Titanium and Titanium alloy welding

Abstract: Titanium and titanium alloys and weldingcharacteristics of the material, and for welding of titanium and titanium alloyoxidized easily, cracks, chips Stomatal weld defects, a welding test. Can leadto titanium and titanium alloy welding process norms continue to explore, aswell as the process of testing the rational analysis of the problem, summed upof titanium and titanium alloy welding technology and operation of keycharacteristics.  

First, the classification of titanium and titanium and features.    

Domestic industrial pure titanium haveTA1, TA2, TA3 three, the difference is that hydrogen and oxygen with nitrogencontent of impurities, these impurities so that to strengthen the industrialpure titanium, but a significant reduction in plastic. Despite the strength ofpure titanium is not high, but the plasticity and toughness of the fine,especially with good low temperature impact toughness; At the same time hasgood corrosion resistance. Therefore, the material used for the chemical industry,petroleum industry, in fact, more than 350 ℃ for the following working conditions.According to the state of titanium at room temperature annealing organizations,titanium alloys can be divided into three types: α-type titanium alloy, (α + β)type titanium alloys and β-type titanium alloy. α-titanium alloy, theapplication is more TA4, TA5, TA6-based Ti-AI alloys and TA7, TA8 type Ti + AI+ Sn alloy. This alloy at room temperature, the intensity can be achieved931N/mm2, but at a high temperature (500 ℃) in the performance of stability and goodweldability. β-type titanium alloy application in China is low, the scope ofits application to be further expanded.

Second, the welding of titanium and titanium alloy.    

Titanium and titanium alloy welding performance, hasmany notable characteristics, the characteristics of these welding of titaniumand titanium alloy as a result of physical and chemical properties of thedecision.

1. Gas and impurities in the performance of pollution on theeffects of welding  

At room temperature, titanium andtitanium alloy is relatively stable. However, when the pilot, in the weldingprocess, the liquid droplet and molten pool of metal with a strong absorptionof hydrogen, oxygen, nitrogen’s role, but also in solidstate, these gases haveoccurred in their role. With increasing temperature, titanium and titaniumalloy to absorb hydrogen, oxygen, nitrogen also significantly increased thecapacity of about around 250 in the absorption of hydrogen, from the beginning of 400 ℃ toabsorb oxygen, from 600 to start to absorb nitrogen, these gases be absorbed, will be a direct resultof emvierment of welded joints, welding quality is affected very important factor.  

(1) the impact of hydrogen is hydrogen gas impurities on themechanical properties of titanium of the factors that most affected. Changes inhydrogen weld weld impact properties of the most significant impact, mainly because of joints with increased hydrogen content, the weld plate orneedle-like precipitation increased TiH2. TiH2 low intensity, the needle likesheet or the role of Guardian cases HiH2 to the gap, with a significantreduction in impact properties; weld volume change on the strength of hydrogenincreases and the reduction of the role of plastic is not very significantwhen.  
 
(2) the effects of oxygen in the titanium oxide phase of the α and β would havea higher degree of melting and the formation of deep-space solid phase, theright to use the  crystal titanium wound seriously distorted, so as toenhance the hardness of titanium and titanium alloy and intensity, so that wasa significant reduction in plastic. In order to ensure the welding performanceof the get-away, in addition to the course in welding and the welding seam toprevent heat-affected zone by the main fat oxidation, at the same time shouldalso be restricted in the basic metals and the oxygen content of wire.

(3) the effects of nitrogen in more than 700 ℃ hightemperature, nitrogen and plays the role of the occurrence of titanium to formtitanium nitride hard brittle (riN) and the formation of nitrogen and titaniumsolid solution when the gap caused by the degree of lattice distortion andNorway, than is the amount of oxygen is even more serious consequences,therefore, nitrogen weld titanium to improve the tensile strength, hardness andreduce the plastic weld performance significantly more than oxygen.  

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4) the impact of carbon-carbon titanium and titanium alloy isalso a common impurity in the experiment showed that when the carbon content of0.13%, the carbon from deep in the α-Ti,  the weld strength, some increasethe limit, plastic drop some, but not oxygen The role of nitrogen strongly.However, when further increase the carbon content of weld time, weld mesh thereTiC, their number increased with the increase in carbon content, so thatplastic weld the sharp decline in the role of stress in the weld cracks easily.Therefore, the titanium and titanium alloy base metal carbon content of notmore than 0.1% carbon content weld does not exceed the carbon content of parentmaterial.  
         
2. Welded joints crack problem  

Titanium and titanium alloy welding,the welded joints have a very small possibility of thermal cracks, because oftitanium and titanium alloy in the S, P, C and so little impurity content, from S, P to form the low melting point eutectic not appear in thegrain boundary, in addition to a small effective range of crystallizationtemperature, titanium and titanium alloy solidification contraction of a smallamount of weld metal will not produce hot cracking. Time of titanium andtitanium alloy welding, heat affected zone can be cold crack, characterized bycracks in the solder a few hours or even longer after the delay, known ascrack. After research had shown that such cracks in the welding process and thespread of the hydrogen bomb. Welding process of hydrogen from high-temperaturedeep pool to the lower temperature heat affected zone of proliferation,increase in hydrogen content to make the area TiH2 increased precipitation,increased brittle heat-affected zone, and precipitation as a result of hydridevolume expansion when the stress caused by the larger , together with thehydrogen atom to high-stress parts of the area and gathered the proliferation,resulting in the formation of cracks. Cracks to prevent this delay approach isto reduce the main source of hydrogen welded joints, invoices, they stay deepspace to suppress the fire to deal with.

3. Weld issues of stomatal  

Welding of titaniumand titanium alloy, the porosity is a problem often encountered. The rootcauses of the formation of pores is due to the impact of the results ofhydrogen. Weld metal porosity mainly affects the formation of the fatiguestrength of joints. Process to prevent the generation of porosity measures are:(1), the protection of pure neon gas purity should not be less than 99.99% (2),complete removal of surface welding, welding on the surface oxidation oforganic matter such as skin oil. (3), imposed on a good pool of gas protection,control of traffic flow and argon flow rate, to prevent the generation ofturbulence phenomena, the impact of the protective effect. (4), the correctchoice of welding process parameters to increase the residence time of deep poolof the right to use the bubbles escape, to be effective in reducing theporosity.  
     
Third, titanium tungsten plate manual TIG welding test  

Welding oftitanium and titanium alloy is the most widely used in production of tungstenplate welding, argon welding method of filling a vacuum applications are alsocommon. Argon arc welding in air protection and cooling effect, the arc heat ismore concentrated, high current density, heat-affected zone of small,high-quality welding.  

1. Of titanium and titanium alloywelding, when the temperature is higher than 500 ℃ ~ 700 ℃, it is easy to absorb gas in the air, hydrogen andnitrogen, seriously affecting the quality of welding. Therefore, the welding oftitanium and titanium alloy, the temperature of the olten pool and theDepartment of a comprehensive letter (400 ℃ ~ 650 ℃ and above) ofthe weld area needs to be protected, for this purpose, titanium and titaniumalloy welding must take special protective measures, that is, the use of spraywelding moment of a larger size in order to expand the area of gas, when thenozzle is not sufficient to protect weld seam zone near the high-temperaturemetals, the great impact the protection of drag argon cover charge. Weld andthe near-seam zone is to protect the effectiveness of the color standard fin.Silver said that the protection of the best white, yellow, minor oxide, isgenerally permitted. Surface color should be consistent with the Table (1)provides.

 Taking into account the practicality of the project touse, efficient, and we were prepared to cover a simple drag. Figure (a), argongas from the inlet into the distribution pipes, through the distribution ofpore directly into the protected areas. This trailer cover, the protectiveeffect of welding is not a good weld is dark blue. According to analysis of thedistribution of air from the tube directly into the protected areas. Airflow isnot very uniform, smooth, so that the protection of poor high-temperaturesolder is oxidized Road. Therefore, we further improve the structure of thetrailer cover, Figure (b), argon gas from entering the distribution pipe intothe pores by the drag at the top under the back cover; through the perforatedplate, porous plate gas mainly from the role of screening and distribution, sothat argon gas flow more stable, better welding protection, welding River Roadwas silver or yellow. L drag cover charge for a long 40 ~ 100mm Material forbrass. Titanium and titanium alloy welding, the weld should also be noted thatthe protection of the north, taking into account the welding distortion, wehave adopted a slotted copper plate fixed charge approach to the protection ofargon, in order to weld the back line to the full protection, but also in worsein porous brass plus one, so that by the brass argon into the hole evenprotected areas, protection of the effect of a good weld back was silver-white.Tungsten plate manual TIG welding process and parameters of choice (1)pre-solder wire Weldment and surface quality of the mechanical properties ofwelded joints have a significant impact on clean-up must be rigorous. Iron andtitanium wire can be mechanical cleaning and chemical cleaning methods. 1) buttwelding machine according to the quality of cleaning or pickling do not ask formuch Weldment difficulties that can be used fine stainless steel wire brush orsandpaper to clean, but it is best to use carbide scraper yellow plate, removethe oxide film. 2) chemical cleaning before welding of the test piece and thefirst wire for pickling, acid lotion available HF5% + HNO335% melt water. Rinsewith clean water after acid washing and drying facilities immediately after thewelding. Or with acetone, ethanol, carbon tetrachloride, methanol and otherclean plate and both sides of the groove (with the 50mm), wire surface, thefixture in contact with the plate part. (2) the choice of welding equipmentdeposit of titanium and titanium alloy plate welding tungsten should be usedwith a downward trend in the characteristics of high-frequency DC arc weldingpower supply, gas delivery and the time delay of not less than 15 seconds, toavoid been welding oxidation, pollution. (3) the choice of welding materialpurity argon should not be less than 99.99%, dew point -40 ℃in the following, the mass fraction of the total impurities < 0.001%.When the argon pressure in the cylinder to 0.981MPa should stop using, toprevent the impact on the quality of welded joints. In principle, should bechosen with the same basic titanium metal wire, and sometimes in order to holdhigh-weld metal, but also may choose to use a slightly lower intensity than thebasic metal of the wire. (4) the choice of groove form of the principle ofminimizing the welding and welding of metal layers. With the rise of the numberof welding, weld the cumulative increase in inspiratory home, as well as affectthe performance of welded joints and welding of titanium and titanium alloy asa result of weld pool at a larger size, so the sample billing V-70 ~ 80 ° bevel. (5) sample group and the positioning of the welding in order to reduce thewelding distortion, welding position before welding, general weldingpositioning spacing 100 ~ 150mm, length of 10 ~ 15 mm. Positioning of the wireused in welding, welding parameters and conditions for gas shielded welding ofwelded joints should be the same. Gap 0 ~ 2mm, blunt edge 0 ~ 1.0mm. (6) choiceof welding parameters such as the table (2) below, we pass under the differentprocess of comparing the performance of welded joints, and explore the moreappropriate specification of the welding process.

    Technology (1), welding current of150A, 170A, 180A, this parameter measures welding, welded joint surface,showing a dark blue, gold plain that joint oxidation of the more serious, donot meet the technical requirements, this process desirable.  

    Technology (2), welding current isrelatively lower for the 120A, 150A, 160A, this parameter measures welding,weld surface showing a gold, purple, deep yellow, X-ray detection withoutflaws, but failed in the mechanical properties of bending test on welded jointsa significant reduction in plastic, not the technical requirements, thisprocess equally undesirable.  

     Technology (3), welding currentof 95A, 115A, 120A, this parameter measures welding, weld surface was white,light yellow, X-ray detection without flaws, but the mechanical properties ofbending test pass, also in line with the requirements of tensile strength,welding joint performance of the technical requirements, the more appropriatetechnology. Welding of titanium and titanium alloy, the tendency to have largecrystal material, a direct impact on the mechanical properties of weldedjoints. Therefore the choice of welding parameters to take into account notonly the weld metal oxidation and the formation of pores, grain coarseningshould also consider factors, it should be smaller as far as possible thewelding heat input, process (1), (2), as a result of welding larger factors,resulting in joint oxidation than the process (3) serious. Metallographic andmicro-experimental results show that the degree of coarse grain joints than theprocess (3) serious. Therefore, poor mechanical properties of weldedjoints.  

    The choice of gas flow in order toachieve a good protective effect in which the flow is too large is not easy toform a stable laminar flow, and increasing the cooling rate of weld to weldsurface layer of α phase occur more frequently, as well as micro-cracks caused.Trailer cover of argon gas flow is low, showing a different weld coloroxidation; and when traffic is particularly heavy, the main nozzle willinterfere with the role of air currents. Weld the back flow of argon gas cannot be too much, otherwise they will affect the first layer of a positiveprotective effect of gas weld. (7) of titanium and titanium alloy GTAWoperation essentials 1) hand-welding, the welding wire and welding piecesshould be maintained between the smallest angle (10 ~ 15 °). Pool along the frontof a smooth wire, even into the pool may not be out of the end wire argonprotected areas. 2) welding, the torch is not for the basic horizontal swing,when the need to swing, the lower the frequency, magnitude and it is not toomuch swing in order to prevent the impact of the protection of argon. 3)breaking arc and weld the end, it is necessary to continue to pass theprotection of argon gas until the weld metal and heat affected zone cooled tobelow 350 ℃torch can be removed. (8) the quality of test 1) to check compliance with theappearance of GB/T13149-91. 2) deep injury-ray consistent with JB4730-94. 3) inline with the mechanical properties of test GB/T13149-91.

 
Fourth, the conclusions  

1, titanium and titanium alloy weldinggas protection issues that affect the quality of welded joints of the primaryfactors.  

2, titanium and titanium alloy weldingshould be small as far as possible heat input.  

3, TA2 GTAW should strictly control thesource of hydrogen to prevent the emergence of cold cracking at the same timeshould pay attention to prevent the emergence of stomata.  

4, as long as the welding process instrict accordance with the requirements of welding facilities, and to takeeffective measures to protect the gas, you can access high-quality weldedjoints.