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Advantage Online: 2004 Archives

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This article may be distributed and reproduced, but only by following the requirements stated in Permission to Reproduce.



Figure 1 - This frame is made by welding two C-channels together.


Figure 2 - The front rail on this vehicle is made with a hydroformed part.

September 13, 2004 -Hydroforming is a process that forms a tubular piece of metal into a desired shape with the use of dies and a pressurized liquid. This process has been around for a long time, although the ability to form metal thick enough to be used for automotive structural parts application is only about 15 years old.

Vehicle makers are using the hydroforming process to make parts that require strength and durability, as well as parts with complex shapes. Frames are an area where the hydroforming process has been used quite frequently. The advantage to using this process is primarily weight savings. A thinner wall material can be used without compromising the strength when compared to a welded box or C-channel (see Figure 1). Another advantage is a complex shape can be made with one piece of metal without having to weld various pieces of metal together. This enables the vehicle maker to build a frame with a modular design that is welded together. Previously, areas with a change in shape required joining pre-bent frame members together with GMA (MIG) welding. With hydroforming, this step is eliminated (see Figure 2). Additional advantages include improved corrosion resistance due to having less welded seams, and greater predictability of how collision energy will be absorbed. When collapse zones are incorporated into the hydroformed part, engineers can predict at what rate the vehicle will collapse during a collision (see Figure 3).

The first high-volume vehicle that used a hydroformed structure was the 1997 Chevrolet Corvette. Along with this vehicle came several sectioning procedures where a portion of the frame could be replaced in the area of damage without necessitating a complete frame replacement. Since this time, General Motors, along with Ford and DaimlerChrysler have incorporated the use of hydroformed steel on the frames of trucks and SUVs, as well as for roof rail supports and pillars.


Figure 3 - The front rail on this hydroformed frame has crush initiators to make the frame collapse more predictably.


Figure 4 - The front frame horn is frequently damaged during a collision.


Figure 5 - This replacement front frame horn attaches to a hydroformed frame.


Figure 6 - This frame has welded plates for reinforcement.

Hydroformed Metal-Is It Repairable?

Collision repair technicians are concerned that the hydroformed frames may have special considerations for repair or repair limitations. Although the rigidity and strength may create a challenge when straightening these frames, vehicle makers have made an effort to make replacement parts available for repairs to the areas that are frequently damaged in a collision (see Figure 4). Repair options offered by vehicle makers include sectioning and replacement of frame parts at factory seams. These replacement part kits generally include a procedure for the repair (see Figure 5).

Examples of hydroformed parts with sectioning or complete part replacement procedures include:

  • 2004 Dodge Durango
    – Front frame and upper fender rail shotgun
  • 2002 and newer Dodge Ram 1500
    – Front frame and upper fender rail shotgun
  • 1999 and newer GM C/K series truck
    – Front frame crush cap and rear module
  • 2000 GM C/K Sport Utility Vehicle (Tahoe/Suburban)
    – Front frame crush cap
  • 2002 GM S/T Sport Utility Vehicle

Although the kink vs. bend rule applies to hydroformed frames, access to the inside of the frame to remove visible deformation is very limited due to the tubular design of hydroformed parts. Unless the damage is located near the end of the structural part, access to the backside to allow pushing the damage outward is typically not an option. If a slightly collapsed area needs to be raised from the backside of the rail assembly, a small hydraulic ram may be used if there are no inner obstructions in the part. In areas where a ram cannot be used, removing the damage is often limited to using welded-on plates. One of the concerns with weld-on attachments for pulling is the concern of the heat from the welding process affecting the integrity of the part.

To date, many hydroformed parts on truck frame assemblies have been made from mild steel, but applications of high-strength hydroformed parts are becoming more common to provide a stronger, more rigid vehicle. The 2004 Ford F-150 and the 2004 Dodge Durango are two vehicles that are constructed with frame parts made with high-strength hydroformed steel. Also, some vehicle makers are incorporating the use of external welded-on reinforcement plates on portions of the frame to strengthen the frame assembly. The 2004 Ford F-150 is an example of a vehicle with a frame that has reinforcement plates (see Figure 6).

Heating Recommendations

Little documentation exists in regard to heating hydroformed parts. Many vehicle makers publish heating recommendations that apply to a specific vehicle, but there are typically no specific recommendations given for the hydroformed portions of the vehicle structure.

Industry wide, the use of heat is continually drawing more scrutiny due to concerns of damage to corrosion-resistant coatings on the metal and the possible changes to the strength that may occur from the effects of heat on the structure.


The use of hydroforming in the assembly process for full-frame vehicles is being used more frequently. While these frames have created a challenge to collision repair technicians, vehicle makers have addressed some of the concerns with the availability of replacement parts. Find out more about sectioning and installing full-frame replacement parts in a coming article in the I-CAR Advantage Online.

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