Metals that make a difference

Toughest frame in GM's history

If you are in the vicinity of a Chevy or GMC dealer, check out the undercarriage of the new Silverado or Sierra pickups. GM engineers claim that the '99-model vehicles have the toughest, most intelligently built light-duty truck frame in the company's history. And that's saying a lot, since the venerable Chevy C/K and Sierra pickups, introduced in 1988, had a frame that set a new industry benchmark, according to automotive experts.

"Not only is the new frame stiffer and more durable, but it also enabled us to make other improvements from ride and handling to body fit and finish," says Ken Sohocki, chief engineer of GM's new full-size truck platform.

To bring it off, GM's frame engineers applied a modular, three-section strategy to the new pickups' construction. It allows the front, mid, and rear sections to be made independently by combining materials and processes that not only meet or exceed performance requirements, but don't impact overall vehicle mass targets.

The design starts with the front-frame rails. Here, the engineers employed new hydroforming technology (see diagram) for producing the rail and engine cross members. With the hydroforming process, the rails start as hollow, round steel tubes bent to general form. High-pressure fluid then forms the tube to just the right shape--from the inside out--while the ends are compressed.

Hydroforming, roll-form/draw-bending, and traditional stamping processes converge in the design of a three-component pickup frame that GM calls the most intelligently built frame in the company’s history.

Hydroforming offers several advantages over conventional boxed-frame technology, say the engineers. They include:

Eliminating about 300 inches of weld from the front section for increased structural rigidity and frame strength, producing more consistent steel-wall thickness during forming.

• Reduction in weight because the process requires fewer steel pieces and welds.

• Improved dimensional control of mounting brackets, including front bumper brackets, leading to an improved front-end sheet-metal fit.

• Eliminating more than 40 lb of steel scrap per frame.

• Absorbing about 35% more energy than a box frame, while "crush caps" contribute to front-end crush performance and reduced repair costs.

The new mid-frame section comes in four lengths to accommodate two wheelbases and two cab styles. It also transfers loads to the front axles and supports such components as the fuel tank, brake lines, and wiring harnesses.

The roll-form/draw-bending process for producing the frame uses a solid, straight piece of lightweight, high-strength steel. It rolls the steel gradually into a lipped C-shape, forming the frame without the need for a more traditional stamping procedure. This allows the steel to retain strength without increasing its mass. Added strength is achieved by rolling over the edges of the frame to reinforce it. In fact, the design increases the mid-rail's properties 65% over its predecessor, while reducing its total mass 27%.

The third part of the modular frame, the rear C-section, mounts suspension components, bolts the pickup box, and hangs the spare tire carrier, bumper, and trailer hitch. Weight-carrying requirements of this section are minimal, therefore, the engineers elected to use a more traditional steel stamping technology to create this section.

Four-step hydroforming process employed in GM’s modular frame design eliminated more than 40 lbs of steel, while improving dimensional control of mounting brackets. Frame absorbs about 35%more energy than a box frame.

To round out the new full-size pickup frame design, the engineers turned to tubular cross members and close-out plates. These components provide enhanced torsional performance over traditional stamping, without driving beaming stiffness into an undesirable range. Welded construction eliminates the need for rivets in many areas.

As a final improvement, the engineers specified the use of a new, high-temperature wax application to better protect the frame from corrosion. With a melting point of 295F, the hot-wax treatment is reportedly 55 degrees higher than that used on the Ford F-150 and GM's 1998 full-size pickups.

Magna International's Cosma Body & Chassis Systems' Formet frame manufacturing facility (St. Thomas, Ontario, Canada) produces and assembles the three-component frames. Automated guided vehicles deliver the stacked frames directly to railcars, where they are shipped JIT to three GM assembly plants.

"Vehicle build time has been reduced because the frame arrives at the final assembly plant in a more finished condition," observes GM's Sohocki. "This design has created a solid foundation for our new full-size pickups, and, at the same time, demonstrated GM's role as an industry leader."

Micro-arcs form ultra-hard aluminum

An electrolytic surface conversion and coating process developed by Almag Al Ltd. (Jerusalem, Israel) seems certain to challenge such established aluminum finishing systems as plasma coating, hard-coat anodizing, and hard-chrome plating in the near future. Named the Almag Process, the system resembles anodizing--it takes place in an electrolytic bath and is energized by electric power--but the similarity ends there.

For example, the batch is silicon-based, rather than an acid solution, and the power source is ac rather than dc. Moreover, the electrical charge is applied by a specific waveform.

In the process, racked workpieces go into a bath held at 95F (35C) and 800V ac is applied. In the first few seconds, a thin layer of aluminum hydroxide forms on the workpiece surface.

Activated by multiple dielectric breakdowns on the aluminum hydroxide layer, a series of "micro-arcs" is generated at the surface to form a ceramic layer. The layer extends to the smallest, most remote cavities in such applications as a fine screw thread or piping.

This complex physical/chemical reaction results in a hybrid aluminum hydroxide/silicon oxide ceramic coating. In about 40 minutes, a 50-micron coating forms. This, Almag's Patrick Elbeze says, proves sufficient for most industrial applications.

What sets the Almag process apart from similar processes, Elbeze notes, is the hardness and abrasion resistance of the newly formed surface. Measured using the Vickers system, hardness falls in the 3,200 range. That puts the coating in the same hardness class as sapphire. Wear resistance measures about four times that of hard anodized aluminum, according to Elbeze.

The process also strengthens thin aluminum sections. An aluminum wire measuring 1-mm thick, protected with 100 microns of Almag coating, takes on the stiffness of steel wire of equivalent diameter. Treated aluminum parts can operate at temperatures up to 662F (350C), imparting thermal and electrical insulation to the material. Dielectric strength measures more than 20V per micron of coating thickness, while the electrical resistance of a 100-micron coating approaches 100 M{OMEGA} or greater.

"Our coating stubbornly resists attack by seawater, oils, acids, and alkali," says Elbeze. "It even shrugs off salt spray exposure for 1,000 hours or more."

Polish the coated surface to a mirror finish and you achieve a very low coefficient of friction. You may not even need a lubricant in bearing applications, Elbeze says. Impregnating the surface with a tough, slippery polymer or lacquer, which fills in the crystalline interstices, further decreases friction.

Omega Metal Laboratories conducted independent tests on the coating (see table). The results show that an Almag-process aluminum part tests out at 4.4 times the abrasion resistance of hard-anodized aluminum, 2.7 times that of tempered steel, and twice that of hard-chrome-plated aluminum.

"Our process is 10 times better than plasma-coated aluminum at roughly the same processing cost," Elbeze says. "What's more, our processing equipment costs one-quarter that of plasma equipment."

A basic Almag unit sells for $120,000. Currently, the machines are made in France by Acore, a producer of generators and electrical specialties. Acore has signed an agreement with Robert Moos, a Frenchman who has established Almag France, to distribute the equipment throughout Europe.

"We estimate that during the next six years, Almag coating will capture 50% of the ceramic plasma coating market," predicts Elbeze. This works out to about $5 billion for coated aluminum parts and $3 billion for steel parts replaced by the Almag process.