Shot Peening in the Automotive Industry
in Vol. 11 - September Issue - Year 2010
Shot Peening For Obtaining Nanostructured Surfaces: The Future?
Notwithstanding the long history of shot peening (the first industrial applications are dated more than 80 years ago) this treatment is still rich in tricks and trips and in further developments of industrial interest.
It is true that shot peening is not fully understood and that a lot of research is on course to try to obtain a better knowledge of how shot peening affects the mechanical behaviour of materials; particular research is nowadays oriented to relate the peening parameters to the effects induced on the materials (residual stresses, surface work-hardening, roughness..) and to the (hopefully) better mechanical properties, such as fatigue limit, fretting resistance, pitting, …and to develop simple approaches that can be applied also by technicians, not only academics.
However, in the last few years, one of the most interesting fields of research that involves shot peening is the possibility to obtain nanostructured surfaces by adapting particularly severe peening parameters.
Indeed, it is not really new that by severe plastic deformation it is possible to strongly modify the microstructure of metals and to fragment crystal grains to dimensions up to 100 nm.
It is also not new that a structure with nanograins seems to have superior mechanical properties (wear, fretting, fatigue…). But that is not all, since nanostrucutred surfaces are able to increase the diffusion of elements during thermo-chemical treatments (i.e: nitriding). And the tests performed show that the nanostructure is stable at high temperature, thus making the treatment appealing also for high-temperature applications.
What is interesting of the application of shot peening for obtaining nanostrucutred surfaces, is the ability of this treatment to be used in industrial applications with high production rates, and automotive is probably the first one. In fact, different technological processes have proven to be suitable for obtaining nano grains, but all of them can be used in labs, not in industrial plants. Shot peening does not have this disadvantage but a lot of work has to be done to fully understand the mechanisms that lead to the nanograin formation, to relate the peening parameters to the characteristics of the nanostrucutred layer and to assess the actual mechanical properties of the so treated parts. Until now it is clear that the kinetic energy and the treatment time are the most relevant parameters that affect the generation of a nanostructured layer: by increasing the impact energy and the peening time, in fact, it is possible to accumulate the plastic deformation (that is the cause of the grain fragmentation) up to values that make possible the grain refinement.
Although we can obtain this effect through increasing the kinetic energy both by increasing the velocity of the shots and by increasing their dimension, the effect on the surface will not be the same. The time can be varied also for increasing the depth of the nanostructured layer of material: which are the optimized parameters that can be used to obtain superior properties with respect of the application of interest (fatigue, contact fatigue, wear,…)?
And what about residual stresses; their role is still primary in determining the mechanical properties of the peened parts?
Until now the research has been mainly focused on particular classes of materials: Ti alloys, Al alloys, some kinds of steels and not much more. Besides, the research was oriented in assessing the possibility of obtaining nanostrucutred surfaces with conventional peening plants. The results were positive and encouraging: shot peening is able to form nanograins and the mechanical properties of the so treated materials are really superior and promising. But still a lot of work has to be done.
Also the name of the process is not fixed: some call it high energy shot peening (HESP), others severe shot peening (SSP) or NanoPeening®.
We are far from full understanding and knowing the process and a lot of important classes of material have to be tested, but the treatment seems to be very promising in many engineering fields, and automotive is among them: it is also attractive from an environmental point of view, one of the most important industrial requisites for the XXI century.
Why not think of developing research on large scale, involving academics, companies and engineering?
Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
Author: Mario Guagliano