Main defects and repair methods of titanium alloy welding
(1) When titanium and titanium alloys are welded, the possibility of hot cracking in the welded joints is very low because the content of impurities such as S, P and C in titanium and titanium alloys is very small and the low-melting eutectic formed by S and P Not easy to appear on the grain boundaries, combined with a narrow range of effective crystallization temperature, titanium and titanium alloy shrinkage is small, the weld metal does not produce hot cracks. However, when the titanium and titanium alloys are welded, cold cracking may occur in the HAZ, which is characterised by the occurrence of cracks within a few hours or more after the welding, as delayed cracks. During the welding process, hydrogen diffuses from the high temperature deep pool to the lower temperature heat-affected zone. The increase of hydrogen content increases the number of TiH2 precipitated in the zone and increases the heat affected zone embrittlement. In addition, due to the volume expansion of hydride precipitates,, Coupled with hydrogen atoms to the high-stress areas of the region to spread and gather, resulting in the formation of cracks.
(2) when titanium and titanium alloy welding, stomata is often encountered problems. The basic reason for the formation of stomata is the product of the hydrogen effect. The formation of pores in the weld metal mainly affects the fatigue strength of the joint. Hydrogen is the main cause of bitter cracks and stomatal formation. Because hydrogen is less than 300 C, the solubility in the α phase is small and the limit solubility is only 0.002% at room temperature. When the weld or HAZ is cooled below 300 C after welding, supersaturated hydrogen is precipitated as titanium hydride (γ phase). Volume increases and produces intergranular stress, the stress development can cause intergranular micro-cracks. Intergranular microcracks will expand into cracks under the action of external stress.
Determination of repair methods
Titanium alloy welding, when the temperature is higher than 500 ~ 700, it is easy to absorb oxygen in the air, hydrogen and nitrogen, seriously affecting the quality of welding. Therefore, the titanium alloy welding, weld pool full and high temperature parts (400 ~ 650 above) weld zone must be strictly protected. To this end, titanium and titanium alloy welding must take special protective measures. Therefore, the use of argon arc welding method and the use of spray size larger welding torque to expand the area of Gas protection zone, when the nozzle is not enough to protect the weld seam and near seamless high temperature metal, the need to add argon protection dragging.
Welding preparation and groove selection
(1) The surface quality of weldment and welding wire has a significant influence on the mechanical properties of welded joints. Before welding can be the initial test pieces and wire pickling. Rinse with water, immediately after drying welding. Titanium plate bevel and its sides (within 50mm, respectively) were wiped with acetone, ethanol, carbon tetrachloride, methanol, etc., the surface of the wire, and the part of the tool holder in contact with the titanium plate.
(2) the choice of welding equipment [3]. Titanium and titanium alloy argon arc welding should be accompanied by a decline in the characteristics of high-frequency arc ignition of the DC TIG welding power source, and delaying the time of not less than 15 s, to avoid welding oxidation, pollution. Therefore, using WSM-315 IGBT inverter DC pulse welding machine.
(3) the choice of welding materials. Argon purity should not be less than 99.99%, dew point below -40, relative humidity less than 5%. When the argon cylinder pressure drops to 0.981 MPa, it should be abandoned. Filler wire generally uses the same material, in order to improve the plasticity of the joint, available welding wire TC3 slightly lower than the base metal alloy, the welding wire: TC3.
(4) level forms of choice. In principle, attempt to reduce the number of welding layers and welding metal. As the number of welding layers increases, the cumulative amount of suction in the weld seam increases, influencing the performance of the welded joint. And because of welding titanium and titanium alloy weld pool larger size, so the weldment V-type 70 ~ 80 bevels.
The correct choice of welding process parameters completely removes the weldment surface, wire surface oxide, oil and other organic matter. Control of argon flow and flow rates to prevent turbulence, affecting the inflatable protective effect. The use of manual TIG welding titanium welding crack approach feasible, you can get satisfactory results.
