Analyzing Titanium Extrusion: Key Factors Influencing Metal Flow

Hot extrusion plays a crucial role in titanium material processing. However, the unique physicochemical properties of titanium and titanium alloys make this process far more complex than that of aluminum alloys, copper alloys, or even steel. The uniformity of metal flow directly affects the quality of extruded products. Today, we will delve into the key factors influencing metal flow during titanium material extrusion.

 

 

The Challenges Arising from Titanium's Inherent Characteristics

 

Titanium rods and titanium alloy billets have low thermal conductivity, a characteristic that poses significant challenges during hot extrusion. When the extrusion barrel temperature reaches 400 degrees Celsius, the temperature difference between the surface and inner layers of the billet can reach 200-250 degrees Celsius. Coupled with the effect of gas absorption strengthening, the metal on the surface and in the center of the billet exhibits significant differences in strength and plasticity, resulting in extremely uneven deformation during extrusion. This leads to substantial additional tensile stress on the surface layer, which is the root cause of cracks and fissures on the surface of extruded products.

 

Furthermore, titanium material, at 980°C and 1030°C, forms a fusible eutectic with iron-based or nickel-based alloy mold materials, causing severe mold wear. Therefore, lubricants are currently required for the extrusion of titanium alloy bars.

 

Core Factors Affecting Metal Flow

 

Extrusion Method

Different extrusion methods significantly affect the uniformity of metal flow:

Reverse extrusion is superior to forward extrusion because it changes the direction and degree of friction between the metal and the extrusion cylinder, reducing frictional resistance to metal flow and allowing for smoother flow.

Cold extrusion results in more uniform metal flow than hot extrusion. During cold extrusion, the metal is in a cold state, with high deformation resistance but a stable internal grain structure, leading to relatively uniform deformation across different parts. During hot extrusion, the metal is at a high temperature, reducing deformation resistance, but uneven temperature distribution can easily lead to uneven flow.

 

Lubricated extrusion is better than unlubricated extrusion. The lubricant forms a lubricating film between the metal and the mold, reducing friction and resistance, resulting in more uniform metal flow. Therefore, the extrusion method primarily affects metal flow by altering friction conditions.

 

Extrusion Speed

Increasing the extrusion speed exacerbates the unevenness of metal flow. This is because excessive speed prevents the metal from fully deforming and coordinating its flow, leading to uneven internal stress distribution. For example, during high-speed extrusion, the metal near the inner wall of the extrusion cylinder flows slowly due to high friction, while the metal in the center flows quickly, creating a significant difference.

 

Extrusion Temperature

As a key factor, increased extrusion temperature reduces the billet's deformation resistance but exacerbates uneven metal flow. If the extrusion cylinder and die are heated too low, the temperature difference between the outer and inner layers increases, further worsening the uneven flow. Because the plasticity and strength of the metal differ at different temperatures, high-temperature areas exhibit better plasticity and faster flow, while low-temperature areas show the opposite. Furthermore, the better the metal's thermal conductivity, the more uniform the temperature distribution on the billet's end face, resulting in more uniform metal flow.

 

Metal Strength

Under the same conditions, higher metal strength leads to more uniform flow. High-strength metals have strong internal grain bonding, better transmitting stress and enabling coordinated deformation across different parts; low-strength metals, due to weak grain bonding, are prone to localized uneven deformation.

 

Die Angle

The die angle (the angle between the die end face and the central axis) has a significant impact on metal flowability. A larger die angle results in more uneven metal flow. This is because a large die angle causes uneven resistance distribution as the metal passes through the die; the metal near the inner wall of the die experiences greater resistance and flows more slowly, while the center experiences the opposite. However, when using a multi-hole die for extrusion, if the die holes are arranged reasonably, the metal flow in each hole can become more uniform, improving the overall flow.

 

Deformation Degree

Both excessive and insufficient deformation degrees lead to uneven metal flow. If the deformation degree is too small, the internal stress of the metal is low, insufficient to promote sufficient flow, easily resulting in localized areas of no deformation or insufficient deformation. If the deformation degree is too large, the internal stress is too high, leading to defects such as cracks in the metal. Additionally, the metal near the deformation zone flows faster than that further away.