1. Low thermal conductivity characteristics: When the titanium alloy hot extrusion barrel temperature is 400℃, the temperature difference between the surface and inner layer of the billet can reach 200-250℃, resulting in a significant strength gradient and plasticity difference between the surface and the center. The superposition effect of thermal stress and deformation stress can easily lead to surface tensile stress concentration, which becomes the cause of extrusion cracks.
2. Phase change sensitive characteristics: Metal flow behavior is closely related to the phase state. When extruding in the α or α+β phase region (700-950℃), the flow uniformity is significantly better than that in the β phase region (above 950℃). This phase dependence directly affects the process window design.
3. Interface reaction characteristics: At high temperatures of 980-1030℃, titanium is easy to form a low-melting eutectic phase with die steel, and graphite-based or glass-based lubricants must be used. This characteristic is significantly different from the extrusion process of aluminum and copper alloys.
Key factors for process parameter control
1. Optimization of the extrusion method
Reverse extrusion can reduce friction resistance by 30-40% compared with forward extrusion
Lubricated extrusion can improve metal flow uniformity by more than 50% compared with the non-lubricated process
The Cold extrusion process can eliminate the influence of thermal gradient, but is limited by the deformation resistance of the material
2. Temperature field control
The preheating temperature of the extrusion barrel/die needs to be maintained at ≥300℃ to reduce thermal shock
β phase extrusion needs to strictly control the temperature rise rate ≤15℃/s
The use of gradient heating technology can control the axial temperature difference of the billet within ±20℃
3. Dynamic parameter matching
Optimal extrusion speed range: 1/3-1/2 of aluminum alloy (usually 5-15mm/s)
The degree of deformation should be controlled at 60-85%, corresponding to a titanium alloy hot extrusion ratio of 4:1-7:1
When designing a multi-hole die, it is necessary to ensure that the hole spacing is greater than 1.5 times the die hole diameter
4. Die system design
Optimal die angle selection range: 45°-65°
Sizing belt length must be ≥ (0.15-0.25) D (product diameter)
Surface roughness must be below Ra0.4μm
Process optimization direction
The friction coefficient can be reduced to 0.06-0.08 through composite lubrication technology (graphite + nano-ceramic coating), and the surface crack rate of the product can be controlled below 0.3% with the isothermal extrusion process. The latest research shows that electromagnetic field-assisted extrusion can improve metal flow uniformity by 18-22%, providing a new way to break through the existing technical bottleneck.
This study provides a theoretical basis for the formulation of titanium alloy extrusion process specifications. Through multi-parameter coordinated regulation, it can effectively improve metal flow uniformity and increase the yield rate to more than 90%, which has important engineering application value.
