Manuel Ludwig

Fundamentals of EMPT-Welding

A well-suited solid state welding process for treatment of tubular structures is the electromagnetic pulse welding technique (EMPT): A pulsed magnetic pressure loads the structure to be welded within a few microseconds and accelerates one of the both contact partners (the so called “flyer”) onto a stationary one. When the flyer strikes the stationary contact partner, contact normal stresses far above 1000 MPa act on the interfacial zone between flyer and stationary part. As a result of these high interfacial loads, a layer of several micrometers thickness next to the interface is severely plastically strained. Hence, the oxide layers covering both contact partners are cracked. These chipped oxide particles are blown out of the joining area by a so called “jet”. This jet is caused by the air between the two joining partners being compressed and accelerated due to the movement of the flyer. The result of both phenomena –the oxide chipping by severe plastic deformation of the interfacial zone and the particle blow out caused by the jet– is a pure metallic interfacial zone, loaded by contact normal stresses. The conjunction of the highly reactive metallic surface and the contact normal stresses establishes a metallic bonding, whose strength equals at least the strength of the weaker contact partner. This report presents the results of a collaborative research project between the Institute for Production Engineering and Forming Machines (PtU) and PSTproducts GmbH. Experimental welding analysis is accompanied by numerical work for the study of the underlying mechanisms of solid state welding with respect to interfacial plastic deformation and contact loads. Additional metallographic work gives insight into the microscopical structure of the interfacial joint zone.

Simulation of Dynamic Lubricant Effects in Sheet Metal Forming Processes

Tribology plays an important role in sheet metal forming processes relating to near net shape production processes and achievable surface qualities. Nearly every process is realized by using characteristic lubricants affecting the tribological system to achieve the desired results. Deterministic structures on sheet surfaces can result in less friction and higher drawing ratios. This is caused by hydrostatic pressures build up in closed lubricant areas and hydrodynamic pressures due to the lubricant motion especially in thin fluid films [1, 2, 3]. Friction mechanisms in the mixed lubrication regime are not fully understood till today. The numerical simulation of flows in lubricant pockets and their influence on surface evolution are promising ways to gain more knowledge of the lubricant behavior in tribological systems. Therefore, this paper shows results of combined numerical and experimental approaches. The described simulations of closed lubricant pockets on surfaces identify influencing parameters. Strip drawing experiments are done to verify the simulations. The influence and the importance of local pressures due to viscous effects in the lubricant are considered as well as the necessity to use fluid-structure-interactions to simulate the behavior of lubricants in the tribological system.

Surface Evolution and Lubricant Distribution in Deep Drawing

Deep drawing is one of the most important processes applied in industrial production. Here the Finite-Element-Method (FEM) is an important tool in the development and optimization process. One aspect to optimize simulations is to consider real friction behavior. Thus the friction phenomenon has to be describable. In addition to contact normal pressure and velocity the surface topography and the lubricant amount have a great influence on friction. This paper illustrates the influence of surface evolution in real, inhomogeneous processes on the lubricant distribution. For this a rectangular cup with four different corner radii is used to evaluate local surface topographies and lubricant amounts in deep drawing. The lubricant amount is measured by fluorescence technique and the surface topography is evaluated by a confocal white-light microscope. Due to hydrodynamic effects the lubricant is squeezed out and displaced to adjacent regions. Further hydrostatic pressures built up in closed lubricant pockets force the lubricant to stay in the forming zone to bear a part of the load. In free forming zones without contact between the sheet and tool the surface roughens due to grain dislocations in the microstructure. This paper also presents the results of lubricant distribution and surface evolution by varying the initial lubricant amounts and drawing depth. It can be recognized that the different corner radii of the rectangle cup have a great influence on the surface evolution and lubricant distribution. Moreover it can be clearly seen that surface parameters correlate with the lubricant amount. By means of the described evaluation it is also possible to correlate these values with load histories consisting of contact pressures and strain evolution, evaluated in FEM. All the results contribute to a better understanding of the friction behavior in deep drawing and point out the inhomogeneous character of friction.