Heat Conduction Welding Research Articles

Process Studies on Laser Welding of Copper with Brilliant Green and Infrared Lasers

Copper materials are classified as difficult to weld with state-of-the-art lasers. High thermal conductivity in combination with low absorption at room temperature require high intensities for reaching a deep penetration welding process. The low absorption also causes high sensitivity to variations in surface conditions. Green laser radiation shows a considerable higher absorption at room temperature. This reduces the threshold intensity for deep penetration welding significantly. The influence of the green wavelength on energy coupling during heat conduction welding and deep penetration welding as well as the influence on the weld shape has been investigated.

Bifocal Hybrid Laser Welding -More than a Superposition of two Processes

The shortage of resources demands a consequent lightweight design with aluminium alloys. Process stability and a high seam quality in welding processes of this material are difficult to achieve. Bifocal hybrid laser welding (BHLW) demonstrates how this material can be welded reliably without defects. The used hybrid laser system consists of a Nd:YAG laser and a high power diode laser (HPDL). This leads to a high welding penetration in combination with a high seam quality. This publication demonstrates that the BHLW process is more than a coupled deep penetration welding (DPW) process with a heat conduction welding (HCW) process. The synergetic effect is a better quality in the entire welding seam.

Utilizing Laser Power Modulation to Investigate the Transition from Heat-Conduction to Deep-Penetration Welding

In laser welding the two regimes of heat-conduction welding and deep-penetration welding are distinguished. In order to characterize these two process regimes, the shown approach uses a sinusoidal laser power modulation together with timeresolved coaxial back-reflection measurement and high-speed imaging. Thereby the lower laser power levels lead to heatconduction welding whereas the higher levels result in deep-penetration welding. By analyzing the measured signals for different modulation parameters and on different processed materials, detailed information about the threshold between the two welding regimes is gained and in addition the material-dependent performance within those regimes can be specified.

Laser Joining Applications Highest Efficiency Using Diode Lasers

Laser technology is becoming more and more the only method of joining which can stand up to the high demands of modern production processes in the automotive industry, such as high reproducibility, minimal heat-affected zones and the special esthetic requirements made on the seams. In addition to Nd:YAG lasers, which have already established themselves in this field, the use of diode lasers is now becoming increasingly common. The principal reasons for this are the financial benefits in combination with the high beam quality and the reliability of modern diode laser systems as seen, for example, in brazing and heat conduction welding in the mass production applications of the automotive industry.

Aluminum alloy welding by using a high power direct diode laser

Aluminum alloys are the focus of increasing attention from the automobile industry because of their light weight, high formability, and easy recyclability. In this report, a 4 kW high power direct diode laser was used to examine the weldability of aluminum alloys of AA5022 and AA6016. A heat conduction welding type bead shape was produced with a very smooth bead surface. Full penetration butt welding was possible up to a welding speed of 12 m/min for 1 mm thick AA5022 and 6 m/min for 1 mm thick AA6016. Welding phenomena were observed with a high speed video camera. Very quiet welding phenomena were observed, with no spattering or laser plasma formation.

Experimental study on welding characteristics of CO2 laser TIG hybrid welding process

The experiments of CO2 laser TIG paraxial hybrid welding with 4 mm thick AISI 321 stainless steel sheet have been performed. The arc images and welding characteristics have been investigated with different energy ratios between laser and arc. The experimental results indicate that the hybrid welding is similar to laser welding and has also two welding mechanisms: deep penetration welding and heat conduction welding. Because of the effect of keyhole induced by laser, the arc root can be stabilised and compressed, and the current density and the penetration depth are all increased significantly, which show the characteristics of deep penetration welding. However, when the current is increased to a critical value, the laser induced keyhole disappears and the arc expands obviously, which decreases the penetration depth, so that the welding mechanism has been changed from deep penetration welding to heat conduction welding. Furthermore, the effects of distance between laser beam and electrode, pulsed laser and hybrid manners on hybrid welding characteristics have also been studied.

Energy Coupling Rate in Aluminum Welding by CO2 Laser and Nd:YAG Laser.

The energy coupling rate during welding of Al-Mg-Si alloys with a CO2 laser and a Nd:YAG laser was measured calorimetrically. The coupling rate obtained with the Nd:YAG laser was higher than that with the CO2 laser for both heat conduction welding and keyhole welding. The coupling rate for CO2 laser welding changed steeply at the transition from a heat conduction welding state to a keyhole welding state. Coupling rates under the keyhole welding state match the Gouffe-Dausinger model that determines the coupling rate based on the weld depth and focal spot size. This model includes a constant figure that is dependent on laser beam absorptivity to molten metals and plasma in the keyhole. It is thought that the coupling rate in the heat conduction state depends on the absorptivity of the laser beam to molten metals. Absorptivity to plasma in the keyhole also affects the coupling rate in the keyhole welding state.

Marangoni effect in laser deep penetration welding of steel

In welding, the resulting weld-seam geometry may vary significantly although using constant process parameters and steels with the same material number. One likely reason for this are small variations in the concentration of sulfur, phosphorus, oxygen, and other chemical elements that are well within the tolerance of the standard of a specific alloy. These substances act as surfactants and even marginal changes strongly effect the temperature-dependent coefficient of surface tension. In simulations of conventional electric arc welding and laser heat conduction welding, the effect of the temperature-dependent coefficient of surface tension (Marangoni effect) has been identified as one of the primary driving forces of the liquid melt. In laser deep penetration welding simulations this effect has been widely neglected, so far. In this contribution, simulations of flow fields in the weld pool resulting from different temperature dependencies of the coefficient of surface tension are presented. The simulations show the same effect in laser deep penetration welding that is already known in gas tungsten arc welding. Consequently, it becomes clear that the chemical specifications for many steel alloys are too crude for reliable predictions of the seam geometry in laser deep penetration welding.

Manufacturing with novel high-power diode lasers

Direct applications of high-power diode lasers (HPDLs) like hardening, heat conduction welding of metals, and joining of polymers have already been demonstrated also in the industrial environment. Relatively low intensities in the range of 10/sup 3/ W cm/sup -2/ are sufficient for these applications. While the commercial HPDL systems are built on the basis of diode laser bars with 40 W output power, in the meantime a record continuous-wave-output power of 267 W per bar has been demonstrated. The achievable higher output power per bar will lead to enhanced applicability of HPDLs and thus to a further steep increase of their industrial use. Improved packaging technology, multiplexing the emission of single bars and coherent coupling as well as promising new diode laser structures like Z-shaped broad area emitters, is discussed. In this paper, emphasis is laid on the potential applicability of commercial HPDLs for metal working with elevated intensities up to 10/sup 5/ W cm/sup -2/, like oxygen cutting and the worldwide first deep-penetration HPDL-weld up to a sheet thickness of 6 mm in stainless steel. These results have been predicted by proper theoretical modeling. Strong reduction of phase space dimension takes place in convective-diffusive-type free boundary problems typical for thermal processing. This property makes it possible to construct approximate finite-dimensional dynamical systems being solvable with controlled error. Numerical solutions of the full problem are used to investigate the quality of the approximate model. Observable quantities of the technical processes like signals from monitoring devices are part of the solution and solutions to the inverse problem are given.

Effect of welding conditions on transient response of temperature and penetration in automatic TIG welding

Transient responses of temperature and penetration to a step change in welding conditions were investigated using FEM calculation of three-dimensional heat conduction and TIC welding of mild steels. Correlations between welding parameters and transient response of temperature and penetration were clarified through the comparison of experimental results with calculated ones. In addition, the temperature distribution and heat flow in the workpiece were also analyzed, from which the main factors of welding parameters dominating the transient response were discussed.Temperature and penetration showed different behaviors to a step change in welding parameters, and their transient responses were characterized by their variation rates from the former quasi-state to later one. With a step change in welding velocity, the characteristics of transient response of temperature showed identical behaviors regardless of the variation of step change and heat input. The same response was also observed in the transient change of penetration. The main factor dominating the transient response to the step change in welding velocity was the heat flow direction. The transient response to a step change in welding current or heat input showed the different characteristics depending upon the variation of heat input, which was caused from the heat flow rate in the workpiece.

自動ＴＩＧ溶接の過渡応答特性に及ぼす溶接条件の影響

Transient responses of temperature and penetration to a step change in welding conditions were investigated using FEM calculation of three-dimensional heat conduction and TIC welding of mild steels. Correlations between welding parameters and transient response of temperature and penetration were clarified through the comparison of experimental results with calculated ones. In addition, the temperature distribution and heat flow in the workpiece were also analyzed, from which the main factors of welding parameters dominating the transient response were discussed.Temperature and penetration showed different behaviors to a step change in welding parameters, and their transient responses were characterized by their variation rates from the former quasi-state to later one. With a step change in welding velocity, the characteristics of transient response of temperature showed identical behaviors regardless of the variation of step change and heat input. The same response was also observed in the transient change of penetration. The main factor dominating the transient response to the step change in welding velocity was the heat flow direction. The transient response to a step change in welding current or heat input showed the different characteristics depending upon the variation of heat input, which was caused from the heat flow rate in the workpiece.