While the aluminum shells of Ford’s pickups have gotten a lot of attention recently, the frame is still a steel beast. The 2017 F-Series Super Duty truck line’s frame is 95% high-strength steel, 6x more than the previous model.
Higher-strength steels are easy for automakers to justify. The stronger the steel, the less you need, so structures become lighter and more fuel efficient. And even with heat-treatments to boost strength, steel commodity prices are still much lower than aluminum and other lightweight metals.
Dave Park, global business segment director – automotive at welding technology company Lincoln Electric, says as companies increase their use of advanced steels in body frames and chassis components, they will have to reconsider some of their classic welding techniques.
“The high-strength steels used in automotive construction, both body and chassis components, are high strength by virtue of their heat treatment, rather than being heavily alloyed,” Park says. “When you take an arc weld, or to a somewhat smaller extent a spot weld, you destroy locally that thermal treatment.”
The concept of a heat-affected zone (HAZ) surrounding a weld, the area where weld heat can alter the mechanical properties of the base metal, isn’t new. However, Park says the increase in heat-treated, strong steels is forcing manufacturing engineers to reconsider welding practices.
Until recently, much of the high-strength steel in the auto world had gone into body-in-white structures, the basic shell of the vehicle. Park says those structures are typically held together with hundreds of small spot welds. While there is softening around each weld, the large number of welds reinforces the structure.
Heat-treated engine cradle
Mercedes-Benz wanted to reduce weight and improve strength on a 14kg aluminum engine cradle for C- and E-Class cars. Steel and automotive components supplier Benteler International AG developed a 12kg high-strength steel structure that was 45% less expensive and offered more crash-impact mitigation.
Dr. Andreas Frehn, head of materials technology for automotive chassis and modules at Benteler, says the supplier formed the cradles using softer steel, then heat-treated it after welding to improve hardness and crash resistance.
“The hardness stability in the weld seam is more important in the air quench-and-tempered condition,” achieved with post-weld heat treatment, Frehn says. The process worked well for Mercedes, and the company continues to use steel engine cradles in its current lineup, though another supplier is providing them.
Frehn adds that Benteler is studying other weld-seam hardening technologies as the use of heat-treated metal continues to rise, such as shot peening and hammering. Engineers are also studying TiG remelting, a process used primarily on cast parts to selectively heat-treat portions of a structure.
Benteler International AG
Structural and chassis components, on the other hand, tend to be arc-welded with long, continuous seams, creating a critical joint that cannot be allowed to fail.
“You need to know where the HAZs are. The classic solution is to design the joint to be in compression, rather than tension,” Park says. “However, with the vehicle frame, there’s no way you can design every weld joint to be in compression.”
Designers are using finite-element-analysis simulation tools to study how arc-welded seams with HAZ softening will perform in crashes and other extreme events. Park says software designers should continue perfecting those tools so automakers can get the most efficient designs that compensate for softening affects.
Some manufacturers and suppliers are looking at mechanical methods of strengthening metal after welding (see sidebar above). Park says some of those technologies are promising, but adding process steps means more cost and production time.
The big draw for high-strength steels is the ability to use thinner, lighter metal without sacrificing capacity or crashworthiness. But as gauges get thinner, HAZ issues and other problems get more extreme.
“Chassis components that used to be 2mm, 3mm, 4mm, we’re now seeing those parts below 1mm,” Park says. “As you reduce these component thicknesses, the strengths of the joint are impacted.”
Careful design and simulation can create structures that handle strains, but material issues come into play at those thinner gauges – corrosion being the biggest.
“If you have a 4mm chassis part and it rusts 1mm, that’s no big deal, you’ve still get 3mm of structure left. If you have a 1mm chassis part, and it rusts 1mm, you’ve got nothing,” Park says. “So when an engineer sees that thin gauge, the first thing he says is, ‘Let’s galvanize that.’ So they add a zinc coating.”
Zinc vaporizes at a much lower temperature than the melting point of steel, so as manufacturers arc-weld seams in galvanized metal, zinc bubbles through the molten metal, creating pores that can weaken the joint. The most effective way to combat that porosity, Park adds, is to slow down welding – not a popular solution in high-volume automotive production.
“Instead of making 50 parts per hour out of a $5 million equipment cell, a supplier’s down to 25 parts per hour. To make rate, you might have to double the amount of equipment,” Park says.
Going slower allows more time for the zinc vapor to escape the weld before the metal hardens. But productivity suffers.
Park notes that automakers have dealt with all of these issues with spot welding for body-in-white structures, so designers and engineers are aware of the challenges. The difference with arc welding is the length and importance of the seams.
“If you have a failure on a 3mm-long slot weld on a body-in-white component, that’s part of 150 welds. It might create a rattle or you might never know it,” Park says. “You have a failure of a 20cm weld on an engine cradle, you’ll know it.”
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or firstname.lastname@example.org.