Titanium alloys are widely used in aerospace, medical devices, and high-end equipment manufacturing due to their high specific strength, excellent corrosion resistance, and good biocompatibility. However, the hot extrusion process of titanium alloy bars faces numerous challenges, with significantly higher complexity compared to aluminum, copper, and steel alloys. Based on metal flow dynamics and industrial practices, this article systematically analyzes the key issues and countermeasures in the hot extrusion process of titanium alloys.
一,Analysis of process difficulties and mechanisms
1. Temperature difference stress due to low thermal conductivity
Titanium alloy has a low thermal conductivity (about 6.7 W/(m·K)), which is only 1/3 of aluminum alloy and 1/5 of steel. During the hot extrusion process, if the temperature of the extrusion cylinder is 400°C, the temperature difference between the surface layer and the core of the billet can reach 200–250°C. This significant gradient results in:
The surface metal forms a "hard shell" with high strength and low plasticity due to rapid cooling.
The core metal maintains a high temperature and high plasticity state;
The deformation of the inner and outer layers is uncoordinated, resulting in additional tensile stress, which is the main cause of surface cracks.
According to statistics, the surface crack rate of unoptimized titanium alloy bars is as high as 35%, while similar aluminum alloy products are usually less than 5%.
2.Phase change sensitivity and flow inhomogeneity
The α+β/β phase transition temperature of titanium alloy significantly affects the flow behavior of the material:
Extrusion in the β phase area (above the phase transition point): good fluidity, but prone to surface defects such as orange peel;
Extrusion in the α+β phase region (below the phase change point): the metal shows a layered flow, and the difference in the flow rate of the surface center can reach 20%–30%, resulting in excessive bending.
In industry, the heating temperature is usually controlled in the middle of the α+β phase zone (e.g., 920–950°C for TC4 alloys) to balance surface quality and flow uniformity.
3. Mold-billet interface reaction and wear
At a high temperature of 980–1030°C, titanium alloys are prone to eutectic reactions with iron-based or nickel-based mold materials, forming low melting point phases such as TiFe and TiNi, resulting in mold adhesion wear and peeling. Without the lubrication process, the mold life is only 200–300 pieces; After using glass lubricant, it can be lifted to more than 1500 pieces.
The core functions of lubricants include:
High temperature isolation: form a liquid film above 800 °C to block direct contact;
Friction reduction and drag reduction: reduce the friction coefficient from 0.8 to 0.1–0.2;
Inhibition of oxidation: control the thickness of the oxide layer on the surface to avoid defects caused by the embedding of the oxide scale in the matrix.
二,Process optimization and flow control strategy
1. Optimization of extrusion methods and friction conditions
Reverse extrusion: The uniformity of metal flow is increased by 40% compared with forward extrusion, and the "dead zone" is reduced because the friction is consistent with the extrusion direction.
Cold extrusion: suitable for small diameter bars, the flow uniformity is better than hot extrusion, and the standard deviation of flow rate is reduced by 25%;
Composite lubrication: using graphite + oil-based lubricant, the flow unevenness coefficient can be reduced from 0.35 to 0.18.
2. Speed and temperature coordinated control
The increase in extrusion speed (such as 1→5 mm/s) will increase the flow rate difference by 3 times, which needs to be compensated by dynamic speed regulation.
The preheating temperature of the extrusion cylinder and the die (up to 400–450°C and 350–400°C respectively) was controlled to make the temperature difference between the end face of the billet ≤ 50°C and the uniformity of the flow rate increased by 15%.
3. Mold Structure Design
The cone angle of the mold is reduced from 120° to 90°, which can reduce the flow unevenness coefficient by 18%.
The asymmetric porous mold layout of "large central hole and small peripheral hole" is adopted, which increases the peripheral flow rate by 12% and makes the overall balance more balanced.
The total deformation is controlled at 60%–70% to avoid stagnation or cracking due to insufficient (<40%) or excessive (>80%).
三,Typical case: TC4 titanium alloy bar extrusion process optimization
An enterprise reduced the surface crack rate of TC4 bar from 28% to less than 3% through the following comprehensive measures:
Heating system: three-stage heating (600°C→850°C→930°C), the heat preservation time is calculated according to the diameter of 1.5 minutes per millimeter;
Lubrication scheme: 0.2 mm glass lubricant is coated on the surface of the billet, and boron nitride coating is sprayed in the mold;
Speed-temperature linkage: the initial extrusion speed is 1 mm/s, the speed is increased to 3 mm/s when the blank tail enters the deformation zone, and the extrusion cylinder temperature is increased from 400°C to 420°C;
Mold design: 100° cone angle and asymmetrical 6-hole die, the diameter of the center hole is 15% larger than the periphery.
The optimized product quality is significantly improved: straightness increased from 3 mm/m to 1 mm/m, and surface roughness Ra ≤ 0.8 μm in accordance with aerospace standards.
四,Future development direction
1. Intelligent process control
Digital twin technology is introduced to predict the metal flow state through real-time simulation and dynamically adjust the process parameters.
2. Mold material innovation
We have developed gradient composite molds with a cobalt-based alloy surface and titanium alloy core, taking into account high temperature wear resistance and structural lightweight.
3. Ultrasound-assisted extrusion
The use of high-frequency vibration to reduce flow stress is expected to reduce the extrusion force by 20%-30%, further improving the quality and efficiency of molding.
Titanium alloy bar hot extrusion is a typical "temperature-stress-flow" multi-field coupling process. By accurately controlling the phase transition temperature, optimizing the lubrication interface, innovating the mold structure, and introducing intelligent control methods, it can effectively solve bottleneck problems such as cracks and bends, and promote the development of high-end titanium materials in the direction of high-precision, low-cost, and large-scale manufacturing. With the deep integration of material genome and industrial intelligence, the titanium alloy hot extrusion process is moving towards a new stage of "customization and zero defects.