This is not the first time I have written about additive manufacturing and its relationship with shot peening in automotive. If I am not wrong, the last time, I highlighted the problem of the cost related to post-processing of additively manufactured parts, underlining that the final incidence of that cost can reach even 40% of the total cost. This is one of the reasons why AM is not easy to be introduced as a manufacturing technique, at least if we are thinking of AM as a process to manufacture the parts the same way they are built with traditional technologies.

The secret is to redesign the mechanical parts, thinking and taking advantage of additive manufacturing processes: almost no shape constraints, new microstructure design with considerable lightening, and just few waste materials. 

However, talking about structural parts, the problems remain the same:  the presence of internal defects and the surface state, which is not suitable for bearing cyclic load and, finally, to resist fatigue. Automotive is not different. In this case also, we wish we could have light and resistant parts and AM is the right tool for topological optimization or for designing new materials structures that allows to get lightness; however, the fatigue strength of these parts in the as-built condition is quite poor and post-treatments are needed. Shot peening is a strong candidate for that and for increasing the fatigue strength to a reasonable level. However, which are the peening parameters we should consider for these materials? Which is the role of surface finishing, surface work hardening and residual stresses for such different materials and surfaces?

Recent research from me and my group investigates these aspects and the results we have got so far are interesting. We applied shot peening to AlSi10Mg alloy specimens manufactured with Selective Laser Melting (SLM) and tested them under rotating bending load. We considered the following conditions: as-built, heat-treated, shot-peened and heat-treated + shot-peened. The peening parameters were chosen considering the usual values of that material. The results were interesting. The fatigue strength in the as-built condition was negligible and not enough for any practical application. After the heat treatment the fatigue strength was much better, around 80 MPa. With shot peening, the improvement was even better, around 180 MPa. But what is more interesting is that using a heat treatment before shot peening, the fatigue strength dropped down to 100MPa. The post-mortem analysis of the broken samples revealed a different failure mode for the two series, while residual stress measurement before and after the fatigue tests showed that the initial compressive residual stresses were not as high as expected and that after the tests, they were considerably relaxed. 

To cut a long story short, it seems that starting from the as-built surface state typical of SLM and with the parameters we used, the main factor for fatigue alleviation by shot peening is not the residual stress field but the modified surface finishing state. In different terms, the initial conditions of AM parts are quite different with respect to the usual ones, and this factor affects the way shot peening should be done on these elements. 

We need more investigation and research to find the right way shot peening should be done to maximize the performance. At the same time, we learned that also with parts produced with AM processes, shot peening is a powerful tool to alleviate fatigue and is able to open the application of additive manufacturing also for structural parts in automotive. This is a good starting point.

(If you are interested to have the complete results of the research, contact me).

by Mario Guagliano

Contributing Editor MFN and 

Full Professor of Technical University of Milan

20156 Milan, Italy

E-mail: mario@mfn.li