"Preturning" (or "pre-turning") is a crucial preprocessing step in the manufacturing of forged centrifugal impellers, especially for high-performance applications like aerospace engines, turbochargers, and critical industrial compressors.
Here’s a detailed breakdown of what it is, why it's done, and the process.
What is Preturning?
Preturning is the machining operation performed on a forged, rough-shaped billet (called a "mult" or "forging stock") before the final closed-die forging process that forms the impeller.
In simpler terms: You start with a cylindrical forged billet. Before placing it into the final precision forging die, you machine it on a lathe to create a specific preform shape. This pre-shaped piece is then forged into the final impeller.
Why is Preturning Done? (The Key Reasons)
Optimal Material Distribution: The goal is to place the right amount of metal in the right places before the final forge. A uniform cylinder doesn't have enough metal where the impeller's hub is thick and has too much metal where the blades will be thin. Preturning creates a "preform" that closely approximates the volume distribution of the final part.
Improved Forging Quality & Grain Flow:
Reduces Incomplete Filling: Ensures metal flows smoothly into all the intricate details of the blade tips and contours without leaving voids.
Minimizes Flash: Flash is the excess metal that squirts out between die halves. A well-preturned preform reduces flash, saving material and reducing post-forge machining.
Controls Grain Structure: Promotes uniform, radial grain flow that follows the shape of the blades and hub, which is critical for fatigue strength and resistance to centrifugal forces.
Reduces Forging Loads and Die Wear: By starting with a shape closer to the final form, the forging press doesn't have to work as hard to move the metal. This extends the life of the expensive forging dies and allows for the use of smaller presses.
Consistency and Defect Prevention: It helps eliminate forging defects like folds, laps, and underfills, which are common when trying to forge a complex shape from a simple block.
The Typical Process Flow with Preturning:
Start: Raw material (e.g., Titanium alloy, Inconel, high-strength stainless steel) is cut to size.
Upset Forging: The billet may be upset (forged to increase diameter and shorten length) to break the original cast grain structure and improve workability.
Preturning (The Key Step): The upset or cylindrical billet is mounted on a lathe or CNC turning center.
A preform shape is machined into it. This shape is carefully calculated by process engineers using forging simulation software.
It typically looks like a "top-hat" or a stepped cylinder, with more mass in the central hub area and less in the outer rim/blade region.
Critical dimensions like preform diameter, hub thickness, and corner radii are tightly controlled.
Final Closed-Die Forging: The pre-turned preform is heated to the forging temperature (e.g., 900°C for Ti-6Al-4V) and placed into a precision, multi-part die set. A high-tonnage press (mechanical or hydraulic) forces the dies closed, causing the metal to flow into the intricate blade cavities.
Post-Forge Processes: The forged impeller is then heat-treated, descaled, and undergoes extensive CNC machining to achieve final aerodynamic and dimensional tolerances (blade profiles, bore, balance, etc.).
Visual Comparison: Without vs. With Preturning
| Feature | Forging from a Simple Cylinder | Forging from a Pre-turned Preform |
|---|---|---|
| Starting Shape | Uniform cylinder | Engineered "preform" shape |
| Material Flow | Excessive, chaotic lateral flow | Controlled, radial flow |
| Flash | Significant, wasteful | Minimal |
| Risk of Defects | Higher (underfills, folds) | Much Lower |
| Die Stress | High, concentrated | Lower, more distributed |
| Final Grain Flow | May not optimally follow blade contours | Ideally aligned along blades and hub |
Summary
Preturning is a strategic investment in material, time, and engineering that pays off by enabling the reliable, high-quality, and cost-effective forging of complex centrifugal impellers. It is a standard best practice in the industry for critical rotating components where integrity, performance, and safety are paramount.
In essence, it's the step that ensures the forging process itself is set up for success, transforming a lump of metal into a near-net-shape component with the right internal structure to spin at tens of thousands of RPM for thousands of hours.
