The use of lightweight materials is predicted to grow significantly within the automotive sector, from 29% of vehicle content in 2010 to 67% of vehicle content by 2030 according to the McKinsey & Company, “Lightweight, heavy impact” study. The majority of material content will come from Advanced High Strength Steel (AHSS) at 38% with the remaining lightweight content being a mix of aluminum, 12%; magnesium, 5%; plastics, 11.5%; and carbon fiber, 0.5%.
The advantage of AHSS is that it is typically compatible with the existing manufacturing infrastructure and processes. However, rather than pitting steel against other lightweight offerings, the key will be finding a way to make these multiple materials work together. Implementing multimaterial solutions can be a costly trial and error method at times as the industry endeavors to overcome the challenges associated with multimaterial usage, such as joining issues and corrosion prevention.
In general, lightweight materials mean thinner gages. As material gets thinner, joining—especially with screws and rivets—becomes more difficult. Currently, the primary methods for single-point joining of lightweight materials include resistance spot welding (RSW), toggle-locks, rivets, and self-piercing fasteners and rivets. While self-piercing riveting works well when joining lower strength steels with aluminum, it isn’t suitable for joining aluminum to ultra-high-strength steel as it can create a stress riser or a fracture in the material. Toggle-Lok processes can be simple and affordable, but have less strength than RSW. And, in most cases, mechanical joining techniques are combined with adhesive bonding to increase the static and fatigue strength of joints and prevent corrosion of joints caused by the contact between the dissimilar metals.
Although RSW is the typical strategy for joining steel stampings together into a completed body, it has been problematic when applied to aluminum. Additionally, when applying RSW to the joining, dissimilar materials present numerous challenges. Issues can appear due to different melting points, different chemical structures, the formation of intermetallics and corrosion problems.
Friction welding, a type of solid-state joining, creates mechanical friction between workpieces in relative motion to one another, heating the materials until they reach a plastic state (nonmelting) at the joint interface. The materials are then forged together by force, creating a joint. It offers numerous benefits over other joining techniques including elimination of filler metal or flux, higher quality joints, a very small heat affected area, and no coarse grain formation.
A proven joining methodology, friction welding can be applied as friction spin (or rotary) welding, linear friction stir welding (LFSW or FSW) or refill friction stir spot welding (RFSSW), as well as multiple variants of each approach. While most rotary friction welding is used on round, symmetrical parts, linear friction stir welding and RFSSW allow solid-state welds on a wider range of part geometries.
A major advantage of friction welding is that it allows dissimilar materials to be joined. In fact, nearly half of the welds made through friction welding are for joining of dissimilar materials. Normally the wide difference in melting points of the two materials would make it impossible to weld using traditional techniques, and would require some sort of mechanical connection. Friction welding provides a “full-strength” bond with no additional weight.
As a variant of friction stir welding, RFSSW has become a focus as a solution for spot welding aluminum and dissimilar materials. It shows great potential to be a replacement of single-point joining processes like resistance spot welding and riveting.
Refill friction stir spot joining is similar in principal to LFSW, although generally applied as a joining technology for overlapping or stacked sheet material. Both techniques use a rotating tool with a specially designed pin and shoulder. However, with LFSW the tool travels along a seam between two metal plates versus the tool staying in one spot in friction spot joining.
Coldwater Machine first began development of its friction welding solutions in 2003, originally developing and integrating friction spin welding solutions. Since then, it has designed dozens of its SpinMeld systems for installation at a variety of Tier suppliers and OEMs in both automotive and nonautomotive markets. Given the increasing use of lightweight materials, and especially aluminum in automotive body applications, Coldwater has applied this friction welding experience to the challenge of spot welding of aluminum and dissimilar materials.
In 2014, the SpotMeld RFSSW solution, based upon technology developed and patented by Helmholtz-Zentrum (Geesthacht, Germany) was announced. For integration of the technology into high-production environments, Coldwater continues to partner with the Helmholtz Institute and weld-head provider Harms & Wende.
SpotMeld uses a three-piece tool to join two or more faying surfaces. Basically, heat is generated between the tool and materials being mated to create a soft, plastic-like region. Coldwater has had success in spot welding aluminum (1000–7000 series), magnesium, nonferrous and dissimilar sheet materials, making SpotMeld a viable alternative to single-point joining processes like resistance spot welding, laser welding and riveting.
In addition to its ability to join dissimilar and lightweight materials, benefits include high-quality joints with a small heat affected zone, consistency in weld duplication, as well as being environmentally cleaner and safer with no filler material, spatter, smoke, radiation or shield gasses.
Compared to laser welding and other aluminum-joining techniques, SpotMeld is an easier process to fixture and it’s more tolerant of imperfections. It also is designed to have the same basic footprint and work envelope as a resistance spot-weld robot, putting it in a familiar context for OEMs. Additionally, the electricity cost is much lower than that of resistance welding since there is no need for the huge current. The only utility cables are the servo lines that connect to the servomotor and some water cooling for the tools.
Coldwater’s RFSSW process consists of five phases:
A major advantage of this approach is that the tool doesn’t fully penetrate through the bottom sheet, leaving a smooth surface with potential for use on exterior body panels. Alternative friction spot welding techniques typically use a solid pin that does not retract, so the pin advances partially into the sheet, a little more than halfway through the joint, leaving a surface that has some material offset on it. Additionally, this creates a 3–4 mm hole on the center in the center of the weld.
Currently, Coldwater can join a stack-up of materials from 0.8 to 8 mm, weld dissimilar aluminums in one stack and join multiple sheets across the edge of a panel.
Three sheets of 3.0-mm material 6000 series + one sheet of 1.5 mm 5000 series
The foray by manufacturers into new lightweight materials is certainly not going to subside. Coldwater is staying ahead of the curve by focusing on the development of solid-state joining technologies for high-production environments, especially in the areas of refill friction stir spot welding. This year, its development partner will have a system installed at a low-volume exotic vehicle manufacturer in addition to two other RFSSW systems ordered for a helicopter manufacturer, all in Europe.
To date, the repeatability and viability of joining aluminum to aluminum and aluminum to magnesium has been well documented. Next on the agenda is investigating the feasibility of joining aluminum to carbon-fiber materials and aluminum to steel.
Bob Rich is Solid State Joining Center Director and Mike Spodar is Senior Welding Engineer at Coldwater Machine Co. (Coldwater, OH).
This article was first published in the 2016 edition of the Motorized Vehicle Manufacturing Yearbook.
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