(1)When titanium and titanium alloys are welded,the possibility of hot cracking in welded joints is very small.This is because there are few impurities such as S,P and C in titanium and titanium alloys.The eutectic with low melting point formed by S and P is not easy to appear on grain boundaries,and the effective crystallization temperature range is narrow.The shrinkage of titanium and titanium alloys during solidification is small,and the weld metal will not produce hot cracking.However,when titanium and titanium alloys are welded,cold cracks may occur in the HAZ,which is characterized by delayed cracks occurring within hours or longer after welding.During the welding process,hydrogen diffuses from high temperature deep pool to low temperature heat affected zone,and the increase of hydrogen content increases the amount of TiH2 precipitated in the zone and increases the brittleness of heat affected zone.In addition,the larger tissue stress is caused by the volume expansion of hydride precipitation,and the hydrogen atoms diffuse and accumulate to the high stress area,resulting in cracks.

(2)Porosity is a common problem in the welding of titanium and titanium alloys.The fundamental reason for the formation of stomata is due to the effect of hydrogen.The formation of pore in weld metal mainly affects the fatigue strength of the joint.Hydrogen is the main cause of cold cracking and pore formation.Because hydrogen is less than 300 C,its solubility in the alpha phase is very small,and its ultimate solubility is only 0.002%at room temperature.When the weld or heat-affected zone is cooled below 300 C after welding,supersaturated hydrogen precipitates in the form of titanium hydride(gamma phase).Volume increases and intergranular stress occurs,which leads to intergranular microcracks.Intergranular microcracks will expand into cracks under external stress.

Determination of Repair Method

When the temperature of titanium alloy is higher than 500~700 C,it is easy to absorb oxygen,hydrogen and nitrogen in air,which seriously affects the welding quality.Therefore,when titanium alloy is welded,the weld zone of the full weld pool and high temperature position(400~650 C above)must be strictly protected.For this reason,special protective measures must be taken when titanium and titanium alloys are welded.Therefore,the argon arc welding method is adopted,and the welding moment with larger size is used to expand the area of gas protection zone.When the nozzle is not enough to protect the high temperature metal in the welding seam and near seam area,the argon protective tow cover should be added.

Pre-welding preparation and groove selection

(1)The surface quality of welding parts and wires has a great influence 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 titanium plate and its sides(within 50 mm,respectively),welding wire surface,tool holder and the contact part of titanium plate.

(2)Selection of welding equipment[3].DC argon arc welding power source with descending external characteristics and high frequency arc initiation should be selected for titanium and titanium alloy argon arc welding,and the delay time is not less than 15 seconds,so as to avoid being oxidized and polluted during welding.So WSM-315 IGBT inverter DC pulse argon arc welding machine is adopted.

(3)Selection of welding materials.The purity of argon should not be less than 99.99%,the dew point should be below-40 C,and the relative humidity should be less than 5%.When the pressure in the argon cylinder drops to 0.981 MPa,it should stop using.In order to improve the plasticity of the joints,the filler wire TC3,which is slightly lower than that of the base metal alloy,is usually made of homogeneous material.In this welding,the filler wire TC3 is used.

(4)Choice of groove form.In principle,minimize the number of welding layers and welding metals.With the increase of the number of welding layers,the accumulative gas absorption of the weld increases,which even affects the performance of the welded joint.Because the weld pool size of titanium and titanium alloy is large,single V-shaped 70-80 degree groove is used.

Correct selection of welding parameters can thoroughly remove the surface of welding parts,oxide skin and oil stain on the surface of welding wire.The flow rate and velocity of argon should be well controlled to prevent turbulence and affect the effect of gas-filled protection.It is feasible to treat cracks by manual TIG welding of titanium alloy,and satisfactory results can be obtained.

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