Vol. 15
March Issue
Year 2014

Shot Peening in the Automotive Industry

in Vol. 15 - March Issue - Year 2014
Cars, New Materials and Shot Peening

Mario Guagliano

In recent years we've been assisting stronger and stronger competition in the automotive industry. We know about the changes that are on course in the field, with a progressive reduction of the number of independent car manufacturers and a global market characterized by a bad trend in Europe and a good one overseas. Indeed, there are many factors that make a car manufacturer competitive or not. One of these is the development of light cars, and for sure, the development of new frames is really important to meet this goal.
Aluminium alloys today are used for cars and new high-strength steel has been developed to contrast the diffusion of aluminium in the automotive industry; by using high-strength steels, we are able to reduce the thickness of the structural parts and compensate for the higher density of steel with respect to aluminium. The application of these materials requires addressing new technological solutions to derive the maximum benefits. For instance, we know that welding aluminium needs particular care and the definition of a strict technological cycles. The same can be said for high strength steels.
And one of the most important issues related to these advanced materials is the ability to correctly weld them. We know that multiple-incident welding is the critical point of a structural part, especially when the applied loads are cyclic, thus inducing fatigue damage.
Bearing these facts in mind, it is clear the application of shot peening on these welded details can greatly improve their mechanical behaviour and make them more efficient from a structural point of view. What is the effect of shot peening on welded details?
We know that the effect of shot peening is mainly related to its ability to introduce compressive residual stresses in the surface layer of material, and to cause surface work hardening of virtually the same layer.
These two factors are even more important for structural welding. In fact, these kinds of details are characterized by tensile residual stresses induced by the welding itself and by a series of micro defects, acting as a sort of crack initiation points. The final result is that the fatigue S-N curves of welded details are much poorer than the curves of the base material.
By considering these facts, it is worthwhile considering the application of shot peening to improve the performances of high-strength steel-welded parts and to obtain a fatigue strength aligned with that of monolithic parts. And, if applied in combination with some other treatment to modify the weld profile, its effectiveness can be increased, since shot peening will act on a surface where defects and imperfections have been already removed, and the weld profile has been geometrically improved.
Nevertheless, it is true that if shot peening is applied without carefully thinking about the parameters to be used, it is hard to obtain the desired improvement of the mechanical performances. The choice of the peening parameters depends on many factors; the material, the geometry, the applied loads, the initial surface state, and so on. It is not a good option to use the parameters defined for basic application of welding to high-strength steels or aluminium alloys: the results would be below expectations. For obtaining the best results by the application of shot peening, a fine investigation about the effects of this treatment is needed. This means that studies should be done both from a theoretical point of view, to understand the way shot peening modifies the surface state and to define some model to assess how the peening parameters affects the final result, and with experimental tests, in order to investigate the effective result in terms of fatigue life and strength.
This would also help the development of design codes focussed on the application of shot peening on welded structures and on the criteria to choose the correct treatment parameters, with the final result of widening the application of shot peening on high-tech welded details.
Last but not least, it would make easier the diffusion of shot peening in other fields where these materials are used.

Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
E-mail: mario@mfn.li                                   

Author: Mario Guagliano