Titan has one of the two main stable phases (allotropic modifications), characterized by the structure of the crystal lattice:
- α-Titanium has a hexagonal crystal lattice (fine structure) at temperatures up to 882 ° C;
- β-Titanium with a body-centered lattice (coarse structure) at temperatures above 882 ° C.
A number of alloying elements and impurities, called α-stabilizers (aluminum, nitrogen, tin, oxygen, etc.), increase the temperature of polymorphic transformation of titanium and thus stabilize the α-phase and broaden the range of α-titanium. Alloying elements, called β-stabilizers (chromium, molybdenum, vanadium, manganese), help to preserve β-titanium with decreasing temperature. Depending on the composition of the alloy components titanium alloys are conventionally divided into α-alloys , beta alloys- and α + β-alloys (see figure below).
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| The dependence of the structure of titanium alloy on the temperature and the content of alloying elements |
For two-phase α + β-alloys are alloys OT4, VT3, VT4, VT6, VT8, VT14. Two-phase alloys with a small amount of β-stabilizers (for example, alloys OT4) is not thermally hardening and well welded, and alloys, where the β-structure can be preserved by cooling to room temperature (for example, alloys VT6 and VT14), heat-hardening and welded worse. Due to heat treatment (quenching and artificial aging), their strength can be increased up to 1400 MPa at satisfactory ductility.
Alloys with β-phase, for example, VT15, heat-hardened and welded worse. They are prone to grain growth and the emergence of cold cracking.
Difficulties in the welding of titanium and solutions
The main difficulties in the welding of titanium due to its high chemical activity with respect to gases (oxygen, nitrogen, hydrogen) during heating and melting.
At room temperature, titanium reacts with oxygen, stabilizing α-phase by the reaction of Ti + O 2 = TiO 2 to form a surface layer with high hardness - alfinirovannogo layer - which protects the titanium from further oxidation. When heated to a temperature of 350 ° C and higher titanium absorbs oxygen to form various oxides (from Ti 6 O to TiO 2 ) with high hardness, strength and low ductility. As the oxidation of oxide film changes color from yellow-gold to deep purple, passing into white. These colors are in the weld zone characterize the quality of the protection of the metal during welding.
At temperatures above 500 ° C titanium actively reacts with nitrogen to form nitrides that increase hardness and strength of metal, but reduce the ductility. Before welding, you must completely remove the surface layer of titanium, the increased amount of saturated oxygen (alfinirovanny layer), and nitrogen, as in contact with particles of a given layer in the weld metal becomes brittle, cracks appear cold. Permissible content of nitrogen in titanium is up 0.05%, oxygen - up to 0.15%.
Hydrogen, even in small quantities significantly worsens the properties of titanium. He is actively absorbed by the titanium at a temperature of 200-400 ° C. As the temperature starts to separate hydrogen from titanium and burns. At lower temperatures, the hydrogen content is also reduced, but the titanium hydride TiH 2 contribute to the formation of pores and delayed fracture of titanium - the appearance of cold cracking after a long time after welding. Allowable concentration of hydrogen in titanium up to 0.01%.
Careful protection of the metal gas saturation is required not only for the molten metal, but also for areas of solid metal with a temperature of 400 ° C and above. This is usually achieved through the use of flux, and flux metal pads, special protective gas bags. On the evidence of reliable protection of the shiny surface of the metal after welding, the protection of the poor - the yellow-blue color, gray raids.
Welding of titanium and its alloys is performed filler metal, similar in composition to the base metal, for example, wire VT1-00. Usually, before welding wire is subjected to vacuum (diffusion) annealing to remove hydrogen. The edges are prepared by mechanical means, plasma or oxy-fuel cutting, followed by removal of a saturated gas metal edges machined. Surface and the adjacent edges of the base metal and filler wire is carefully cleaned by etching or by mechanical means.
Titanium has low thermal conductivity, and therefore the butt joints, resulting from the welding consumable electrode in an argon atmosphere, have a distinctive conical shape with a deep penetration. Therefore, for some designs require the imposition of additional seams on the edges of the main seam (fillet welds) or welding in helium to produce a wider weld.
The main methods of welding titanium and its alloys:
- arc welding with inert gas and non-consumable electrode melting;
- submerged arc welding;
- Electroslag welding;
- electron-beam welding;
- welding.
Arc welding of titanium in inert gases can be performed ittrirovannym nonconsumable tungsten electrode and lantanirovannym (manual or mechanized welding) and the consumable electrode (semi-automatic or automatic welding). As the inert gas argon is used premium, high-purity helium or a mixture of argon and helium.
Protection of metal in the welding process can be carried out in the following ways:
- on the air with an inert gas supply nozzles with special elongated nozzle (50 cm) to increase the protection zone and the gas supply from the back side of the weld through a special lining;
- on the air with the help of local chambers, nozzles, protective welding zone and the welded part of the site, while the reverse side of the seam can be protected by the gas through the liner;
- by placing all welded unit in a sealed chamber with a controlled atmosphere.
The most popular welding of titanium tungsten electrode in the air. It operates in the conventional apparatus for automatic argon-arc welding of non-consumable electrode with direct current straight polarity. At a special welding torch nozzle is secured to protect the inert gas from the air with the temperature of the metal parts of 250-300 ° C and above. The sizes of these sites are usually determined by calculations based on formulas of heat propagation in metals during welding. The best protection is achieved by placing the nozzle in the reticulate-porous material to provide a laminar flow of inert gas. Reverse side of the seam to protect the use of special nozzles and pads.
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| Welding of titanium non-consumable electrode in air (using special pads for supplying inert gas from the back side seam) |
TABLE. Suggested modes of argon welding of titanium sheet tungsten electrode (flow rate of argon through the burner 13-18 l / min, the reverse side of the weld - 2-2.5 l / min)
| Metal thickness (mm) | Diameter (mm) | Current (A) | Welding speed (m / h) | |
| tungsten electrode | filler wire | |||
| 0.3–0.7 | 1.6 | – | 40 | 55 |
| 0.8–1.2 | 1.6 | – | 60–80 | 40–50 |
| 1.5–2.0 | 2.0 | 2.0–2.5 | 80–120 | 35–40 |
| 2.5–3.5 | 3.0 | 2.0–2.5 | 150–200 | 35–40 |
When TIG welding of titanium thickness greater than 4 mm is commonly used V-shaped, X-shaped or ryumkoobraznaya cutting edges. To improve the performance of tungsten electrode welding, the following methods:
- submerged arc welding;
- penetration welding of cutting;
- pulsed-arc welding;
- Welding of flux;
- welding flux cored wire filler;
- Cutting a slit in the welding;
- Welding with magnetic stirring the weld pool
- and others
Using pulsed-arc welding (when the current supply to the zone of the arc is short-lived pulses) may be a wider range of joint resize, reduce the level of residual stresses, to reduce deformation of welded structures, reduce the heat-affected zone, as well as to reduce the crystallite size and porosity in the weld joint.
When welding on the flux-paste-type AN-TA caused to a thin layer on the surface of the workpiece edge, the lower currents can be welded without the cutting edge metal thickness up to 12 mm. This technology allows to increase the depth of penetration, reduce distortion of welded structures, change the shape of penetration, reduce the heat-affected zone, reduce the likelihood of pore formation and burn-through. Has the same advantages and welding with flux cored wire as a filler.
When welding with magnetic stirring of the metal weld pool with the help of an external magnetic field decreases the chemical heterogeneity and porosity of the weld metal. When welding in a slit cut (in the narrow gap) reduces the consumption of expensive and scarce materials and increases productivity.
Welding consumable electrode (wire) is satisfied when the thickness of titanium and its alloys are more than 3 mm in the lower position on the DC reverse polarity modes, which ensure the transfer of atomized metal electrode.
TABLE. Welding of titanium and its alloys, consumable electrode (wire) in inert gases
| The diameter of the electrode (mm) | Welding current (A) | Arc voltage (V) | The thickness of the butt joints, welded without any cutting edge (mm) | Welding speed (m / h) | Flight of the electrode (mm) | The gas flow rate (l / min) |
| in argon | ||||||
| 0.6–0.8 | 150–250 | 22–24 | 4–8 | 30–40 | 10–14 | 20–30 |
| 1.0–1.2 | 280–320 | 24–28 | 5–10 | 30–40 | 17–20 | 25–35 |
| 1.6–2.0 | 340–520 | 30–34 | 8–12 | 20–25 | 20–25 | 35–45 |
| 3.0 | 480–750 | 32–34 | 14–34 | 18–22 | 30–35 | 40–50 |
| 4.0 | 680–980 | 32–36 | 16–36 | 16–18 | 35–40 | 50–60 |
| 5.0 | 780–1200 | 34–38 | 16–36 | 14–16 | 40–45 | 50–60 |
| in helium | ||||||
| 0.6–0.8 | 150–250 | 28–32 | 4–6 | 30–40 | 10–14 | 30–40 |
| 1.0–1.2 | 280–320 | 32–36 | 4–8 | 30–40 | 17–20 | 35–45 |
| 1.6–2.0 | 340–520 | 38–40 | 5–10 | 20–25 | 20–25 | 70–90 |
| 3.0 | 480–750 | 42–48 | 10–28 | 18–22 | 30–35 | 80–100 |
| 4.0 | 680–980 | 46–50 | 12–32 | 16–18 | 40–50 | 100–120 |
| 5.0 | 780–1200 | 46–52 | 12–32 | 14–16 | 45–55 | 100–120 |
To improve the performance of welding titanium and its alloys, consumable electrode inert gas is used pre-heating wire passing current and pulse-arc welding (for instance, a semi-automatic with a decrease in heat input welding in the 2-2.5 fold increase in performance in 2 - 3 times), and welding in a slit cut (which allows to reduce the consumption of expensive materials).
Arc welding of titanium submerged
Titanium and its alloys can be welded under anoxic fluxes ANT 1, ANT-3 with a thickness of the metal 2,5-8 mm, and ANT-7 for the greater thickness of the metal. Before welding, flux calcined at a temperature of 200-400 ° C, so that moisture content did not exceed 0.05% by weight. Welding is performed using standard equipment on the DC reverse polarity.
The resulting welds are not inferior in strength and ductility of the base metal and have a more fine-grained structure than when welding in inert gases. This method is cost-effective metal thickness greater than 6-8 mm.
TABLE. Welding of titanium and its alloys, consumable electrode (wire) submerged ANT-1 (welding speed is 50 m / h)
| Metal thickness (mm) | Diameter of electrode wire (mm) | Current (A) | Voltage (V) | Wire feed speed (m / h) |
| one-sided welding on the remaining lined | ||||
| 2–2.5 | 2 | 190–220 | 34–36 | 167–175 |
| 4–4.5 | 2 | 300–320 | 34–38 | 221–239 |
| 4–5 | 3 | 310–340 | 30–32 | 95–111 |
| double-sided welding | ||||
| 8 | 3 | 310–370 | 30–32 | 135–140 |
| 10 | 3 | 340–360 | 30–32 | 150–155 |
| 12 | 3 | 350–400 | 30–32 | 160–165 |
| 15 | 3 | 390–420 | 30–32 | 175–180 |
In electroslag welding of titanium and its alloys are used plate electrodes of the same composition as the base metal, thickness of 8-12 mm and a width equal to the thickness of the workpiece. We use high-melting fluxes ANT-2 ANT-4, ANT-6, which must be pre-ignited at a temperature of 200-400 ° C to a moisture content in the flux did not exceed 0.05% by weight. To protect the cooling of the metal and slag bath from the air in the gap between the sliders and the water-cooled argon workpiece is fed at the rate of 5.12 l / min at a thickness of 30-120 mm of metal.
The resulting welds on properties close to the base metal and have a coarse texture. Electroslag welding of titanium parts for the effective thickness over 40 mm.
TABLE. Modes of electroslag welding of titanium forgings plate electrode with a flux of ANT-2 (voltage of 16-18 V)
| Metal thickness (mm) | The thickness of the plate electrode (mm) | The clearance (mm) | Current (A) |
| 30–50 | 8–10 | 23–25 | 1200–1600 |
| 50–80 | 8–10 | 23–25 | 1600–2000 |
| 80–100 | 10–12 | 24–26 | 2000–2400 |
| 100–120 | 10–12 | 24–26 | 2400–2800 |
Electron beam welding of titanium and its alloys provides fine-grained structure of the weld metal and the protection of the gases. It is used for thicknesses up to 160 mm. In some cases, to prevent the appearance of pores and discontinuities is applied to the welding of horizontal placement of the beam.
See also:
argon-arc welding (argon welding)
electroslag welding
welding wire
welding equipment
welding flux
Welding of metals:
welding of cast iron
welding of aluminum
welding of titanium
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