With the development of aero-engines towards high thrust-to-weight ratio and stronger working stability, core components—especially impellers in compressor and fan systems—impose increasingly stringent requirements on the strength of the materials, thermal stability, and light weight. Under such background, titanium alloys (such as Ti6Al4V) are widely used in impeller manufacturing due to low weight, high strength, and superior corrosion resistance. Meanwhile, the complex geometry, precision demands, and harsh service environment of impellers also challenge their manufacturing processes with great challenges.
Introduction
Three-axis machining within my working experience has exposed path limitations on working with complex flow channels, small fillet transition zones, variable-thickness blades, and other zones, prone to causing tool interference, residual material accumulation, or compromised surface integrity. Therefore, five-axis (linkage) CNC machining technology is the mainstream fashion for the high-quality manufacturing of aviation titanium alloy impellers. Reasonable multi-axis path design can not only completely liberate the structural degrees of freedom of the machine tool but also significantly improve cutting stability and tool life and become the guarantee key of part accuracy and overall process controllability.
Structural Complexity and Machining Difficulties of Titanium Alloy Impellers
From a structural dimension, aviation impellers typically have the following characteristics:
- Irregular three-dimensional curved surfaces: Such as space free curves and variable-curvature surfaces formed by leading edges, trailing edges, and ventral/dorsal surfaces.
- Narrow flow channels: Highly narrowed blade clearances limit tool entry directions;
- Small fillets at blade roots: Form and location tolerances need to be closely controlled, where errors of machining (easily) converge;
- High-precision and high-surface quality requirements: Contour error generally needs to be kept in the range of ±0.01 mm, and surface roughness Ra needs to be optimized to 0.4 μm.
Materially, titanium alloys (such as TC4) mainly present the following issues during machining:
- High cutting temperature: Low thermal conductivity causes cutting heat to concentrate at the front of the tool and subsequently the development of local heat accumulation and (easily) leading to accelerated tool wear;
- High material elasticity and severe springback: Impacting forming precision and (easily) causing unstable cutting depth;
- Strong work hardening tendency: The workpiece becomes harder throughout the process of machining, increasing cutting resistance;
- Easy tool adhesion and built-up edge formation: Damaging machining surface quality and process stability.
Therefore, how to break through the two complexity of geometry and materials by utilizing multi-axis path strategies has become the major proposition in the titanium alloy impeller manufacturing process.
Basic Principles of Multi-Axis Path Design
Five-axis linkage machining provides extremely high accuracy and flexibility to complex curved surface parts but will bring an enormous increase in path design dimensions. Geometric reasonableness and dynamic flexibility of the path directly affect machining efficiency, surface quality, and tool life in extremely complex part machining, i.e., aircraft impellers and turbine guide vanes. Thus, in many projects I have been involved in, we have always followed the four main principles of “interference safety, load uniformity, path smoothness, and scientific machining sequence” when planning a path strategy, and optimized the effects of path execution continuously through the integration of simulation tools with on-the-job feedback.
Avoiding Interference and Overcutting: The Core Goal of Dynamic Posture Control
The most important problem in multi-axis path planning is spatial interference, especially in areas such as deep blade flow channels and hub connection areas, where the machining space is closely restricted, and physical interference between tool shank and workpiece or path self-intersection can occur 不慎 (carelessly). To avoid such threats, one must take advantage of the full potential of the A/C axis linkage in facilitating real-time tool posture adjustment so that the cutting direction of the tool is optimal while spatial avoidance is achieved. Also, the spatial location of fixtures and tooling also needs to be in the realm of collision detection while executing the path simulation to prevent machinability being jeopardized on account of poor clamping system design. CAM systems that have interference prediction and posture correction modules (such as UG NX CAM) easily have an upper hand in this aspect.
Uniformity of Cutting Load: Emphasizing Both Residual Control and Tool Entry Strategies
Ensuring load is balanced for multi-axis machining is important to ensuring machining stability and tool life. I would myself prefer applying a three-dimensional equal residual strategy combined with an inclination-initiated path to dynamically regulate cutting thickness in different regions of curvature in order to avoid edge chipping or vibration marks due to abrupt local load spikes. At the same time, in order to enter hard-to-reach areas such as the bottom of the flow channel and the blade tip, spiral tool entry, constant cutting depth, or composite path of “Z-axis tool entry + XY constant speed” can be used to enter and leave smoothly, reducing instantaneous load impact. In addition, by including force cutting simulation or thermal distribution analysis modules within path simulation, finding the points of heat concentration and stress concentration ahead of time helps to optimize feed rhythm and cooling schedules and achieve load balancing in the process window.
Path Continuity and Smoothness: Suppressing Vibration Marks from Posture Changes
Abrupt posture changes of tools during five-axis machining paths are typically the first reason behind trajectory breakpoints, machining vibration, and surface defect. Therefore, I always emphasize the laws of path continuity and smoothness control while trying to maintain the lowest posture change rate and shortest path jump in the process of cutting. This is typically achieved with the assistance of G2/G3-level smooth transitional curves between path segments, multi-axis continuous spiral tool entry techniques, and a hybrid contour and variable-step path strategy. For example, use of a spiral contour approach for machining the blade ventral surface can avoid Z-axis direction changes continuously; in regions of small diameter, a mode of fixed tool axis angle + dynamic adjustment of speed for feed guarantees the path does not kink or jump. Such continuity design in the path not only improves machining stability but also allows easy compliance with the subsequent surface finish needs.
Scientific Arrangement of Machining Sequence: Constructing Geometric Integrity in Stages
Path planning is not just an issue of “how to go” but also “where to go first.” Reasonable planning of the machining sequence is the basis of deforming accumulation, path interference, and clamping instability avoidance. The general idea is “from thick to thin, from exterior to interior, from primary to secondary,” i.e., processing the surface of the contour envelope and high-rigidity areas in the first place, then processing thin walls, cavities, or nesting structures in sequence. In impeller-type parts, I am employed to machine the largest blade root shape first, and proceed to the center of the blade body, and fine processing is performed last for the blade tip junction with the hub. For heat deformation sections in workpieces, natural cooling or simulated aging stages should also be incorporated so as not to cause dimensional drift from superposition of thermal stress relief. In addition, when performing multi-clamping or direction change processes, the machining coordinate system should be converged to ensure that the path and clamping processes are optimized in parallel, improving overall machining accuracy.
Typical Path Strategies and Implementation Methods
For five-axis machining of complex impellers, the quality of path strategy has direct influence on machining efficiency, surface quality, and tool life. Due to the endless change of curvature of blade shape, tiny flow channel space, sharp angles, and unbalanced wall thickness, it is necessary to adapt path strategies to local geometric features and integrate them into a high-degree-of-freedom five-axis linkage mechanism in order to achieve dynamic control. Systematically discussed below are four prevailing strategies’ implementation paths and engineering values: flow channel machining, transition area treatment, equal residual control, and inclination-driven.
Five-Axis Side Milling Paths for Flow Channel Areas
In conventional flow channel profiles such as aviation impellers or gas turbine guide vanes, ventral and dorsal areas are mostly composed of free-form surfaces, narrow cutting spaces, and high curvature changes, susceptible to interference or unbalanced cutting force. Therefore, side-edge contour and equidistant paths have become the mainstream method. The core strategy is to model the tool path spacing in the Z-direction with optimal contour layering having a residue less than 0.1 mm to reduce the repair work burden in the finish operation. At the same time, introducing an automatic tool axis tilt adjustment algorithm in accordance with the surface normal can make it possible to adjust the tool posture continuously with curvature change, without tool dragging or root interference. This method is a path continuity and tool load balance combination, and has brought good stability and surface quality to some complicated flow channel impeller machining pieces.
Normal Inclination Paths for Blade Root Transition Areas
The transition region from blade root to wheel disc usually has small-radius fillets and groove structures, and these are the most significant regions of stress concentration and deformation sensitivity. When surface machining in this region is done along traditional vertical paths, it is easily going to cause over-load at the tool bottom, producing vibration marks, tool marks, or even micro-cracks. For this purpose, I prefer applying standard inclination paths between 20° and 45°, and perform interpolation machining with the tool inclined by maintaining a constant or ever-changing angle in the direction of the tool axis. As for the selection of the tool, ball-end mills or tapered ball mills are typically used in combination with a small step pitch strategy to offer smooth connection within small-curvature transition areas. Constantly changing the tool normal entry angle through the five-axis swinging capability can effectively scatter bottom cutting heat concentration and local force (fluctuation), serving as a core link to high-reliability impeller machining.
Three-Dimensional Equal Residual Strategy
The classical contour paths are generally not able to accurately reflect the true residual condition after machining, resulting in residuals cumulating in some curvature mutation areas and thus inducing surface quality oscillation or even overcutting risks. Three-dimensional equal residual path strategy forms a spatial model of the previous tool path residual thickness and tool geometrical information, conducts real-time residual distribution analysis in the machining area being processed, and dynamically plans the position of the next tool path accordingly based on this, providing “uniform load” cutting throughout the process. It not only improves cutting force continuity but also significantly reduces surface stress concentration, which makes it particularly suitable for surface repair and error compensation in finishing. In practice, equal residual paths are largely used as high-performance functional blocks in costly CAM systems (e.g., NX CAM, PowerMILL), and combined with margin visualization analysis and simulation verification, which can greatly improve path quality and safety.
Inclination-Driven Path Generation
In the inclination-driven path method, path smoothness and posture modification are improved by limiting the extent of angle change between the tool and the surface normal. This method sets up an angle maintenance interval (e.g., 10° to 20°) during the path planning stage, keeping the stable posture of the tool axis and gradually changing along the machining trajectory, avoiding big-angle mutations or “flips”, therefore evading trajectory jumps, error accumulation, or breakpoint interference. This method is particularly required in machining impeller inner cavities with limited space and high-curvature change rates. By inclination-controlled control, not only is machining stability improved, but also the direction of tool chip removal controllability and risk of chip clogging and surface scratching are prevented.
Key Points of Simulation and Post-Processing
When working in engineering practice, I always pay great attention to verification of path simulation. By using software platforms such as HyperMill, UG NX CAM, or PowerMill, simulated paths are verified as follows:
- Collision and interference detection: Spatial simulation of workpiece, tool shank, and tool axis;
- Machine tool simulation verification: Validation of path motion feasibility in target machine tool (e.g., HERMLE C42) in conjunction with platforms such as VERICUT;
- Post-processing matching: Conversion of the path into G-code that adheres to the target CNC system (e.g., Heidenhain or Siemens) with the correct instructions aligned with the machine tool interpolation logic;
- Optimization of tool entry and exit strategies: Employing circular interpolation, spiral tool entry, etc., to minimize path entry influence, which is especially important in the initial contact phase of titanium alloys.
Conclusion
The design of multi-axis machining paths for aerospace titanium alloy impellers is a system engineering integrating advanced manufacturing theory, numerical control path algorithms, material behavior control, and numerical simulation. On the basis of only a thorough knowledge of the geometric characteristics and material behavior of impellers, the integration of multi-axis machine tool capabilities and tool process knowledge, can high-efficiency, high-precision, and high-reliability manufacturing of impellers be achieved.
