Building a BIW (Body In White)

The typical mass-produced unibody / monocoque body structure is an assembly of many hundreds of pressed-sheet steel panels and brackets, mainly welded together by some 3 - 5000 resistance welding (single point / spot welds) plus many other resistance welded fasteners.

This stiff-shell structure is called a body in white (BIW) and typically accounts for one-third of a vehicle's total weight.

Over the past 20 years, there has been a substantial effort to reduce vehicle body weight by the use of high strength (HS) steels and aluminium alloys.

Even higher strength steels have now been developed and are beginning to be used, particularly in components where there is a high demand on safety performance. The steels of interest, generally referred to as ultra-high strength (UHS) steels, include dual phase (DP), transformation induced plasticity (TRIP), complex phase (CP), martensitic and boron-alloyed steels.

Do these high strength steels Affect Spot Welding?

Resistance spot welding is still the main joining process in the automotive industry. Consequently, development of suitable welding schedules, to achieve the required joint mechanical properties, has become the key issue for maximising the benefits of using these materials.

The welds made must not affect the intrinsic performance of the steel body parts For example, if a weld is too weak the part will function correctly up to the point where the weld fails. This compromises the structural integrity of the whole body unit in respect of occupant safety.