Titanium alloy raw materials for aviation forgings are generally used as rods, and large forgings such as wheel discs, magazines, integral blade discs and fan blades are generally used as large-size rods, while small-size rods are used for small pressurised gas blades and turbine blade forgings. With the advanced engine tends to use the whole blade disc, the whole leaf ring structure form, the corresponding forging and bar size increase, control of large-size bar organisation uniformity is essential to ensure the quality of forgings, need to choose the right forging equipment, optimize the design of forging process. For TB12 and TiAl alloy ingots, due to the cast metal forging deformation resistance, process plasticity is low, sensitive to the deformation temperature, easy to forging cracks, ingots should be used in high temperature extrusion billet process for the preparation of large-size rods, not only to improve the deformation uniformity, to ensure that there is enough deformation, but also improve the production efficiency of the rods and the stability of the batch.
The microstructure and crystallographic structure of titanium alloys are the main factors affecting the mechanical properties due to the anisotropy of the α-phase. Controlling the morphology of the microstructure and the uniformity of the microstructure and texture of forgings not only improves the average level of performance, but also improves the creep-fatigue interaction performance of the parts, i.e., load-bearing fatigue performance, and reduces the dispersion of the performance data of the parts from batch to batch. For these new high-temperature titanium alloys, especially TiAl alloys, the introduction of ordered structure makes the weaving problem more complicated and important, and the influence on the high and low circumferential fatigue performance and load-holding fatigue performance is also more complicated. The organisation and structure should be strictly controlled during the preparation of bars and forgings.
Due to the continuous improvement of the performance level of advanced engines, the whole leaf disc, the whole leaf ring has become the development trend. Integral disc blade structure is complex, poor channel openness, thin blade, bending and twisting, poor rigidity, easy to deform, the design of its geometric accuracy level, the level of comprehensive quality requirements are increasingly high, machining and surface integrity of the guarantee becomes more and more difficult[30] . For the small size of the blade of the compressor whole leaf disc and the whole leaf ring, the leaf type is generally processed using high-speed CNC milling method, control parts processing deformation, vibration finishing stress relief technology to improve the parts surface residual stress distribution, followed by part of the blade surface grinding and abrasive flow polishing, leaf size precision, leaf error is less than 0.1mm, blade surface roughness Ra to 0.2μm level, improve the surface of parts. The surface roughness Ra of the blade reaches the level of 0.2μm, which improves the surface quality and surface integrity of the parts. Electrochemical methods should be used to process the profile of TiAl alloy blade.
The above four types of materials are still in the engineering research and trial stage, and the accumulated performance data are not sufficient, which affects the design selection and strength calculation of materials and components. Compared with ordinary titanium alloys, these four types of high-temperature titanium alloys have lower plasticity, fracture toughness, impact toughness, large notch sensitivity, crack tip of the stress through the local plastic deformation and the ability to decline is poor. In particular, TiAl alloy, with a fairly low room temperature tensile plasticity and fatigue crack extension resistance, but in close to 700 ℃ will significantly improve, and the initial creep deformation rate is large. According to the characteristics of these materials, the design and development of scientific and reasonable technical specifications, play the thermal strength at the same time, should ensure that there is enough plasticity, pay full attention to the fracture properties of the parts. Engine design selection and strength calculation, need to establish a complete material design performance database. For the low plasticity of TiAl alloy, according to the material properties, to determine a reasonable component design and life-span method, as well as cost-effective supply chain. Reasonably control the design stress level of TiAl alloy structures to avoid significant stress concentrations and improve surface integrity. Scientific evaluation of the flame retardant properties of these titanium alloys is also critical. In addition, whether the whole leaf disc or the whole leaf ring, when used at high temperatures, there is a temperature gradient on the same part, one part of the material will constrain the deformation of the other part of the material, under the action of the temperature gradient will cause thermal stresses, affecting the fatigue performance of the component and the reliability of the use of the component.
In fact, there is no high fatigue limit for titanium alloy materials. The U.S. Engine Structural Integrity Program (EngineStructuralIntegrityProgram, ENSIP) 1999 and 2004 editions require that the high fatigue life of titanium engine components should reach 109 weeks at a minimum. With the decrease of the acting stress, the fatigue cracks tend to emerge from the surface to occur in the interior. For 600 ℃ high temperature titanium alloy integral leaf disc, titanium matrix composite material integral leaf ring and TiAl alloy blade, because the fatigue performance of the blade is very sensitive to the vibration stress, should be fully investigated for its ultra-high fatigue behaviour and performance. Reasonable selection of appropriate surface strengthening means, such as laser impact strengthening and low plasticity polishing, etc., in order to improve the ultra-high fatigue performance of the blade, to prevent blade failure caused by internal damage and catastrophic failure.
