Common straightening methods for wire rods and titanium tubes include tension straightening, sinusoidal straightening, pressure straightening, etc.
Tension straightening is a processing method that applies a longitudinal tensile force exceeding the yield limit of the material to the workpiece to produce plastic extension and realize the correction of shape defects, also known as drawing straightening. During straightening, the tensile stress exceeding the yield limit of the material is applied to the titanium rods, titanium wires, and titanium tubes with wavy shape defects before and after the tension leveler. The tensile stress is superimposed on the original residual stress of the titanium rod. At the place where the extension is large, a part of the tensile stress is offset, so that the actual deformation stress is reduced, and the plastic extension is small during straightening; while at the place where the original extension is small, due to the tension The superposition of stress, the actual deformation stress increases, and the plastic extension increases during straightening. As a result, after the tension, each part of the straightened workpiece extends evenly, and the wave-shaped defect is thus eliminated.
In production, the sinusoidal straightening method is widely used for rods, wires, and pipes with simple cross-sections. Titanium tubes and titanium rods are processed on a skewed roller straightening machine. The number of rollers of the straightening machine is more than 4 (usually 5~29 rollers), and its working principle is to continuously and repeatedly bend the workpiece at three points by each roller, thereby gradually reducing the variation range of the residual curvature of the workpiece.
Sine straightening is often used in conjunction with pressure straightening. First, the workpiece with a large curvature is straightened by a pressure straightening machine and then straightened by skewed rollers. The straightening effect mainly depends on the pressure of the straightening machine and the inclination angle of the roll. The magnitude of the pressure depends on the yield strength and bending degree of the alloy material. If it is a high-strength titanium alloy, the straightening pressure should be larger when the bending degree is larger, and vice versa. The size of the roll inclination depends on the diameter of the workpiece, and the workpiece with a large diameter should be larger than the small straightening inclination. After straightening, unqualified workpieces should be returned for re-straightening. Titanium tubes that cannot be straightened should be sent to a tension leveler for straightening.
The basic principles of roller straightening are:
(1) Generally, the smaller the roll diameter and the more the number of rolls, the higher the straightening accuracy; the smaller the roll distance value, it is beneficial to the biting of the workpiece and the establishment of the straightening process.
(2) The main function of the first few rollers of roller straightening is to reduce the difference in the residual curvature of the workpiece along the length direction, and the main function of the last few rollers is to reduce the residual curvature to make it tend to be uniform.
(3) The quality of straightening mainly depends on the reasonable determination of the reverse bending rate of the workpiece under each roller. The first few rolls (second and third rolls) are selected to have a large inversion rate, and the inversion rate on the subsequent rolls is determined according to the maximum residual curvature that can completely straighten the preceding adjacent rolls.
(4) The larger the hardening coefficient 7 is, the more difficult it is to straighten the material. At this time, a larger inversion rate, more straightening rollers and a smaller roller diameter should be selected.
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