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Titanium plate welding technology and measures to prevent the formation of porosity

Author : jason Date : 6/14/2020 8:36:15 PM
 Titanium plate welding technology and measures to prevent the formation of porosity

 

1. Gas pollution

   (1) Prepare for welding. Strictly clean the surface of the weld to prevent the intrusion of hydrogen, oxygen and nitrogen.

   (2) Select an accurate argon gas flow meter to control the gas flow. The choice of gas flow rate is subject to the good protection effect. The flow rate of argon gas has a considerable impact on the protection. Excessive flow rate is not easy to form a stable gas flow layer. Instead, turbulent flow is formed in the protection zone, so that harmful gases are immersed in the melt. Pool, so that the surface of the weld is prone to micro cracks. Too little air flow makes the protection in place and fails to achieve the protection effect. When the flow of argon gas in the drag hood is insufficient, the welding seam shows a different oxidation color.

   (3) Strengthen weld protection. During welding, the end of the welding wire must not be moved out of the argon gas protection zone; when the arc is broken and the welding seam ends, the argon gas protection should be continued until the metal in the welding seam and heat affected zone cools below 100 before removing the welding gun.

2. Cracks in the welded joint

When titanium is welded, the possibility of thermal cracks in the welded joint is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is very small. The low-melting eutectic formed by S and P is not easy to appear at the grain boundary. In addition, the effective crystallization temperature range is narrow, the shrinkage of titanium and titanium alloys during solidification is small, and the weld metal will not generate thermal cracks.

  Titanium welding, cold cracks may appear in the heat affected zone, which is characterized by cracks generated within hours or even longer after welding. Studies have shown that this crack is mainly caused by the influence of carbon and hydrogen and excessive cooling rate. The way to prevent such delayed cracks is mainly to reduce the source of hydrogen and carbon in the welded joints, and to protect the weld area before welding to prevent contamination by harmful impurities.

   Secondly, the temperature between layers should be strictly controlled. Under the premise of ensuring good fusion, the welding should be performed with low heat input as much as possible, that is, the fusion ratio should be reduced.

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

3. Porosity in the weld

   Porosity is a defect that is easy to produce when welding titanium plates. The main reason is the result of the influence of hydrogen. The surface of the plate and welding material is not clean. The moisture and grease on the operator's gloves, the sand particles and the flying dust grinded by the angle grinder are all sources of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint.

  Technical measures to prevent the formation of pores are:

   (1) The protective argon gas should be pure, and the purity should not be less than 99.99%. The air guide tube should be reinforced plastic tube, not rubber tube.

   (2) Thoroughly remove the scale and other organic substances on the surface of the welding equipment and the surface of the welding wire.

   (3) Apply good gas protection to the molten pool, control the flow rate and flow rate of argon gas, prevent turbulence, and affect the protection effect.

  (4) Correctly select welding process parameters, increase the residence time of the weld metal in the weld pool, make the air bubbles escape, and effectively reduce the porosity.

   (5) Welding with a small heat input, preferably pulsed argon arc welding, can improve the joint plasticity, reduce overheating and coarse grains, reduce deformation, but also increase the penetration depth and reduce the occurrence of pores.

4. Preparation before welding

It is best to separate an area at the welding site of titanium. Non-professionals are not allowed to enter to protect the cleanliness of the area. All welding personnel should wear clean work clothes, knitted gloves or thin sheepskin gloves, and not cotton gloves. Welding in the tank, the shoes should be covered with shoe covers.

The inner and outer surfaces of the bevel and its sides within 50mm and the surface of the welding wire should be cleaned of oil, and the oxide film, burrs and surface defects should be removed by mechanical methods such as fine file austenitic stainless steel wire brush and milling cutter. The cleaning tool should be dedicated and maintained Clean; the surface after mechanical cleaning should be degreased with sulfur-free acetone or ethanol before welding. Chloride solvents such as trichloroethylene and carbon tetrachloride are strictly prohibited, and cotton fibers must not be attached to the groove surface. According to the technical requirements, test was carried out on the welding environment before welding, and an "iron ion" pollution test was carried out around the weld, and the welding was carried out after passing the test.

The quality of the cleaning will directly affect the generation of welding cracks. When the cleaning quality is not good, a gray-white getter layer will be formed on the base material and the surface of the welding wire, resulting in the generation of welding cracks and pores, so pay attention to when cleaning The following points:

   First pickling. The pickling solution is 2%~4%HF+30%~40%HN03+H20 (residual). Finally rinse with clean water and blow dry, scrub with acetone or alcohol before welding.

   The welding wire generally needs to be dehydrogenated after pickling, and then degrease with acetone before welding.

   Oxide degreasing is strictly prohibited.

   Do not wear rubber gloves when cleaning, so as to avoid the rubber and degreasing solvent react to make the welding seam produce air holes.

   welded joint after cleaning

   must be welded immediately, and should not be placed for a long time (generally not more than 4h), otherwise it needs to be re-cleaned according to the previous steps.

The protective cover used for welding titanium is very important. A skilled welder does not have a good protective cover. Even if the weld is beautifully welded, it will be oxidized and discolored after welding. This weld is still unqualified and must be scrapped.

5. Operation manual of titanium manual tungsten argon arc welding

   1) During manual argon arc welding, the minimum angle (10~15°) should be kept between the welding wire and the welding equipment. The welding wire is smoothly and evenly fed into the molten pool along the front end of the molten pool, and the end of the welding wire must not be moved out of the argon protection zone.

   2) During welding, the welding torch basically does not swing horizontally. When swinging is required, the frequency should be low and the swing amplitude should not be too large to prevent affecting the protection of argon gas.

   3) When the arc is broken and the welding seam ends, the argon gas protection should be continued until the welding seam and the metal in the heat affected zone have cooled to below 100 before removing the welding gun.

   4) The quality of the cleaning will directly affect the generation of welding cracks. When the cleaning quality is not good, a gray-white getter layer will be formed on the surface of the base metal and the welding wire, resulting in the generation of welding cracks and pores.

6. Basis for selection of process parameters

   1) If the welding current is large, the welding is very smooth, but the grains of the weld are thick and the performance is poor, and large residual stress is likely to occur around the weld. Therefore, the upper limit of the welding current should be controlled. When determining the current of the fillet weld of the cover plate, the first pass is self-melting and the second pass is adding wire. The welding current is 130-140A.

   2) Arc voltage selection The arc voltage is actually to choose the length of the arc. The commonly used argon arc welding machine does not have a voltage selection button. The arc should be as short as possible to control the arc voltage between 14-16V.

   3) Welding speed The thermal conductivity of titanium material during welding is very poor, and the current and speed must be matched well, otherwise the fusion is not good. After testing, when the welding current is 130-140A, the welding speed is 160-170mm/min is more appropriate.

   4) Determination of argon flow

  Titanium should be cooled in the argon protective atmosphere within 100mm of the weld area behind the weld pool (including the front and back of the weld and heat affected zone), otherwise the weld will be oxidized and discolored, which will degrade performance. After field test adjustment, the final determined argon flow rate is: nozzle 10-11L/min, holding cover 12-13L/min, backside 3-5L/min.

5) Titanium plate argon arc welding uses a DC argon arc welding power source with reduced external characteristics and high-frequency arc starting. The tungsten pole tip should be ground into a conical shape and horizontal rotation flat welding is used. The welding torch should be supplied with gas in advance, and the current attenuation device and gas delay protection device should be used when extinguishing the arc. The delayed gas delivery time should be no less than 15 seconds to avoid oxidation and pollution of the weld.

7. Assembly

   In order to reduce welding deformation, tack welding is required before welding. The tack welding seam should use the same welding materials and welding process as the formal welding, that is, the welding wire, welding process parameters and gas protection conditions should be the same as those of the welding joint. Before welding, remove the oxide layer, oil stain, moisture, rust, etc. on the surface of the welding wire and the welding groove and on both sides, and should be welded by a qualified welder. Position welding is welded on the base weld, and the length of the weld should be 10-15mm, the spacing is 100-150mm, and the height should not exceed 1/3 of the wall thickness. The gap is 0-1mm, and the blunt edge is 0-1.0mm.Welds must not have defects such as cracks, porosity, slag inclusions, and discoloration due to oxidation. When defects are found, they should be eliminated in time.

 

 

 

 

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