Titanium alloy tubes can be divided into two major categories based on their production methods: seamless tubes and welded tubes. Titanium alloy seamless tube rolling can be categorized into large-diameter seamless tubes and small-diameter seamless tubes based on diameter. Generally, seamless tubes with a nominal diameter greater than 200mm are considered large-diameter seamless tubes, while tubes with a nominal diameter greater than 200mm are considered small-diameter seamless tubes.
The main production methods for titanium alloy seamless tube billets are piercing, extrusion, and cross-rolling. The piercing extrusion method generally involves directly extruding a titanium ingot heated to a certain temperature into a hollow cup in an extrusion cylinder using lubrication. Cross-rolling piercing, on the other hand, uses rollers and fixed guides to rapidly pass the titanium alloy billet through a fixed mandrel to produce a seamless tube billet.
The perforated extrusion method offers greater flexibility in the production of tube billets and can produce tube billets of various specifications. Due to the large deformation during the extrusion process, the grains can be broken, achieving a fine-grained strengthening effect. Furthermore, this method exhibits triaxial stress during the extrusion process, enabling the production of tube billets from metals with poor plasticity. However, the perforated extrusion method consumes a large amount of metal during tube billet production, resulting in significant wear and tear on the dies used. Furthermore, the equipment used is relatively complex, resulting in high investment costs and difficulty in ensuring product surface quality. The perforated extrusion method can be further divided into forward extrusion and reverse extrusion. Reverse extrusion is often used for tubes with large diameters and deformation resistance. K∙Srinivasan et al. [verified the feasibility of extruding commercially pure titanium tubes using open dies.] The results showed that open-die reverse extrusion required less force than forward extrusion, and that titanium tubes produced by open-die reverse extrusion had higher surface quality and less lubricant consumption. However, this method is only suitable for the manufacture of shorter pipes. The cross-rolling piercing method reduces metal and die loss during tube production, maintains product surface quality, and requires less lubrication. The equipment used is also simpler, resulting in lower investment costs. The production cost of this method is approximately 50-70% of that of the piercing and extrusion method. However, the cross-rolling piercing method also has significant disadvantages: the tubes produced are relatively uniform, have limited flexibility, and exhibit low deformation, making them unsuitable for thinner-walled tubes.
Titanium alloy seamless tube rolling. Depending on the rolling conditions, titanium alloy seamless tubes can be categorized as hot rolling or cold rolling. Hot rolling, as the name suggests, involves rolling the tube at a specific temperature, generally below the recrystallization temperature. Cold rolling, on the other hand, involves rolling the tube at room temperature. Hot rolling generally causes significant wear on mill components such as the die and rolls, and lacks the precise dimensional accuracy of cold rolling. Therefore, cold rolling is a more suitable method for producing titanium alloy seamless tubes. Cold rolling can be categorized as two-roll cold rolling or three-roll cold rolling, depending on the rolling mill used. The groove cross-section of the rolls in a two-roll mill changes gradually, and the mandrel and tube billet rotate and advance intermittently during the rolling process. Two-roll cold rolling can achieve large diameter reductions and high production efficiency, but the equipment is relatively complex, roll replacement is difficult, and the finished tube has poor gloss and dimensional accuracy. A three-roll mill has three rolls spaced 120 degrees around the tube billet. The groove cross-sections of all three rolls are uniform, forming a circular groove. During the rolling process, a crankshaft and rod system drives the slideway and roller frame in reciprocating linear motion. Three-roll cold rolling equipment is relatively simple, with easy tool replacement and uniform deformation. The finished tube surface quality is good, but the rolling deformation is small during the rolling process, resulting in lower production efficiency.
Rolling process parameters significantly affect the performance and quality of the tube. Process parameters for titanium alloy seamless tube rolling include rolling deformation, number of rolling passes, feed rate, and rolling speed.
(1) Deformation Deformation has a certain influence on the rolling force required for rolling pipes, the surface quality of rolled pipes, the deformation thermal effect, and the microstructure and properties of the pipes after rolling. Qi Yunlian et al. studied the influence of cold rolling deformation on the microstructure and properties of Ti-1300 titanium alloy pipes and found that when the cold rolling deformation increased from 20% to 30%, its tensile strength and plasticity were improved, with the strength increasing from 1207.5MPa to 1223.5MPa and the elongation increasing from 12.75% to 13.25%. (2) Feed rate. Feed rate not only affects the production efficiency of titanium alloy seamless pipe rolling but also affects the surface quality of the product. Too small a feed rate will lead to low productivity, while too large a feed rate will cause defects such as burrs, ovals, and uneven wall thickness in the pipe, which seriously affect the quality of the pipe. (3) Rolling Passes
During the rolling process of titanium alloy seamless tubes, it is usually not possible to produce the required specifications of the tube in one pass. Multiple rolling passes are required to obtain the required product. Although increasing the number of rolling passes can reduce the deformation required for each pass, thereby obtaining a tube with more uniform wall thickness, too many rolling passes will lead to increased production costs, and the inclusions generated during the rolling process will damage the surface quality of the tube during the multiple rolling passes.
