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Classification, characteristics and welding properties of titanium and titanium alloy

Nov 07, 2018

First. Classification and characteristics of titanium and titanium alloys

Domestic industrial pure titanium has three kinds of TA1, TA2 and TA3. The difference is that the content of impurities containing hydrogen and oxygen is different. These impurities make industrial pure titanium strengthen, but the plasticity is significantly reduced. Although the industrial pure titanium is not high in strength, it has excellent plasticity and toughness, especially good low temperature impact toughness, and good corrosion resistance. Therefore, this material is mostly used in the chemical industry, the petroleum industry, etc., and is actually used in working conditions below 350 °C. According to the room temperature structure of the annealed state of the titanium alloy, the titanium alloy can be classified into three types: an α type titanium alloy, an (α + β) type titanium alloy, and a β type titanium alloy. Among the α-type titanium alloys, TA4, TA5, and TA6 Ti-AI alloys and TA7 and TA8 Ti+AI+Sn alloys are widely used. The alloy has a strength of 931 N/mm 2 at room temperature, and is stable at high temperatures (below 500 ° C) and has good weld ability. The application amount of β-type titanium alloy is small in China, and its scope of use needs to be further expanded.

Second, the welding performance of titanium and titanium alloy
The welding properties of titanium and titanium alloys have many remarkable features. These 
welding characteristics are determined by the physical and chemical properties of titanium and
titanium alloys.
1. Influence of gas and impurity pollution on welding performance
At normal temperature, titanium and titanium alloys are relatively stable. However, in the test,
during the welding process, the liquid droplets and the molten pool metal strongly absorb
hydrogen, oxygen and nitrogen, and in the solid state, these gases have reacted with them.
With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen,
oxygen and nitrogen also rises obviously. At about 250 °C, hydrogen is absorbed, and oxygen
is absorbed from 400 °C, and nitrogen is absorbed from 600 °C. After being absorbed, it
will directly cause the brittleness of the welded joint, which is an extremely important
factor affecting the welding quality.
(1) Hydrogen is the most serious factor affecting the mechanical properties of titanium in the 
presence of hydrogen. The change of hydrogen content in the weld has the most significant effect
on the impact performance of the weld. The main reason is that the amount of hydrogen bombs in
the weld increases, and the sheet-like or needle-like TiH2 precipitated in the weld increases.
The strength of TiH2 is very low, so the action of the sheet-like or needle-like HiH2 is not
ched, and the impact performance is significantly reduced; the effect of the change in the
hydrogen content of the weld on the strength and the plasticity is not very obvious.

 


(2) Effect of oxygen Oxygen has a high degree of melting in the α phase and β of titanium, and can form a gap solid phase. The crystal wound of titanium is severely distorted, thereby increasing the hardness of titanium and titanium alloy. And strength, so plasticity is significantly reduced. In order to ensure the performance of the welding joint, in addition to the welding process and the welding in the heat-affected zone, the oxygen content of the base metal and the welding wire should be limited.

 (3) Effect of nitrogen At a high temperature of 700 ° C or higher, nitrogen and titanium play a dramatic role to form brittle hard titanium nitride (riN) and the degree of lattice turbulence caused by the formation of interstitial solid solution between nitrogen and titanium is The effect of the amount of oxygen is more serious. Therefore, nitrogen improves the tensile strength and hardness of the industrial pure titanium weld and reduces the plasticity of the weld more than oxygen.

(4) Effect of carbon carbon is also a common impurity in titanium and titanium alloys. Experiments show that when the carbon content is 0.13%, the carbon is deep in α-titanium, the weld strength limit is somewhat improved, the plasticity is somewhat decreased, but less than oxygen. The role of nitrogen is strong. However, when the carbon content of the weld is further increased, the mesh TiC appears in the weld, and the amount increases with the increase of the carbon content, so that the plasticity of the weld is sharply decreased, and cracks are likely to occur under the welding stress. Therefore, the carbon content of the titanium and titanium alloy base material is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base material.

2. Weld joint crack problem
When titanium and titanium alloys are welded, there is little possibility of hot cracks in the 
welded joints. This is because the content of impurities such as S, P, and C in titanium and
titanium alloys is small, and the low-melting eutectic formed by S and P is not easy to appear.
On the grain boundary, plus the effective crystallization temperature interval
Narrow, titanium and titanium alloys have a small amount of shrinkage when solidified, and weld
metal does not cause hot cracks. When titanium and titanium alloys are welded, cold cracks may
occur in the heat-affected zone, which is characterized by cracks occurring several hours or
more after welding as delayed cracks. Studies have shown that this crack is related to the
diffusion of hydrogen bombs during welding. During the welding process, hydrogen diffuses from
the high temperature deep pool to the lower temperature heat affected zone. The increase of
hydrogen content increases the amount of TiH2 precipitated in the zone, increases the
brittleness of the heat affected zone, and causes large microstructure stress due to
volume expansion during hydride precipitation. In addition, hydrogen atoms diffuse and
aggregate to the high stress sites in the region, so that cracks are formed. The way to
prevent such delayed cracks is mainly to reduce the source of hydrogen in the welded joints.
3. Porosity problems in the weld
When welding titanium and titanium alloys, pores are a common problem. The root cause of the
formation of pores is the result of hydrogen influence. The formation of pores in the weld
metal mainly affects the fatigue strength of the joint. The main measures to prevent the
generation of pores are:
(1), the protection of helium should be pure, the purity should be no less than 99.99%
(2) Thoroughly remove organic matter such as scale oil on the surface of the weldment and the
surface of the wire.
(3) Apply good gas protection to the molten pool, control the flow rate and flow rate of argon
gas to prevent turbulence and affect the protection effect.
(4), correctly select the welding process parameters, increase the use of the deep pool 
residence time in the bubble escape, can effectively reduce the pores.

 


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