Aluminum metal has a melting point of only 660°C. Generally speaking, a lower melting point facilitates stretching. However, why is the actual stretching process so challenging? Kailai Machinery, drawing on 30 years of experience in custom metal parts processing, will help you answer this question.

Aluminum's melting point exacerbates the problem of sticking during stretching.

· Aluminum's melting point is only around 660°C, far lower than that of common metals like steel. This property is not advantageous during stretching. The constant friction between the mold and the aluminum material during stretching rapidly generates heat. Due to aluminum's low melting point, this localized temperature increase can easily soften the material surface.

· The softened aluminum material comes into direct contact with the microscopic uneven structure of the mold surface. Combined with aluminum's inherent low yield strength and easy adhesion, adhesion quickly occurs, resulting in "mold sticking," which can cause scratches and roughening on the workpiece surface, and in severe cases, even tearing of the material.

· In contrast, high-melting-point metals are less likely to soften rapidly under the same friction conditions, and their risk of adhesion is far lower than that of aluminum. This is one of the core reasons why aluminum's low melting point actually increases the difficulty of stretching.

Aluminum's Crystal Structure and Oxide Film Amplify Tensile Resistance

· Aluminum has a face-centered cubic lattice structure with a high number of slip systems. In theory, this should provide excellent plasticity, but this structure actually presents challenges during stretching. Aluminum has a high solid solubility with the mold metal and a strong alloying ability. This facilitates atomic interactions during stretching, further increasing the tendency to adhere.

· Aluminum rapidly forms an oxide film approximately 4nm thick in air. This film is hard and brittle, tightly bonded to the substrate, and prevents it from deforming synchronously with the material during stretching. This hard and brittle oxide film intensifies friction with the mold and is prone to cracking under tensile stress, forming scratches or cracks, significantly increasing processing difficulty.

· The presence of this oxide film, combined with aluminum's low melting point, means that the stretching process must contend with both softening and adhesion and the frictional resistance caused by the oxide film. These dual challenges significantly increase processing difficulty. 

Thermal Effects Caused by Low Melting Point Undermine Tensile Stability

· Although aluminum has excellent thermal conductivity, frictional heat generated during the drawing process is much faster than it can be dissipated. Furthermore, due to its low melting point, heat accumulation easily reaches the material's softening temperature. This softened aluminum's strength drops dramatically, making it susceptible to localized excessive deformation under tensile stress, leading to dimensional deviations or cracking in the workpiece.

· Temperature fluctuations also affect aluminum's plasticity. When the temperature approaches its melting point, the material's plasticity fluctuates abnormally, causing otherwise uniform tensile deformation to become unstable, increasing the difficulty of process control.

· In contrast, high-melting-point metals experience less impact on material properties due to temperature fluctuations during drawing, resulting in far greater processing stability than aluminum, further highlighting the detrimental impact of aluminum's low melting point on the drawing process.

The compositional characteristics of aluminum alloys used in actual drawing further complicate matters.

· Currently, aluminum materials used in industrial drawing are almost all aluminum alloys (such as 6-series and 5-series aluminum alloys), not pure aluminum. While the addition of alloying elements such as magnesium, silicon, and copper to aluminum alloys improves the material's strength and corrosion resistance, it also alters the aluminum's inherent plasticity, leading to a decrease in tensile properties.

Alloying elements form secondary phase particles (such as Mg₂Si and CuAl₂) within the aluminum matrix. These particles are relatively hard and hinder the sliding of metal atoms during stretching, increasing the material's deformation resistance. This requires greater tensile force to achieve plastic deformation and can easily lead to stress concentration around the particles, causing cracking.

Also, the addition of alloying elements slightly lowers the melting point of aluminum alloys (some alloys even melt below 600°C). Combined with the aforementioned issues of softening and adhesion due to their low melting point, aluminum alloys face both increased deformation resistance due to increased strength and thermal effects due to their low melting point during stretching. These dual factors make stretching significantly more difficult than pure aluminum and other high-melting-point metals.

Aluminum's Material Properties Place Strict Requirements on Lubrication

The core role of lubrication in the stretching process is to isolate the material from the die and reduce frictional heat generation. However, aluminum's low melting point, prone to adhesion, and the presence of hard, brittle second-phase particles in aluminum alloys prevent common lubricants (such as motor oil and peanut oil) from forming a stable lubricating film.

• Common lubricants have poor cooling properties and insufficient extreme pressure and anti-wear properties. They quickly fail under the high-temperature, high-pressure environment of aluminum drawing. Not only do they fail to address adhesion, but they can also cause increased friction due to oil film rupture, leading to workpiece damage. Furthermore, the hard, brittle second-phase particles can scratch the lubricating film, further impairing lubrication.

• Specialized aluminum drawing oils are essential to meet these requirements. These oils must possess excellent lubrication, extreme pressure, and cooling properties, forming a durable protective film on the material surface while also addressing scratches from aluminum alloy particles. However, the selection and adaptation of specialized oils themselves add complexity to the drawing process.