Gr23 Titanium Bar Rolling Process Research

Gr23 titanium alloy is a high-performance material commonly used in aerospace technology, and its light weight, high strength, good toughness, and corrosion resistance are its most notable characteristics. With regard to the manufacture of automobiles, titanium alloy is the only metal material with high strength, low density, excellent fatigue and corrosion resistance, and low modulus of elasticity, so it is very ideal for the manufacture of automobiles. In addition, titanium alloy is also commonly used in medical equipment, chemical equipment, military (military TC4 titanium rods, titanium plate) and sports equipment (titanium golf clubs) materials and other categories. Due to the complexity of the processing technology, so the material price is expensive, which is the main reason for the formation of high product prices. Titanium alloy and many alloy metals, also by titanium and some useful metal components to meet the requirements of the material performance, but its main components of titanium accounted for more than 90%. Commonly used in aerospace technology are two alloys, both Ti6AL4V (6% aluminum, 4% vanadium, 90% titanium) and Ti3AL2.5V (3% aluminum, 2.5% vanadium, 94.5% titanium).

TC4 titanium alloy has good room temperature and high temperature mechanical properties, is widely used in the manufacture of aerospace structural components and aero-engine fans, drums, etc., has accounted for more than 50% of the total amount of titanium alloy for aviation. Many load-bearing structural parts of the aircraft, especially the variable cross-section structural parts require raw materials titanium rods with good notched stress fracture properties, the purpose is to ensure that the parts in the state of high stress concentration has a strong inhibition of crack initiation and expansion of the ability not to fracture failure. The upper limit of the specification of this type of TC4 titanium alloy bar in the relevant domestic technical standards is >220mm, and at present, there is no public report on the research of the preparation process of TC4 titanium alloy bar requiring notched stress fracture performance at home and abroad. It is generally believed that higher hydrogen content or poorer organization uniformity in TC4 titanium alloys will reduce their room temperature notch stress fracture properties (1). In industrialized mass production, the phenomenon of unqualified notch stress fracture performance of such bars due to improper process is also more common.
When the middle and lower rolls over the material, the rolling force of the middle rolls continue to impact, so that the slanting wedge of the anti-loosening device quickly failed, in the rolls of the fixed stable state is destroyed; adjusted hole type, with the middle rolls of the same stable state is destroyed and changed, affecting the stability of the size of titanium rods, and ultimately affect the quality of the finished product; in the rolling process, in order to prevent the structure of the glue wood tile heat wear, the cup should be poured with cooling water on the glue wood medium for cooling and lubrication! In the rolling process, in order to prevent the heat and wear of the gluing tile structure, we have to pour cooling water on the gluing medium for cooling and lubrication, and the cold water is often sprayed onto the surface of the rolls, which makes the surface of the billet and the overflow drop very quickly, especially when rolling titanium and titanium alloys, due to the poor thermal conductivity of titanium and titanium alloys, the difference in the temperature inside and outside of the bad material is even greater, which is not conducive to the uniform deformation of billet, and the rolling accuracy is not ensured; the slanting wedge is fixed on the side of the friction resistance is large, and the electricity consumption is also large in the rolling, and even if we adopt the motor of 630kW, the bogging down phenomenon also occurs frequently, affecting production. After changing the structure of 250 rolling mill from rubber tile to bearing structure, the size of rolled titanium and titanium alloy bars is stable, the external surface quality is improved, and the yield rate is increased.

In accordance with the current technical standards to provide a super-specification range of 350mm diameter TC4 titanium alloy bar, under the premise of ensuring that the mechanical properties of the indicators remain unchanged, the level of ultrasonic flaw detection is required to be increased from the original smaller specifications of the bar of GB/T5193 in the B level to the A level, and therefore carried out a special demand for large-size TC4 titanium alloy bar preparation of the relevant process research. The low-fold organization of <^50mm TC4 titanium alloy bar in forging state prepared by two processes. As can be seen from Figure 1, the uniformity of the low-fold organization of TC4 titanium alloy bars prepared by Process 1 is poor, showing the transition from the fuzzy crystals at the edge to the semi-sharp crystals at the center; the uniformity of the low-fold organization of bars prepared by Process 2 is good, and the whole specimen is in the fuzzy crystal state. This indicates that the use of process 1 for forging, ingot and intermediate billet heart organization is not enough to break and refine the degree of its total deformation is directly related to the small amount of deformation, due to the titanium alloy deformation resistance is larger, coupled with large size titanium bar billet size, a single straight uprooting deformation is also difficult to ensure that the billet heart deformation is sufficient. And process 2 make full use of 4500t fast forging machine large tonnage forging pressure, make large size billet in the two-phase area after upsetting and pulling deformation, to ensure the forging permeability of the billet, and the use of anvil pulling to reduce the billet deformation of the "dead zone", so that the billet of the different parts of the billet are fully deformed, and get a good crushing and refinement of good consistency of the organization.
(1)Adopting the forging process of opening billet in p-phase area and upsetting + straight drawing in two-phase area, we can prepare TC4 titanium alloy bars with large specifications of 350mm apricot which can meet the technical requirements of supplying in terms of organization, performance and flaw detection level.
(2) Primary a-phase iso-axialized good organization is conducive to improve the room temperature notch stress fracture performance, the direction of strong consistency of the short bar a-phase organization will reduce the notch stress fracture performance.

The buckled 50mm TC4 titanium alloy bars obtained by forging using two processes were cut along the longitudinal direction to a length of 75mm for ordinary annealing treatment at two temperatures, with annealing regimes of Ml (720ux2h/AC) and M2 (790t:x2h/AC), respectively. Observe the low magnification organization of the bar after forging with the naked eye; respectively, intercept the metallographic specimens along the transverse direction at the upper 1/2 radius of the bar lining in the forging state and after the annealing treatment, and observe the microstructure using a 0LMPUS optical microscope. In the annealing treatment of the bar 1/2 radius along the longitudinal line cutting to get the specimen blanks, machined to meet the standard room temperature tensile and notch stress fracture performance of the test specimens, using lnStron4507 tensile testing machine and DN2 notch tensile testing machine on the specimens for mechanical properties testing, observation of notch stress fracture specimen notch region of the microstructure. S0NIC-138VFD ultrasonic flaw detector was used to carry out ultrasonic non-destructive testing on the finished TC4 titanium alloy bars obtained by forging with the two processes.