Nowadays with the severely competitive business environment, limited sources of materials and high cost of manufacturing, the importance of maintenance and repair is self-evident. Researchers from the Polytechnic University of Milan (Politecnico di Milano, POLIMI) and the Massachusetts Institute of Technology (MIT) are investigating capabilities of an emerging low-temperature coating technique for repairing damaged structural parts. A new study has shown that shot peening can improve the load bearing capacity of structural coatings. The research team includes: Mario Guagliano, associate professor at Politecnico di Milano, Atieh Moridi and Seyyed Mostafa Hassani-Gangaraj, Polimi graduates and currently post-doctoral researchers at MIT.

The cold gas dynamic spraying, or simply cold spray, is a progressive step in the development of high kinetic energy coating processes. Similar to the other thermal spray methods in principle, it follows the trend of increasing particle spray velocity and reducing particle temperature as in the high velocity oxygen fuel (HVOF) process, but to a more extreme level. Since the development of the process, a wide range of materials from metals, ceramics and polymers to composites and nanostructured powders have been cold sprayed [1]. Cold spray technology is applicable to many purposes such as design against fatigue or corrosion [2,3]. Cold spray high deposition rate and minimal undesired reactions make it a potential candidate for additive manufacturing. In addition, the process can be used to restore damaged parts and extend their life span [4]. Similar material deposition is often considered for repair applications.
For repairing load-bearing components, the challenge is that the repaired part must retain bulk material properties to withstand service loads. Fatigue causes the majority of mechanical failures. This justifies the great significance of the understanding/improving fatigue behavior of coated components. The main possible drawback of coatings in general is their detachment form substrate in service.
The substrate surface preparation and coating post-treatments could be effective pathways to hinder the problems associated with coating detachment. It can also influence the coating deposition efficiency, modify the residual stress, and thus affect the resultant fatigue performance.
In the present investigation, the effect of shot peening as pretreatment on the fatigue behavior of hybrid-treated specimens (shot peening + cold spray deposition) was studied.
Work hardening, roughness alteration and generation of residual stress are three important effects of shot peening. Work hardening of the substrate can assist in coating adhesion by inducing additional plastic deformation of particles upon impact. Roughness alteration can enhance mechanical interlocking and residual stress can extend the component life span by retarding crack propagation. In addition, shot peening can also reduce/remove the inherent oxide layer present on the surface of the metallic materials, which in turn improves the interface quality.
Experiments were performed on hourglass-shape Al-6082 specimens. This is a medium strength alloy with excellent corrosion resistance. It is known as a structural alloy and has the highest strength of the 6000 series Al alloys. Shot peening was performed with both conventional and severe parameters [5-7] prior to cold spray deposition. The peening intensity measured on "Almen A" strip was 6 and 8 for shot peening (SP) and severe shot peening (SSP) respectively. SP and SSP were performed with 100% and 800% coverage at Peen Service Srl, Bologna, Italy. Eight hundred percent coverage means that the time and thus number of impingements has been increased 8 times as compared to the conventional peening that provides 100% coverage.  
Powder material for deposition was considered the same as the substrate material. The coatings were deposited using a CGT- Kinetic® 4000 commercially available high-pressure system equipped with standard type-33 PBI nozzle. The process gas is nitrogen and its temperature was kept at 350°C while gas pressure was approximately 30 bars. Standoff distance was set to 20 mm and all coatings were deposited with a single pass of the gun. Fatigue tests were performed with stress ratio R = -1 and frequency of 20 Hz at room temperature using rotating bending fatigue machine. Four different series were tested. These are: 1. Base material (BM), 2. Cold spray coating, (C) 3. Shot peening followed by cold spray (SP+C), and 4. Severe shot peening followed by cold spray coating (SSP+C).
Figure 2 shows the fatigue strength obtained for the above-mentioned four series. Cold spray coating alone increased the fatigue strength by 15%. The best result was obtained for severe shot peening followed by cold spray, which increased the fatigue strength up to 26% in comparison to the base material. These improvements are promising notwithstanding the tests were done on smooth specimens with a limited stress gradient. Higher improvement can be expected for the notched specimens.
SEM observations of specimens after failure are presented for cold spray and one of the hybrid treated samples (SSP+C) in Figure 3. The same features as SSP+C were observed for SP+C. The differences between fracture surface of cold sprayed and hybrid treated specimen are both in terms of crack initiation and propagation. In case of cold spray coating, the crack initiated at the interface between the coating and the substrate. The boundaries of individual particles, as preferential sites for crack propagation, can be observed within the coating. This shows that intercrystalline crack propagation mechanism is dominant in this case (Figure 3a). There are also signs of delamination of coating from the substrate. In contrary, the fractured surface in the hybrid-treated specimen shows that the fatigue crack initiated on the surface of the coating and not at the interface between the coating and the substrate. In addition, the crack propagated through the coating by combination of transcrystalline and intercrystalline mechanisms. However, the transcrystalline mechanism is dominant in these cases. When shot peening was performed before cold spray coating, the boundary between the coating and substrate is barely visible (Figure 3b). The coating remained attached without any signs of delamination, which shows the contribution of coating to the fatigue load bearing.
In conclusion, severe/conventional shot peening was efficient as pretreatment to improve fatigue strength of cold spray coating. Three main factors contributed to such improvement in the fatigue behavior: I) Well-adhered coating without signs of delamination from the substrate; II) crack initiation from the surface instead of interface as well as crack propagation mainly by trancrystalline mechanism and not through particle boundaries; and III) compressive residual stress developed by shot peening process.
This indicates that the application of shot peening before cold spray can play a valuable role in improving the fatigue performances of coated parts, widening the applications of both shot peening and cold spray.

References:

[1] Moridi A, Hassani-Gangaraj SM, Guagliano M, Dao M. Cold spray coating: review of material systems and future perspectives. Surf Eng 2014;30:369-95. doi:10.1179/1743294414Y.0000000270.
[2] Hassani-Gangaraj SM, Moridi A., Guagliano M. Critical review of corrosion protection by cold spray coatings. Surf Eng 2015;00:1743294415Y.000. doi:10.1179/1743294415Y.0000000018.
[3] Ghelichi R, MacDonald D, Bagherifard S, Jahed H, Guagliano M, Jodoin B. Microstructure and fatigue behavior of cold spray coated Al5052. Acta Mater 2012;60:6555-61.
[4] Champagne V, Helfritch D. Critical Assessment 11: Structural repairs by cold spray. Mater Sci Technol 2015;31:627-34. doi:10.1179/1743284714Y.0000000723.
[5] Hassani-Gangaraj SM, Moridi A, Guagliano M, Ghidini A, Boniardi M. The effect of nitriding, severe shot peening and their combination on the fatigue behavior and micro-structure of a low-alloy steel. Int J Fatigue 2014;62:67-76.
[6] Hassani-Gangaraj SM, Moridi A, Guagliano M, Ghidini A. Nitriding duration reduction without sacrificing mechanical characteristics and fatigue behavior: The beneficial effect of surface nano-crystallization by prior severe shot peening. Mater Design 2014;55:492-8.
[7] Hassani-Gangaraj SM, Cho KS, Voigt H-JL, Guagliano M, Schuh CA. Experimental assessment and simulation of surface nanocrystallization by severe shot peening. Acta Mater 2015;97:105-15.

For Information:
Prof Mario Guagliano
Dr. Atieh Moridi
Dr. Seyyed Mostafa Hassani-Gangaraj
E-mail: mario.guagliano@polimi.it
amoridi@mit.edu
mhassani@mit.edu