VOL. 15 July ISSUE YEAR 2014

Shot Peening in the Automotive Industry

in Vol. 15 - July Issue - Year 2014
Shot Peening and Cold Spray, A Synergic Combination!
Mario Guagliano

Mario Guagliano

I am currently involved in a research project presented in the last call of the FP7 program of the European Union. Indeed the project is not directly related to the automotive industry; it was presented and accepted in the call "Aeronautics" and deals with new maintenance and repair methods by using the so called "cold gas dynamic spray", or simply cold spray. Anyway, the concepts of the project are more general, and give me the opportunity to introduce cold spray, a novel and promising coating technique with many advantages with respect to other more traditional coating techniques.
And, first of all, quite general and easily applicable to the automotive industry: I know that a colleague of mine working in a well known Canadian university is developing a project for assessing the application of cold spray to automotive light alloys, like Al alloys and Mg alloys to obtain improved and customized properties.
In brief, in cold spray, powders are accelerated through a De-Laval nozzle and then are impacted on a target material. If the impact velocity exceeds a threshold value, the powder remains attached to the impacted surface and the coating can be formed. So, cold spray is very attractive, because it can be applied to quite a number of material combinations, and can be applied both to metals and plastics, a basic requirement being the ductile behaviour of the powders (but there are many innovations that are going to enlarge the possible applications of cold spray).
The reader now is wondering what cold spray has to do with shot peening. Indeed, cold spray is a peening process, even though not finalized to obtaining residual stresses but to form a coating, and there are a number of possible interactions that make shot peening attractive to be applied together with cold spray.
The critical velocity value is influenced by many factors related to the state of the surface: surface roughness and even residual stresses are among them.
A logical consequence is that shot peening applied before cold spray would be a valid tool to decrease the value of the critical velocity. That is to say, that if shot peening is applied, we can get the coating with less energy demand and improved coating efficiency.
There are already some scientific papers concerning this interaction between cold spray and shot peening (some of them were written by my group and me) that confirm this effect, but work is still in progress to find the optimal shot peening parameters with respect to the combination powder-target material of interest.
Besides, the application of shot peening has also relevant (positive) consequences on the properties of the cold sprayed surfaces, and this is very important if you want to use cold spray for parts that must carry some load and are a structural part of a system. We know that compressive residual stresses improve fatigue strength of metal parts: the application of shot peening before cold spray would induce a better fatigue strength, making the process much more attractive for application on machine elements.
Furthermore, many times the interface between the coating and the treated surface is the weakest point and fatigue failure (and not only fatigue failures) starts from there. The roughness induced by shot peening is able to increase the adhesion of the coating, thus providing another advantage to cold sprayed parts.
I could go on describing other positive aspects of shot peening with respect to cold spray (maybe I will in some next column) but my space is taken up and I stop here; but not before having underlined that cold spray in the automotive industry has many perspectives and a careful study of how to apply shot peening to improve the process and its results can be an interesting way to widen the application of shot peening in this field.

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