Titanium and titanium alloy welding technology:
Titanium and titanium alloy have good corrosion resistance, in oxidation, neutral and chloride ion medium, its corrosion resistance is better than stainless steel, sometimes even ordinary austenitic stainless steel 1Cr18Ni9Ti 10 times.Industrial pure titanium has good plasticity, but low strength, good low temperature performance, its linear expansion coefficient and thermal conductivity are not big, which will not bring difficulties to welding.Titanium alloy has high specific strength, good toughness and weldability. It is widely used in aerospace industry.Titanium and titanium alloy are classified into alpha titanium alloy, beta titanium alloy and alpha + beta titanium alloy according to their annealed structure in the current standards of China, respectively expressed by TA, TB and TC.In the petrochemical industry in the pressure vessel equipment, the brand TA2 industrial pure titanium is used in the majority.
Welding characteristics of titanium and titanium alloy
(1) the contamination of impurities causes embrittlement
Titanium is an active element, especially in the welding temperature is very easy to absorb nitrogen, hydrogen, oxygen, so that the hardness and strength of the weld increased, plastic, toughness decreased, resulting in embrittlement.Carbon will also form hard and brittle TiC with titanium, which is easy to cause cracks.Therefore, titanium and titanium alloy welding must be effective protection against air or other factors pollution.Therefore, the welding of titanium and titanium alloy cannot be carried out by gas welding or electrode arc welding. Otherwise, the joint cannot meet the welding quality requirements. Generally, argon protection or welding under vacuum can be used.
(2) welding phase change caused by the plastic decline of the joint
Commonly used industrial pure titanium for alpha alloy, welding due to the poor thermal conductivity of titanium, small specific heat, high temperature residence time long, slow cooling speed, easy to form a thick crystal;If the use of accelerated cooling, and easy to produce acicular alpha tissue, will also reduce the plasticity.
(3) produce welding cracks
The probability of welding thermal cracks in titanium alloy welding is very small. Only when the quality of welding wire or base material is not a problem can thermal cracks occur.The cold crack caused by hydrogen should be avoided during the welding of titanium alloy. During the welding, hydrogen in the weld pool and the base material in the low temperature zone diffused to the heat affected zone, resulting in the increase of hydrogen content in the heat affected zone and the occurrence of delayed cracks in the heat affected zone.
Porosity is the most common welding defect in titanium and titanium alloy.Pores will be generated if the surface of welding wire or base material is not cleaned cleanly or argon is not pure. Therefore, the purity of shielding gas-argon shall be above 99.99%. The surface of welding wire and workpiece shall be pickled and cleaned with clean water before drying.
Argon tungsten arc welding of titanium and titanium alloy
Tungsten electrode argon arc welding is the most widely used in the welding of titanium and titanium alloy. For thicker workpiece, it can also be used in the melting electrode argon arc welding. For some important equipment in the aerospace industry with strict technical requirements, vacuum electron beam welding is often used.
(1) the selection of welding wire.The selection of welding wire should keep under normal welding seam after welding of the state of the tensile strength of not less than the parent metal annealing standard tensile strength limit state, the state of the weld after welding plastic and corrosion resistant performance of parent metal or not under annealing conditions with the parent metal, welding performance is good, can satisfy the requirement of titanium vessel manufacture and use.
The upper limit of the standard content of nitrogen, oxygen, carbon, hydrogen, iron and other impurities in the welding wire should be much lower than the limit of the standard content of impurities in the parent material.It is not allowed to cut strips from the welded base material to be used as welding wire. The welding wire in appendix D of JB/T 4745-2002 titanium welding vessel shall be used as welding wire for titanium vessel.Welding wires with impurity elements not higher than the other standards listed in appendix D of JB/T 4745-2002 May also be used
In general, according to table 10-8, the corresponding welding wire brand can be selected according to the brand of the welding base material, and it can be verified by the welding process qualification of appendix B in JB/T 4745-2002.
Different grades of titanium welding, generally according to the corrosion resistance of better and lower strength level of the parent material to choose the wire materials.
(2) the selection of protective gas.The purity of argon used for welding shall not be lower than 99.99%, and the dew point shall not be higher than -50℃, and it shall conform to the provisions of GB 4842-1984.Bottle argon should not be used when the pressure is less than 0.5MPa.
(3) tungsten electrode.Cerium tungsten electrode is recommended for argon arc welding.The electrode diameter shall be selected according to the welding current, and the electrical extreme shall be conical.
The key to argon arc welding of titanium and titanium alloy is to separate the high temperature area of welding from the air. In order to effectively protect the welding torch, it is extremely important that the nozzle, the towing hood and the back protection device pass the argon gas with appropriate flow.Near and weld seam area color is a sign that the measure of protection, silvery white, buff said protection effect is good, deep yellow to light oxidation, under normal circumstances is allowed, gold and purple said moderate oxidation, dark blue said serious oxidation, is absolutely not allowed, as for the gray said weld has been bad, must be discarded rewelding.