Introduction to the processing of components by machine hammer peening

Compared to shot peening or deep rolling, machine hammer peening for strengthening components is a relatively new process that is currently only used in special cases. However, the process offers great advantages, since the subsurface area of a component is affected more deeply, with the process not requiring any complex machine technology. The subsurface area can be influenced to a depth of 2-5 mm without placing any particular stress on the machine. We will discuss below why this is the case and how machine hammer peening works in principle.

As the name suggests, machine hammer peening (MHP) is a pulsed mechanical surface treatment using a machine-guided tool. This puts the process somewhere between shot peening and deep rolling. In shot peening, blasting grains hit the surface with an impulse. Process integration into a CNC machine, however, is practically impossible. Deep rolling, on the other hand, can be integrated into any CNC machine. However, the rolling element is always in continuous contact with the surface. Forming speeds are lower and there is no pulsed forming. 

With MHP, a tool whose geometry is initially completely free is accelerated via an impact system and struck against the surface at a high frequency. Depending on tool type and manufacturer, there are different ways of accelerating the tool. Pneumatic, electromagnetic or piezoelectric systems can be used. In these cases, an additional energy source is required. When using a pneumatic-mechanical tool concept this is not necessary, because, as with a hammer drill, a swash plate is set in motion via rotation, which then drives an impact system. The movement of pistons and cylinders builds up pneumatic pressure and accelerates the piston. This then hits the tool insert, which is accelerated towards the surface.

The process itself is the same for all tool systems and is essentially described by the parameters impact frequency, impact energy and impact distances/degrees of coverage. A VDI standard (VDI 3416 Sheet 1) is currently being prepared for the process, which describes and defines the process with all process parameters. In addition to these main parameters, the shape of the tool insert is decisive. It influences the contact surface and therefore the pressure and thus the deformation of the surface at a given impact energy. Spherical tips with a defined radius are common. However, other shapes with or without a surface structure are also possible.

The kinematics of the process usually meander across the surface. This means that a surface is filed down. Feed velocitiy and impact frequency are selected in such a way that the individual impacts overlap in feed direction. Overlaps of 30 - 80% are common. The smaller the overlap, the more productive the process. The next path then follows with a defined path offset. Here too, the path offset is selected so that the impacts overlap.

The image shows surface topography images for the machining of 42CrMo4 with a hardness of 47 HRC. An impact energy of E_kin = 120 mJ and three different tip diameters were used. It is clearly visible that a rather structured surface remains. A rolled surface would have a much more even structure. However, the image also shows that impact energy, radius and material have to be synchronized. With a diameter of d = 50 mm, the surface is hardly affected by the same impact energy. 

However, this does not apply to the formation of residual stresses. Although there is no visible effect on the surface when using these parameters, compressive residual stresses are still formed. Hammer peening therefore also has an effect in depth. In this case too, residual stresses are greater if plastic deformation is also visible on the surface. The next figure shows the residual stress depth curves for the impact head with a diameter of d = 12 mm. It can be clearly seen how the depth of compressive residual stresses increases with increasing impact energy. Although it appears that the level is comparable, as the curves are very similar for a difference between 30 and 185 mJ, the depth must be considered here. At E_k = 185 mJ, the depth effect is approx. 1.75 mm, whereas at 30 mJ it is only approx. 1 mm. 

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