Mechanism of surface modification using machine hammer peening technology Friedrich Bleicher a, *, Christoph Lechner a , Christoph Habersohn a , Ernst Kozeschnik b , Beatrix Adjassoho b , Heinz Kaminski b a Institute for Production Engineering and Laser Technology, Vienna University of Technology, Vienna, Austria b Institute of Materials Science and Technology, Vienna University of Technology, Vienna, Austria Submitted by Guenter Pritschow (1), Stuttgart, Germany. 1. Introduction The need for improved surface integrity and enhanced functional performance of manufactured components has acted as a driving force in the development of new production methods and high performance manufacturing technologies [1]. Machine hammer peening (MHP) can be regarded as one of these novel technologies for surface treatment. The process is based on the oscillating motion of an axially guided plunger. The actuator, which provides this movement, can be based on different working principles (e.g. an electro-magnetic actuator system) and is attached to a machine tool or a robot. Due to the NC-controlled positioning of the hammering tip it is possible to machine the surface by well-directed impacts (Fig. 1b). Hence the ability of performing a controlled surface modification distinguishes the MHP process from comparable technologies like shot peening, where it cannot be guaranteed that the impact intensity is uniform across the machined area [2]. However, the surface modification by machine hammer peening is a direct result of the process itself like it is given in shot peening or roller burnishing. Thus the above mentioned technologies can be classified as directly operating processes (Fig. 2). In contrast, indirect processes are based on the creation and induction of shock waves to modify the material properties as presented in [2–4]. This paper seeks to explore the effects of surface modification by the machine hammer peening process (Fig. 1). Apart from experimental tests a model based description of the actuator system and the surface modification leads to a better under- standing of the process mechanisms. Parameters which can be adjusted to influence the process are given in Table 1 and are illustrated in Fig. 1c. 2. Process modelling 2.1. Actuator for machine hammer peening Fig. 1a illustrates the basic design of the MHP device. The ram, which is guided by two linear bearings, is driven axially by a plunger coil. A sinusoidal current in the coil pushes the ram onto the material surface and lifts it back again. Additionally in the absence of current a return spring moves the plunger in an upper position to ensure a defined starting stroke. For basic investigation of the plunger motion a unidimensional model of the system was built in Matlab–Simulink (Fig. 3). This model is based on the analytical mechanical system of Newton’s laws of motion, the equivalent circuit diagram of a plunger coil and the Lorentz force. m  ¨ x ¼ F friction þ F plunger coil þ F reaction þ F spring þ F impact (1) The parameters for modelling the return spring c f , the inductance L, the resistance R of the coil and the viscous damping d f as well as the motor constant k g are identified by measurement. In Fig. 3 the parameter u represents the sinusoidal voltage power supply of the actuator coil by the controlling system. The punch on the workpiece surface and the reaction force due to the impact on the backside of the housing are described by spring-damper-systems. The simulation confirms measurement results that the plunger gets most of its kinetic energy by the rebound on the workpiece and on the backside of the housing, respectively. To keep the process stable and energy efficient it is necessary to use these additional accelerations. As an enhanced approach the material behaviour was described by the use of the Zerilli–Armstrong material model (3) as presented in [5]. The compressive stress and reaction force are calculated, as follows, for single time steps by the projected impact area a pr . The CIRP Annals - Manufacturing Technology 61 (2012) 375–378 A R T I C L E I N F O Keywords: Residual stress Roughness Machine hammer peening A B S T R A C T The process of machine hammer peening is based on an actuator, which moves an axially guided hard metal ball with an oscillating movement to a workpiece surface. This actuator could be attached to any kind of machine tool, even to a robot. Because of the well-directed impacts on the surface it is possible to structure and modify the material and its surface in various ways. The main positive effects of machine hammer peening are the induction of compressive residual stresses, the hardness increase of the upper surface layer and a reduction of surface roughness up to mirror-like surfaces. ß 2012 CIRP. * Corresponding author. Contents lists available at SciVerse ScienceDirect CIRP Annals - Manufacturing Technology journal homepage: http://ees.elsevier.com/cirp/default.asp 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.139