One way to reduce CO2 emissions is to develop renewable energies. Energy conversion plants contain many mechanical components made of stainless steel, which have to withstand complex load collectives. However, the performance of such components is limited due to inadequate mechanical strength and fatigue properties, especially in the highly loaded surface layer.
One way to improve these properties is to use forming surface layer treatments (SLT), such as electrodynamic machine hammer peening (MHP). MHP is a cold micro forging process that machines metallic surface layers at high frequency using a spherical plunger. The MHP tool is usually guided by machining centers. Due to the innovative character of robot-guided MHP, there are currently still fundamental knowledge deficits in the area of process kinetics, contact mechanisms and the surface layer and fatigue properties of hammered materials. MHP machining of stainless steels is still largely unexplored.
The subject of this work is, in addition to the development of a knowledge base on MHP process kinetics and the influence of industrial robots as MHP carrier systems, the analysis of contact mechanisms and surface layer and fatigue properties of metastable austenitic stainless steel. In a first step, an electrodynamic MHP system was modified with inline force and distance sensors, characterized, and a databased numerical MHP kinetic model was developed using machine learning methods. In a second step, single-impact phenomena were investigated experimentally by means of a drop tower test rig with regard to geometric and microstructural mechanisms as a function of the energy input and supplemented by numerical FE simulations of the plastomechanical material behavior. In a third step, cause-effect relationships between the energy input and the material-physical mechanisms were determined by means of robot-based MHP tests and synthesized with findings from dislocation-dynamic FE simulations to form a design model. In the final step, the fatigue behavior of hammered workpieces was evaluated in rotating bending tests in accordance with the developed model.
Technologie der Fertigungsverfahren
Increasing Fatigue Strength of Austenitic Stainless Steel Using Machine Hammer Peening
Machine hammer peening (MHP) is a forming process that specifically improves the surface integrity of metallic materials. However, the process variant of robot-based electrodynamic MHP as well as MHP machining of metastable austenitic stainless steels has been poorly researched so far.The present work is therefore dedicated to the analysis of the cause-effect relationships between MHP process kinetics, surface integrity and fatigue behavior of AISI 304 stainless steel.