Introduction

Over the past few decades, ceramic beads have become a standard in the shot peening industry, as part of the available solutions alongside cast steel shots, steel cut wires, or glass beads [1]. Avoiding contamination on non-ferrous alloys and ensuring a smooth surface thanks to consistent shape and hard media, are some of the main reasons for choosing ceramic beads. Due to their properties, ceramic beads are also widely used in finishing, from mould cleaning to providing an aesthetic satin finish.

The standard ceramic beads are zirconia-silica composites, with a Vickers hardness of 700HV, a density of 3.8 g/cm3, and size ranging from 10µm to 1mm. For shot peening, these media follow specifications such as AMS2431/7. A new generation of ceramic beads is now available with a significant gap in material properties: density of 6 g/cm3 and hardness of 1200HV [2]. These new beads are made from stabilized zirconia, which is recognised as an exceptional ceramic regarding its mechanical properties, in particular high toughness compared to other structural ceramics [3]. The higher density provides higher kinetic energy and easier use in the wheel turbine, while the higher hardness means much better efficiency for shot peening hardened steels, and the better mechanical performance allows for use of the media for much longer or at higher impact velocity.

In this study, we evaluated of such high-density beads of various sizes through shot peening trials on different alloys, focusing in particular on fine sizes, following some previous work on fine shot peening [4,5,6,7].

Materials and methods

High density ceramic beads from Saint-Gobain Zirpro were tested: Microshot YZ of size 100µm (YZ100) and 300µm (YZ300), and Zirshot HDC of size 600µm (ZHDC600). Flat strips of different alloys were shot peened: titanium TA6V, and 16MnCr5 carburised steel.

Shot peening is performed with compressed air equipment at an 85° angle, with different Almen intensities corresponding to pressures from 1 bar to 3 bar, and a 200% coverage. After shot peening, the roughness is measured using a linear profilometer and a confocal microscope. The surfaces are also observed by SEM for a qualitative assessment of the defects. The residual stress profiles are then measured by X-Ray diffraction.

Experimental results

Surface roughness

For all conditions, the surface presented a homogenous coverage of smooth indents with well-pronounced edges. This can be observed with the confocal microscope on Figure 1, or with SEM on Figure 2. As expected, the diameter of the indents is directly related to the media size, with a typical size factor of one third. No inclusion nor bead fragment were observed on the surface. Such surfaces are typical of what is obtained with ceramic beads, even if more often observed only on relatively soft alloys (e.g. aluminum alloys).

Roughness measurement showed some rapid evolution as the Almen intensity was increased, for a given media. As can be seen on Figure 3 however, at different media size it was possible to maintain a low roughness by using moderate pressure corresponding to the lower end of the Almen intensity achievable by each media. The roughness creation was attributed to deeper indents even if the diameter was mainly controlled by the bead size.

Residual stresses

The residual stress profiles exhibit common features, typical of profiles obtained by shot peening a material softer than the media, with very intense stresses already at the surface [3]. This is particularly noticeable for the carburised samples (Figure 4), which is specific to such ceramic beads, as all metallic shots are at best at the same level of hardness.

Even when using the lower pressures that gave lower roughness, we observed the same level of stresses at the surface, while the depth became shallower.

For titanium alloy TA6V, the stress profiles were similar to those obtained with steel shots or standard ceramic shots of the same size and Almen intensity. Since in all cases the hardness of the media is higher than that of the treated part, the plastic deformation that occurred followed a similar pattern.

For carburised steel, the stress profiles are much more intense than for metal shots of equivalent size at the same intensity, and the most striking difference is the higher stress at the surface.

Conclusions

High density/high hardness ceramic beads of different sizes were evaluated for shot peening TA6V titanium alloy and 16MnCr5 carburised steel. Fine sizes of 100µm and 300µm beads were tested, along with 600µm media. We observed some typical surface signatures of ceramic beads with well-defined indentations even on the harder substrate. Residual stresses showed a high level of compressive stress, which is a characteristic of such media on very hard metals.

This evaluation confirms the interest in this new generation of ceramic beads as they offer new opportunities to control the surface topography and the residual stress profile. The inert nature of the beads makes them good candidates for non-ferrous alloys, and the outstanding mechanical properties may be of interest for hardened steel, e.g., as second-stage fine peening, or for treating thin parts.

References

[1] Baiker, S., et al.: Shot peening: a dynamic application and its future (2006).

[2] Cabrero, J., et al.: High density ceramic shot for peening application. ICSP-17 Proceedings, (2017), pp. 62-67.

[3] Wohlfahrt H., The Influence of Peening Conditions on the Resulting Distribution of Residual Stress. ICSP-2 Proceedings (1984), pp.316-331.

[4] Kikuchi S. et al.: Effects of Ultrafine Particle Peening on Fatigue Properties of ASTM 5056 Aluminum Alloy, ICSP (2014).

[5] Sugimoto K. et al.: Effects of fine particle peening on fatigue strength of a TRIP-aided martensitic steel, International Journal of Fatigue, Vol. 100 (2017), pp.206-214.

[6] Bagherifard et al.: Fatigue Strength of a Low-Alloy Steel Shot Peened with Ultra Fine Hard Steel Media, ICSP (2014).

[7] Oguri, K: Fatigue life enhancement of aluminum alloy for aircraft by Fine Particle Shot Peening (FPSP), Journal of Materials Processing Technology, Vol. 211 (2006), pp.1395-1399.

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