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Numerical and Experimental study of texture, residual stresses, and eccentricity evolution in drawn copper tubes

Subject Area Primary Shaping and Reshaping Technology, Additive Manufacturing
Synthesis and Properties of Functional Materials
Term from 2015 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 282209677
 
Final Report Year 2019

Final Report Abstract

To find a general rule for the tubes’ behavior concerning eccentricity different drawing steps were investigated varying the tilting angles, offset values, or a combination of tilting and offset. Measurements of the wall-thickness of the tubes were performed prior to each experiment. Tilting results of copper tubes have shown that it is not only possible to affect the eccentricity, it is also possible to control it, whether for reduction or targeted thickening of the tube. Using -5° tilting angle resulted in ca. 50% reduction after the first drawing step, where the standard drawing (0°) resulted only in 18% reduction of eccentricity. Using positive tilting angles, the eccentricity was increased about 50% (for +5° tilting) and a local thickening of the 푀푎푥 position resulted. Although most producers want to decrease the eccentricity because of material costs and product’s weight, some applications favor thickening of the tube. Introducing tilting to the tube drawing process, however, did not change the force needed for drawing. After analyzing the effect of tilting/offset on the eccentricity of the drawn tubes, to know whether the tilting and/or offset can influence the developed RSs in a destructive way, the evolution of the RSs due to the introduced tilting and/or offset was investigated. Two different methods were chosen; hole drilling and neutron diffraction methods. Hole drilling results, which were measured on the surface of the tubes, showed that in the case of using a 5° tilting (both, negative or positive) the achieved RSs are less than the result of the standard drawn tube. In another word, using +5° tilting angle resulted in reduction of the RSs. Same exact behavior has been seen by the three other materials investigated, as well. Hole drilling is a fast and quite inexpensive method, but limited to the surface of the tube. In contrary using the neutron diffraction method, the whole wall-thickness of the tube can be measured (the measurements were done using SALSA instrument at ILL in Grenoble/France); however, it is an expensive method. The measurements done with neutron diffraction method were used also to prepare required data (input as well as validation data) for the simulation. To be able to follow the development of the RSs during deformation and to be able to investigate the RSs inside the die, special samples were prepared. For this reason, the drawing process was interrupted, and IDT samples were prepared, which all three main zones were existing in these tubes. The benefit of such sample was that the as-received sample, the part of the tube which is inside the die, and the drawn tube could be measured in one shot. Considering the deformation zone of the -5° tilting condition, it can be stated that the hoop RSs changes significantly compared to the axial ones, which shows that in the deformation zone, there is mass flow from 푀푎푥 side of the tube to the 푀푖푛 side. Tube drawing investigations with different tilting/offset values on materials of different qualities with different starting RSs and initial textures are time and cost intensive. Moreover, not all combination can be studied experimentally. To overcome this problem and also to have a better understanding of the process, it was decided to develop a simulation model containing all relevant properties of the as-received materials – such as eccentricity, RSs, initial texture, mechanical properties – and therewith analyzing more complex situations. Moreover, using the developed model, it was possible to compare the texture evolution of the drawn tubes with and without titling, without performing time consuming neutron, synchrotron, or electron measurements. The results of the FEM simulations were validated using eccentricity, RSs, stress-strain diagrams, and texture. Using the developed model, it is possible to study the tube drawing process having different parameters, such as different materials, different amount of reduction, and various tilting/offset values. Within the framework of the planned project, the knowledge to be developed will enable the small and mediumsized enterprises of the above-mentioned industrial branches to supply market-oriented, optimized solutions for the plant, pipeline construction, and automotive areas by exploiting the possible material gain and ensuring narrower and more homogeneous material properties. In addition, new product fields with strong demand potential, such as heat exchanger tubes as well as bearings, high-precision bushes and guide tubes for the automotive industry, are developed. This results in an effective increase in the performance and competitiveness of these companies in the German market. The economic importance of the planned research project is confirmed by the great interest of various companies who have actively supported the project. Moreover, the developed simulation methodology will be used to study other metal forming processes, to be able to generalize the existing model.

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