Soeren Mueller

Experimental Investigations of Friction Carried out with the Tribo-Torsion-Test and Frictional Modelling

Complex frictional effects occur during the extrusion process between the extrusion die and the extruded material. The recently developed Tribo-Torsion-Test is used to measure friction under thermo-mechanical conditions similar to the extrusion process. Investigations have not been carried out only with nitrided but also with two different chemical vapour deposited CVD coated samples. In the context of this study the Tribo-Torsion-Test is introduced and laboratory results are presented.

A New High Speed Friction Test for Extrusion Processes

A new axial friction set up for high speed friction was successfully developed and tested at the Extrusion Research and Development Center of TU Berlin. The friction behavior between the Aluminium alloy EN-AW 6060 and the hot-working steel 1.2344 at 300 °C and 400 °C was studied. The experiments were carried out with a relative friction speed from 0.1 to 50 mm/s, and a normalized contact/normal pressure of σn/kf ~ 0 and 1.5. The friction stress curves were depicted and compared with the Tresca friction model. A similar tendency between them was found, however all the theoretical results were around 15 and 30 % higher than the experimental results. In general, higher friction forces were found at 300 °C. Moreover, the effect of normal stress on the friction stress could be studied. By increasing the normalized normal stress from 0 to 1.5 kf the friction stress increases around 30 % at 300 °C and 10 % at 400 °C. A strong adhesion effect was observed especially at 400 °C, which represents around the 90 % of the total friction stress. The friction results can be reproduced using a friction factor of m= 0.85 at 300 °C and m= 0.7 at 400 °C.

Coextrusion and Mechanical Characterization of Aluminum Coated Mg-Profiles

Due to their low density and high specific strengths Mg-alloys provide an excellent potential to be used for light weight constructions. However, their equilibrium potential is very low resulting in relatively low corrosion resistance, especially in contact with other, more noble metals. In order to separate Mg from a corrosive environment hybrid billets with Al-alloy coating and AZ31-core were coextruded. Thus, the extrusion and coating of Mg-profiles can be done in a single production step, resulting in aluminum coated Mg-profiles. The influence of the extrusion ratio as well as of different die angles on the formation of diffusion layers at the interface was investigated. Furthermore the phases formed in the diffusion zones were analyzed using EDS and synchrotron XRD. Additionally, FEM-simulations were conducted in order to reveal the material flow of core and shell material during the forming process and to identify differences in using different die angles. The FEM-results were verified by comparison with the real extrusion experiments. Finally, the shear strengths of the produced compounds were evaluated in push-out tests.

Compression Creep at 240°C of Extruded Magnesium Alloys Containing Gadolinium

For uses at high temperatures, magnesium alloys containing rare earths have proven to be very suitable. High proportions of melting precipitates contribute to strengthening, even at temperatures above 200°C. If these alloys are extruded, their creep resistance rises even further due to the resulting fine-grained structure. In this paper, magnesium alloys with 10% Gd and additional small amounts of La and Nd are compared with WE43 for compression creep at temperatures of 240°C and stresses between 80 and 150 MPa. The minimum creep rates are determined and the stress exponent evaluated in accordance with the Norton equation. By calculating the threshold stress, the true stress exponents are determined.

Finite Element Analysis of Material Flow and Characterization of Tube Dimensions of Indirectly Extruded Tailored Aluminum Tubes

In this work the extrusion of tailored aluminum tubes is investigated. Therefore, a stepped mandrel was applied and moved in axial direction in order to vary the tube’s wall thickness. Since surface irregularities were observed on the tube’s surface in transition areas between the different wall thicknesses, the material flow was analyzed by FEM in order to clarify the origin of the surface defects. It was revealed that the inwardly directed material flow in combination with the lack of inward resistance when the mandrel step is moving in the region of the die bearing causes the geometric inhomogenities.

Influence of temperature and sliding speed on the subsurface microstructure evolution of EN AW-6060 under sticking friction conditions

The microstructure evolution of the friction boundary layer of the aluminum alloy EN AW-6060 was investigated. Sticking friction tests at different temperatures and sliding speeds were carried out. A severe deformation below the friction surface was observed by means of LOM and EBSD mapping. Thus, the thickness variation and the grain structure of the high deformation zone could be described. Fibrous structure was observed at 300 °C and 400 °C, while equiaxed grains with high misorientation angle (>15°) were generated at higher temperatures. Additionally, abnormal grain growth and coarse grains were detected at high sliding speeds (10 mm/s, 42 mm/s) at 450°C and 500 °C respectively.The microstructure evolution of the friction boundary layer of the aluminum alloy EN AW-6060 was investigated. Sticking friction tests at different temperatures and sliding speeds were carried out. A severe deformation below the friction surface was observed by means of LOM and EBSD mapping. Thus, the thickness variation and the grain structure of the high deformation zone could be described. Fibrous structure was observed at 300 °C and 400 °C, while equiaxed grains with high misorientation angle (>15°) were generated at higher temperatures. Additionally, abnormal grain growth and coarse grains were detected at high sliding speeds (10 mm/s, 42 mm/s) at 450°C and 500 °C respectively.

Microstructural Characterization of Indirectly Extruded EN AW-6060 Square Tubes with Axial Variable Wall Thickness

Today, innovative lightweight constructions increasingly demand for profiles with higher strength and stiffness. In this investigation an axially moveable stepped mandrel allowed the manufacturing of load adapted (tailored) aluminum tubes with axial variable wall thicknesses by the extrusion process. Thus, on the one hand it is possible to produce thick-walled sections for highly stressed areas and on the other hand save profile weight by applying reduced wall thicknesses in areas with lower loads. Varying the extrusion ratio along tube direction affects the product velocity as well as the profile exit temperature und thus the microstructure in different tube sections. At high temperatures and high strain rates the microstructures revealed very large grains due to static recrystallization. However at low temperatures and low strain rates dynamic recovery lead to a microstructure dominated by fibrous grains. Small equiaxed grains were also found indicating geometric dynamic recrystallization.