Silverado frame rails
General Motors uses hydroforming to produce frame rails for its vehicles including the Silverado pickup and the aluminum pieces found in the Corvette. This process makes stronger, lighter and to more accurate dimensions than typical welded multi-piece components.
In tube hydroforming, pressure is applied with hydraulic fluid inside a tube that is held by dies that dictate the final shape of the part. This strain hardens the part and makes it conform the desired shape.
Designing hydroformed parts is challenging because it is difficult to assess the forces the part will undergo and what the strength of the final part. Modern computing in conjunction with finite element analysis (FEA) are required to ensure that the desired result is achieved.
Finite Element Analysis
To assess how a complex shape like a hydroformed frame rail will flex under loads at the various mounting points, engineers divide the shape of the object into hundreds or even thousands of smaller, well understood shapes called finite elements. While these shapes only add up to approximate the original shape, because the discrepancies are so small the resulting predictions of stress and strain affecting the object are quite accurate.
Whereas the majority of a car’s chassis will be made from stamped parts which are then fastened together, hydroforming begins with tubes which are then “inflated” from the inside. In other words, the part expands to fit its die, rather than the die closing upon it.
FEA breaks the structure of a car part (or structural components of the entire car) into finite elements to which known estimations can be applied. This lets computers calculate how the car will deflect under load or during a collision.
Using high pressure hydraulic fluid during hydroforming ensures consistent pressure applied to the entire part from within and provides predictable results.
Hydroforming is similar to blow molding that is used to make hollow plastic parts and glass bottles.