Titanium alloy impellers are significant to the key equipment such as high-performance aero-engines, ship propellers, and high-speed compressors. Due to their excellent specific strength, high-temperature stability, and corrosion resistance, titanium alloys (such as TC4) have become the preferred choice for advanced thin-walled rotary structural parts. However, such material has low thermal conductivity, strong work hardening property, and high thermal sensitivity when undergoing processing, thus it is liable to severe thermal deformation and residual stress concentration during heat treatment and CNC machining and hence affects the geometric accuracy, fatigue life, and overall service performance.
Introduction
In aerospace, energy equipment, and others, titanium alloy impellers, as significant parts with complicated structure and high accuracy, have long been enduring harsh service conditions of high-speed rotation, high temperature, and high pressure. Their quality of manufacture has a direct impact on the performance and reliability of the entire system. Specifically α+β titanium alloys such as Ti-6Al-4V have wide use in complex impeller structural parts (such as turbine discs and axial compressor blades) for their high overall mechanical properties, low weight, and adequate corrosion resistance. But at the same time, the low thermal conductivity of titanium alloy materials (just 1/4 of steel) and high chemical activity make them highly sensitive to accumulation of heat locally under machining and heat treatment, thus resulting in severe thermal deformation and residual stress problems.
In engineering technical applications, I found that titanium alloy impellers due to the thin thickness and nonuniform thickness of the wall would suffer excessive deformation after machining, and dynamic balance is difficult to meet the standard. Residual stress has also emerged as a potential source of danger affecting fatigue life. As a result, the manufacturing process needs to begin from the material thermo-mechanical response mechanism, systematically optimize the manufacturing process, and achieve both accurate control over thermal deformation and effective regulation of residual stress.
Analysis of Evolution Mechanism of Thermal Deformation and Residual Stress
Since they are high-speed rotating parts, titanium alloy impellers suffer extremely complex thermo-mechanical coupling behaviors during processing, heat treatment, and other joints at high speeds, which will easily lead to thermal deformation accumulation and multi-source residual stress release, influencing not just dimensional accuracy but also the uniformity and service stability of end-products to a great extent. Therefore, deep understanding of its generating mechanism and evolution process is the priority in improving processing quality and process stability.
Essential Causes of Thermal Deformation
Titanium alloys have material properties of low thermal conductance and high thermal expansion coefficient and therefore highly vulnerable to thermal accumulation and difficult to diffuse in cutting and easy to form a strong non-uniform temperature field. especially in high-speed milling, large quantities of transient thermal input generated by aggressive friction between the tool and the workpiece are highly localized in the cutting zone, causing the area to become quite hot, while the uncut area has slight temperature rise, resulting in a thermal gradient.
Such uneven heating and cooling promotes the material to develop gradient thermal expansion, thus triggering asymmetric internal stress distribution. As machining ceases, material rapidly quenches from elevated to room temperature under a superposition of plastic deformation and elastic rebound, and leaves the impeller with geometric deformation irreversibly in local areas of the impeller, manifesting itself in the form of bulging, warping, or eccentricity. Especially for slender structures with a blade thickness of 0.5-1.5 mm, the effect of thermal deformation amplification is particularly obvious and quality defects such as “flanging” or “free-edge warping” are likely to occur when inproper heat input happens.
Main Sources and Influence Paths of Residual Stress
Residual stress, as another main inducement of deformation after machining impellers, has multiple superposition sources. First, during the cutting process, the severe shearing and friction between the tool and material will generate tensile or compressive stress in the surface layer of the workpiece. The stress distribution tends to be concentrated tens of microns below the machining surface and is a major internal cause of “slow deformation” or “late warping” after machining.
Second, in the heat treatment process, due to the disparity between heating and cooling rates at thick and thin part connections, thermal stress is fiercely accumulated in such locations and ultimately causes transverse or longitudinal shrinkage deformation. In addition, during clamping, if fixture design is illogical or clamping force is inhomogeneously distributed, clamping-induced stress is commonly generated in local areas, especially in flexible areas or free edges. Besides, titanium alloy blanks may also have initial forming residual stress with forging and heat shaping. If they are input into the machining link without sufficient stress relief treatment, they are (highly prone to) reactivated and released in the following processes and thereby amplify the final geometric error.
These stresses do not operate in isolation but are superimposed and interlaced between different processes and stages to appear ultimately as deformation, stress-release cracking, local structural instability, etc. Statistical data reveal that in inspection of finished products from typical aviation titanium alloy impellers, about 40% of the geometric deviations are due to the effect of thermal deformation and release of residual stress. Therefore, in the process planning of high precision, it needs to be treated as an important control objective and thought through from multiple aspects such as material pretreatment, process path planning, fixture structure, and tool parameter configuration.
Technical Paths and Practical Strategies for Thermal Deformation Control
Thermal deformation control in the machining of titanium alloy impellers has always been a central issue in high-precision manufacturing. Due to the low thermal conductivity and large thermal expansion coefficient of the material, it is not difficult to locally concentrate machining heat inside, leading to shape fluctuation or structural instability. Thus, it is necessary to establish a systematical control path from multiple aspects such as process, clamping, simulation, and feedback control. In my view, building a complete technical structure of “prediction-simulation-feedback-compensation” for full-process technical means is the core to achieve high-consistency processing quality.
Optimization of Process Path and Machining Parameters
To control the thermal deformation from its source, it should start with two aspects: processing route and parameter setting. First, the methodology of layered multi-pass processing should be applied, spreading the processing process into three phases: “roughing-semi-finishing-finishing”, removing materials progressively and releasing stress. Especially in thin blades or sections with a fluctuant cross-section, both small cutting depth and a high feed should be used as the parameter in order to reduce the heat accumulation effect per unit time.
In addition, the tool path arrangement should try to follow the principle of symmetric processing. By cross tool setting or symmetric tool entry, heat input balance left to right is guaranteed to prevent warping and deformation caused by over-heat on one side. At the same time, create tool materials with high rake angle and low friction coefficient (for example, ultrafine-grained cemented carbide with AlTiN coating), and a high-pressure cooling system (50-70 bar) to form an effective removal and cooling mechanism, and restrain the local explosion of the sources of machining heat to the utmost extent.
Design of Flexible Clamping System
Reasonable clamping not only guarantees positioning, but also acts as a “buffer zone” to control thermal deformation. When machining titanium alloy impellers with high thermal deformation risk, the flexible fixtures with elastic limit capabilities must be utilized, allowing parts to possess (micro) displacement when heated, to minimize warping due to internal stress without sacrificing positioning accuracy.
In addition, using the thermal deformation prediction model, by means of finite element simulation data, the size of support point position and the size of clamping force must be fairly distributed in order to form a dynamic adaptive clamping scheme. In high-end applications, an online laser measurement system can be incorporated to real-time obtain the position offset and temperature field distribution of the impeller surface and input them back to the control system in order to real-time compensate for the tool path or clamping mode, realizing on-machine closed-loop compensation for thermal deformation.
Processing Compensation Mechanism Driven by Thermal Simulation
With the ongoing development of simulation processing and multi-physics field modeling technology, the pioneering role of thermal simulation in deformation control becomes increasingly prominent. Under the simulation software ANSYS, Simufact, and Deform, one can establish a full-process simulation system involving “processing-cooling-heat treatment-assembly” to obtain the temperature field, the evolution of stress, and the structural response trend of each processing stage in advance. For critical zones (e.g., blade roots, free edges, and shell joints), the NC tool path can be pre-corrected based on the prediction model, and active thermal error compensation strategies can be employed. Following a few cycles, the stable empirical correction database can be established to guarantee stable accuracy and geometric consistency for mass production of batch impellers.
Control and Elimination Technology of Residual Stress
In practice, I have found that it is difficult to completely eliminate residual stress by process optimization alone, and diversified physical fields or strengthening methods need to be introduced to break the mechanism of stress accumulation from the micro level.
Thermal Aging Treatment
Placing the impeller within 450-650℃ for a few hours and slow cooling can significantly relieve the thermal stress formed in the crystal lattice, which is one of the most prevalent and reliable methods of stress relief in modern aviation manufacturing.
Ultrasonic Impact Peening (UIP)
High-frequency mechanical vibration is used to induce micro-plastic deformation on the surface of the impeller, converting the original tensile stress into compressive stress, which realizes stress field reconstruction while improving fatigue life.
Laser Shock Peening (LSP)
The plasma shock waves are excited with high-energy short-pulse laser to create a deep compressive stress region on the surface layer, which is particularly in application for high-stress concentration regions like blade roots and channel corners, and possesses advantages of non-contact and high control accuracy.
Segmented Heating Process Technology
As shown from the experiment of TC4 titanium alloy forgings, the “preheating-insulation-heating” segmented control method can significantly optimize the temperature field distribution of forgings, reduce the maximum temperature difference to 89℃, and restrain the deformation tendency effectively. At the same time, the maximum residual stress is reduced from 165MPa through continuous heating to 74MPa, verifying the outstanding merit of segmented heating in internal stress control.
Conclusion
Thermal deformation control and elimination of residual stress of titanium alloy impellers are major technical issues in high-performance manufacturing process. Coupling rational process route planning, advanced measurement compensation tools, high-precision clamping system, and scientific stress release tools can systematically improve part quality and service stability.
