Problems encountered in welding titanium pipes and repair methods

The porosity is often encountered in the welding of titanium pipes. The root cause of the formation of stomata is the result of hydrogen influence. The porosity of weld metal mainly affects the fatigue strength of joint. Hydrogen is the main cause of cold crack and porosity formation. Because hydrogen is less than 300 ° C, the solubility in the alpha phase is very small, and the solubility at room temperature is only 0.002%. When the weld or heat-affected zone is cooled below 300 ° C after welding, the susaturated hydrogen is precipitated in the form of titanium hydride (gamma phase).

The volume increases and the intergranular stress is generated, and the development of this stress will cause intergranular micro-cracks, which will expand into cracks under the action of external stress. When titanium tubes are welded, the possibility of hot cracks in welded joints is very small, because the impurity content of S, P, C and other impurities in titanium tubes is very small, the low melting point eutectic formed by S and P is not easy to appear on the grain boundary, and the effective crystallization temperature range is narrow, the shrinkage amount of titanium tubes is small during solidification, and the weld metal will not produce hot cracks. However, when titanium pipes are welded, cold cracks may appear in the heat affected zone, which is characterized by cracks occurring several hours or even longer after welding called delayed cracks. During the welding process, hydrogen diffused from high temperature deep pool to low temperature heat affected zone, and the increase of hydrogen content increased the amount of TiH2 precipitation in the zone, which increased the brittleness of the heat affected zone. In addition, due to the volume expansion of hydride precipitation, greater structural stress was caused, and hydrogen atoms diffused and accumulated to the high-stress parts of the zone, resulting in the formation of cracks.

When welding titanium pipes and other materials, when the temperature is higher than 500~700 ° C, it is easy to absorb oxygen, hydrogen and nitrogen in the air, which seriously affects the welding quality. Therefore, when welding titanium alloy, all aspects of the weld pool and high temperature parts (400~650℃ above) must be strictly protected.

For this reason, special protective measures must be taken when welding titanium pipes. Therefore, argon arc welding method is adopted, and welding moment with larger spray size is used to expand the gas protection area. When the nozzle is not enough to protect the weld and high temperature metal near the seam area, argon protection drag cover should be added.

Correct selection of welding process parameters to remove the solder surface, the surface of the wire oxide, oil and other organic matter. Control the flow rate and velocity of argon gas to prevent turbulence and affect the effect of inflation protection. It is feasible to treat the crack of titanium alloy by manual argon tungsten arc welding, and satisfactory results can be obtained.

The selection of pre-welding preparation and groove is:
(1) Selection of welding materials. The purity of argon should not be less than 99.99%, the dew point is below -40℃, and the relative humidity is less than 5%. When the pressure in the argon cylinder drops to 0.981 MPa, it should be stopped. The filler wire is generally made of homogeneous material, in order to improve the joint plasticity, the welding wire Ti-5Al-4V titanium can be slightly lower than the base metal alloy, and the welding wire: Ti-5Al-4V titanium is used.

(2) The surface quality of welds and wires has a great impact on the mechanical properties of welded joints. The specimen and wire can be pickled before welding. Rinse with clean water and weld immediately after drying. Use acetone, ethanol, carbon tetrachloride, methanol, etc. to wipe the groove of the titanium plate and its two sides (within 50 mm respectively), the surface of the welding wire, and the part of the tool clip in contact with the titanium plate.

(3) Selection of welding equipment. The DC argon arc welding power supply with external descent characteristic and high frequency arc initiation should be selected for titanium tube argon arc welding, and the delayed gas delivery time should not be less than 15s to avoid oxidation and pollution during welding. Therefore, WSM-315 IGBT inverter DC pulsed argon arc welding machine is adopted.

(4) Selection of groove form. In principle, minimize the number of welding layers and welding metals. With the increase of the number of welding layers, the cumulative suction capacity of the weld increases, and even affects the performance of the welded joint. And because the welding pool size of titanium pipe welding is large, so the welding parts open a single V-type 70~80° groove.