The welding performance of titanium coil and heat exchanger has many remarkable characteristics, which are determined by the physical and chemical properties of titanium and titanium alloy.
1. Influence of gas and impurity pollution on welding performance
Titanium and titanium alloys are stable at room temperature.However, the test results show that during the welding process, liquid droplet and molten pool metal have strong absorption of hydrogen, oxygen and nitrogen, and in the solid state, these gases have acted with it.With the increase of temperature, the absorption capacity of hydrogen, oxygen and nitrogen of titanium and titanium alloys also increases significantly. Hydrogen absorption starts at about 250℃, oxygen absorption begins at 400℃ and nitrogen absorption begins at 600℃. After these gases are absorbed, embrittlement of welding joints will be directly caused, which is a very important factor affecting welding quality.
2. Welding joint crack problem
Titanium and titanium alloy welding, the welding joint of hot cracking formation of possibility is very small, it is because the titanium and titanium alloy in the impurities such as S, P, C, and low melting eutectic formed by S, P is not easy to appear on the grain boundary, combined with effective crystallization temperature range is narrow small, titanium and titanium alloy solidification shrinkage small, weld metal does not produce hot crack.During welding of titanium and titanium alloys, cold cracks may appear in the heat-affected zone, which is characterized by the fact that the cracks occur several hours or even longer after welding, so it is also called delayed cracks.The results show that the crack is related to hydrogen diffusion during welding.During the welding process, hydrogen diffused from the high temperature deep pool to the heat-affected zone at a lower temperature. The increase of hydrogen content increased the amount of TiH2 precipitated from the zone and increased the brittleness of the heat-affected zone. In addition, due to the volume expansion during the precipitation of hydride, the larger tissue stress was caused.The main way to prevent this kind of delayed crack is to reduce the source of hydrogen in welding joints.
3. Porosity in welding seams
When welding titanium and titanium alloy, porosity is often encountered.The root cause of stomata formation is the result of hydrogen influence.The porosity of weld metal mainly affects the fatigue strength of joint.The main technological measures to prevent the generation of pores are:
1) the protective gas should be pure and the purity should not be lower than 99.99%
2) thoroughly remove the surface of the welding parts, the surface of the wire oxidation skin oil and other organic matter.
3) good gas protection should be applied to the molten pool to control the flow and velocity of argon to prevent turbulence and affect the protection effect.
4) choose the welding process parameters correctly and increase the residence time of the molten pool to facilitate the bubble escape.