One Factors affecting welding quality of titanium materials
Influence of gas impurities on weld metal properties
Titanium has a high chemical activity, and has a high affinity with oxygen and nitrogen in the air.At a lower temperature, titanium interacts with oxygen to form a dense oxide film. With the increase of temperature, the thickness of the oxide film becomes thicker. Over 600℃, titanium begins to absorb oxygen and dissolve oxygen into titanium.At higher temperatures, the activity of titanium increases dramatically and reacts violently with oxygen to form an oxide of titanium.Titanium begins to absorb hydrogen above 300℃ and nitrogen above 700℃.The result of oxygen and nitrogen pollution to titanium is that the strength and hardness of titanium are increased and the plasticity is decreased.The influence degree 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 decreases less.This is the brittleness caused by hydride, known as "hydrogen embrittlement".Hydrogen is also the cause of weld porosity.
During the melting welding process, the molten pool is like a small metallurgical furnace, and the molten metal is exposed to the atmosphere.If the corresponding protective measures are not taken to make the molten metal titanium and air insulation, oxygen, nitrogen, hydrogen and other gas elements will melt into titanium, forming brittle oxide or nitrous compounds, resulting in a sharp reduction in the weld metal plasticity, tensile strength increased, serious cases will occur brittle fracture, plasticity is equal to zero.
Influence of other impurities on weld metal properties
Other impurities refer to impurities that may melt into the molten pool in addition to gaseous impurities.It may be caused by unclean welding operation environment, dirty gloves touching the titanium welding parts left grease, cotton yarn scrubbing joints before welding, wadding that may be left by groove, rust, water and other organic matter that may be generated when the welding environment is mixed with steel welding production.These pollutants decompose oxygen, hydrogen, nitrogen, carbon and other elements under the action of arc high temperature, and then dissolve in 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 crystal lattice of titanium along with the crystallization of molten pool, causing distortion and distortion of the crystal lattice of titanium, thus changing the mechanical properties of titanium.
Some trace elements are dissolved in titanium in small amounts, which is fine if not more than the allowable amount, and sometimes desirable.But excessive content of impurity elements is not allowed, especially organic impurities, there are all kinds of harm and no benefit, this is because these impurities in addition to the titanium welding mechanical properties become poor, reduce and corrosive, but also the source of porosity in the weld.
Microstructure of weld metal and joint heat affected zone
Titanium is a metal with allotrope transformation.The solid state transformation of tissue began at 882.5℃.The crystal structure below 882.5℃ is a dense row of hexagonal structure, called alpha titanium;At temperatures higher than 882.5 ° c, alpha - structured titanium transforms into body-centred cubic - structured beta titanium.The transformation process is completed in an "instant" when the molten pool changes from a liquid state to a solid state.However, the length difference of the "instant" still affects the crystal form of the molten pool, and the longer the "instant" is, the more favorable it is for the growth of columnar crystals.Due to the high melting point (1668℃), large heat capacity and poor thermal conductivity, etc., the welding seam is affected by the energy of the welding line and the quality of forced cooling of the welding seam, and the "instantaneous" retention of the welding seam at high temperature is different.The slightly longer "instant" provides conditions for the crystal growth of the molten pool and the heat-affected widening of the joint.This is also one of the important causes of plastic decrease of welded joint.The tensile strength fracture of the joint often occurs in the heat affected zone of the weld.In order to reduce this adverse effect, titanium welding as far as possible to use a softer welding specification, that is, smaller welding line energy and faster cooling speed.
Porosity is a common and difficult defect to avoid in titanium weld
The mechanism of stomatal formation is that the gas dissolved into liquid metal in the welding process forms bubbles through diffusion, dissolution, nucleation and growth.Due to the rapid solidification and crystallization of the molten pool, the growing bubbles will remain in the solid metal in the form of pores before they can escape the liquid metal.Gas such as hydrogen and CO, which are produced by arc thermal action, mainly come from organic pollutants.Sometimes, the welding parts and welding materials are cleaned and cleaned thoroughly before welding, and the argon gas protection effect is also ideal, but there are still pores in the welding seam.The practical experience of titanium experts shows that moisture in the air has a great influence on welding.In the experiment, under the welding environment with relative humidity less than 40%, the welding seam was basically not found.In the environment with relative humidity over 90%, there are many and large bubbles in the weld.The result shows that the humidity of air is one of the important reasons of stomatal formation.
Two Welding method of titanium materials
Manual argon tungsten arc welding
Tungsten argon arc welding of fusion arc welding, is to use the electric arc between the tungsten electrode and the welding workpieces is hot melt welding seam and welding pieces of fused together, the welding process can add wire can not add wire, tungsten electrode and molten pool, the weld seam near area and add the melting end of the wire should be in the protection of argon gas.
Non-contact high frequency arc initiation is generally adopted in welding, and the arc length is controlled at 1.0 ~ 1.5 times of electrode diameter.For fillet weld, the arc length can be slightly longer, and the welding nozzle is tilted back (anti-welding direction) by 75 degrees.Welding current is the most important technical parameter of arc welding, which has direct influence on weld depth, weld speed, deposited metal quantity and weld quality.Argon tungsten arc welding titanium commonly used positive welding power source, that is, positive electrode connection welding parts, negative electrode connection welding.The thermal energy generated by the forward welding arc is 30% concentrated on the tungsten electrode, while 70% of the thermal energy is concentrated on the welded piece. Therefore, compared with the reverse welding method, the melting depth is relatively deep.From the beginning of arc ignition to arc quenching, the arc must be matched with the time of argon supply and gas cessation, that is, gas supply before arc ignition and argon gas suspension after arc quenching must be delayed.
Protective gas is ejected from the welding nozzle to cover the whole length of tungsten electrode and arc melting pool area from air pollution.The gas in common use is the inert gas argon or helium.Argon has a small thermal conductivity and does not decompose and absorb heat under the action of arc. Therefore, argon has a small heat loss and a low arc voltage, which is about 8 ~ 15V.The protection effect is not only important to the purity of the shielding gas (greater than 99.98%), but also related to the geometric size design of the welding nozzle, which ensures that the argon flow ejected from the welding nozzle is laminar flow rather than turbulent flow.In general, the height of the welding nozzle is 1.5 times the diameter of the nozzle.
Three Argon tungsten arc welding process
Joint and groove
In titanium welding, a variety of joint forms, such as butt, lap, corner, tube and plate welding.Plate thickness is generally 1.0 ~ 10mm, there are different thickness plate joint.Joints and grooves are important for obtaining good weld quality.
Clean before welding
Titanium welding parts and welding wire (filler wire) are easy to be contaminated, such as titanium production process used by the residual lubricant and oxide film, grease, paint, coating, fingerprints, etc.If these pollutants are not removed before welding, they will decompose into harmful impurities dissolved in weld metal by arc thermal action during welding, which will have a negative impact on weld quality.