Dieless drawing of metal wires with laser assistance

In the conventional glide drawing process the diameter of a wire or rod-like workpiece is reduced by drawing it through a conical die. Since the diameter reduction per drawing step is limited, usually many steps are required in order to achieve the desired diameter reduction. In the beginning seventies a modification of the glide drawing process, called "dieless drawing", was developed which did not require drawing tools any more. In the "classical" realization of dieless drawing the material is locally heated up by an induction coil and cooled immediately behind the heating zone. By applying a drawing force to this workpiece, deformation and thus the required diameter reduction appears in the heated zone. Since in this case the local heating is sufficient to define the position of deformation no mechanical tool is required. As the drawing tool which usually defines the diameter ratio is missing this ratio can be chosen in a flexible way by adjusting the velocities of the rod or wire in front of and behind the forming zone. To summarize, the dieless drawing process shows some advantages over the conventional wire drawing process: - simple change of the wire size - greater diameter reduction in one forming step - production of rods or wires with varying diameter over the length - no lubricants required - no environmental drawbacks as in the case of oil or salt baths for heating - high-temperature materials can be processed (e.g. tungsten, molybdenum) - thermomechanical treatment can be achieved In the framework of this project fundamental investigations on the heating by a high power laser beam shall be performed. This new process is expected to provide some additional advantages compared to conventional heating sources: - excellent control of the spatial and temporal distribution of power input (e.g. change of the cross sectional form by controlling the spatial distribution of the laser irradiation or compensation of unwanted asymmetries due to convectional cooling) - reasonable costs of process feasible (use of diode lasers) Within the framework of this project, the performance of laser induced dieless drawing shall be demonstrated and optimized. As laser source the CO2 laser will be applied. By using different arrangements for beam forming and focusing the influence of the spatial distribution of the laser beam power density on the deformed cross section shall be investigated. For the experiments a continuous drawing apparatus consisting of a torque and rpm controlled winding device and a wire feed with controllable brake will be constructed. This apparatus together with the laser shall be controlled by a personal computer. The experiments shall start with wire of steel, aluminum and copper with good forming properties. In a later step materials with poor cold working properties such as highly alloyed steel, tungsten and molyb-denum shall be investigated. The different processing results will be investigated under metallographical aspects in order to get information about the influence of laser treatment on the material. Moreover, numerical investigations of the process will be carried out in view of obtaining a better understanding of the influences of the various process parameters. For this purpose a finite- element- calculation shall be carried out which considers the coupling between the tempera-ture field and the plastomechanical processes in the region locally heated by the laser. These results will be used for optimizing the control algorithms for the drawing apparatus.

Conventional wire drawing is used to reduce the diameter of a wire by drawing it in several steps through conical drawing dies of successively decreasing diameters. Wear of the drawing dies and small deformation ratios per step because of friction and the tensile strength of the wire material are the major restrictions of this process. In the present project a dieless wire drawing process was investigated which avoids the above problems by replacing the drawing dies by a laser beam, acting as local heat source. The deformation ratio in this case is determined by the speed of the wire in front of and behind the heated zone. This process allows to control the diameter reduction by adjusting process parameters instead of changing the tool geometry. In addition the increased forming temperature and the lack of frictional forces in the forming zone reduce the drawing force which allows increased forming ratios per drawing step. In the present work, a wire drawing apparatus was developed which uses a 1 kW semiconductor laser as heat source and a closed-loop control of the drawing force and the temperature in the forming zone. The drawing apparatus was equipped with a speed controlled electric motor together with a controllable brake for applying the drawing forces. For measuring the speed of the wire in front of and behind the forming zone digital resolvers in contact with the wire were used. In addition a force transducer was connected to a wheel pressing perpendicularly at the wire. The temperature was measured in a contactless way by using a pyrometer. For a defined axial heat profile the wire was cooled by compressed air or a compressed air/water mixture immediately behind the laser heated zone. The closed-loop control was realized by using a PC with analog and digital input and output cards running the graphical software package LabVIEW. The force and temperature control was numerically modeled in order to find optimum parameters for the closed- loop control system. A FEM analysis of the laser assisted wire drawing process was done in order to support the experimental investigation. It could be verified that for the used drawing speeds one single laser is sufficient for a heat distribution with cylindrical symmetry and thus a circular cross section of the wire. The experimental and theoretical results were in good agreement and showed the advantages of dieless wire drawing with laser heating over the conventional wire drawing using drawing dies. The new process has been demonstrated for wires form copper, brass, steel, aluminum and titanium. It was possible to reduce the cross section of the wires by up to 30 % in one step, maintaining a circular cross section. This makes the described process especially interesting for materials with poor cold forming properties or high tensile strength.