Papers by Author: Alexander Wanner

Abstract: The effect of defined preloading in the tensile and compressive regime on the near surface (residual) stress distributions, which result from laser surface hardening, is systematically studied in-situ, i.e. under the applied preload and after unloading. Samples made of steel grade AISI 4140 are defined surface hardened by means of a high-power diode laser (HPDL) system during uniaxial compressive elastic loading at-300 MPa as well as during uniaxial elastic tensile loading at 300 MPa using a custom designed 4-point-bending device, which can be mounted on an X-ray diffractometer. The results of X-ray stress analysis were compared to data derived for a sample state unaffected by any preload. Without external loading compressive residual stresses are induced inside the process zone that are balanced by tensile residual stresses outside this zone. The investigations show that external loading in the tensile and compressive regime has a strong impact on the resulting lateral residual stress distribution in loading direction. The results further indicate that undesirable tensile residual stresses outside the process zone can even be suppressed by using a defined appropriate preloading in the tensile regime.

Abstract: The tool and workpiece surface layer states of the tribosystem uncoated WC-Co cutting tools vs. normalised SAE 1045 workpiece material are studied in detail for a dry metal cutting process. Within the system the cutting parameters (cutting speed, feed rate, cutting depth) determine the wear state of the cutting tool and the resulting surface layer state (residual stress) in the workpiece. As the built-up edge can be used as a possible wear protecting layer [1] the influence of built-up edge and wear behaviour of the cutting tool was examined with respect to the workpiece surface layer state for knowledge based metal cutting processing. Small compressive stresses (-60-80 MPa) are induced in the surface layer, that are nearly homogeneous for the highest built-up edge, which lead to the lowest tool wear in combination with lowest cutting temperature.

Abstract: The effect of processing atmosphere on the microstructure and residual stresses are studied for laser surface hardening on steel samples of grade AISI 4140. Samples were hardened in air, vacuum and inert gas atmosphere (Helium) by means of a stationary laser beam. A high-power diode laser (HPDL) system was used in combination with a custom-designed process chamber. Residual stress distributions in lateral and in depth direction were analysed after laser processing by means of X-ray diffraction according to the well known sin² - method. X-ray residual stress analyses were supplemented by microscopic investigations of the local microstructure. The results indicate a widening of the compressive stressed region in lateral as well as in depth direction by surface hardening in inert gas atmosphere compared to laser surface hardening in air or vacuum atmosphere. This is due to the local heating flux distribution during the laser assisted heat treatment which is strongly affected by the processing atmosphere an leads to an extension of the hardening zone when using helium as inert gas.

Abstract: Internal load transfer in an interpenetrating metal/ceramic composite has been studied in this work using energy dispersive synchrotron X-ray diffraction. One of the samples was loaded in tension and the other one in compression. In each case, the sample was first loaded into the elastic-plastic regime, unloaded to zero stress, and reloaded beyond the prior maximum stress. Results show that at stress amounts greater than 100 MPa aluminum deforms plastically and the load is transferred to alumina and silicon. Unloading and reloading typically show reverse plastic deformation, Bauschinger effect and strain hardening in aluminum.

Abstract: Broaching is an important technique for creating tooth structures in mechanical components. In the present work, the effects of the broaching process on the material state in the near surface region at the root of the tooth was analyzed. The studies were carried out on broached plates made from case hardening steel SAE 5120. The cutting speed and machining condition (cooling lubricant, dry machining) were varied. During broaching with a TiAlN coated tool the cutting forces were monitored. Subsequently, the local residual stresses at the root of the tooth were determined using X-ray diffraction. Further, surface roughness and micro hardness measurements as well as microstructure analysis complement the results. The results indicate that cutting forces have a high influence on the development of the residual stress state at the machined surface whereas no significant effect on changes in surface hardness and microstructure could be observed. Dry cutting with relatively high cutting speeds (≥ 30m/min) result in low cutting forces and hence in high tensile residual stresses in broaching direction.

Abstract: The effects of laser surface hardening of steel samples on the microstructure and residual stresses were determined for single as well as multiple laser pulses. Samples made of steel grade AISI 4140 were hardened by means of a high-power diode laser (HPDL) system using either single or multiple laser pulses resulting in single as well as repeated austenite-martensite transformations. The hardening was carried out in a specially designed process chamber allowing laser surface treatment in inert atmosphere in order to avoid oxide scale formation. The residual stress distributions in lateral and in depth direction were analysed by means of X-ray diffraction for samples hardened by up to 27 laser pulses. Residual stress analyses were carried out by means of the sin²y - method. The results indicate the extension of the hardened zone in lateral and in depth direction with an increase in the number of applied laser pulses. This evolution is connected with significant changes in the local residual stress distributions.

Abstract: An innovative experimental set-up for fast X-ray diffraction analysis on polycrystalline materials has been established at the synchrotron radiation facility ANKA (Karlsruhe, Germany). Key components of the set-up are two fast microstrip line detectors arranged symmetrically around the incident beam in the backscatter region. The capabilities of the set-up are tested by means of in-situ heat treatment experiments on SAE 4140 steel samples. In this feasibility study the heat was introduced by means of either a heating stage or by means of a gas tungsten arc welding torch. It will be shown that the evolution of thermal and elastic lattice strains can be monitored at a sampling rate of up to 4 Hz at a bending-magnet synchrotron beamline. Since the sampling rate may be increased further at a insertion device synchrotron beamline providing higher photon flux, our setup appears to be feasible for monitoring laser treatments in real time.

Abstract: A new class of metal/ceramic composites has recently been developed. A porous ceramic preform, the pore structure of which is created via a freeze-casting technique, is melt-infiltrated with metallic alloy via sqzeeze-casting. The microstructure of the composite has lamellar-like domains with geometrical characteristics which are dependent on the manufacturing parameters. The aim of our study is to find a good micromechanical model in order to deduce the mechanical properties of the single domains and of the whole material as a function of the microstructural geometry and the material parameters of the ceramics (alumina) and the alloy (Al-Si eutectic). Firstly, the statistical analysis of polarized light microscopic micrographs of the cross section of the specimen was performed. Domains with the same orientation of lamellae, so-called single domains were detected, selected and measured. The material modeling was performed by a two-step homogenisation procedure using a combination of different micromechanical models. Predicted material properties were compared with ultrasonic measurements for a single domain and for the whole microstructure.

Abstract: Pearlitic steel and pure tungsten specimens were ground using a table-type grinding machine. The thin surface layers affected by the grinding process were characterized using focussed ion beam milling and microscopy. The strongly graded zone altered due to severe plastic deformation and recrystallisation was found to be less than 3m thick. The microstructure in that zone depends on the grinding parameters. Using synchrotron X-ray diffraction, the residual stresses were measured for penetration depths ranging from 0.25 m to 9 m. Based on the approach by Dölle and Hauk, the residual shear stresses were separated from the residual normal stresses. In pearlitic steel, residual shear stresses of opposite sign were observed in the two phases (ferrite and cementite) and found to be compensating each other, while shear stresses were proved to be absent in single-phase tungsten. These results underline that residual shear stresses caused by severe plastic shear deformation exist only as micro-stresses.

Abstract: In the work presented here the residual stress states of sintered iron (ASC 100.29) were studied after mechanical surface treatments. This included the investigation of the stability of compressive residual stresses of deep rolled sintered iron at different angles relative to the rolling direction at quasistatic and cyclic bending loading. An increase of the compressive residual stress in the transversal deep rolling direction at fatigue loading was found and will be discussed in this presentation.