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The effect of gas impurities on the properties of weld metal titanium has high chemical activity and high affinity with oxygen and nitrogen in air. At lower temperature, titanium interacts with oxygen to form a dense oxide film. With the increase of temperature, the thickness of oxide film thickens. Titanium begins to absorb oxygen and dissolve oxygen into titanium at more than 600 ℃. No matter how high the temperature is, the activity of titanium increases sharply and reacts violently with oxygen to form titanium oxide. Titanium began to absorb hydrogen above 300 ℃ and nitrogen above 700 ℃. The result of the contamination of titanium by oxygen and nitrogen is that  The property is sharply reduced, the tensile strength is improved, and the brittle fracture will occur under severe conditions, and the plasticity is equal to zero.

Effect of other impurities on weld metal properties 

Other impurities refer to impurities that may melt into the molten pool except for gas impurities. Its source may be the welding operation environment is not clean, wearing dirty gloves touch titanium welding parts left oil stains, before welding with cotton yarn scrub joints, groove may leave cotton floc, welding production environment and steel welding production mix may produce rust, moisture and some other organic matter, etc. 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 (TiO 2) is formed.  TiH2 TiN TiC)。 As the molten pool is crystallized into the crystal lattice of the titanium, the crystal lattice of the titanium is distorted and distorted, so that the mechanical property of the titanium is changed. Some trace elements are dissolved in a small amount of titanium, and if the amount does not exceed the allowable range, it is sometimes desirable but the excess impurity element content is not allowed, especially the organic matter impurities, which is beneficial because the impurity elements in addition to the mechanical properties of the titanium welding are poor, the corrosion is reduced, and the root causes of the air holes in the welding seam

 Organization change of weld metal and joint heat affected zone

Titanium is a metal with isomorphous transformation. The solid state transformation of microstructure begins at 882.5 ℃. The crystal structure below 882.5 ℃ is dense hexagonal structure, called α titanium, and when the temperature is higher than 882.5 ℃, the titanium with α structure is transformed into β titanium with body centered cubic structure. This transformation process is accomplished by the instant change of molten pool from liquid to solid. However, the difference of “instantaneous” length still has an effect on the crystallization form of molten pool, and the longer the “instant” is, the more beneficial it is to the growth of columnar crystals. Because titanium has the characteristics of high melting point (1668 ℃), large heat capacity and poor heat conduction, the welding seam is subjected to welding line energy and weld strength. However, the effect of refrigeration on the “moment” of the weld at high temperature is different. The “moment” provides the conditions for the columnar crystal growth of the molten pool and the widening of the heat effect of the joint. The “moment” provides the conditions for the columnar crystal growth of the molten pool and the widening of the heat effect of the joint. This is also one of the important reasons for the plastic decline of welded joints. The tensile strength fracture of the joint often occurs in the heat affected zone of the weld. In order to reduce this adverse effect, the softer welding specification is adopted as far as possible, that is, the welding line energy is smaller and the cooling rate is faster.

Porosity is a common and unavoidable defect in titanium weld.

The mechanism of pore formation is that the gas dissolved into liquid metal in the welding process forms bubbles through diffusion, desolubilization, nucleation, growth and so on. Because the solidification and crystallization rate of the molten pool is very fast, the growing bubbles remain in the solid metal in the form of pores when it is too late to escape the liquid metal. The gases such as hydrogen and CO, which are formed by pores, mainly come from organic pollutants and are produced by arc heat. Sometimes the welding parts and welding materials are fully cleaned and cleaned 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 moisture in air has a great influence on welding. In the experiment, In the welding environment where the relative humidity is less than 40%, the weld seam is basically not found; in the environment where the relative humidity is more than 90%, there are many and large bubbles in the weld. Fully explain the humidity of the air is one of the important causes of stomata. 

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