Welding shafts to other components can be tricky when concentricity matters, as it does in a rotating assembly. As the melted material cools within these circumferential welded joints, shrinkage differences tend to "pull" the shaft out of alignment, explains Richard Trillwood, CEO and founder of Electron Beam Engineering Inc., a specialist in precision electron beam and laser welding (www.electronbeamwelding.com). His company recently found a way to improve these troublesome joints while working on aftermarket automotive turbochargers.
These turbochargers feature a tight concentricity tolerance between their hot-side impellers (often Inconel) and shafts (usually low-carbon steel). "These impellers spin at thousands of rpm, so they can't tolerate any misalignment of their shaft," says Trillwood. The exact tolerance differs with the size and design of the turbocharger, but Trillwood says it falls "within a few thousandths of an inch" in all the designs that have passed though his shop.
The makers of aftermarket turbochargers have tried various means of creating this important joint, including friction welding and electron beam welding. According to Trillwood, both methods have had their problems with concentricity. "Friction welding can introduce enough heat into the joint to cause distortion," he says. And correcting that distortion has required extensive and costly finish machining and an operation to straighten the shafts. Electron beam welding, the method the OEM suppliers currently favor, can also suffer from its share of shrinkage-induced misalignment. In Trillwood's shop, which has been working on turbochargers for about six years, hitting his customers' concentricity targets has until now required frequent manual adjustments to the electron beam equipment by a knowledgeable operator.
Trillwood's new method for maintaining concentricity avoids both the secondary straightening operation and the manual adjustments by creating the joint in two carefully controlled passes of the welding head. The first pass establishes the joint, while the second aligns it. The alignment takes place through a "re-welding" of selected joint segments at 0.050 to 0.250 inches deep, depending on the turbocharger design and the magnitude of misalignment. As this second weld shrinks, its helpful shrinkage corrects the detrimental shrinkage of the first pass.
Proprietary software controls the entire process, including not only when the second weld is applied but also the weld power and angle of the welding head. "We developed software that automates what an experienced operator had to do manually in the past," he says.
Trillwood developed this process improvement specifically for turbocharger applications. But he believes it has much wider implications. "It applies whenever a circumferential weld requires a secondary straightening operation," he says.