Introduction

Research in the field of peening treatments as well as solid particle erosion has shown that blasting particles and fragments of blasting particles can penetrate the workpiece under certain conditions: Brown et al. observed that erosion removal cannot be detected at an impact angle of 90° for aluminium AW 1100. Instead, a typical hill and valley surface topography for erosion treatments could be observed as well as numerous embedded and fragmented blasting particles [1]. The fact that blasting particles could also stick in the workpiece surface during micro peening is shown by the study by Ando et al. The penetration of particles of iron, zirconium, quartz glass and fly ash led to an increase in microhardness of the aluminium alloy A6063S as target material [2]. Similar tests on tin have shown that the penetration depth of the blasting particles can be increased by increasing the process temperature [3].
Penetrated ceramic particles in the surface of the metal leads to functionally-graded metal matrix composites (FGMMC). FGMMCs are distinguished by the goal of improving highly stressed areas by local reinforcement or a graded coating. The advantages of MMCs in general and FGMMCs in particular are high specific strength and stiffness. MMCs also promise improved creep, fatigue, as well as wear properties [4]. Many processes are known to manufacture graded MMCs. A first classification provides the distinction between constitutive, homogenizing and segregating processes [5]. Each of these processes has a generally high degree of complexity. In addition, subsequent modification cannot be realised at all, or only with high effort. Exceptions are coating processes such as laser beam spraying and cold gas spraying.
The following study shows that composite peening is a further possibility to embed ceramic particles into the surface of metallic workpieces. It also offers the possibility of local and subsequent reinforcement.

Materials and Methods

Technically pure aluminium (AW 1050) is used as target material. The grain size of the ceramic blasting particles made of aluminium oxide is F600, which is linked with a weight-averaged particle size distribution of 9.0 µm ± 1.0 µm. The ceramic particles have an angular and splintery shape as shown in Fig. 1.

The schematic setup of the composite peening system is shown in Fig. 2. The Comco AccuFlo microblasting system is connected to the blasting nozzle by a flexible polymer hose. The nozzle is moved over the samples by means of a CNC machine from Isel. The micro blasting system as well as the temperature controller are connected with the control unit. Here, the program ProNC provided by Isel can control every component of the composite peening system. The heating unit provides temperatures close to the melting point of the aluminium alloy at 660 °C.

Composite peening has a large range of process variables. In previous investigations, parameters, namely temperature, pressure and number of operations, have been varied. The nozzle has a diameter of 0.7 mm and is aligned perpendicularly to the sample surface. The distance between two peening paths is 1 mm while the working distance from nozzle to sample is 10 mm. The feed rate of the nozzle is 8 mm/s. These parameters result in an overlap of the peening paths up to 80 %. An Almen intensity of almost 0.05 mmN and multiple particle impact are observed.

Results

Fig. 3a) shows a SEM image of the surface of a composite peened sample with a pronounced hill-valley profile. A preferred orientation along the peening path is not visible. In this case, the dark areas represent the hills and the bright areas represent the valleys respectively. The aluminium oxide particles are mainly found in the valleys, as the detailed view in Fig. 3b) shows. The high contrast between aluminium and ceramic particles can be explained by charging effects. It can also be observed that the ceramic particles are significantly smaller than their initial size of 10 µm.

For a detailed study of the penetration depth, micrographs are prepared. A wavy surface profile can also be seen, as shown in Fig. 4a). The contrast between aluminium and ceramic particles is clearly visible by using back-scattered electrons. Analogous to Fig. 3, the alumina regions can also be found in the valleys. These discontinuous areas consist of many nanoscale particles, as can be seen in Fig. 4b).

Conclusions

Due to the modification of the micro-blasting process by a heating unit, it is possible to embed ceramic blasting particles up to a depth of 30 µm in an aluminium matrix at high homologous temperatures. The ceramic particles are mainly located in the valleys of the characteristic hill-valley profile. Since the aluminium oxide particles are significantly smaller after processing, it can be assumed that multiple impact will lead to fragmentation of already embedded particles. This consideration will be the subject of future investigations.

References

[1] Brown, R., Kosco, S., Jun, E.J.: The effect of particle shape and size on erosion of aluminium alloy at 90° impact angles. Wear, 88(2), 181-193 (1983).
[2] Ando, M., Kitano, H., Usami, H., Endo, T.: Applicability of fine particle peening on surface modification of aluminum alloy. In: The 10th International Conference on Shot Peening procedure, pp. 223–227 (2008).
[3] Seitz, M., Reeb, A., Klumpp, A., Weidenmann, K.A.: Composite peening-a novel processing technology for graded reinforced aluminium matrix composites. Key Eng. Mater. 742, 137–144 (2017).
[4] Allison, J.E., Jones, J.W.: Fatigue Behavior of Discontinuously Reinforced Metal-Matrix Composites. In: Suresh, S., Mortensen, A. Needleman. A. (eds.) Fundamentals of Metal Matrix Composites, pp. 269-294. Butterworth-Heinemann, Stoneham (1993).
[5] Kieback, B., Neubrand, A., Riedel, H.: Processing techniques for functionally graded materials. Mater. Sci. Eng., A 362, 81–106 (2003).
[6] Seitz, M.: Influence of the process parameters on the penetration behaviour of ceramic particles in Composite Peening. In: Schulze, V. (eds.) Symposium Mechanical Surface Treatment 2019: 8th Workshop Machine Hammer Peening, pp. 85-99. wbk Institute of Production Science, Karlsruhe (2019).
[7] Seitz, M., Weidenmann, K.A.: Influence of the process parameters on the penetration depth of the reinforcing phase during composite peening for the production of functionally graded metal matrix composites. Key Eng. Mater. 809, 73–78 (2019).

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