If you read a magazine or a paper, no matter if this is general purpose, technical, scientific, or whatever, you will find some contribution about the technologies able to get elements and goods by adding instead of subtracting material. To make a short story short, I am talking about the 3D printing technologies, used both for polymers and for metal alloys.
Indeed the technologies used for plastics or metals are quite different due to the strongly different physical nature of these families of materials: this makes different also the way these technologies are named. Usually, if we talk about 3D printing we are thinking of polymers while if we talk about additive manufacturing processes, the materials of interests are metals alloys.
In the present contribution, I want to stay on the latter and consider metals. In this case, additive manufacturing means the technologies that start from the powders building 3D objects by adding layer-upon-layer of material. There are different processes that can be used with this aim; most of them are laser-based.
Probably there is no need to remark on the advantages of additive manufacturing: you do not need large productions, and you strongly reduce the scrap (and this is quite important for expensive materials, difficult to machine).
Additive manufacturing makes it also possible to design and produce complex shapes with a favourable ratio between weight and strength: the shapes obtained by topological optimization, once considered a good academic exercise not relevant from a practical point of view, can be now produced, giving a great contribution to lightness, energy saving and safety of systems in different industrial sectors. Indeed, by this kind of process it is possible to get complex internal shapes that cannot be obtained by using other traditional technologies.
This is true also in the automotive field, where the ever-increasing requests of reduction of weight, reliability and reduced consumption are among the most important attributes to remain competitive and respect the strict environmental international standards.
These considerations justify the great attention and expectation about additive manufacturing processes and the significant investments that the automotive actors (as well as the main actors in other industrial field) are dedicating to AM.
But, how about the properties of the parts obtained by these processes?
Indeed the knowledge about the properties of the AM parts is not exhaustive so far. What is known is that generally the surface finishing is not so good and needs further finishing before final use.
If we focus the attention on mechanical properties like fatigue and wear, a lot of work has to be done to achieve a significant amount of data and understanding. Let me say that shot peening will play an important role also in this field. The ability of additive manufacturing processes to obtain complex shapes and rather poor surface finishing makes it somewhat likely that fatigue cracks, as well as other damage mechanisms, start from these points, while it is quite unusual and rare that it happens in parts traditionally machined. And shot peening is one suitable candidate to avoid this kind of damage. Also, as regards the external surfaces, we know that shot peening and its variants are the most flexible treatments to improve the surface state.
On the other hand, we know that shot peening is not a black box and its success depends on the correct setup of the process parameters. These latter are strongly dependent on the material and the geometry of the treated part: it is not possible just to apply the usual process, but it is necessary to investigate and understand the role that hardening, residual stresses, surface roughness and their relationship with the process parameters play. Important investments are needed but the results could be highly beneficial.

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