Titanium and titanium alloy material characteristics and weldability, and for titanium and titanium alloy welding is prone to oxidation, cracks, stomata welding defects, weldability test. Through continuous exploration of the titanium and titanium alloy welding process specifications, as well as a reasonable analysis of the problems in the test process, summarize the characteristics of titanium and titanium alloy welding process and operating essentials.
Classification and characteristics of titanium and titanium
There are three types of industrial pure titanium: TA1, TA2, and TA3. The difference lies in the content of oxyhydrogen and nitrogen impurities. These impurities strengthen the industrial pure titanium, but the plasticity is significantly reduced. Despite its low strength, industrial pure titanium has excellent plasticity and toughness, especially good low-temperature impact toughness; at the same time, it has good corrosion resistance. Therefore, this material is mostly used in chemical industry, petroleum industry, etc., in fact, it is mostly used in working conditions below 350 ℃.
Titanium alloys can be divided into three types according to the room temperature structure of the titanium alloy in the annealed state:
Α-type titanium alloy, (α + β) -type titanium alloy and β-type titanium alloy.
Among the α-type titanium alloys, the Ti-AI alloys of TA4, TA5 and TA6 types and the Ti + AI + Sn alloys of TA7 and TA8 types are mostly used. The strength of this alloy can reach 931N / mm2 at room temperature, and it has stable performance at high temperature (below 500 ℃) and good weldability.
Β-type titanium alloys are used less in China, and their scope of use needs to be further expanded.
Weldability of titanium and titanium alloys
The welding properties of titanium and titanium alloys have many significant characteristics. 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 room temperature, titanium and titanium alloys are relatively stable. But in the test table, during the welding process, the liquid droplets and the pool metal have a strong absorption of hydrogen, oxygen, and nitrogen, and in the solid state, these gases have interacted with them. As the temperature rises, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also increases significantly. It begins to absorb hydrogen at about 250 ° C, absorb oxygen from 400 ° C, and absorb nitrogen from 600 ° C. After being absorbed, it will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the welding quality.
1.1 Effect of hydrogen
Hydrogen is the most serious factor affecting the mechanical properties of titanium among gas impurities.
The change of weld hydrogen content has the most significant impact on weld impact performance. The main reason is that as the hydrogen content of the weld increases, the flake or needle TiH2 precipitated in the weld increases. The strength of TiH2 is very low, so the effect of sheet or needle-shaped HiH2 is notched, and the combined impact performance is significantly reduced; the effect of changes in the hydrogen content of the weld seam on the improvement of strength and the reduction of plasticity is not very obvious.
1.2 Effect of oxygen
Oxygen has a high degree of melting in both the alpha phase and beta of titanium, and can form a gap solid and deep phase. The crystal wound using right titanium is seriously twisted, thereby increasing the hardness and strength of titanium and titanium alloys, making the plastic Significantly reduced. In order to ensure the performance of the welding connection, in addition to strictly preventing the main oxidation of the weld and the heat affected zone during the welding process, the oxygen content in the base metal and the welding wire should also be limited.
1.3 The effect of nitrogen
At high temperatures above 700 ° C, nitrogen and titanium react strongly to form brittle and hard titanium nitride (TiN) and the degree of lattice distortion caused by nitrogen and titanium forming interstitial solid solution is more than the consequence of the amount of oxygen It is more serious, therefore, nitrogen is more significant than oxygen in improving the tensile strength and hardness of industrial pure titanium welds and reducing the weld plasticity.
1.4 Impact of carbon
Carbon is also a common impurity in titanium and titanium alloys. Experiments have shown that when the carbon content is 0.13%, the depth of carbon in alpha titanium increases the weld strength limit somewhat and the plasticity decreases somewhat, but it is not as strong as oxygen and nitrogen. However, when the carbon content of the weld is further increased, the network of TiC appears in the weld. The amount of TiC increases with the increase of carbon content, which causes the weld plasticity to decrease sharply and cracks are likely to occur under the action of 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 seam does not exceed the carbon content of the base material
2 Welding joint crack problem
When welding titanium and titanium alloys, the possibility of thermal cracks in the welded joints is very small. This is because the contents of impurities such as S, P, and C in titanium and titanium alloys are very small, and the low melting point eutectic formed by S and P is not easy to appear in On the grain boundary, the effective crystallization temperature range is narrow, and the shrinkage of titanium and titanium alloy during solidification is small, and the weld metal will not produce thermal cracks.
When welding titanium and titanium alloys, cold cracks can appear in the heat-affected zone, which is characterized by the occurrence of cracks called delayed cracks for hours or longer after welding. 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 in the hydrogen content increases the amount of TiH2 precipitated in this zone, increasing the brittleness of the heat affected zone. In addition, due to the volume expansion when the hydride precipitates, the larger tissue stress In addition, hydrogen atoms diffuse and accumulate to the high-stress parts of the area, so that cracks are formed. The way to prevent this delayed cracking is mainly to reduce the source of hydrogen in the welded joints. When invoicing, also stay in the air to suppress the fire.
3 Porosity in the weld
When welding titanium and titanium alloys, pores are often encountered. The root cause of the formation of pores is the result of the influence of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint.
Technical measures to prevent the formation of pores are:
3.1 The neon gas must be pure and the purity should not be less than 99.99%
3.2 Thoroughly remove the scale, grease and other organic substances on the surface of the weldment and the surface of the welding wire.
3.3 Apply good gas protection to the molten pool, control the flow and flow rate of argon gas to prevent turbulence, and affect the protection effect.
3.4 Correct selection of welding process parameters, increase the right to use the residence time of the deep pool for the escape of bubbles, which can effectively reduce the porosity.
in conclusion
1 The gas protection problem of titanium and titanium alloy welding is the primary factor affecting the quality of the welded joint.
2 When welding titanium and titanium alloys, the heat input should be as small as possible.
3 TA2 manual tungsten electrode argon arc welding, the source of hydrogen should be strictly controlled to prevent the occurrence of cold cracks, and attention should be paid to the prevention of the formation of pores.
4 As long as the welding process is strictly followed and effective gas protection measures are taken, high-quality welded joints can be obtained.