General Motors is poised to pull the trigger on a potentially game-changing manufacturing process to spot weld steel to aluminum. In the first production application for the patented process at GM’s Hamtramck, MI, assembly plant, the advanced spot welding eliminates rivets that join an aluminum bracket to a steel framework to form part of the seatback for the Cadillac CT6.

Engineers claim GM’s steel-to-aluminum welding will be an industry first when it launches later this year. If all goes according to plan with the seatback frame, GM intends to expand the process to the hood of the CT6, a new flagship sedan that currently represents the company’s most aggressive use of multi-material construction.

A specially-designed ridged electrode for the welding-gun tip is a key component of the system, said Blair Carlson, lightweight material processing lab group manager at GM Research & Development. In all, there are 19 patents covering hardware and controls for the process.

The advanced electrode helps to alleviate the physical-properties issues that to now have stymied steel-to-aluminum welding, including the 900º C delta in melting points between the two materials, formation of oxides on the aluminum component that compromise weld integrity and the tendency for a “glassy” layer to form between the two disparate materials, causing brittleness of the weld.

Carlson said cycle time before the welding-gun tip requires dressing currently is about 20 welds. The goal is to move the cycle into the hundreds—this as part of the GM research unit’s mandate for the new welding process to minimize added cost, enhance assembly-plant tooling flexibility for mixed-material vehicles and eventually eliminate altogether the need to rivet steel and aluminum.

Once the mixed-material welding of the CT6 seatback is up to speed, engineers are eying the car’s hood with the intention of welding the pedestrian-impact steel reinforcement to the hood's aluminum “inner.” This will eliminate scores of rivets currently used to join the two pieces, Carlson said.

Advanced joining techniques for the CT6, which employs 11 different materials in the body structure, already have eliminated more than 1400 rivets, which in aggregate add incremental weight and cost. GM estimates the new welding process could eliminate up to 10 lb (4.5 kg) of rivets on some vehicles.

Dual-beam lasers

The news of GM’s imminent deployment of the steel-to-aluminum spot-welding process came at a media event at which engineers also provided updates of other advanced-joining techniques the company currently employs. Those include a refinement to GM's aluminum-to-aluminum laser welding and a new, dual-beam laser that reduces weld spatter, providing a better overall surface finish and eliminating a process formerly required to clean the spatter from the surface.

Another improvement: a redesigned aperture plate that controls reflection on the laser welder. The previous aperture plate had to be replaced after 30 vehicles moved through the body shop. With a more-optimized shape that came at almost no cost, the upgraded aperture plate has been used for more than 3000 vehicles and remains in service, Carlson said.

Summarizing recent whole-vehicle weight reductions, GM said that for seven recently introduced models, lightweighting measures have chopped a total of 2480 lb (1125 kg), including a 700-lb (318 kg) reduction for the 2017 GMC Acadia and 400 lb (181 kg) for the 2016 Chevrolet Camaro. The aggregate savings delivers a CO₂-equivalent reduction of 137,000 metric tons, GM engineers calculated.