As automakers strive to meet National Highway Traffic Safety Administration standards and other global emission mandates, they have put an intense focus on new vehicle technologies that can improve mileage performance. The initial push to improve miles per gallon was centered around new powertrain technologies. But, according to a 2014 study (Figure 1), 49 percent of the respondents identified lightweighting and the use of lightweight structural materials as their key technology focus.
With this focus on lightweighting, aluminum is well-positioned for growth given the various areas of a vehicle where it is a good option. Aluminum offers a balance among the various factors of cost, mass, energy management, safety, formability and styling. Currently, about 80 percent of the aluminum content in a vehicle is from cast content.
A logical place for aluminum content use is in the body. In a typical vehicle, the body-in-white plus the closures: doors, trunk, hood and bolt-down fenders represent 30 percent of the whole vehicle mass. According to Ducker Research, by 2025, 26.6 percent of all the North American body and closure parts on a volume basis will be made of aluminum as compared to 6.6 percent in 2015. Aluminum hood penetration will reach 85 percent and doors will reach 46 percent. Complete bodies will reach 18 percent, driving total aluminum content to 547 lbs per vehicle.
However, with the vast majority of vehicle weight still represented by steel, the ability for steel usage to impact weight, performance and integrity of a vehicle is also significant. While aluminum continues to make inroads in the auto industry, steel is not about to fade away. It has remained the leader in average vehicle content on the road – about 60 percent by weight today – and will continue to be an important material. In fact, despite the 2015 Ford F-150s record use of aluminum, the use of high-strength steel in the frame rose from 23 percent to 77 percent to improve stiffness and durability, while reducing weight.
Rather than pitting materials like steel or other lightweight offerings such as magnesium and composites against each other, an effective lightweighting strategy will likely focus on a multi-material mix. With new materials and new grades rolling out on a continuous basis, advanced lightweight materials use is a key focus for automakers, yet the selection and adoption processes can be a costly trial-and-error method at times.
Joining lightweight materials
In general, lightweight materials mean thinner gauges. 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-lock 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 materials.
The Ford F-150 is the first high-volume vehicle to use large amounts of aluminum. The aluminum body is joined using a few spot welds, but relies on more than 350 ft. of structural adhesive beads to supplement the joint strength provided by 2,000 self-piercing rivets.
The automotive industry typically relies on spot welding to join steel stampings together into a completed body, which has been problematic when applied to aluminum. Engineers at General Motors have been experimenting with a new RSW process that uses a patented multi-ring domed electrode, which, unlike smooth electrodes, has been successful at welding aluminum to aluminum where there is two-sided joint access. With this process, GM hopes to eliminate nearly 2 lbs. of rivets from aluminum body parts, such as hoods, lift gates and doors.
Joining dissimilar materials
Applying RSW when joining dissimilar materials presents numerous challenges. Issues can appear due to different melting points, different chemical structures, the formation of intermetallics and corrosion problems.
In the case of joining aluminum to steel, aluminum has far different properties than steel. It has three times the thermal conductivity, four times the electrical conductivity and requires three times higher welding currents. The melting temperature of aluminum is approximately 650 C while for steel it is about 1,538 C. This means the aluminum melts and flows away before the steel has melted. And, the wide difference in the thermal conductivity and in specific heats of aluminum and steel causes significant thermal stresses.
The most critical factors when joining aluminum to other metals are the metallurgical issues. Under the influence of heat, intermetallic phases are formed at the interface between the two materials. Intermetallics are an intermediate composition between the two primary components with a crystal structure that is different from the primary components. The more heat is applied, the larger the zone containing the intermetallic phases and the poorer the mechanical properties of the joint.
Another issue when joining dissimilar materials is galvanic corrosion. Dissimilar electrically conductive materials have different electrode potential and when they come into contact with an electrolyte, one material can act as anode and the other as cathode. This results in one of the materials corroding preferentially to the other.
A joining alternative
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 (non-melting) at the joint interface. The materials are then forged together by force, creating a joint. It offers numerous benefits over other joining techniques, including the elimination of filler metal or flux, higher quality joints, a small heat-affected zone and no coarse grain formation.
For decades, friction welding has proven to be a successful joining strategy. It can be applied as friction spin (or rotary) welding, linear friction stir welding (LFSW or FSW) and 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, LFSW and RFFSW 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 the 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, however, 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 for single-point joining processes like RSW and riveting.
RFSSW is similar in principal to LFSW, although it’s 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 Co. began developing 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 automotive and non-automotive 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.
Refill friction stir spot welding
In 2014, Coldwater introduced its SpotMeld solution to the market, which has its foundation in the RFSSW technology that was developed and patented by Helmholtz-Zentrum Geesthacht, Germany. To develop the technology for integrating RFSSW into high-volume 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 to 7000 series), magnesium, non-ferrous and dissimilar sheet materials, making SpotMeld a viable alternative to single-point joining processes like RSW, 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.
Coldwater’s RFSSW process consists of five phases:
- Weld head closing.
- Friction Phase – Both the pin and sleeve are placed on the surface of the upper sheet and rotate to generate sufficient frictional heat for plunging.
- Advance Phase – The sleeve advances into the materials and the pin retracts, pulling softened material from the metal sheets into the tool.
- Retract Phase – The sleeve retracts and the pin advances flush with the sleeve, pushing the displaced material back into the hole and forging the ﬁnished weld.
- Weld is complete – The weld head opens.
Compared to laser welding and other aluminum joining techniques, SpotMeld is an easier process to fixture and it’s more tolerant of imperfections. It’s also designed to have the same basic footprint and work envelope as an RSW robot, putting it in a familiar context for manufacturers.
Utility use is another big area for cost savings. The electricity cost is much lower than that of RSW because the need for a huge current is eliminated. The only utility cables are the servo lines that connect to the servo motor and some water cooling for the tools.
A major advantage of the Coldwater system is that the three-piece 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 off-set on it. Additionally, it creates a 3-mm to 4-mm hole in the center of the weld.
Currently, Coldwater can join a stack-up of materials from 0.8 mm to 8 mm, weld dissimilar aluminums in one stack and join multiple sheets across the edge of a panel with the SpotMeld.
On the horizon
The foray by manufacturers into new lightweight materials is certainly not going to subside. And, Coldwater will continue to stay ahead of the curve by focusing on the development of solid-state joining technologies for high-production environments, especially in the areas of RFSSW.
This year, its development partner will have a system installed at a low-volume exotic vehicle manufacturer. That news is in addition to two additional RFSSW systems that have been 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 for Coldwater is investigating the feasibility of joining aluminum to carbon fiber materials and aluminum to steel.