Titanium. Weldability of titanium.
Titanium has a high resistance to temperatures of 450-500 degrees. Since at low density, high corrosion resistance in many corrosive environments, and is increasingly used as structural material in welded structures for various purposes. Separate or joint alloying of titanium with small amounts of technical some elements, such as 3-6,5% g1, 2% Mn , 3,5-4,5% V, 2.5 Cr, 2-3% Sn, greatly increases its strength (up to 100-140 kgf/mm2) (900-1400 MPa) with sufficient ductility. Titanium has a polymorphic transformation at a temperature of 882 deg. C and two allotropic forms: alfa-titanium with a hexagonal lattice at the temperature to 882 degrees. C and beta-titanium, with a body-centered lattice at temperatures above 882 deg. C. A number of elements, in particular aluminum, tin, nitrogen, oxygen, increase the temperature of polymorphic transformation, expanding the region of titanium and alfa-alfa-called stabilizers. Elements such as molybdenum, vanadium, manganese, chromium, help to preserve normal temperature structure of high beta-titanium, called beta-stabilizers. Depending on the combination of alloying components of titanium alloys can be alfa-alloys, beta-alloys and alfa + beta-alloys. alloys with a stable at different temperatures, the structure (technical titanium VT1, alloys VT5 and VT5-1) heat treatment is not hardening, so they have a good weldability. The alloys with a stable beta-structure (such as VT14) also have high thermal stability, high strength, ductility, and also well welded. Two-phase alloys, where the beta-phase exists at high temperatures (such as OT4), heat treatment is not hardening, and alloys, where the beta phase can be preserved by cooling to room temperature (alloys VT6, VT14), heat treatment and hardening welded worse, heat treatment (quenching + artificial aging) can bring the strength up to 140 kgf/mm2 (1400MPa) with satisfactory ductility. main difficulty welding of titanium related to its high chemical activity with respect to gases by heating and melting. Thus, at temperatures of 350 degrees. C and higher titanium absorbs oxygen with formation of structures introduction of high strength, hardness (can be 2 nelly higher than that of titanium) and low plasticity. Oxygen stabilizes alfa-phase and its interaction on the reaction of Ti + O2 = TiO2 to form a surface layer of high hardness which is called alfirovannym layer. When heated to a temperature of 550 deg. With vigor and higher titanium dissolves nitrogen, chemically reacts with it, as a result are often formed maloplastichnye implementation phase (nitrides): Ti + N2 = .5 or TIN 6Ti + N2 = 2Ti3N. Nitrogen, which is in the form of titanium nitride and in the interstitial elements, increases hardness and reduces ductility. The surface layer of titanium is saturated with high amounts of nitrogen and oxygen (alfirovanny layer). Contact this layer of particles in the weld metal leading to the fragility and the formation of cold cracks in connection with nothing before welding it must be completely removed. This sharp increase in strength and lower ductility caused severe restriction of permissible content of these gases in titanium: oxygen up to 0.15% and nitrogen to 0.05%. hydrogen even at low concentrations most strongly affects the properties of titanium. Although the hydrogen content decreases with increasing temperature, the hydrogen in a solid supersaturated solution is released and forms a separate phase - titanium hydride (TiH2), which strongly embrittle titanium and promotes the formation of cold cracks after a long time after welding (delayed fracture). In addition, hydrogen promotes the formation of pores. In connection with this circumstance allowable concentration of hydrogen in the metal is limited to 0.01%, and all necessary steps to eliminate the possibility of hydrogenation of the metal (eg welding wire is subjected to vacuum annealing). In some cases, the suitability of titanium welding pre-estimate of the value of the calculated hardness defining it by the empirical formula where [G] e - the equivalent of oxygen, [O] = E [% O] + 2 [% N] + 2/3 [% S] [O], [N], [C] - the percentage of oxygen in titanium, respectively, nitrogen and carbon. If HB <200 and the hydrogen content does not exceed 0.01%, Titanium Technology has a good weldability. negative effect of saturation of the heated and molten metal gas requires careful protection during swara only molten metal, but also sections of solid metal, heated to a temperature of 400 deg. C and above. Usually it is erased using fluxes, special gas nozzles and the use of the reverse side seam gas protective pads, flux and metal pads. Protection is considered reliable if post-weld metal surface has a shiny surface. Titanium and its alloys are susceptible to thermal cycle of welding, heating and cooling of the metal in the beta-phase grain growth is observed. This contributes to the low thermal conductivity of titanium. Upon cooling and aging can be formed brittle phase. As a result of these processes reduces the plastic properties of metal and there is inhomogeneity of the weld. When welding titanium and its alloys are used filler metal, similar in composition to the base metal. In many cases, positive results are obtained when using a wire VT1-00. To remove the wire is typically hydrogen diffusion (vacuum) annealing. The preparation of the edges are mechanically, oxy-fuel or plasma cutting, followed by removal of the metal rich gas machined edges. Before welding, the surface and the adjacent edges of the base metal and the electrode wire is carefully cleaned mechanically or by etching. Due to the low thermal conductivity of titanium butt joints in welding consumable electrode in argon have a distinctive conical shape with a deep penetration, which makes it necessary for some designs blend fillet sutures (stitches on the edges of the additional primary suture), or go to welding in helium in order to improve the external shape of the weld reinforcement (over a wide seam). The electrical resistivity of titanium is approximately 4 times greater than that of iron, so the flight consumable electrode should be relatively small.
See also:
Welding of Titanium Metal
Welding of Aluminum
Titanium
Welding Magnesium
Welding of Titanium
Copper
Welding of Copper
Refractory Metals
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