Weld Bonding VS. Adhesive Bonding

There are clear differences between weld bonding and adhesive bonding that must be considered in discussions of vehicle manufacturing and repair with adhesives.

The American Welding Society defines weld bonding as a resistance spot welding process variation in which the spot weld strength is augmented by adhesive at the faying surfaces.

Adhesive bonding is a method for joining similar or dissimilar materials relying on the adhesion properties between the adhesive and adhered surface. Ford Motor Company at present only utilizes weld bonding in the manufacture of steel body structures, not adhesive bonding.

Current advertising of aftermarket structural adhesives appears to stress safety and crash performance. Suppliers indicate they have conducted crash tests of manufacturer's vehicles, compared the performance of the vehicles repaired with structural adhesives vs. vehicles repaired with MIG plug welding, and concluded that the crash performance is essentially the same. Aftermarket suppliers imply that because their selected vehicle crash tests were apparently successful, their repair materials and procedures are suitable for other spot-welded sheet metal parts on all other makes and models (except structural parts), under all collision scenarios, all environmental and operating conditions and all possible ages of vehicle, and will completely restore the structural integrity of the vehicle.

Ford Motor Company does not accept the above logic.

Ford vehicles are designed and engineered to meet or exceed Federal Motor Vehicle Safety Standards (FMVSS) and Ford Motor Company cannot confirm that the use of structural adhesives alone will restore compliance to FMVSS requirements. Welded steel body joints (width, gaps, etc.) on Ford designed vehicles have been optimized for spot welding, not for all-adhesive bonding. Ford cannot verify that the use of adhesives alone in these joints without welding will produce the strength, stiffness, dimensions, etc. of the original production vehicle.

Ford currently uses structural adhesives with spot welding to improve Noise/Vibration/Harshness (NVH) and durability, not for safety. It cannot be implied from the aftermarket advertising that the use of adhesives in place of MIG plug welds will restore all of these characteristics. Aftermarket adhesive suppliers appear to be focusing on lap shear as the primary characterization of adhesive strength. Other important factors that are not clearly advertised are modulus, peel strength, toughness, and crack propagation under both static and dynamic load conditions. Because it is not known how these aftermarket adhesives perform to these criteria under all operating conditions, as compared to production spot welds and repairs using MIG plug welding, Ford cannot confirm the effectiveness of these adhesives in meeting crashworthiness, NVH, sealing, and durability requirements.

Aftermarket testing to date apparently has not addressed the potential of adhesive bonds developing non-visible cracks during minor collisions. This could result in significant bond failures through crack propagation or peeling in subsequent collisions.

Ford is concerned about the strength of the correlation between accelerated laboratory coupon (small panel) testing and field data. Aftermarket evaluations under the complete range of possible environmental conditions (temperature, humidity, road salt, time, etc.) appear to be limited to coupons at best and have not addressed accelerated durability or temperature extremes.

Ford has not seen evidence that the quality of these repairs with these materials is robust enough to all the potential variations in technician skill level, environmental conditions, types of repairs, etc. that will be encountered in the field. For example, adhesive performance is dependent on a consistent adhesive bond line thickness. Since mating panels on damaged vehicles will likely require straightening before a replacement panel is bonded on, and given the spring back characteristics of high strength steels, we do not believe that body shops have consistent capability of reproducing production weld-bonded joint gaps with adhesives alone.

Based on our internal investigation to date, we believe that insufficient information exists to validate claims that repairs completed exclusively with structural adhesives will restore pre-accident strength, durability and reliability characteristics under all environmental and operating conditions.

There is one exception to this recommendation: replacement of outer door panels with structural adhesives and hem flanging is an accepted industry practice.