一、factors affecting the welding quality of titanium materials            

1.Influence of gas impurities on weld metal performance            

Titanium has high chemical activity and high affinity with oxygen and nitrogen in the air. At a lower temperature, titanium interacts with oxygen to form a compact oxide film. With the increase of temperature, the thickness of oxide film thickens. When the temperature is over 600 ℃, titanium begins to absorb oxygen and dissolves oxygen into titanium. If the temperature is higher, the activity of titanium will increase sharply and react violently with oxygen to form titanium oxide. Titanium begins to absorb hydrogen above 300 ℃ and nitrogen above 700 ℃. The result of titanium contamination by oxygen and nitrogen is that the strength and hardness of titanium increase while the plasticity decreases. The effect of nitrogen is greater than that of oxygen. The content of hydrogen in titanium from 0.01% to 0.05% will make the impact toughness of weld metal decrease sharply, but the plasticity will decrease less. This is the brittleness caused by hydride, which is often called "hydrogen brittleness". Hydrogen is also the source of weld porosity.            

In the process of melting and welding, the molten pool is like a small metallurgical furnace, and the molten metal is exposed to the atmosphere. If no corresponding protective measures are taken to isolate the molten titanium from the air, oxygen, nitrogen, hydrogen and other gas elements will melt into the titanium, forming brittle oxides or nitrides, which will sharply reduce the plasticity of the weld metal, improve the tensile strength, and in serious cases, brittle fracture will occur, with plasticity equal to zero.

2. Influence of other impurities on weld metal performance            

Other impurities refer to the impurities that may melt into the molten pool except for gas impurities. Its sources may be unclean welding operation environment, greasy dirt left by touching titanium weldment with dirty gloves, joint scrubbing with cotton yarn before welding, cotton wool left by groove, rust, moisture and other organic matters produced by mixing welding production environment with steel welding production, etc. These pollutants decompose oxygen, hydrogen, nitrogen, carbon and other elements under the action of arc high temperature, and then dissolve in the molten titanium. When the amount of these elements exceeds the solubility in titanium, the corresponding compound (TiO2 TiH2 tin TIC) is formed. These compounds enter into the lattice of titanium with the crystallization of the molten pool, which causes the lattice distortion and distortion of titanium, thus changing the mechanical properties of titanium.            

Some trace elements can be dissolved into titanium in small amount. If the amount does not exceed the allowable range, it is possible, and sometimes we hope. However, excessive impurity element content is not allowed, especially organic impurity, which is harmful to all but useless. This is because these impurity elements not only make the mechanical properties of titanium welding worse, reduce and corrode, but also are the source of porosity in the weld.

3. Microstructure change of weld metal and heat affected zone of joint            

Titanium is a metal with allotrope transformation. At 882.5 ℃, the solid-state transformation of the tissue began. The crystal structure below 882.5 ℃ is close packed hexagonal structure, which is called α - titanium. When the temperature is higher than 882.5 ℃, the α - structure of titanium changes into the body centered cubic structure of β - titanium. This transformation process is completed in an instant when the molten pool changes from liquid to solid. However, the difference of the "instant" length still has an effect on the crystal form of the molten pool, and the longer the "instant" is, the better the columnar crystal growth is. Because titanium has the characteristics of high melting point (1668 ℃), large heat capacity and poor heat conduction, the weld is affected by the energy of welding line and the forced cooling of weld, and the "instant" of weld retention at high temperature is different. The "instantaneous" slightly longer length provides conditions for the growth of columnar crystal and the widening of joint thermal effect. This is also one of the important reasons for the plastic degradation of welded joints. The tensile strength fracture often occurs in the heat affected zone of the weld. In order to reduce this adverse effect, the soft welding specification should be used as much as possible, i.e. smaller welding line energy and faster cooling speed.

4. Porosity is a common and unavoidable defect in titanium weld            

The formation mechanism of porosity is that the gas dissolved in the liquid metal in the welding process forms bubbles through diffusion, desolvation, nucleation, growth and other processes. Due to the rapid solidification and crystallization of the molten pool, the growing bubbles remain in the solid metal in the form of pores before they can escape from the liquid metal. The gases such as hydrogen and Co, which are produced by the action of arc heat, are mainly from organic pollutants. Sometimes, the weldment and welding material are cleaned and cleaned sufficiently before welding, and the effect of argon protection is also ideal, but there are still pores in the weld. The practical experience of titanium experts shows that the moisture in the air has a great influence on welding. In the experiment, when the relative humidity is less than 40%, no weld is found; when the relative humidity is more than 90%, there are many and large bubbles in the weld. It is fully explained that the humidity of air is one of the important reasons for the formation of pores.

二、Welding method of titanium

1. Manual TIG welding            

   Tungsten argon arc welding is to use the arc generated between the tungsten electrode and the weldment to heat melt the joint of the weldment and fuse the weldment together. During the welding process, the welding wire can be filled or not, and the tungsten electrode, the weld pool, the near seam area of the weld and the melting end of the filled wire should be protected by argon.  Generally, non-contact high-frequency arc striking is adopted for welding, and the arc length is controlled within 1.0-1.5 times of electrode diameter. The arc length of fillet weld can be slightly longer, and the welding nozzle is inclined 75 degrees backward (reverse welding direction). Welding current is the most important technical parameter of arc welding, which has a direct impact on weld penetration, welding speed, deposited metal amount and weld quality. The welding power source of TIG welding titanium is commonly used in the positive connection method, i.e. the positive electrode is connected with the weldment and the negative electrode is connected with the welding handle. 30% of the heat energy generated by the forward welding arc is concentrated on the tungsten electrode, and 70% of the heat energy is concentrated on the weldment, so the penetration is deeper than that of the reverse welding. From the start of arc striking to arc extinguishing, the arc must match with the time of argon supply and gas stopping, that is to say, the gas supply before arc striking must be in advance, while the argon gas after arc extinguishing must be in delay.    

          

   2. Protective gas            

   Protective gas is sprayed from the welding nozzle to cover the whole length of tungsten electrode and the molten pool area of arc melting to avoid air pollution. The commonly used gas is argon or helium. The thermal conductivity of argon is small, and there is no decomposition heat absorption under the action of arc, so the heat loss of argon is less, and the arc voltage is low, about 8-15v. In addition to the purity of the shielding gas (more than 99.98%), the protection effect is also related to the design of the geometric dimension of the welding nozzle, that is to say, it can ensure that the argon flow from the welding nozzle is laminar rather than turbulent. Generally, the height of the welding nozzle is 1.5 times of the nozzle diameter.

   三、TIG welding process            

1. Joint and groove            

In titanium welding, there are all kinds of joint forms, such as butt joint, lap joint, angle joint, tube plate welding, etc. The plate thickness is generally 1.0-10 mm, and plates of different thickness are connected. Joints and bevels are very important to obtain high quality welds.     

2. Cleaning before welding            

Titanium weldment and welding wire (filler wire) are easy to be polluted, such as lubricant residue, oxide film, oil stain, paint, coating, fingerprint, etc. If these pollutants are not removed before welding, they will decompose harmful impurities and dissolve in the weld metal under the action of arc heat during welding, which will have a bad impact on the weld quality.      

3. Specification for manual TIG welding of titanium                 

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