Analysis of inner surface defects of pure titanium extruded pipe blank

Aiming at the pit defect on the inner surface of pure titanium extruded pipe blank, the composition, phase and cause of formation of the second phase on the surface of cracked copper cladding and the surface of titanium pipe blank after pickling were analyzed by metallography, energy spectrum and XRD.

The results show that the second phase on the contact surface between copper cladding and pure titanium ingot is cu3ti; The main reason for pit defects is that when the lubrication conditions are poor, the increase of extrusion ratio (from 14.2 to 22.9) causes the increase of friction heat and deformation heat, which increases the local temperature of copper clad and pure titanium pipe blank (> 850 ℃), and the opposite reaction of Ti Cu liquid occurs in a short time (< 10 s), resulting in the tearing and falling off of copper clad. The wall thickness of the torn copper cladding is large, and the flow velocity is less than that of the titanium pipe blank. Its irregular shape forms a large number of point or strip pits on the inner surface of the titanium pipe blank. It is proposed that improving the hot extrusion process parameters and reducing the friction heat and deformation heat are effective measures to eliminate the inner surface defects of pipe blank.

Introduction

The red copper cladding used in the hot extrusion of pure titanium pipe blank can prevent the oxidation and pollution of ingot and the bonding of metal titanium to the extrusion die. After extrusion, the red copper cladding is easy to be removed by pickling, and the copper can also be recycled. Therefore, this process is widely used in China. Since the installation and commissioning in 2006, the 16.3 Mn double acting horizontal oil extruder in the engineering center of Northwest Nonferrous Metals Research Institute has used φ 120 extrusion barrel to produce 45 × 6. More than 500 tons of pure titanium pipe blank, with good product quality. In order to improve the product yield and production efficiency, 150 extrusion cylinders were used to produce 45 in 2008 × 6 and ∅ 56 × 4 pipe blank to increase the extrusion ratio from 14.2 to 22.9 and 24.3 respectively. With the increase of extrusion ratio, a large number of point or strip pit defects begin to appear on the inner surface of pipe blank, and the depth of serious cases can be more than 0.4 mm, as shown in Figure 1. Deep pits cannot be eliminated by rolling, resulting in a large number of pipe blanks or even a whole batch of scrap.

After normal extrusion, the copper cladding completely covers the titanium pipe blank and fits closely with the titanium pipe blank, and the wall thickness of the copper cladding is uniformly thinned to 10 ~ 20 μ m。 However, during production, it was found that the copper cladding of the inner hole of the extruded ingot was seriously damaged and fell off after extrusion, some were flake and some were tubular. As shown in Fig. 2, the copper cladding was not fully stretched and deformed, and the thickest was up to 0.8 mm. Through the observation of the damaged and fallen copper cladding, it is found that most of the surface in contact with the titanium pipe blank is silver white, and there are dotted ablation holes in some parts. There are serious oxidation color and ablation holes at the broken edge of the copper cladding. Abnormal phenomena such as breakage and falling off of inner hole copper sheath, pit defect, copper and titanium adhesion of extrusion needle, unstable extrusion force and excessive residual pressure generally occur at the same time, and the severity of various phenomena has the same trend. In this paper, the relationship between cu3ti on the surface of damaged copper cladding and the inner hole defects of pipe blank and the preventive measures are discussed through the analysis of metallography, energy spectrum and XRD.

20210905143244 54328 - Analysis of inner surface defects of pure titanium extruded pipe blank

Fig. 1 photo of pit defects on the inner surface of pipe blank Fig. 2 photo of damaged copper sheath

Metallographic analysis of damaged copper cladding

The metallographic photos of the damaged copper sheath are shown in Figure 3. Fig. 3 (a) is a cross-sectional photo of the silver bright part. It can be seen that the silver bright surface of the copper sheath is actually a layer of continuous second phase, which exists only on the surface in contact with the ingot, and no second phase is found on the surface in contact with the copper sheath and the extrusion needle. The thickest part of the second phase layer is about 20 μ m. There are several vertical cracks on the surface directly to the matrix. It can be seen from Fig. 3 (b) that the second phase layer is closely combined with the matrix. Thickness is only 20 μ The brittle fracture of the second phase of M proves that it is a brittle phase. After normal extrusion, the copper cladding should be thinned evenly, but figure 3 (c) shows that the copper cladding has been seriously folded, and on the side where the second phase exists, the second phase goes deep into the deepest folding, indicating that the folding occurs after the generation of the second phase.

20210905143259 91044 - Analysis of inner surface defects of pure titanium extruded pipe blank

Fig. 2 metallographic photograph of damaged copper sheath

Conclusion

  • (1) The second phase on the contact surface between copper cladding and pure titanium ingot is Cu3Ti; The main reason for pit defects is that when the lubrication conditions are poor, the increase of extrusion ratio (from 14.2 to 22.9) causes the increase of friction heat and deformation heat, which increases the local temperature of copper clad and pure titanium pipe blank (> 850 ℃), and the opposite reaction of Ti Cu liquid occurs in a short time (<10 s), resulting in the tearing and falling off of copper clad. Due to poor lubrication, the flow velocity of the torn thick copper sheath is less than that of the titanium pipe blank, and its irregular shape forms a large number of point or strip pit defects on the inner surface of the titanium pipe blank.
  • (2) The hot extrusion process parameters should be improved to reduce friction heat and deformation heat and avoid Ti Cu reaction, so as to eliminate the fracture of copper cladding and inner surface defects of pipe blank.

Source: Network Arrangement – China Titanium Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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