Shielding gas

About: Shielding gas is a research topic. Over the lifetime, 6697 publications have been published within this topic receiving 58668 citations.

Porosity formation mechanism and reduction method in CO2 laser welding of stainless steel

Abstract: A large amount of porosity is liable to be formed in high power CO2 laser welds of stainless steels. The formation mechanisms of porosity have not been satisfactorily understood up to the present. In this study, therefore, microfocused X-ray transmission in-situ observation system was utilized to observe the behavior of a keyhole and the formation situations of bubbles and pores during high power CO2 laser welding. It was observed that a keyhole fluctuated in geometry unstably and vibrated up and down dynamically, and accordingly many bubbles were frequently generated in the bottom part of a molten pool from the tip of the deep keyhole. Almost all bubbles were captured into spherical pores by solidifying solid-liquid interface during floating up near the bottom part of the weld pool. Moreover, it was revealed that partial penetration welding with a forward inclination of a laser beam or in N2 shielding gas, and full penetration welding in any shielding gas were beneficial to the reduction and prevention of porosity because the formation of bubbles was suppressed or prevented.

Hybrid Laser-arc Welding of 17-4 PH Martensitic Stainless Steel

Abstract: PH stainless steel has wide applications in severe working conditions due to its combination of good corrosion resistance and high strength The weldability of 17-4 PH stainless steel is challenging In this work, hybrid laser-arc welding was developed to weld 17-4 PH stainless steel This method was chosen based on its advantages, such as deep weld penetration, less filler materials, and high welding speed The 17-4 PH stainless steel plates with a thickness of 19 mm were successfully welded in a single pass During the hybrid welding, the 17-4 PH stainless steel was immensely susceptible to porosity and solidification cracking The porosity was avoided by using nitrogen as the shielding gas The nitrogen stabilized the keyhole and inhibited the formation of bubbles during welding Solidification cracking easily occurred along the weld centerline at the root of the hybrid laser-arc welds The microstructural evolution and the cracking susceptibility of 17-4 PH stainless steel were investigated to remove these centerline cracks The results showed that the solidification mode of the material changed due to high cooling rate at the root of the weld The rapid cooling rate caused the transformation from ferrite to austenite during the solidification stage The solidification cracking was likely formed as a result of this cracking-susceptible microstructure and a high depth/width ratio that led to a high tensile stress concentration Furthermore, the solidification cracking was prevented by preheating the base metal It was found that the preheating slowed the cooling rate at the root of the weld, and the ferrite-to-austenite transformation during the solidification stage was suppressed Delta ferrite formation was observed in the weld bead as well no solidification cracking occurred by optimizing the preheating temperature

Prediction of the laser-induced plasma characteristics in laser welding: a new modelling approach including a simplified keyhole model

Abstract: The characteristics of the laser-induced plasma encountered in laser welding are investigated using a new three-dimensional modelling approach. A simplified keyhole model is employed to couple with our previous plasma plume model, and thus both the plasma inside a blind keyhole and the plasma plume issuing from the keyhole can be treated simultaneously. Investigations include the effects on the laser-induced plasma characteristics of many factors, including the velocity of metal vapour leaving from the keyhole bottom, the velocity of the shielding gas injected coaxially with the laser beam, the velocity and location of the assisting gas injected laterally with respect to the workpiece, and the energy absorption and radiation heat loss of the laser-induced plasma. Typical computed distributions of temperature, velocity and vapour concentration within the plasma are presented with the continuous-wave CO2 laser welding of iron workpiece as the calculation example. It is shown that the high-temperature core of the laser-induced plasma is mostly located inside the blind keyhole or near the keyhole top for the cases under study. The metal-vapour/shielding-gas momentum ratio plays an important role in determining the height of the plasma plume, and the plume height decreases with increasing shielding-gas velocity. The laterally injected assisting gas may also significantly affect the laser-induced plasma characteristics and thus can be used to control the unfavourable effect of the laser-induced plasma on the laser welding process. The predicted temperatures of the laser-induced plasma are reasonably consistent with corresponding experimental data.

Fume formation rates in gas metal arc welding

Abstract: An improved fume chamber was constructed, and fume rates were measured with unprecedented precision for both steady- and pulsed-current welding of mild steel using 92% argon/8% CO2 shielding gas. Compre- hensive fume maps were constructed de- picting fume rates over a wide range of currents and voltages. Fume generation was generally lower under pulsed-cur- rent conditions. Theoretical arguments explaining this difference are presented.