Welding of titanium and titanium alloy

This paper describes the best said titanium and titanium alloy materials characteristics and weldability, and prone to oxidation, cracks, porosity chips for titanium and titanium weld defects in the weld, the welding test carried out. Can continue to explore the welding process specifications of titanium and titanium alloys, as well as a reasonable analysis of the problems arising in the test process, summed up the welding process characteristics and operating essentials of titanium and titanium alloys.
1. Classification and characteristics of titanium and titanium
Domestic industrial pure titanium has three kinds of TA1, TA2, TA3, the difference is that the content of impurities containing hydrogen and oxygen is different, these impurities make industrial pure titanium strengthen, but the plasticity is significantly reduced. Although the industrial pure titanium is not high in strength, it has excellent plasticity and toughness, especially good low temperature impact toughness, and good corrosion resistance. Therefore, this material is mostly used in the chemical industry, the petroleum industry, etc., and is actually used in working conditions below 350 °C.
Titanium alloys can be classified into three types according to the room temperature structure of the annealed state of titanium alloy:
Om titanium alloy, (W + type titanium alloy and B type titanium alloy.
Among the titanium alloys, TA4, TA5, TA6 Ti-AI alloys and TAY, TA8 Ti+AI+Sn alloys are widely used. The alloy has a strength of 931 N/m 2 at room temperature, and is stable at high temperatures (below 500 ° C) and has good weldability.
The application of B-type titanium alloy in China is relatively small, and its scope of use needs to be further expanded.
Second, the weldability of titanium and titanium alloy
The welding properties of titanium and titanium alloys have many remarkable features. These welding characteristics are determined by the physical and chemical properties of titanium and titanium alloys.
2. Crack problem of welded joints When welding titanium and titanium alloys, there is little possibility of hot cracks in welded joints. This is because the content of impurities such as 5, P, C in titanium and titanium alloy is very small, and the formation of 5, P is low. The melting point eutectic is not easy to appear on the grain boundary, and the effective crystallization temperature interval is narrow. When titanium and titanium alloys solidify, the shrinkage is small, and the weld metal does not generate hot cracks.
When titanium and titanium alloys are welded, cold cracks may occur in the heat-affected zone, which is characterized by cracks occurring several hours or more after welding as delayed cracks. Studies have shown that this crack is related to the diffusion of hydrogen bombs during welding. During the welding process, hydrogen diffuses from the high temperature deep pool to the lower temperature heat affected zone. The increase of hydrogen content increases the amount of TiH2 precipitated in the zone, increases the brittleness of the heat affected zone, and causes large microstructure stress due to volume expansion during hydride precipitation. In addition, hydrogen atoms diffuse and aggregate to the high stress sites in the region, so that cracks are formed. The way to prevent such delayed cracks is mainly to reduce the source of hydrogen in the welded joints.
3. Porosity problems in welds When welding titanium and titanium alloys, the pores are often encountered. The root cause of the formation of pores is the result of hydrogen influence. The formation of pores in the weld metal mainly affects the fatigue strength of the joint.
The main measures to prevent the generation of pores are:
(1), the protection of helium should be pure, the purity should be no less than 99.99%
(2) Thoroughly remove organic matter such as scale oil on the surface of the weldment and the surface of the wire.
(3) Apply good gas protection to the molten pool, control the flow rate of ammonia gas and flow rate to prevent turbulence and affect the protection effect.
(4), correctly select the welding process parameters, increase the use of the deep pool residence time in the bubble escape, can effectively reduce the pores. [next]
Third, titanium plate manual tungsten plate ammonia arc welding test
The application of titanium and titanium alloys in welding production is at most tungsten plate ammonia arc welding, and the application of vacuum ammonia filling welding methods is also very common. Under the protection and cooling of ammonia flow, the Arc arc welding arc is concentrated, the current density is high, the heat affected zone is small, and the welding quality is high.
1. When welding titanium and titanium alloys, when the temperature is higher than 500'C -700 °C, it is very easy for OA to collect air, hydrogen and nitrogen in the air, which seriously affects the welding quality. Therefore, when welding titanium and titanium alloys, the weld zone of the molten pool and the high temperature part (400 °C and above 650 °C) must be strictly protected. For this reason, special protection measures must be taken when welding titanium and titanium alloys. A welding torch with a larger spray size is used to enlarge the area of ​​the gas protection zone. When the nozzle is insufficient to protect the weld and the high temperature metal in the near seam area, an ammonia protection trailer is required.
The color of the weld and the near seam area is the target of the protective effect. Silver white indicates the best protection and yellow is slightly oxidized, which is generally allowed. The surface color should conform to the table (the seal regulations take into account the practicality and high efficiency of the project. We have prepared a simple drag mask. As shown in Figure (a), ammonia gas enters the distribution tube from the air inlet and passes through the distribution tube hole directly. Entering the protection zone. With this type of tractor, the welding protection effect is not very good, and the weld bead is dark blue. According to the analysis, the airflow enters the protection zone directly from the distribution pipe. The airflow is not very uniform and stable, so the high-temperature weld bead protection is not good. Oxidation. Therefore, we have further improved the structure of the tractor. As shown in Figure (b), ammonia gas enters the distribution tube from the air inlet and then returns to the top of the tractor. Through the porous plate, the porous plate mainly functions as a gas screen and distribution. the ammonia flow more stable, better protection of the welding bead silvery river or yellow. tow cover length L of charge 40 .m fly made of brass.
When ammonia arc welding of titanium and titanium alloys, attention should also be paid to the north side protection of the bead. Considering the welding deformation, we use the method of slotting and fixing the copper pad to protect the ammonia. In order to make the back of the bead fully protected, A porous copper tube is added to the slag to make the atmosphere enter the protection zone evenly through the copper tube hole, and the protection effect is good, and the back surface of the weld bead is silvery white.
Manual tungsten plate ammonia arc welding process and parameters selection (1) Preparation of weldments and wire surface quality before welding have a great influence on the mechanical properties of welded joints and must be strictly cleaned. Titanium plate and iron wire can be mechanical cleaning and chemical cleaning methods.
1) Mechanical cleaning Butt welding is difficult to weld or has difficulty in pickling. It can be wiped with fine sandpaper or stainless steel wire brush, but it is best to scrape the titanium plate with hard alloy yellow to remove the oxide film.
2) Before the chemical cleaning, the test piece and the welding wire can be pickled first. The acid washing liquid can be HF5% HH0335% water melt. After pickling, rinse with clean water, and then dry and then apply welding. Or use acetone, ethanol, carbon tetrachloride, methanol, etc. to wipe the portion of the titanium plate groove and its sides (each within 50m), the surface of the wire, and the workpiece and the titanium plate.
(2) Selection of welding equipment Ammonia arc welding of titanium and titanium alloy gold tungsten plate should use DC ammonia arc welding power source with reduced external characteristics and high frequency arc starting, and delay the refueling time of not less than 15 seconds to avoid welding. Oxidation and pollution.
(3) Selection of welding materials The purity of ammonia should be no less than 99.99%, the dew point should be below 40 °C, and the total mass fraction of impurities should be less than one. .001%, when the pressure in the ammonia cylinder drops to 0.981MPa, it should be stopped to prevent the quality of the welded joint from being affected. In principle, a titanium wire having the same basic metal composition should be selected, and sometimes a wire having a slightly lower strength than the base metal may be used in order to grip the weld metal of the weld.
(4) Selection of groove form The principle is to minimize the number of weld layers and weld metal. With the increase of the number of welding layers, the cumulative suction of the weld increases, which affects the performance of the welded joint. Because of the large size of the weld pool when welding titanium and titanium alloys, the test piece is opened VE270 80. Groove.
(5) Pairing and positioning welding of test pieces In order to reduce the welding deformation, the positioning welding is performed before welding. The general positioning welding pitch is 100 150. The length is 1015. The welding wire, welding process parameters and gas protection conditions for tack welding shall be the same as those for welding joints.
(6) Selection of welding parameters We have explored the appropriate welding procedure specifications by comparing the performance of welded joints under different processes. [next]
Process (1), welding current is 150A, 170A, 180A, according to this parameter welding, welding joint surface, showing dark blue, gold color, indicating that the joint oxidation is more serious, does not meet the technical requirements, this process is not desirable.
Process (2), the welding current is relatively reduced to 120A, 150A, 160A, according to this parameter welding, the surface of the weld is golden purple, deep yellow, the flaw detection of the line is flawless, but the mechanical performance bending test is unqualified, indicating welding The plasticity of the joint is significantly reduced, and the technical requirements are not met, and this process is also undesirable.
Process (3), welding current is 95A, 115A, 120A, according to this parameter welding, the surface of the weld is silver white, light yellow, the flaw detection of the line is flawless, but the mechanical properties of the bending test are qualified, the tensile strength also meets the requirements. The performance of the welded joint meets the technical requirements, and this process is suitable.
When titanium and titanium alloys are welded, there is a tendency of coarse crystals, which directly affects the mechanical properties of the welded joints. Therefore, the selection of welding process parameters should not only consider the oxidation of the weld metal and the formation of pores, but also the factors of grain coarsening. Therefore, the welding heat input should be used as much as possible. The process (sealing, (2), due to welding specifications) The large factor causes the joint oxidation to be more serious than the process (3), and the microscopic metallographic experiment results show that the joint grain coarseness is also more serious than the process (3), so the mechanical properties of the welded joint are poor.
The choice of gas flow rate is based on good protection effect. Excessive flow rate is not easy to form a stable laminar flow, and the cooling rate of the weld is increased, so that the surface layer of the weld has more time and causes microcracks. When the flow rate of ammonia in the tractor is insufficient, the weld seam exhibits different oxidized color; when the flow rate is too large, it will interfere with the air flow of the main nozzle. The ammonia flow rate on the back side of the weld is not too large, otherwise it will affect the gas protection effect of the first layer weld on the front side.
Handmade Tungsten Arc Arc Welding Practice for Titanium and Titanium Alloys 1) For manual ammonia arc welding, the minimum angle between the wire and the weldment should be kept as small as possible (10150). The welding wire is smoothly and evenly fed into the molten pool along the front end of the molten pool, and the end of the welding wire shall not be removed from the ammonia protective zone.
2) When welding, the welding torch basically does not swing horizontally. When it needs to swing, the frequency should be low, and the swinging amplitude should not be too large to prevent the protection of ammonia gas.
3) When the arc is broken and the weld is finished, the ammonia gas protection should be continued until the weld and the heat affected zone metal are cooled to below 350'C before the torch can be removed.
l) Quality inspection The seal inspection conforms to GB/T13149-91.
2) The ray deep injury conforms to JB4730-94,
3) The mechanical properties test conforms to GB/T13149-91.
Fourth, the conclusion
1. The gas protection problem of titanium and titanium alloy welding is the primary factor affecting the quality of welded joints.
2. When welding titanium and titanium alloys, try to use a small heat input.
3. When TA2 manual tungsten arc welding, the source of hydrogen should be strictly controlled to prevent the occurrence of cold cracks, and attention should be paid to prevent the generation of pores.
4. As long as the welding process is strictly in accordance with the welding process requirements and effective gas protection measures are taken, high-quality welded joints can be obtained.

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