The welding performance of titanium equipment and titanium alloys has many significant characteristics. These welding characteristics are determined by the physical and chemical properties of titanium and titanium alloys.
1. The influence of gas and impurity pollution on welding performance
- At room temperature, titanium and titanium alloys are relatively stable. However, the test results show that in 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 starts to absorb hydrogen at about 250 ° C, absorbs oxygen from 400 ° C, and absorbs nitrogen from 600 ° C. These gases After being absorbed, it will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the welding quality.
Detailed explanation of welding problems of titanium alloy-titanium equipment
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, plus the effective crystallization temperature range
Narrow, the shrinkage of titanium and titanium alloy when solidified is small, and the weld metal will not produce thermal cracks. When titanium and titanium alloys are welded on time, cold cracks can appear in the heat-affected zone, which is characterized by cracks that occur several hours or longer after welding, so they are also called delayed cracks. Studies have shown that this crack is related to the diffusion of hydrogen 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 to reduce the source of hydrogen in welded joints.
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. The main technological measures to prevent the formation of pores are:
- The protective gas should be pure, and the purity should not be less than 99.99%
- Thoroughly remove the scale and other organic substances on the surface of the weldment and the surface of the welding wire.
- Apply good gas protection to the molten pool, control the flow and flow rate of argon gas, prevent turbulent flow, and affect the protection effect.
- correct selection of welding process parameters, increase the residence time of the molten pool to facilitate the escape of bubbles.
