Quality Control and Pore Prevention Measures for Welding of Small Diameter Titanium Alloy (TA2) Pipes
Titanium and titanium alloys have the characteristics of low density, high strength, and good corrosion resistance and are widely used in fields such as aviation, aerospace, and chemical machinery. During the construction process of titanium alloy pipes, if the pipe diameter is small（ Φ 12 Φ 100mm), the difficulty of welding construction will increase. To improve the quality of welding small diameter titanium alloy pipes and prevent the generation of air holes, this article introduces the preparation work in terms of environment and materials before welding, as well as preventive measures during welding construction, and elaborates on the welding quality inspection methods.
Table of Contents
Titanium alloy pipes have excellent mechanical properties under high temperature, room temperature, and ultra-low temperature conditions, with low density, high strength, and excellent corrosion resistance in various media. They have been widely used in aerospace, petrochemical, and mechanical manufacturing fields. However, when the diameter of titanium alloy pipes is small (φ 20-φ 100mm), it will increase the difficulty of welding. The article summarizes the problems encountered and measures taken to construct titanium pipes in a certain project to provide a reference for similar titanium pipe welding construction.
1. Characteristics of titanium and titanium alloys in welding operations
Titanium and titanium alloy materials themselves have the following characteristics in welding operations:
- (1) Titanium metal has the characteristics of low density, high specific strength, high melting point, good heat resistance, good corrosion resistance, good resistance to high temperature, room temperature, and ultra-low temperature, and non-toxic and nonmagnetic.
- (2) Titanium is a chemically active metal with extremely strong reduction ability, especially in high-temperature environments where it can combine with various elements such as oxygen, nitrogen, and carbon and can also extract oxygen from some metal oxides.
- (3) If impurities exist in titanium, its mechanical properties will be greatly affected, and interstitial impurities (such as oxygen, nitrogen, carbon, etc.) can greatly reduce the plasticity of titanium and improve its strength. As a structural material, Titanium possesses excellent mechanical properties under high temperature, low temperature, and rapid cooling and heating conditions through the addition of alloy element types and strict control of alloy element content. Inclusions are allowed in industrial titanium alloys. As the content of impurities such as TA1, TA2, and TA3 increases, their strength and hardness will significantly increase, but their plasticity or toughness will decrease.
- (4) Titanium alloys have different characteristics in different temperature ranges during welding. When the temperature is above 300 ℃, 600 ℃, and 700 ℃, they correspond to the characteristics of rapid hydrogen absorption, oxygen absorption, and nitrogen absorption, with the most sensitive being oxidation in air.
- (5) The combination of Ti and H generates TiH2. It is a binary low-density hydride (solid form) with a density 0.86 times that of titanium (Ti density of 4.51g/mm3, TiH2 density of 3.91g/mm3), typically appearing in the form of needle tips or flakes in the weld bead. Therefore, the needle like defects in RT images are usually not hydrogen pores but TiH2. TiH2 exhibits a small and blurry feature in RT images, especially when the edges are not as clear as pores. If there needs to be more understanding of this low-density hydride, it is easy to mistake it for a pore.
2. Preparation before welding
Considering the high activity of titanium alloys, it is necessary to carefully prepare the environment, materials, and groove preparation before welding.
2.1 Preparation of welding environment
(1) The welding site should be set up in a clean and dry room or a dedicated welding workshop as much as possible. The indoor temperature should not be lower than 5 ℃, and the airflow rate should not be too high. The rubber should be laid on the ground.
(2) The welding platform or operating frame should be stainless steel. If other materials with iron ion pollution sources, such as carbon steel, are used, tape or other items should be used to fully wrap the welding platform or operating frame, and the tabletop should be covered with rubber.
(3) Prepare to weld protective towing covers that match the specifications of the base material and other small tools used during welding, as shown in Figure 1.
Figure.1 Protective towing cover
(4) The grinding discs, cutting blades, reamers, polishing wheels, files, etc., used during construction should be of stainless steel grade.
(5) The work clothes worn by operators should be kept clean and avoid the presence of oil and dust as much as possible. Welding gloves should be made of degreased white fine gauze and kept clean, as shown in Figure 2.
2.2 Material preparation
- (1) Base material inspection: Pipes and pipe fittings should have corresponding certificates of conformity. The appearance of the base material and accessories should not have any inclusions or double skinning.
- (2) Welding material inspection: The welding wire should match the base material and have corresponding qualification certificates.
- (3) Argon gas inspection: The purity of argon gas shall not be less than 99.99%, and the gas supplier shall provide corresponding certificates of conformity and inspection reports. Considering the influence of factors such as vacuum pumping and filling by the gas supply party, it is advisable to use canned liquid argon as much as possible and equip it with on-site gasification devices.
2.3 Preparation of Groove
- (1) The groove at the end of the pipe should be machined as much as possible.
- (2) When cutting, cutting, and polishing the groove on site, excessive force should be avoided, and grinding should be carried out at the designated time to prevent excessive local heat.
- (3) The grooved surface cut and polished with a resin grinding wheel should be polished again with a stainless steel reamer.
- (4) Within 40mm of the inner and outer surfaces of the pipe end groove, a reamer should be used to remove the passivation film.
- (5) Before pairing, use a polishing wheel to remove burrs and debris from the groove.
- (6) Before assembly, it is necessary to carefully check whether the oil stains in the end face of the pipe mouth are cleaned thoroughly to prevent them from being absorbed by the molten pool due to heating during welding.
- (7) Before pairing, clean the groove end face with acetone or anhydrous alcohol.
3. Assembly welding
Strengthen gas protection and process control during the assembly welding process to prevent the generation of defects (such as pores). The specific requirements are as follows:
(1) Tack welding should be carried out under argon-filled conditions.
(2) Before positioning welding, the gap and misalignment of the welding junction should be carefully checked. The gap should be controlled within 2.5-3.5mm, and the misalignment should be controlled within 1mm, as shown in Figure 3.
Figure.3 Measurement of groove gap before welding
(3) It is best to use the one drop spot welding method for positioning welding to avoid polishing the positioning welding points during formal welding.
(4) Before formal welding, if there are impurities (such as dust, sweat stains, etc.) on the groove during assembly, it is necessary to clean it again with acetone.
(5) The triple protection of argon filling in the pipe, welding handle, and drag cover are indispensable, and there is a matching flow rate (usually, the flow rate range of the welding tool nozzle is 13-16L/min; the flow rate range in the pipe is 8-16L/min, and the flow rate range of the protective drag cover is 15-30L/min). The argon filling in the pipe should fully replace the air, and the protective gas of the welding torch and towing cover should be opened 5-10 seconds before the arc striking, as shown in Figure 4.
(6) Moderate cooling. In addition to using as little heat input as possible during welding, welding of small-diameter pipes should also be stopped in stages to ensure that the base material does not overheat.
(7) During welding, the welding tool should not swing left or right as much as possible. The towing cover should not immediately detach from the weld bead and weld pool after each intermediate stop and final welding. At this time, gas supply protection should be continued. Only when the temperature drops below 250 ℃ can the towing cover be considered for detachment.
(8) When stopping welding in the middle, the melting end of the welding wire also needs gas protection. If the melting end is moved out of the gas protection zone, the oxidized part of the front end of the welding wire should be cut off during re-welding.
(9) When welding, two people should operate, one person welding, and one person cooperating. The cooperating personnel should have rich experience in titanium pipe construction and a strong sense of responsibility.
(10) Cooperating personnel should always check the working status of welding equipment, argon gas meters, gas bands, argon filling plugs, and drag covers, cooperate with the cleaning of welding wires, and cut off the oxidation welding wire heads to ensure smooth welding.
Figure.4 Schematic diagram of the triple shielding gas system during the welding process
4. Appearance inspection
The appearance of welded pipe joints can determine whether the quality meets the requirements.
(1) The appearance and internal penetration of the weld bead are well formed, with a smooth and uniform transition, meeting the geometric size requirements, and without surface defects such as pores, inclusions, incomplete penetration, cracks, and undercuts.
(2) The color inspection of the welding corner and welding area surface is shown in Table 1.
Table.1 Comparison Table of Weld Protection, Oxidation Effect, and Color
|Protective Effect||Degree Of Oxidation||Color (Oxide Film)|
(3) Titanium pipe joints can be preliminarily inspected based on the color of the surface oxide film, and “good” or above is considered a qualified protective effect. For joints with unsatisfactory protective effects, they should be treated according to the actual situation. Generally speaking, when the oxidation effect is relatively light, acid pickling treatment or stainless steel wire brush can be used to brush the oxidation area until the metal is silver white to prevent the internal metal from further oxidation. For heavy oxidation effects, the welded junction should be cut off and reassembled for welding.
The weld morphology during the welding process of small diameter titanium alloy pipes is shown in Figure 5.
Figure.5 Effect diagram during the welding process
- (1) Protective gas – Argon should have strict quality assurance to ensure that the purity of argon is not less than 99.99%.
- (2) The welding area (groove and its inner and outer sides) and the surface of the welding wire should be thoroughly cleaned of dirt, such as oxide skin and grease, to ensure the cleanliness of the operating surface.
- (3) Properly control the change of welding temperature to prevent the molten pool and the base metal in the Heat-affected zone from absorbing nitrogen, hydrogen, oxygen, other gases, iron, carbon, and other elements.
- (4) The adhesive tape pasting the groove should be manageable, as it may cause cleaning difficulties. Instead, use less adhesive paper tape.
- (5) A good welding process should maintain the molten pool’s clear and stable state. If phenomena such as small explosions are observed, it is necessary to consider whether there are pores. The possible reasons for this may be insufficient purity of the protective gas, unclean welding surface, failure to cut off the oxidized part of the welding wire head, and high molten pool temperature.
Author: Sun Liqun