Papers by Author: Dirk Bormann

Abstract: Due to its low density and good mechanical properties Magnesium holds a high potential for design applications. The investigations discussed herein focus on the use of extruded magnesium sheets as semi-finished products e.g. for subsequent processing by roll forming. Special interest is given by the hcp-structure of magnesium which reduces forming abilities. Extrusion experiments using AZ31 were carried out to investigate the influence of different die geometries (S = sharp-edged and R= radius), billet temperatures (TB= 350°C and 390°C) and extrusion speeds (vext= 2.31 m/min and 9.24 m/min) on mechanical properties, grain structure and texture of said alloy. Two types of sheets with the dimensions of 80 x 1 mm and 80 x 2 mm respectively were produced using the institutes 10 MN extrusion press make SMS MEER. Sheet material with a thickness of 1 mm rolled from cast billets has been used for reference. Microstructural analysis focused on recrystallization behavior and grain size as well as grain distribution within the sheets. Tensile tests in extrusion or rolling direction as well as 45° and 90° to the same showed a strong influence of the specimen orientation on the mechanical properties. Additional Erichsen cupping tests demonstrated the stretch-forming capacity of the different test set-ups.

Abstract: Owing to their mechanical properties, metallic foams possess the outstanding ability to considerably improve a structure's stiffness and energy absorption capacity with low increases in weight. In the research results from the sub project A4 "Foam filled, rolled, closed profiles” of the CRC 675 "Creation of high strength metallic structures and joints by setting up scaled local material properties" introduced here, both the manufacture as well as the reinforcement of magnesium foams, which are produced by means of powder metallurgy, are described. The potential for increasing their strengths using reinforcements are demonstrated and the results of mechanical tests are presented. In addition to this, research results are presented which have contributed to achieving the main objectives of developing a combined technology for producing profiles which are locally reinforced using magnesium foam. The developed technology is characterised by integrating the foaming process into the roll forming process.

Abstract: Resorbable magnesium alloy implants for osteosynthetic surgery would be advantageous to common implants of titanium or surgical steel as a second surgery for implant removal would become unnecessary. To influence the degradation progress, surface modifications are sensible. As plates and screws were used to stabilize fractures, the degradation behavior of threaded cylinders is of particular interest. Therefore each eight solid MgCa0.8 alloy cylinders (3 x 5 mm) with smooth and sandblasted surface, respectively, and eight screw-shaped, threaded MgCa0.8 cylinders (thread pitch 1.25 mm, length 5 mm) were inserted into the medial femoral condyle of adult New Zealand White rabbits. Implantation periods were three and six months, within which the animals were examined daily. To evaluate a possible gas generation radiographs were taken weekly. After euthanasia the bone-implant-compound was scanned in a µ-computed tomograph (µCT80, ScancoMedical). All implants were well tolerated. Smooth implants degraded slowly. The cross sectional area did not reduce obviously after three months implantation duration and only mildly after six months. Sandblasted implants showed the fastest degradation progress after both implantation periods with the most obvious generation of gas. Threaded cylinders revealed pitting corrosion at the thread pitches. They degrade faster than smooth implants but slower than sandblasted cylinders. In summary, surface modification influences the degradation behavior of resorbable magnesium alloy implants. Contrary to common materials, smooth surfaces seem to be favorable. Thread pitches of screw-shaped implants show pitting corrosion. To what extend this result affects future applications of resorbable screws has to be examined in further investigations.

Abstract: Current research focuses on magnesium based alloys in the course of searching a resorbable osteosynthetic material which provides sufficient mechanical properties besides a good biocompatibility. Previous studies reported on a favorable biocompatibility of the alloys LAE442 and MgCa0.8. The present study compared the degradation process of cylindrical LAE442 and MgCa0.8 implants after 12 months implantation duration. Therefore, 10 extruded implants (2.5 x 25 mm, cross sectional area 4.9 mm²) of both alloys were implanted into the medullary cavity of both tibiae of rabbits for 12 months. After euthanization, the right bone-implant-compound was scanned in a µ-computed tomograph (µCT80, ScancoMedical) and nine uniformly distributed cross-sections of each implant were used to determine the residual implants´ cross sectional area (Software AxioVisionRelease 4.5, Zeiss). Left implants were taken out of the bone carefully. After weighing, a three-point bending test was carried out. LAE442 implants degraded obviously slower and more homogeneously than MgCa0.8. The mean residual cross sectional area of LAE442 implants was 4.7 ± 0.07 mm². MgCa0.8 showed an area of only 2.18 ± 1.03 mm². In contrast, the loss in volume of LAE442 pins was more obvious. They lost 64 % of their initial weight. The volume of MgCa0.8 reduced clearly to 54.4 % which corresponds to the cross sectional area results. Three point bending tests revealed that LAE442 showed a loss in strength of 71.2 % while MgCa0.8 lost 85.6 % of its initial strength. All results indicated that LAE442 implants degraded slowly, probably due to the formation of a very obvious degradation layer. Degradation of MgCa0.8 implants was far advanced.

Abstract: The investigation of the basic principles for the production of foamed out sections using magnesium foam for support structures by including the foaming process into the cold forming of sections to produce indiviually locally strengthened components is the subject of this research project. To absorb tensile stress, the metal foam will be strengthened with three-dimensional branched struts of high-tensile materials. The quantification of the influence of locally introduced foaming elements on e.g. stiffness alterations and the influence of the resonance frequency of the total structure will be effected by destructive but particularly also by nondestructive tests.