Machine surface hammering
The mechanical surface hammers (. Engl machine hammer peening , MHP , also: . Hard Knock . Knock shock compression ) is a manufacturing method for mechanical surface treatment of metallic materials . The high-frequency blows of a hammer tool result in surface smoothing as well as induction of work hardening and residual compressive stresses . The MHP has its industrial origins in tool and mold construction for the surface smoothing of drawing tools in the automotive industry, but is now used in a wide variety of ways.
Mode of action
With the MHP, a mostly spherical hammering tool hits a workpiece surface at high frequency. The hammering tool is moved along the workpiece surface by a robot or a processing machine, so that a row or a field of plastic impressions is created. Electromagnetic, pneumatic and piezoelectric systems are currently used to drive the hammering tool . If the hammer tool oscillating with the frequency f is guided against a metal surface, the kinetic energy E of the hammer tool is converted into an elasto-plastic deformation work when it hits it. After relieving the contact, e.g. B. during the return stroke, a plastic impression corresponding to the geometry of the hammer remains on the surface. The plastic deformation smoothes out surface roughness. At the same time, strain hardening and internal compressive stresses are induced. The frequency and processing speed define the distance between the impressions. In this way, both very smooth and defined structured surfaces can be created.
Process variants
Four excitation technologies can currently be used to generate the oscillating movement.
In the pneumatic system (P-MHP) a piston, which is movable within the tool system, is set into oscillation by applying a stream of air, which then transfers its kinetic energy to the ram or the hammer tool. The advantages of the pneumatic variant of the process are the high level of reliability and ease of use. Depending on the application, the inability to set the oscillation frequency and the permanent contact of the hammer tool with the surface can be viewed as disadvantageous.
In the electro-mechanical hammer system (E-MHP), the plunger is excited by applying an electrical voltage to a coil connected to the plunger, which is set in a defined oscillation due to the magnetic field of a permanent magnet positioned around it, similar to a loudspeaker. The advantages of the E-MHP are the adjustable frequency between 20 and 500 Hz and the high impact energy. This enables a wide range of applications and creation of tailor-made surfaces even with high-strength materials. Depending on the application, the time-consuming commissioning and the required installation space can be viewed as disadvantageous.
With "Piezo-Peening" the hammer head is made to vibrate by a piezo crystal to which a high, pulsating DC voltage is applied. The vibrations are forced. The piezo crystal is typically excited by high-voltage amplifiers. Due to the controllable conduct of the experiment, the boundary layer conditions achieved are easily reproducible. Typical processing frequencies are around 200 - 500 Hertz and typical amplitudes are around 10 - 20 µm. A functional test facility is located at the Karlsruhe Institute of Technology.
With (purely) mechanical hammering, no external energy supply is necessary. The mechanical drive is provided by the rotating machine spindle and is comparable to the drive concept of a percussion drill or hammer drill. Mechanical drive concepts achieve an impact energy of 50 mJ to a maximum of 2000 mJ and a maximum impact force of 1000 N to a maximum of 8000 N. The hammer oscillates with an impact frequency between 330 and 400 Hz and up to 225 - 300 Hz with a higher impact force. This enables residual compressive stresses up to approx. 800 MPa and residual compressive stress depths (zero crossing) up to 4.5 mm (material: Inconel 718, outsourced, Rp0.2 = 827 MPa).
use cases
The pneumatic and electro-mechanical hammer principle are currently widely used in industry. Both are mainly used in the automotive industry for smoothing drawing tools. However, both processes are currently being used in other industries in which the smoothing of the components (e.g. plastic injection molding industry), the structuring of surfaces (e.g. the hydrodynamic plain bearing industry), work hardening and the induction of residual compressive stresses (e.g. Turbine blade) are advantageous.
Smoothing of tool surfaces
The mechanical smoothing of drawing tool surfaces enables, on the one hand, an increase in process stability and, on the other hand, the shortening of the process chain. This was demonstrated using the toolmaking of an automobile manufacturer as an example.
Reduction of friction and wear
Through the defined introduction of surface structures, hydrodynamic lubricant pocket effects can be used, which can reduce friction and wear during deep drawing or in plain bearing applications (e.g. camshafts).
Increase in flexural fatigue strength
Due to the induction of compressive residual stresses and strain hardening, the mechanical surface hammering leads to an increased flexural fatigue strength compared to the unhammered state.
Classification in the group of mechanical surface treatments
The machine surface hammering belongs such. B. shot peening or deep rolling to the group of mechanical surface treatments. These methods lead to an improvement in the surface and surface layer properties through repeated local plastic deformation of the workpiece surface.
In shot peening, the abrasive is usually accelerated by means of a wheel blasting machine or compressed air blasting system and thrown against the surface to be treated. The points of impact of the individual elements of the abrasive are not defined here. To indicate how often a point on the surface is hit by the abrasive, the degree of coverage is used as a characteristic value. Another process that is related to mechanical surface hammering is deep rolling. Deep rolling is characterized by a continuously applied rolling pressure on the surface and the associated deformation of surface layers close to the surface. A simple rolling over, but also multiple rolling over of a roller track is possible here. The pressures and reaction forces that occur on the tool require the tool holder to be very rigid.
As with the processes mentioned, mechanical surface hammering also results in the contact between the hammer head and the workpiece exceeding the flow limit of the material and thus plastic deformation. The advantage of the method is that the effects described can be produced in a controlled and deterministic manner in the surface layer of the workpiece. In contrast to the processes mentioned above, a hammering tool can be integrated very well into machining centers and robot-supported process chains . The reason for this lies in the simplicity of the tools used for machine surface hammering. A compressed air or power supply is usually sufficient to operate the tools on the market. A targeted setting or distribution of the processing intensity is also given locally via the process parameters of hammering. Using special hammer heads with a micro-tip, it is also possible to introduce microstructures into the workpiece surface during the machining process. These serve as lubricant reservoirs when the machined workpiece is used later and also improve the tribological properties of the surface.
Research, development and standardization
MHP is used successfully in industry today. Since it is a comparatively new metalworking process, it is currently (2016) the subject of numerous research projects. In Germany, the P-MHP is being researched by the Institute for Production Technology and Forming Machines at TU Darmstadt . The E-MHP is examined by both the PtU and the WZL machine tool laboratory at RWTH Aachen University . The IAM materials science in Karlsruhe is intensively dedicated to piezo peening. P-MHP and E-MHP are also examined at the Institute for Manufacturing Technology and High-Power Laser Technology IFT at the Vienna University of Technology.
Every year towards the end of the year, a forum takes place which brings together the users of the MHP from industry and research and provides information on the state of the art through lectures from industry and research. The so-called Machine Hammer Peening (wMHP) workshop alternates between Darmstadt, Vienna, Aachen and Karlsruhe.
A VDI committee on the subject of surface hammering has emerged from the activities of the workshop members. The aim of the VDI committee was to create a VDI guideline for standardizing the terminology and describing the process properties. The VDI guideline was adopted on December 3, 2015 by the VDI committee.
See also
literature
- M. Steitz: Tribologically favorable surface structuring of deep-drawing tools by means of mechanical surface hammering. PhD thesis TU Darmstadt. Shaker Verlag, Aachen 2016, ISBN 978-3-8440-4691-5 .
- D. Trauth: Tribology of Machine Hammer Peened Tool Surfaces for Deep Drawing. PhD thesis TU Aachen. Apprimus Verlag, Aachen 2016, ISBN 978-3-86359-424-4 . (Abstract)
- C. Habersohn: Analytical and simulative consideration of a surface hammering process. PhD thesis. TU Vienna. 2015. (PDF)
- C. Lechner: Surface modification using the technology of impact compaction (machine hammer peening). PhD thesis. TU Vienna. 2014. (PDF)
- J. Wied: Surface treatment of forming tools by tapping. PhD thesis. TU Darmstadt. 2011. (PDF)
Web links
Individual evidence
- ↑ a b P. Groche, M. Steitz: Shortening the process chain in toolmaking - integration of processes for machine surface smoothing. In: Workshop technology online. wt, Springer VDI Verlag, Düsseldorf, 101 (10), 2011, pp. 655-659.
- ↑ a b J. Wied: Surface treatment of forming tools by tapping. PhD thesis. TU Darmstadt, 2011.
- ↑ a b C. Lechner: Surface modification using the technology of impact compaction (machine hammer peening). 2014.
- ↑ M. Oechsner, J. Wied, J. Stock: Influence of Machine Hammer Peening on the Tribology of Sheet Forming. In: Advanced Materials Research. Vols. 966-967, 2014, pp. 397-405.
- ^ F. Lienert, J. Hoffmeister, V. Schulze: Residual Stress Depth Distribution after Piezo Peening of Quenched and Tempered AISI 4140. In: Materials Science Forum. Vols. 768-769, 2013, pp. 526-533.
- ↑ Product flyer from Ecoroll, as of August 22, 2016, initially available from mail@ecoroll.de
- ^ M. Steitz, P. Stein, G. Groche: Influence of Hammer-Peened Surface Textures on Friction Behavior. In: Tribology Letters. 02, 2015, pp. 1–8.
- ↑ D. Trauth, A. Feuerhack, P. Mattfeld, F. Klocke: Analysis of the velocity distribution of an elliptic surface structure manufactured by machine hammer peening. In: Tribology Letters. 60 (19), 2015, pp. 18–31.
- ↑ D. Trauth, F. Klocke, D. Welling, M. Terhorst, P. Mattfeld, A. Klink: Investigation of the Surface Integrity and Fatigue Strength of Inconel 718 after Wire EDM and Machine Hammer Peening. In: International Journal of Material Forming. 2015.