Abstract: The welding process of TA16 and TA18 titanium alloy conduits for aeroengine was studied. The manual argon arc welding of titanium alloy conduit was studied.

Key words: aero-engine; Titanium alloy conduit; Welding parameters

CLC Number: TG456.7 Document CODE: A Article ID: 1009-14X (2019) 04-0045-01

1 introduction

Titanium alloy has the characteristics of low density, high strength and corrosion resistance. As a new material, titanium alloy pipe is widely used in the field of aerospace, and the proportion of titanium alloy pipe in the aero-engine pipeline is increasing. Other, titanium alloy is a very lively metal at high temperature of oxygen, hydrogen, nitrogen and other gases has a great affinity, absorbed and dissolved gas ability is very strong, especially in the welding process, the ability with welding temperature rises, the performance is especially intense, when welding need to absorb oxygen, hydrogen, nitrogen and other gases and dissolved control, avoid the abandonment of the product, This brings great difficulty to titanium alloy pipe welding.

Titanium alloy pipe manual argon arc welding

2.1 Weldability of titanium alloy conduit

(1) embrittlement of welded joints

At room temperature, titanium reacts with oxygen to form a dense oxide film, which makes it have good chemical stability and corrosion resistance. Under high temperature, especially in the welding process, titanium and oxygen, hydrogen, nitrogen reaction speed, when the molten pool in the invasion of oxygen, hydrogen, nitrogen and other harmful gases, welding joint of the plasticity, toughness and surface color has the obvious change, especially in the more than 882 ℃, sub grain growth tendency is serious, martensite structure are formed when cooling, As a result, the strength, hardness, plasticity and toughness of the joint are decreased, the tendency of overheating is serious, and the joint is seriously embrittlement. Therefore, in titanium alloy welding, the pool, droplet and high temperature zone, whether front or back, should be comprehensive and reliable gas protection.

(2) stomata

Porosity is the most common defect in titanium and titanium alloy welding, mainly near the fusion line. Hydrogen is the main cause of stomata. During welding, titanium has a strong ability to absorb hydrogen (even stronger at high temperature), but its solubility decreases significantly with the decrease of temperature, so hydrogen dissolved in liquid metal often has no time to escape and gathers near the fusion line to form pores.

(3) Delay crack near the seam area

Titanium alloy after welding for a period of time. Cracks (delayed cracks) are often found near the crack zone. The reason is that hydrogen diffused from the high-temperature molten pool to the low-temperature heat-affected zone. With the increase of hydrogen content, the amount of TiH2 precipitated increases, which increases the brittleness of the heat-affected zone. In addition, the microstructure stress caused by the volume expansion of precipitated hydride finally leads to the generation of cracks.

2.2 Welding requirements and precautions for titanium alloy conduits

(1) Try to set up a special welding workshop, indoor smoking is strictly prohibited, the environment should be kept clean and dry, and air convection should be strictly controlled.

(2) Welders wear clean overalls and fat-free gloves when welding, and do not touch parts with bare hands.

(3) The welding area and the surface of the welding wire shall be degreased with acetone.

(4) High purity protective argon, the purity is not less than 99.99%. During welding, the front and back of the weld bead shall be protected according to the value specified in the technical specification.

(5) During the welding process, the flow of argon gas in the tube and the nozzle of the welding tool should be kept constant to prevent the phenomenon of convex and concave in the weld pool forming in the tube.

(6) When welding, short arc welding should be adopted as far as possible, and small welding line energy should be used.

(7) The gap is less than 30% wall thickness when the nozzle is positioned for spot welding. As far as possible, each weld shall be welded once.

(8) During welding, the welding tool shall not swing from side to side, and the molten end of the welding wire shall not be moved out of the gas protection zone. When starting the arc, air should be sent in advance for 10-15s. When stopping the arc, the welding torch can not be lifted immediately. The air supply should be delayed for 15-30s until the temperature drops to below 250℃.

2.3 Welding Process

2.3.1 Cleaning before welding.

The occurrence of welding defects is closely related to the surface cleanliness of welding parts and wires. Before welding, clean oil, water, oxide film and other dirt within 15 ~ 20mm of the edge of the pipe joint and the surface of the welding wire. The cleaning method can be chemical (pickling) or mechanical (stainless steel brush removal) to remove the surface oxide. Before welding, acetone or alcohol should also be used to scrub. After cleaning, the welding parts must be welded within 24 hours, otherwise it needs to be cleaned again. Wire pickling after the best vacuum dehydrogenation treatment, before welding with acetone degreasing.

2.3.2 Gas protection. During the welding of titanium pipe joint, in order to prevent the welding joint from being polluted by harmful gases and elements at high temperature, the welding seam must be protected by argon gas, and the purity is not less than 99.99%. Argon gas flow is shown in Table 2-1.

2.3.3 Selection of welding process parameters.

(1) selection of welding wire. Filling wire grade should be selected according to the base metal, generally using the principle of homogeneity with the base metal, sometimes in order to improve the joint plasticity, can also choose a slightly lower than the base metal alloying degree of the welding wire. Select the diameter of the welding wire according to the thickness of the base metal, as shown in Table 2-1.

(2) Selection of power supply and polarity. Titanium and titanium alloy welding generally adopts DC manual tungsten argon arc power supply, and its polarity connection method adopts DC direct connection.

(3) The choice of tungsten pole. Tungsten electrode diameter is selected according to the thickness of titanium alloy tube wall, generally between 1.0-3.Omm, tungsten extreme should be ground into 25° ~ 45° cone.

(4) Welding current and other parameters are shown in Table 2-1.

reference

[1] Chen Zhunian. Manual for Welding Engineers. China Machine Press, Beijing, 2002.1, ISBN 7-111-09859-5.

[2] XU Chuxiong. Welding Process 500 questions. China Machine Press, Beijing, 1997.2, ISBN 7-111-05202-1.

[3] Editorial Committee of Chinese Aeronautical Materials Manual. Chinese Aeronautical Materials Handbook. Standards Press of China. Beijing, 2001.8, ISBN 7-5066-2439-7.

[4] Cui Zhongxin, Liu Beixing. Metalology and principles of heat treatment. Harbin Institute of Technology Press. Harbin, 2004.2, ISBN 7-2603-1334-5.

[5] Hu Shao-quan, ZHANG Xue-de, ZHONG Shan. Welding technology. Review Committee of aeronautical mechanics teaching materials.