melting behavior of aluminum play a decisive role in the casting quality.

(1)  Melting and holding temperature setting

The melting temperature of aluminum alloy is usually controlled at 50-100°C above the melting point, that is, 680-750°C (adjusted according to the alloy type):

1) Temperature is too low (such as only 20°C above the melting point):

The melt has poor fluidity, which can easily lead to insufficient filling of the casting (such as lack of meat in thin walls) or "cold shut" defects due to inclusion of unmelted particles.

2) Temperature is too high (such as over 800°C):

The oxidation of aluminum is intensified (forming Al₂O₃ film), and the melt absorption volume increases (mainly absorbing hydrogen), and dispersed pores (diameter 0.1-1mm) are formed after cooling. In industrial practice, for every 100°C above the melting point, the oxide inclusion defect rate increases by 20%, and the hydrogen solubility increases by 3 times.

(2) Casting behavior and casting quality

The low melting point gives aluminum alloy excellent fluidity (flow length can be up to 3 times that of steel), making it suitable for manufacturing complex structures with wall thickness ≤1mm (such as mobile phone middle frame, automobile gearbox housing). However, it should be noted that:

1) Mold temperature matching:

The difference between the pouring temperature and the mold temperature (superheat) determines the cooling rate of the melt. Taking ADC12 as an example, when the superheat is controlled at 80-100°C, the grain size of the casting can reach less than 50μm, and the mechanical properties (tensile strength ≥230MPa) are the best; if the superheat is insufficient, shrinkage cavities (diameter 2-5mm) are easily formed due to localized rapid solidification.

2) Prevention of thermal cracking:

Aluminum alloy will produce thermal cracking due to shrinkage stress in the late solidification stage (10-30°C below the solidus temperature). The risk needs to be reduced by optimizing the mold structure (such as increasing the fillet radius ≥3mm) and controlling the cooling gradient (temperature difference ≤50°C/m).

(3) Alloy composition design and melting point control

The addition of alloying elements will change the melting behavior of aluminum through the "eutectic reaction":

1) Lowering the eutectic point:

For example, the eutectic point of aluminum-silicon alloy is 577°C (silicon content 12.6%), which is lower than the melting point of pure aluminum 83°C, so that the alloy can be melted at a lower temperature, which is suitable for die casting process (mold withstand temperature ≤ 600°C).

2) Widening the melting range:

For example, the melting range of 7075 alloy reaches 160°C (475-635°C), which means that there is a long mushy zone during the solidification process, which is prone to segregation (uneven distribution of alloying elements), and needs to be refined through modification (such as adding 0.02% sodium).

4. Summary: Melting point data is an important basis for processing and process design

The melting point of aluminum (660°C) seems simple, but it is like a "key" that unlocks its diverse applications in the manufacturing field:

● In CNC machining, low melting point and high thermal conductivity form a "double-edged sword": it allows high-speed machining to improve efficiency, but also needs to be vigilant against tool sticking and thermal deformation, and relies on tool coating technology and precise temperature control to achieve precision machining.

● In the casting process, the melting point of aluminum directly determines the melting temperature, pouring parameters and alloy composition design. By controlling the superheat and cooling rate, a balance can be found between thin-walled castings and high strength.

Through the introduction of this article, it can be learned that paying attention to the melting point of aluminum during processing and deeply understanding the interactive relationship between materials and processes can truly achieve "applying technology according to materials" and let aluminum, a "light metal", play a "heavy role" in high-end manufacturing.