Gas pollution problem  

Get ready for welding. Strictly clean the weld surface to prevent the invasion of hydrogen, oxygen and nitrogen.

Accurate argon flow meter was used to control gas flow. The gas flow rate is selected to achieve a good protection effect. The flow rate of argon has a considerable influence on the protection. An excessive flow rate is not easy to form a stable airflow layer, but on the contrary, a turbulent flow is formed in the protection zone, which makes the harmful gas immersed in the molten pool and makes more micro-cracks on the weld surface. The protection is not in place due to too small airflow, and the protection effect cannot be achieved. When the argon gas flow in the towing cover is insufficient, the welding seams show different oxidation colors.

Strengthen protection on weld seam. During welding, the end of the welding wire shall not be removed from the Argon gas protection zone; When the arc is broken and the welding seam is closed, argon gas protection shall be continued until the welding seam and the metal in the heat-affected zone are cooled below 100℃.

Weld joint crack problem

Titanium welding, the welding joint of hot cracking formation of possibility is very small. It is because the titanium and titanium alloy in the impurities such as S, P, C, and so formed by S, P, low melting eutectic difficult in grain boundary, combined with effective crystallization temperature range is narrow, titanium and titanium alloy solidification shrinkage is small, weld metal does not produce hot crack.

During titanium welding, cold cracks may appear in the heat-affected zone, which is characterized by cracks occurring several hours or even longer after welding. The results show that the cracks are mainly caused by the influence of carbon and hydrogen and the rapid cooling rate. The way to prevent this kind of delayed crack is mainly to reduce the source of hydrogen and carbon in the welding joint, and to protect and clean the weld area before welding to prevent the contamination of harmful impurities.

Secondly, the temperature between layers should be strictly controlled. On the premise of good fusion, low heat input is used to reduce fusion ratio as far as possible.

Use small diameter wire, low welding current, narrow bead technology, fast welding. It is best to control the cooling rate around 100℃/s.

Porosity in welds

The porosity is a relatively easy defect caused by titanium plate welding, mainly due to the effect of hydrogen. The surface of plate and welding material is not clean, the water and grease on the operator's gloves, sand grains and flying dust under the angle grinder are all sources of hydrogen. The porosity of weld metal mainly affects the fatigue strength of joint.

The technological measures of process flow to prevent the formation of pores are as follows:  

1. The protection neon gas should be pure, purity should not be less than 99.99%, airway should be reinforced plastic tube, not rubber tube. 

2, Thoroughly remove the surface of welding parts, welding wire surface oxide oil and other organic matters.  

3. Provide good gas protection for the molten pool and control the flow rate and flow rate of argon to prevent turbulence and affect the protection effect. 

4. Correct selection of welding process parameters, increase the residence time of weld metal in the weld pool, make bubbles escape, and effectively reduce pores.

5. Small heat input is used in welding, and pulsed argon arc welding is better, which can not only improve the plasticity of the joint, reduce overheating and coarse crystal, and reduce deformation, but also increase the depth of fusion and reduce the generation of pores.

Weld preparation

The titanium welding site should be divided into a separate area, non-professional personnel are not allowed to enter, in order to protect the area clean, all welding personnel should wear clean work clothes, wearing knitted gloves or thin sheepskin gloves, can not wear cotton gloves, enter the tank welding, shoes should be covered.  

The inner and outer surfaces of the groove and its two sides within 50mm and the surface of the welding wire shall be cleaned of oil stains. The oxidation film, burr and surface defects shall be removed with fine file austenitic stainless steel wire brush, milling cutter and other mechanical methods. After mechanical cleaning, the surface should be degreased with acetone or ethanol without sulfur before welding. It is forbidden to use chlorinated solvents such as trichloroethylene and carbon tetrachloride. Cotton fiber shall not be attached to groove surface. According to the technical requirements, the "Ferrion" test was carried out on the welding environment before welding, and the "iron ion" pollution test was carried out around the welding seam.

The quality of cleaning will directly affect the occurrence of welding cracks, when the quality of cleaning is not good, it will generate a layer of gray suction layer on the surface of the base material and welding wire, leading to the occurrence of welding cracks and pores, so pay attention to the following points:

1. Use pickling first. The acid lotion was 2%~4%HF+30%~40%HN03+H20(margin). Finally, rinse with clean water and blow dry. Before welding, scrub with acetone or alcohol.

2. Welding wire in general need to be dehydrogenated after pickling, before welding with acetone degreasing.

3. Do not use oxide degreasing.

4. Do not wear rubber gloves when cleaning, in order to avoid rubber and degreasing solvent reaction so that weld porosity.

5. Cleaned solder joint

The welding must be finished immediately, and should not be put for a long time (generally not more than 4h), otherwise it needs to be cleaned again according to the previous steps.

The protective cover used in titanium welding is very important. A skilled welder, without a good protective cover, even if the weld welding again beautiful, all oxidation after welding color, the weld is still unqualified, and to scrap.

Operation essentials of titanium manual argon tungsten arc welding 

1. In manual argon arc welding, the minimum Angle between welding wire and welding parts should be kept as far as possible (10-15 °). The welding wire is sent into the molten pool smoothly and uniformly along the front end of the molten pool. The welding wire end shall not be removed from the argon gas protection zone.

2. During welding, the welding gun does not swing laterally. When it needs to swing, the frequency should be low and the swing amplitude should not be too large to prevent the protection of argon.

3. When the arc is broken and the welding seam is closed, argon gas protection shall be continued until the welding seam and the metal in the heat-affected zone are cooled below 100℃.

4. The quality of cleaning will directly affect the occurrence of welding cracks. In case of poor cleaning quality, a gray suction layer will be generated on the surface of the base material and welding wire, leading to the occurrence of welding cracks and pores.

Selection basis of process parameters  

1. If the welding current is large, the welding is very smooth, but the grain size of the weld is large, the performance is poor, and the residual stress around the weld is easy to be generated. Therefore, the upper limit of welding current should be controlled. When determining the current of fillet weld of cover plate, the welding current should be 130-140A after the first self-melting and the second wire addition.

2. Arc voltage is actually to select the length of arc. Common argon arc welders do not have voltage selection buttons, so the arc should be as short as possible to control the arc voltage between 14-16V.

3. Titanium has poor thermal conductivity in welding, so the current and speed should be matched well, otherwise the fusion is not good. According to the test, when the welding current is 130-140A, the welding speed is 160-170mm/min.

4. Determination of argon gas flow

During welding, the titanium welding zone behind the welding pool within 100mm (including the weld and both sides of the heat affected zone) shall be cooled in the atmosphere of argon gas protection, otherwise the welding seam will be oxidized and change color, which will degrade the performance. After field tests, the final flow rate of argon gas was determined as follows: 10-11l /min for the nozzle, 12-13L/min for the housing, and 3-5l /min for the back. 

Argon arc welding of titanium plate adopts DC argon arc welding power source with descending external characteristics and high frequency arc starting. The tungsten extreme part should be ground into a cone, and horizontal rotation flat welding should be adopted. The torch should be supplied with air in advance, and the current attenuation device and gas delay protection device should be used when the arc is extinguished. The delay time of gas transfer should be no less than 15 seconds to avoid oxidation and pollution of the welding seam.

Assembly  

In order to reduce welding deformation, positioning welding shall be carried out before welding, and the positioning weld shall adopt the same welding materials and welding process as the formal welding, that is, the welding wire, welding process parameters and gas protection conditions shall be the same as the welding joint. Before welding, remove oxide layer, grease, moisture, rust, etc. on the surface of welding wire, groove and both sides within 20mm, and shall be welded by qualified welders. Position welding shall be performed on the base weld, the weld length shall be 10-15mm, the spacing shall be 100-150mm, and the height shall not exceed 1/3 of the wall thickness. clearance 0-1mm and blunt edge 0-1.0mm. The weld shall not have defects such as cracks, pores, slag inclusion and oxidation discoloration, and shall be eliminated in time when defects are found.


Welding technology of titanium plate and measures to prevent the formation of pores
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