Texture and stress characterization of a copper tube by neutron, synchrotron and electron diffraction

Abstract

Copper (Cu) was one of the first metals to be used by human beings (gold was the other) and was probably discovered around 6000 B.C. Cu is a very useful material with good properties like very gut thermal and electrical conductivity as well as corrosion resistance. The main applications of Cu and its alloys (like brass and bronze, etc.) are as thermal and electrical conductors, art works, beer tanks, musical instrument, coins, a building material, and is also a constituent of various metal alloys (e.g. Al, Fe, Ti alloys). The present work deals with a DHP-Cu tube. Commercially pure Cu is available in several grades defined by the refining process, the impurities present and the oxygen content. One example of commercially pure Cu is DHP-Cu, which means a Cu variety with low phosphorus content. A typical application of DHP-Cu is as condenser, evaporator and heat exchanger tubes, for air conditioning for refrigeration as well as for plumbing pipes and steam tubes. Rolling, extrusion and drawing are the preferred techniques for manufacturing seamless Cu tubes. Among other criteria, the quality of tube manufacturing can be determined from the tube eccentricity and ovality. Seamless tube production is a combination of both diameter and wall thickness reduction, this leads to a complex residual stress profile and variation in crystallographic texture around the circumference and through the wall thickness. Experiments have been carried out by neutron-, synchrotron-, and electron diffraction to determine texture gradients and residual stress profiles. Neutron diffraction is non-destructive while the other methods need intense sample preparation. A main part of this work was the usage of the robot system at STRESS-SPEC (neutron diffractometer at FRM II) to handle large samples up to 12 kg. Due to the sample geometry (140 mm outside diameter and 10 mm average wall thickness) absorption and volume correction were necessary. The texture was a combination of deformation and dominant recrystallization texture components, namely cube, rotated cube, copper, brass, S and Goss component. The {001}<100> cube component shows a variation of the orientation density around the circumference and a remarkable variation through the wall thickness while the copper component is nearly constant. Here, the temperature plays a major role. The residual strains measured by the robot and by the XYZ-stage agree well; this indicates that the robot is reliable for strain measurements. The residual axial stress was tensile for the outer part of the copper tube and compressive for the inner part. The residual hoop stress has a similar distribution but with lower values. The residual radial stress has quite a low value.