{"id":251,"date":"2022-05-29T01:57:00","date_gmt":"2022-05-29T01:57:00","guid":{"rendered":"https:\/\/mewelding.com\/?p=251"},"modified":"2024-03-18T09:39:40","modified_gmt":"2024-03-18T09:39:40","slug":"welding-titanium","status":"publish","type":"post","link":"https:\/\/mewelding.com\/welding-titanium\/","title":{"rendered":"Welding Titanium"},"content":{"rendered":"\n
Titanium is a useful metal that is used in a wide variety of applications due to its\u2019 excellent properties. It has low density, has good strength, has a lustrous silvery appearance, and has excellent resistance to corrosion. Welding titanium is considered a difficult prospect; however it can be done by following the requisite cleanliness guidelines.<\/p>\n\n\n\n
High strength and corrosion resistance are found in other metals too. But what distinguishes titanium apart from other metals is its\u2019 excellent strength to weight ratio. If you pick two bars: one made of steel, and other of titanium, you would notice that the titanium bar is almost only half of the weight of steel bar. Also, it can operate under a wide temperature range in service.<\/p>\n\n\n\n
Although it is an expensive metal, it is still economical to use it because it lasts for a long time in service. The repair and maintenance needs are not as much as other metals because it does not corrode like other metals.<\/p>\n\n\n\n
Titanium can be combined with other metal such as aluminum, vanadium, molybdenum, etc. to obtain strong light weight alloys for use in aerospace industry, military applications, as well as for industrial processes such as in petro-chemical industry, desalination plants, etc., and for medical applications. Besides this, it also finds use in mobile phones, sports equipment, and jewellery.<\/p>\n\n\n\n
Titanium is a very reactive metal. At room temperature, it combines with oxygen to form a layer of titanium oxide that exists as an impervious, passive coating on its\u2019 surface and prevents further reaction with the ambient conditions. This layer is responsible for the excellent corrosion resistance exhibited by titanium.<\/p>\n\n\n\n
However, from a welding point of view, titanium oxide is not a friendly entity. If mixed with weld metal, it creates discontinuities, and causes an unacceptable weld. It must therefore be removed by cleaning rigorously before welding. In ths article, we shall discuss a few aspects of welding titanium. Before that, let us understand about the commonly used grades of titanium.<\/p>\n\n\n\n
Titanium and its\u2019 alloys can be categorized into four: commercially pure titanium (containing no alloying content), alpha alloys, alpha-beta alloys, and beta-alloys. Some firms consider alpha alloys and commercially pure titanium in the same category.<\/p>\n\n\n\n
Alpha and beta denote the crystal structure of the metal at different temperatures. These structures have different properties that make them suitable for specific applications. However, by adding suitable alloying content, these structures can be made to exist at room temperatures as well. The crystal structure can be precisely controlled by adding suitable alloying elements such as vanadium, aluminum, iron, etc.<\/p>\n\n\n\n
The commonly used grades of commercially pure titanium are ASTM grade 1, 2, 3 and 4. They contain different amounts of oxygen and iron. As these elements, strength increases, but ductility decreases.<\/p>\n\n\n\n
Among these four, Grade 2 has the widest usage because of good corrosion resistance. The weldability of commercially pure titanium grades is good, the cost is less too, but the tensile strength is lower in comparison to other alloyed grades.<\/p>\n\n\n\n
Among alpha-beta alloys, grade 5 is the most widely used grade in all titanium alloys. This grade is made by adding aluminum and vanadium as alloying elements. This increases the tensile strength to almost 120 ksi. The industries that make use of this alloy are power generation, marine engineering, aerospace and off shore industry.<\/p>\n\n\n\n
However, due to the high strength, the formability of the metal is less, and weldability is slightly lower than grade 2.<\/p>\n\n\n\n
Grade 23 is another commonly used grade. It has slightly lower oxygen content. As a result the ductility of the metal is high, in exchange for only a slight reduction in strength.<\/p>\n\n\n\n
As discussed above, titanium is silvery in color, light-weight, lustrous in appearance. The corrosion resistance is very high, which makes it attractive to the designers. The strength to weight ratio is highest among all metals.<\/p>\n\n\n\n
It is very reactive metal, having high affinity for oxygen and other atmospheric gases at high temperatures. Small amounts of impurities make it brittle. Also, impurities raise the ductile to brittle transition temperature of the metal to quite close to the room temperature, which is undesirable. The impurities can enter the metal at high temperatures during welding, so every effort is required to keep the welding zone as clean as possible.<\/p>\n\n\n\n
At room temperature, a layer of titanium oxide can be found on the surface of the metal. This layer is responsible for the famed corrosion resistance of titanium. It is an impervious layer that prevents any further reaction between the metal and ambient media.<\/p>\n\n\n\n
The melting point of the oxide layer is higher than that of the metal. This creates some problems. If the oxide layer is not removed by cleaning before welding, and if some traces of the oxide enter the weld metal, they create discontinuities (not having melted) in the weld metal. This causes the weld joint to show unacceptable indications during radiography. Discontinuities in the weld metal also cause a decrease in the strength and ductility of the weld.<\/p>\n\n\n\n
Welding of titanium and its\u2019 alloys cannot be done by any processes that utilize active gases such as CO2 for shielding. Processes such as oxy-acetylene also cannot be used. The active gases in these processes cause a contamination in the weld metal, as we discussed above, and cause embrittlement of the weld metal.<\/p>\n\n\n\n
Welding of titanium is typically done by gas tungsten arc welding process, although gas metal arc welding is used as well. The welding procedures for titanium involve use of large gas nozzles not just at the torch, but also for trailing shielding and backing shielding. The use of all three shielding (torch, backing, and trailing) effectively shields the metal from any contamination.<\/p>\n\n\n\n
It is not just the weld metal that needs to be shielded from atmospheric gases, but the adjoining base metal too, to the extent where its temperature rises up to 1000\u00b0F (538\u00b0C) during welding, must also be shielded.<\/p>\n\n\n\n
The shielding gases protect the metal from contamination during<\/em> welding. However, the welder must ensure thorough cleaning before starting the welding as well, as only that can eliminate any possible contamination from the base metal itself.<\/p>\n\n\n\n 90% of all advice on welding of titanium revolves around cleanliness. Cleaning the base metal before welding, cleanliness in the atmosphere during welding, cleanliness of the welding wire, purity of the argon gas are therefore very important.<\/p>\n\n\n\n Proper cleaning of the base metal removes every possible contamination of the base metal. This contamination could be scale, oxides, or any other foreign material. Even a small amount of impurity in the weld metal can render it brittle at or near room temperature.<\/p>\n\n\n\n Cleaning strategy depends on the surface condition of the base metal. The base metal can be in three different conditions before welding:<\/p>\n\n\n\n First, no scales are present. Material is directly received from the mill. Cleaning of such a surface can be done by simple degreasing.<\/p>\n\n\n\n Second, light scales are present. This happens when the metal has been subjected to hot forming or annealing at intermediate temperature of less than 1300\u00b0F (704\u00b0C). Cleaning of such surfaces requires pickling. The pickling solution is nitric acid with a concentration greater than 20%. The pickling solution should be applied on the metal surfaces to be welded for 1 to 20 minutes, and then rinsed with water.<\/p>\n\n\n\n Third, heavy scales are present. This happens when metal has been subjected to hot forming, annealing or forging at high temperature.<\/p>\n\n\n\n Cleaning of surfaces having heavy scales has to be done with sand blasting or vapour blasting, or with molten sodium hydride salt baths, or molten caustic baths. Sodium hydride salts should be handled with extra care. Water should not come into contact with large amount of sodium hydride, because it causes formation of explosive hydrogen gas. Due to this reason, sand blasting or vapour blasting is preferred when possible.<\/p>\n\n\n\n Another reason why baths are not preferred is that the operation needs to be done at a high temperature of about 750\u00b0F to 850\u00b0F (399 to 455\u00b0C). Also, there is a chance of hydrogen pick-up during these baths.<\/p>\n\n\n\n The pickling time should be controlled when cleaning is done with salts, to avoid hydrogen pick-up. Pickling should not be done any longer than is necessary. Once heavy scales are removed, pickling with nitric acid should be done as described above for metal surfaces having light scale.<\/p>\n\n\n\n If the metal to be welded has undergone cutting by an oxy-acetylene flame, the presence of heavy thickness of oxides is a given. The cut surface may contain microscopic cracks that occur due to the surface having becomes brittle due to impurities.<\/p>\n\n\n\n The best method to make such a surface ready for welding is to remove the layers of oxides by machining<\/em>, followed by pickling as described above for metals having light scale.<\/p>\n\n\n\n Note:<\/p>\n\n\n\n Nitric acid is used for cleaning the surface before start of welding. Adequate precautions must be adopted while handling the acid. Fumes of the acid should not be inhaled. Acid should be used in a well-ventilated area only.<\/p>\n\n\n\n While diluting the acid, do not pour water in the acid. Instead, pour acid into the water. Do not stir rapidly. Go slow. Use PPEs such as gloves, apron, goggles, while handling acid.<\/p>\n\n\n\n If acid spills on the body, wash abundantly with cold water and seek medical attention immediately.<\/p>\n\n\n\n The range of welding processes suitable for welding titanium and its\u2019 alloys is very narrow. GMAW and GTAW process fit the requirements. Thorough shielding of weld metal and HAZ with an inert shielding gas is possible in both these processes, unlike other processes such as FCAW, SMAW, and SAW.<\/p>\n\n\n\n Welding with both manual method as well as automatic method is possible.<\/p>\n\n\n\n In GTAW welding, the tungsten electrode may be of 2% ceriated type or thoriated type. The size of the tungsten electrode should be sufficient to carry the welding current. The smallest size that will carry the specified welding current should be used.<\/p>\n\n\n\n For a welding current of < 125 amperes, a tungsten electrode of 1\/16 inch (1.6 mm) or lower is recommended. If the welding current is 125 amperes to 200 amperes, a tungsten electrode of 1\/16 inch to 3\/32 inch (1.6 mm to 2.4 mm) diameter may be used. If the welding current exceeds 200 amperes, the diameter of the tungsten electrode should be 3\/32 inch (2.4 mm) or 1\/8 inch (3.15 mm).<\/p>\n\n\n\n The preparation of the tungsten electrode should be done properly. The tip of the electrode should be taper down gradually to a point. The extension of the tungsten electrode beyond the nozzle may be 1.5 times the diameter of the electrode.<\/p>\n\n\n\n The generally used polarity for titanium alloys with GTAW is straight polarity (or DCEN polarity).<\/p>\n\n\n\n The selection of the filler rod depends upon the composition of the base metal. When pure titanium is to be joined, the filler rod is pure titanium too. When an alloy is welded, the filler metal chosen should have the strength of the alloy having next highest strength to the base metal being welded. In other words, the filler rod should be under-matching<\/em>.<\/p>\n\n\n\n The following table lists the recommended welding parameters when welding titanium with GMAW process.<\/p>\n\n\n\nWelding Titanium: Surface Preparation<\/h2>\n\n\n\n
MIG or TIG Welding of Titanium<\/h2>\n\n\n\n