Characteristics of the Hot Extrusion Process for Titanium and Titanium Alloys

The hot extrusion process for titanium and titanium alloy billets is characterized by low thermal conductivity. During hot extrusion, this can cause a significant temperature difference between the surface layer and the inner layer of the billet. When the temperature of the extrusion barrel is 400°C, the temperature difference can reach 200-250°C. Under the combined influence of gas absorption strengthening and a large temperature difference across the billet section, the metal on the surface and at the center of the billet exhibits very different strength and plasticity properties. This results in highly uneven deformation during extrusion, generating large additional tensile stresses in the surface layer, which can lead to cracks and fissures on the surface of the extruded products. The hot extrusion process for titanium and titanium alloy products is more complex than that for aluminum alloys, copper alloys, and even steel, due to the unique physical and chemical properties of titanium and its alloys.

Major Factors Affecting Metal Flow During Extrusion:

1. Extrusion Method:

   • Reverse extrusion results in more uniform metal flow compared to forward extrusion.
   • Cold extrusion leads to more uniform metal flow than hot extrusion.
   • Lubricated extrusion results in more uniform metal flow compared to non-lubricated extrusion.
   • The impact of extrusion methods is realized by altering friction conditions.

2. Extrusion Temperature:

   • Higher extrusion temperatures reduce billet deformation resistance but increase non-uniform metal flow.
   • If the heating temperature of the extrusion barrel and die is too low during extrusion, the metal temperature difference between the outer and central layers increases, leading to more uneven metal flow.
   • Metals with better thermal conductivity exhibit more uniform temperature distribution across the billet end face.

3. Metal Strength:

   • Under similar conditions, higher metal strength results in more uniform metal flow.

4. Die Angle:

   • A larger die angle (i.e., the angle between the die face and the central axis) results in less uniform metal flow.
   • When using multi-hole dies for extrusion, reasonable die hole arrangement can lead to more uniform metal flow.

5. Degree of Deformation:

   • Both excessive and insufficient degrees of deformation result in uneven metal flow.

6. Extrusion Speed:

   • Increasing extrusion speed exacerbates the non-uniformity of metal flow.

Kinetics of Metal Flow in Industrial Titanium Alloys

Studies have shown that the flow behavior of metals varies significantly within the temperature range corresponding to different phase states of various alloys. Therefore, one of the primary factors influencing the extrusion flow characteristics of titanium and titanium alloys is the billet heating temperature that determines the metal phase state.

Extrusion at temperatures in the α or α+β phase regions results in more uniform metal flow compared to extrusion at temperatures in the β phase region. Achieving high surface quality in extruded products is particularly challenging. To date, the extrusion process for titanium alloys must use lubricants. This is primarily because, at temperatures of 980°C and 1030°C, titanium can form eutectics with iron-based or nickel-based alloy die materials, leading to severe die wear.

When graphite lubricants are used, deep longitudinal scratches can form on the product surface due to titanium adhesion to the working parts of the die. When glass lubricants are used for extrusion, a new type of defect called "orange peel" can appear, characterized by cracks in the surface layer of the product. Research indicates that the appearance of "orange peel" is due to the low thermal conductivity of titanium and titanium alloys, which causes the surface layer of the billet to cool rapidly and its plasticity to decrease sharply.