Hot extrusion is one of the main routes for converting titanium and titanium alloy billets into bars, tubes and profiles. The process is attractive because it imposes a favourable compressive stress state and can shape material that is difficult to machine, but titanium behaves very differently from steel or aluminium inside the press. This article examines the characteristics that define titanium hot extrusion and how each one is managed in production.
Characteristic One: Low Thermal Conductivity and Steep Temperature Gradients
Titanium and its alloys conduct heat poorly compared with most structural metals. Heat generated by friction and deformation therefore stays close to the surface instead of diffusing into the tooling and the core. When the extrusion barrel is at 400 C, the temperature difference between the surface layer and the inner layer of the billet can reach 200-250 C.
That gradient creates a real problem: the surface layer is hotter and softer than the core, so the two regions deform at different rates. The resulting non-uniform flow generates large additional tensile strain in the outer skin, and tensile stress at the surface is the root cause of the cracks and tears seen on extruded surfaces.
| Process factor | Typical condition | Effect on extrusion |
|---|---|---|
| Barrel temperature | About 400 C | Surface to core difference of 200-250 C |
| Billet preheat range | 900-1150 C depending on alloy | Controls flow stress and die loading |
| Extrusion speed | Low compared with steel | Limits redundant shear heating |
| Lubrication | Glass or proprietary lubricant | Reduces friction heat and pickup |
| Die preheat | Close to billet temperature | Flattens the thermal gradient |
Characteristic Two: Phase Dependent Metal Flow
Titanium exists in a hexagonal close packed alpha phase at low temperature and a body centred cubic beta phase above the beta transus, which is about 882 C for commercially pure titanium and higher for stabilised alloys. Research on metal flow dynamics shows that the flow behaviour of the metal differs markedly between the temperature ranges that correspond to these states.
Extrusion in the alpha or alpha plus beta phase zone produces more uniform metal flow
Extrusion wholly in the beta phase zone gives lower flow stress but coarser structure and less uniform flow
Billet heating temperature is therefore one of the main factors controlling extrusion fluidity
The practical consequence is that heating temperature, not ram speed, is usually the first parameter adjusted when surface quality or dimensional consistency drifts.
Characteristic Three: High Flow Stress and Narrow Working Window
Titanium alloys retain high strength at temperature, and they oxidise readily above roughly 600 C. The usable window is therefore bounded below by excessive die load and above by alpha case formation and grain growth. Extrusion is normally carried out in a narrow band, with short soak times, protective atmosphere or glass coating, and rapid transfer from furnace to press.
Characteristic Four: Surface Quality and Lubrication Demands
Because the hot surface tears easily when it cools against the die, lubrication and die design carry unusual weight. Glass lubricants form a viscous film that separates billet from tooling, while generous die entry angles and radiused bearings reduce the additional shear that triggers cracking. Post-extrusion descaling and pickling remove the oxide and alpha case layer before further processing.
Characteristic Five: Microstructure and Property Control
Extrusion is also a thermomechanical treatment. By holding the billet in the alpha plus beta field, a fine equiaxed or bimodal structure can be produced that balances strength and ductility. Extruding in the beta field, by contrast, tends to leave a coarse transformed structure with lower fatigue performance, which is why the phase zone is deliberately chosen for critical aerospace and medical product.
FAQ
Q: Why does titanium extrude differently from steel?
Titanium conducts heat poorly and keeps its strength at temperature, so deformation heat concentrates at the surface and the usable temperature window is narrow.
Q: What causes cracks on the surface of extruded titanium?
Non-uniform deformation between the cooler core and the hotter skin produces additional tensile stress at the surface, which opens as longitudinal cracks and tears.
Q: Which phase zone gives the most uniform flow?
Extrusion in the alpha or alpha plus beta zone generally gives more uniform metal flow than extrusion in the beta zone, where flow is coarser and less even.
Q: Why is billet heating temperature so important?
It sets the phase balance and therefore the flow stress and fluidity of the metal, which directly control die load, surface quality and final microstructure.
Q: What is alpha case and why must it be removed?
Above about 600 C titanium absorbs oxygen and nitrogen, forming a hard brittle surface layer that must be removed by descaling and pickling to avoid cracking in service.
