Laser welding differential housings, ring gears

While many automotive lightweighting methods focus on replacing steel and iron with aluminum and composites, there are some vehicle parts that need the durability and strength of ferrous materials. But that doesn’t mean engineers can’t shave a few pounds off iron and steel components.

Housings for differentials – components that manage and transfer engine power to wheels – tend to be cast iron, but the ring gears in the systems are often case-hardened steels. Joining those two materials has generally required mechanical fasteners, but machine tool company EMAG is promoting laser welding for that connection. Its ELC 250 DUO Laser Cell can fuse dissimilar metals, cutting system weights by a few pounds.

Renald Schedewy, applications manager for laser beam welding (LBW) at the company, says fuel economy, power transmission, and noise all improve from LBW differential systems.

“Compared to conventional welding methods LBW has a low heat input per unit length. The damaging of the base material is much lower,” Schedewy says, adding that the technology enables welding cast iron to case-hardened steel.

LBW also allows for deep-penetration weld seams, creating a stronger, more continuous connection between parts. That tighter bond could allow higher torque transmission from vehicle drive shafts to the wheels. And the process’ high circular runout reduces noise throughout the entire transmission system.

Improved technology?

Weight savings are the main draw for laser-welded gear sets for differentials, with EMAG engineers estimating 0.6kg (1.3 lb) to 1.2kg (2.6 lb) in savings per system. More important, that weight comes off rotating powertrain parts. Transmission weight reductions can be more valuable than static-body weight lowering because vehicles will need less engine power to rotate those components. So, lighter transmission parts can let automakers put less-powerful engines in vehicles without sacrificing acceleration and torque. Schedewy says moving away from mechanical fasteners also can lower production costs.

“Beside the immense reduction of weight by eliminating the screws, flanges, etc., a number of machining operations and screwing operations become obsolete,” Schedewy says.

Multi-material challenges

The benefits of welded differential gear sets have been obvious for many years, but technical limitations on arc-welding systems were too big of a roadblock for implementation. Laser welding solves most of those problems by offering precise temperature control.

“LBW has a low heat input per unit length and a high aspect ratio, which result in deep and narrow weld seams. The only limitation is the temperature. The material demands something like, ‘melt me but leave me cold,’” Schedewy explains.

High-temperature welding risks include the formation of cold cracks and brittle failure of components due to a lack of elasticity in the microstructure and construction. High carbon content in steel leads to high hardness, but alloying elements such as magnesium and chromium decrease the critical cooling rate.

“Molybdenum and chromium are carbide formers and additionally increase the hardness,” Schedewy says. “When welding cast iron, especially the spheroidal graphite cast iron, zones result with very high carbon content. Particularly at risk is the heat-affected zone beside the weld seam. Here, the very brittle ledeburite zones (a mix of primary cementite and carbon-saturated austenite) can critically fail when loaded.”

LBW allows the machining cell to weld the two materials with low stiffness without harming the microstructure of the underlying material. EMAG’s system achieves this by using brilliant laser sources combined with controlled heat treatment via induction.

Welding process

ELC 250 DUO uses fixed lasers for focusing, moving the workpiece instead of moving laser heads. That design processes rotational parts in a single setup, enables loading and unloading during the cycle time to reduce idle times, and protects sensitive optical elements.

“The welding head with the laser-fiber cable is the primary tool for LBW. It contains a lot of precision optical elements, which form the laser beam shape. The shape of the laser beam influences the weld seam quality and the welding process,” Schedewy says.

He adds that the laser-fiber core is precisely machined with 10kW or more power flowing through fused silica glass with diameters ranging from 100µm to 600µm.

“Fixed tooling provides protection from welding fumes or mechanical shocks, improves the tool life, and guarantees best quality conditions,” Schedewy says. “Users who have the choice between moving the laser fiber cable or holding it in a position will choose to not wave it around.”

Laser processing takes place on two stations within the ELC 250 DUO, but the cell requires only one laser source. A beam deflector switches the laser between the focusing lenses at the two welding stations, improving use of the laser and increasing system productivity.

By acting as a system integrator, adding components such as automation, parts logistics, clamping devices, part cleaning systems, and ultrasound testing equipment, Schedewy says EMAG tailors its cells to the exact materials and processes used by each manufacturer.

EMAG LLC

www.emag.com

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.