Jong-Do Kim

Dynamics of keyhole and molten pool in laser welding

In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

Dynamics of keyhole and molten pool in high-power CO2 laser welding

A deep cavity called keyhole is formed in the laser weld pool due to the intense recoil pressure of evaporation. The formation of keyhole leads to a deep penetration weld with high aspect ratio. However, a hole drilled in a liquid pool is primarily unstable by its nature and the instability of keyhole also causes the formation of porosity in the weld metal. The porosity formation is one of the serious problems in the very high power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The paper describes the real time observation of keyhole and laser plasma/plume behaviors in the high power CW CO2 laser welding by the high speed optical and X-ray transmission methods, cavity formation process and its suppression measures.

Experimental and theoretical studies on keyhole dynamics in laser welding

The present paper describes the results of high speed photography, acoustic emission (AE) detection and plasma light emission (LE) measurement during CO2 laser welding of 304 stainless steel in different processing conditions. Video images with high spatial and temporal resolution allowed to observe the melt dynamics and keyhole evolution. The existence of a high speed melt flow which originated from the front part of weld pool and flowed along the sides wall of keyhole was confirmed by the slag motion on the weld pool. The characteristic frequencies of flow instability and keyhole fluctuations at different welding speed were measured and compared with the results of Fourier analyses of temporal AE and LE spectra. The experimental results were compared with the newly developed numerical model of keyhole dynamics. The model is based on the assumption that the propagation of front part of keyhole into material is due to the melt ejection driven by laser induced surface evaporation. The calculations predict that a high speed melt flow is induced at the front part of keyhole when the sample travel speed exceeds several 10 mm/s. The numerical analysis also shows the hump formation on the front keyhole wall surface. Experimentally observed melt behavior and transformation of the AE and LE spectra with variation of welding speed are qualitatively in good agreement with the model predictions.The present paper describes the results of high speed photography, acoustic emission (AE) detection and plasma light emission (LE) measurement during CO2 laser welding of 304 stainless steel in different processing conditions. Video images with high spatial and temporal resolution allowed to observe the melt dynamics and keyhole evolution. The existence of a high speed melt flow which originated from the front part of weld pool and flowed along the sides wall of keyhole was confirmed by the slag motion on the weld pool. The characteristic frequencies of flow instability and keyhole fluctuations at different welding speed were measured and compared with the results of Fourier analyses of temporal AE and LE spectra. The experimental results were compared with the newly developed numerical model of keyhole dynamics. The model is based on the assumption that the propagation of front part of keyhole into material is due to the melt ejection driven by laser induced surface evaporation. The calculations predict ...

Weldability and Keyhole Behavior of Zn-Coated Steel in Remote Welding Using Disk Laser with Scanner Head

Zinc-coated steels are widely used in automobile bodies. Laser welding, which offers a lot of advantages over the conventional welding with metal active gas welding, CO2 arc, etc. in terms of improved weld quality, high-speed, and easy automation, has been developed for cars. However, in laser lap welding of zinc-coated steel sheets without gaps, defects such as underfilled beads or porosity were easily formed due to higher pressure of zinc vapor trapped in the molten pool because of the lower boiling point of zinc (1180 K) with respect to the melting point of steel (Fe, 1803 K). Laser lap welding results of two Zn-coated steel sheets have been reported. However, there are not enough data for welding of three Zn-coated steel sheets. Therefore, to understand laser lap weldability of three Zn-coated steel sheets, lap welding of two or three sheets with and without gaps was performed using 16 kW disk laser apparatus with a scanner head, and molten pool motions, spattering, and keyhole behavior during welding...

Spectroscopic studies on laser induced plume of aluminum alloys

The paper describes the features and characteristics of plume induced in the pulsed YAG laser welding of Al-Mg alloys in air and Argon atmospheres. Compared with plumes of steel, stainless steel, Titanium, etc., the dynamic behavior of Al-Mg alloys plume is very unstable and this instability is closely related to the unstable motion of key-hole during laser irradiation. The shape of key-hole fluctuates both in size and shape and its fluctuation period was about 450 μs. This instability has been estimated to be caused by the evaporation phenomena of metals with different boiling points and latent heats of vaporization. Therefore, the authors have conducted the spectroscopic analysis of laser induced Al-Mg alloys plume. In the air environment, the identified spectra are atomic lines of Al, Mg, Cr, Mn, Cu, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectra of AlO, MgO and AlH. It has been confirmed that the resonant lines of Al and Mg are strongly self-absorbed, in particular in the vicinity of pool surface. The self-absorption of atomic Mg line is more eminent in alloys containing higher Mg. These facts have shown that the laser induced plume is relatively a low temperature and high density metallic vapor. The intensities of molecular spectra of AlO and MgO are different each other depending on the power density of laser beam. Under the low power density condition, the MgO band spectrum is predominant in intensity, while the AlO spectrum became much stronger in higher power density. In Argon atmosphere MgO and AlO spectra vanish, but AlH spectrum is detected. The Hydrogen source is presumably the Hydrogen solved in the base metal, adsorbed water on the surface oxide layer or H2 and H2O in the shielding gas. The time average plume temperature at 1 mm high above the surface is determined by the Boltzmann plot and the obtained electron temperature is 3,280 ± 150 K. The electron number density, which has been determined by measuring the relative intensities of the spectral lines of atomic and singly ionized Magnesium, is 1.85 × 10191/m3.The paper describes the features and characteristics of plume induced in the pulsed YAG laser welding of Al-Mg alloys in air and Argon atmospheres. Compared with plumes of steel, stainless steel, Titanium, etc., the dynamic behavior of Al-Mg alloys plume is very unstable and this instability is closely related to the unstable motion of key-hole during laser irradiation. The shape of key-hole fluctuates both in size and shape and its fluctuation period was about 450 μs. This instability has been estimated to be caused by the evaporation phenomena of metals with different boiling points and latent heats of vaporization. Therefore, the authors have conducted the spectroscopic analysis of laser induced Al-Mg alloys plume. In the air environment, the identified spectra are atomic lines of Al, Mg, Cr, Mn, Cu, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectra of AlO, MgO and AlH. It has been confirmed that the resonant lines of Al and Mg are strongly self-absorbed, in particular in the v...

Basic understanding on beam - Plasma interaction in laser welding

In early development days of laser welding, most of engineers and scientists believed that the laser induced plasma was the high temperature and high pressure plasma which could reflect the incident beam by plasma due to the plasma frequency [1]. However, the necessary electron number density to reflect the incident beam is 1019 1/cm3 for YAG laser and 1021 1/cm3 for CO2 laser. These electron number densities are unachievable in the one atmospheric pressure thermal plasma. The maximum electron number density of thermal plasma at 1 atm. is in the order of 1017 1/cm3 which is much less than the equivalent number density of cut-off frequency for infrared lasers such as CO2 and YAG lasers.The paper describes the type of laser induced plasma, behaviors of plasma during welding, spectroscopic characteristics of plasma, measured plasma temperature and electron number density, and possible energy dissipation of beam energy in laser plume. It has been concluded that the plasma induced in laser welding is a low temperature and weakly ionized plume. The energy loss in plasma is interpreted due to Inverse Bremsstrahlung for longer wavelength lasers and to Rayleigh scattering by ultra-fine particles formed in and around plume for shorter wavelength lasers.In early development days of laser welding, most of engineers and scientists believed that the laser induced plasma was the high temperature and high pressure plasma which could reflect the incident beam by plasma due to the plasma frequency [1]. However, the necessary electron number density to reflect the incident beam is 1019 1/cm3 for YAG laser and 1021 1/cm3 for CO2 laser. These electron number densities are unachievable in the one atmospheric pressure thermal plasma. The maximum electron number density of thermal plasma at 1 atm. is in the order of 1017 1/cm3 which is much less than the equivalent number density of cut-off frequency for infrared lasers such as CO2 and YAG lasers.The paper describes the type of laser induced plasma, behaviors of plasma during welding, spectroscopic characteristics of plasma, measured plasma temperature and electron number density, and possible energy dissipation of beam energy in laser plume. It has been concluded that the plasma induced in laser welding is a low tem...