Heat Conduction Welding Research Articles

Effect of focusing condition on laser energy absorption characteristics in pulsed laser welding

Pulsed laser welding of SUS 304 stainless steel under different laser focusing conditions was performed in this study. A multi-information synchronous monitoring system was developed in experiment. The electron temperature signal, audible acoustic signal and acoustic emission signal were detected in pulsed laser welding to characterize the behavior of plasma plume. The fluctuation of physical characteristic parameters was calculated to analyze the influence of focusing condition on the welding thermal effect and laser induced plasma. The results showed that the focusing condition results in different energy absorption behavior of plasma in heat conduction welding mode and deep penetration welding mode. The energy absorption behavior of plasma to laser beam can be characterized by plasma signals and physical characteristic parameters effectively. The keyhole effect and energy density distribution of the laser beam acting on the materials, which closely relate to the focusing condition, are important factors to affect the energy absorption mechanism of plasma plume. With sufficient laser energy density and no keyhole effect, a strong plasma plume may be induced in heat conduction welding mode. High intensity plasma plume above the molten pool promotes the inverse bremsstrahlung absorption effect in laser welding, which shields the effective absorption of materials to laser energy. But the keyhole effect contributes to the absorption efficiency of the materials to the energy of plasma and laser. Once the energy density of laser beam is enough to induce keyhole effect, the inverse bremsstrahlung absorption effect of plasma is attenuated.

Evaluation of material based influences at laser based heat conduction welding of carbon fiber reinforced plastics

Today, fiber reinforced materials are present in a wide field of industrial applications. Short glass fiber reinforced composites are used in automotive, aerospace, and medical sectors. In the last couple of years, fiber reinforced thermoplastics have gained importance as a construction material, especially for lightweight assembly. There are different methods of joining thermoplastic materials such as vibration, resistance, and induction welding. A more advanced process is the laser transmission welding, which can be characterized by its excellent reproducibility, high flexibility, and potential for automation. Laser transmission welding can be applied for joining unreinforced or glass fiber reinforced thermoplastic parts. This welding process was transferred to heat conduction welding for joining thermoplastic carbon fiber reinforced plastics to itself. The goal of these investigations was to determine the influence of the material thickness on the weld seam quality.

Industrial Blue Diode Laser Breaks 1 kW Barrier

AbstractUp to now, infrared lasers have been largely established for the welding of copper, due to the unavailability of alternative continuous‐wave laser sources with a lower wavelength. Despite the low absorption of the wavelength 1 μm on copper, conventional melting by evaporation is possible when using high laser intensities. But especially with thin components, this approach reaches its limits, and only deep penetration welding can be achieved. Laserline's 450 nm diode laser closes this gap and enables process‐reliable heat conduction welding with 1 kW of laser power. The new laser system scores with an unprecedented process stability and new application possibilities.

Monolithic Structure-Optical Fiber Sensor with Temperature Compensation for Pressure Measurement.

In this paper, an optical fiber pressure sensor cascading a diaphragm-assisted Fabry-Perot interferometer (FPI) and a fiber Bragg grating (FBG) is proposed and demonstrated. The sensor comprises an optical fiber, a fused-silica ferrule, and a fused-silica diaphragm. We use a femtosecond laser firstly to fabricate a pit on the end face of the ferrule and then investigate the laser heat conduction welding and deep penetration welding technology for manufacturing the seepage pressure sensor of the all-fused-silica material. We develop a sensor based on a monolithic structured FPI without adhesive bonding by means of all-laser-welding. The pressure characteristics of the sensor have good linearity at different temperatures. Also, the monolithic structured sensor possesses excellent resolution, hysteresis, and long-term stability. The environmental temperature obtained by the FBG is employed to compensate for the difference in seepage pressure at different temperatures, and the difference in seepage pressure responses at different temperatures is shown to be very small after temperature compensation.

Shaped laser beam profiles for heat conduction welding of aluminium-copper alloys

Heat conduction welding is often used where weld seams with a high surface quality and the exact retention of the chemical composition are needed. This study investigates the influence of intensity profiles in laser beam welding with laser powers up to 3.2 kW. The resulting process windows for heat conduction mode welding, the melt pool shape, the process stability and the dynamics, and the produced surface roughness are analysed. Therefore, three different intensity profiles are created with diffractive optical elements and the process dynamics are monitored with two high-speed cameras. Before analysing etched cross sections with respect to microstructural properties, the surface roughness is measured on all produced samples with a laser scanning microscope. With beam shaping, we found that a high peak intensity does not necessarily lead to an instable melt pool and that the distribution itself plays the major role. The use of shaped beam profiles leads to a more stable process and an enlarged heat conduction process regime compared to a defocussed multimode spot of the same size. Applying shaped laser beam profiles surface roughness close to laser polished surfaces can be achieved while also simultaneously enlarging the weld cross section area.

Influence of Surface State in Micro-Welding of Copper by Nd:YAG Laser

Laser welding of copper is characterized by low and unstable light absorption around 1000 nm wavelength. Combination of high thermal conductivity and low melting point makes it difficult to obtain good welding quality and leads to low energy utilization. To improve efficiency and welding quality, a technique to enhance process stability using 1064 nm wavelength Nd:YAG laser has been proposed, and absorption rate and molten volume in laser micro-welding were discussed. Since the surface state of specimen affects absorption phenomena, effects of surface shape and surface roughness were investigated. Absorption rate and molten volume were increased by creating appropriate concave holes and by controlled surface roughness. Stable micro-welding process with deep penetration and good surface quality was achieved for transitional processing condition between heat conduction and keyhole welding, by enhanced absorption rate.

Adhesive-free bonding homogenous fused-silica Fabry–Perot optical fiber low pressure sensor in harsh environments by CO2 laser welding

Abstract In this paper, we present a high-sensitivity homogenous fused-silica low pressure sensor. It was constructed from the commercially available standard silica fiber, fused-silica diaphragm and ferrule through CO2 laser all-welding. A Fabry–Perot (FP) cavity was fully sealed between the ultra-thin fused-silica diaphragm and the ferrule end face, and the ultra-thin diaphragm acts as a pressure transducer. We proposed the laser heat conduction welding for the fused-silica diaphragm, and the laser deep penetration point welding for the fiber and the ferrule. So the monolithic structure fused-silica FP interference optical fiber sensor was achieved. The sensor has a vent hole between the fiber and the ferrule forming during the laser deep penetration point welding, which can effectively reduce temperature dependence. The sensor exhibits excellent stability and repeatability, extreme low temperature dependence with 0.025nm/ o C that is 1/76 of the theoretical value, a sensitivity of 5.15nm/kPa, and a resolution of 4.7Pa. With a monolithic fused-silica configuration, the adhesive-free bonding high-sensitivity fused-silica sensor is expected to be suitable for low pressure measurements in extreme harsh environments.

Effect of relative position in low-power pulsed-laser–tungsten-inert-gas hybrid welding on laser-arc interaction

In laser-arc hybrid welding, the appropriate position between the laser beam and arc will be helpful in forming an intensive interaction and significantly increasing penetration depth. In this paper, first, an orthogonal experiment with three factors and four levels were carried out on the inclination angle of the welding torch, the arc length, and the distance between laser beam and TIG electrode, Dla, to find the optimal positional parameters with the strongest laser-arc interaction. The composite plasma behaviors and interaction mechanism between laser and arc were then analyzed under different position parameters; on this basis, the influence of laser power on the laser-arc interaction was further analyzed. Results reveal that the interaction is a decided difference under different positions. When the distance Dla is small, the arc will be significantly compressed; when the laser acts on the arc center, the pre-heating of the arc will greatly improve the laser absorption rate, a shorter arc length will help to further increase the penetration. When the distance Dla, the arc length, and the welding torch angle are 1.5, 1 mm, and 45°, respectively, the laser-arc interaction reaches its strongest level and the weld penetration is the deepest. In this case, the arc tail is obviously shortened, and the laser-arc composite plasma volume in the central region is greatly increased. Further, when the laser-power percentage is 10%–50%, the weld profile shows the characteristics of laser heat-conduction welding; when the laser-power percentage is 50%–80%, the weld profile shows the characteristics of laser deep penetration welding; and when the laser-power percentage is 80%–100%, the valid penetration is shallow and accompanied by a large splash due to the very short laser duration. But the weld profile should also be characterized by typical laser deep-penetration welding when the duration of laser action is enough long.

Dissimilar joining of AZ31B Mg alloy to Ni-coated Ti-6Al-4V by laser heat-conduction welding process

To improve the metallurgical bonding of immiscible and nonreactive Mg/Ti system, Ni electro-coating was employed acting as an intermediate element by reacting with both Mg and Ti in laser heat-conduction welding process. Influence of laser power on welding quality was investigated and interfacial reaction with participation of Ni coating was analyzed. The elemental diffusion mechanism was clarified with the assistance of numerical simulation. Reliable joints were obtained with suitable laser powers ranging from 1200 W to 1800 W. Continuous reaction layer of Ti2Ni and Ti solid solution formed at the interface. The reaction layer thickness was not monotone increasing with increase of laser power. Mg-Al-Ni ternary phase dispersed in fusion zone at Mg side. Diffusion of electroplated Ni was influenced by a bidirectional mechanism to Mg side and Ti side, which resulted in the variation of reaction layer thickness. The maximum tensile-shear load could reach 144 N/mm (about 53.3% of joint efficiency of base metal AZ31B Mg alloy) when laser power was 1500 W. Joint strength and fracture mode were associated with two aspects: interfacial reaction and weld appearance of Mg side. As a result, reliable joining was realized based on appropriate interfacial metallurgical bonding and no weld defects at Mg side.

A study on plasma plume fluctuation characteristic during A304 stainless steel laser welding

A passive electrical probe device is employed to detect the electrical signals of the plasma in the experiments of YAG laser welding on 304 stainless steels. The high-speed CCD camera device is synchronously triggered to record the plasma plume shape. The corresponding relation between plasma plume shape and electrical signal is found by analyzing the fluctuation of the plasma plume shape and the change of the detected electrical signal. A shape of the plasma plume expands corresponding to the electrical signal declining while a shape of the plasma plume contracts corresponding to the signal electrical increasing. This indicates that the voltage fluctuation of the electrical signal can be used to characterize the expansion and contraction of the plasma plume shape, so the electrical signal can be used to analyze the fluctuation characteristic of the plasma plume. The electrical signals are processed by FFT. A characteristic peak occur on the frequency spectrum of the electrical signal in the deep penetration laser welding. In contrast, the frequency spectrum of the electrical signals did not show a characteristic peak in the heat conduction welding. A method named spectral intensity is proposed to compare the difference between deep penetration laser welding and heat conduction welding. The result demonstrates that the spectral intensities of deep penetration laser welding are larger than those of heat conduction welding.

Investigations on copper welding using a frequency-doubled disk laser and high welding speeds

In this contribution laser welding of pure copper using laser light at the wavelength of 515 nm is discussed. Welds were analyzed with respect to the resulting penetration depth and the resulting quality. Heat conduction welding at high welding speed (> 1 m/s) is observed to be a stable process enabling reliable welds without spatter formation. In order to analyze the thresholds of fusing and evaporation high-speed images of the welding process were made. Analytically calculated threshold parameters were compared to the experimentally determined parameters, including an approach to deal with temperature-dependence of material properties. Furthermore, the transition zone between heat conduction welding and deep-penetration welding was investigated with respect to the resulting welding depth and weld seam width.

Temperature distribution during laser based heat conduction welding of CFRP

For the implementation of carbon fiber reinforced (CFRP) parts, these need to be assembled to more complex structures. Therefore, laser transmission welding was transferred to heat conduction welding for joining thermoplastic CFRP to itself. The goal of these investigations was to determine the influence of the focal point geometry and the main fiber orientation within the CFRP on the temperature distribution at the upper joining member. A set-up was chosen consisting of two thermo cameras in order to measure the process temperatures on top and underneath the upper joining member. Furthermore, the heat affected width was determined and correlated to the process temperatures.

Laser welding of copper using a high power disc laser at green wavelength

The trend towards renewable energies and electromobility is increasing the demand for copper. Laser welding as a flexible and fully automatable process is being more frequently employed to join copper materials. The use of green laser radiation compared to infrared laser radiation has the advantage of a significantly higher absorption coefficient for copper materials. Therefore, high power laser sources emitting at a green wavelength promise a more stable and a more efficient process. In this paper, experimental investigations with a green continuous wave laser radiation at a maximum power of 1 kW are presented. Using design of experiments, the process limits are determined depending on the laser power and the welding velocity. Furthermore, the potential of heat conduction welding using this laser source is investigated.

Simulation of laser welding using advanced particle methods

AbstractThe process of laser welding is modelled using the meshless Lagrangian Smoothed Particle Hydrodynamics (SPH) method. Significant physical effects during the welding process like heat conduction, surface tension, and the occuring phase transitions melting, solidification, and evaporation are considered in the model. By coupling an SPH code with a ray tracer that tracks the propagation of independent light rays in the capillary and calculates the locally absorbed intensity by means of geometrical optics, the laser‐material interaction is represented in detail, even for complex capillary geometries. Therefore, both heat conduction and deep penetration laser welding can be simulated using this co‐simulation approach. The model is able to predict the temperature distribution during the welding process and the dimensions of the resulting weld seam. Furthermore, the vaporisation threshold and deep penetration threshold can be estimated. The numerical results are validated by comparing the obtained threshold values with experimental data and an analytical approximation during seam welding of aluminum. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Advantages of laser beam oscillation for remote welding of aluminum closely above the deep-penetration welding threshold

Laser beam welding of aluminum has been developed into a widely used process in modern car body manufacturing. Following the trend of recent lightweight designs and the use of high strength materials, the application of thinner aluminum sheets is increasing. Due to the stepwise increase of the welding depth at the transition from heat-conduction welding to keyhole welding, the usability of the laser beam as a tool has some limitations. This work has investigated the penetration depth when keyhole welding close to the deep-penetration welding threshold can be reduced in a controlled manner by means of laser beam oscillation.

Experimental and Numerical Investigations of Laser Beam Welding with an Ultra-high Brightness Direct-diode Laser

Laser beam welding of steel sheets with conventional disc and fibre lasers is part of many manufacturing processes, e.g. car manufacturing. In modern times, all manufacturing processes are also evaluated regarding sustainability. Although conventional beam sources have an ever increasing efficiency, direct-diode lasers are said to make a step in the wall-plug efficiency in comparison to conventional beam sources due to a missing brightness-converter. Investigations concerning laser beam welding with ultra-high brightness direct-diode lasers were carried out, showing heat conduction welding as well as deep penetration welding of 22MnB5 steel sheets is possible. Furthermore a beam parameter product of about 8 mm-mrad was measured, showing comparable beam quality to conventional disc and fibre lasers. Metallographic cuts were used for determining the welding penetration depth and cross-section. Especially the cross-sections, as a measure for process efficiency, show almost the same result using a direct-diode laser or a disc laser. Due to the limited laser power of 500W, numerical simulations were used to extend the experimental results.

In Situ Observation of Solidification Conditions in Pulsed Laser Welding of AL6082 Aluminum Alloys to Evaluate Their Impact on Hot Cracking Susceptibility

The influence of laser pulse parameters on solidification conditions and hot crack formation in pulsed laser welding of Al6082 aluminum alloys was studied with the aid of high-speed cameras capturing visible and infrared radiation. Hot cracking was evaluated with respect to strain rate, strain, and metallurgical outcome. The strain rate was approximated by the product of interface velocity and temperature gradient at the interface. The temperature gradient decreases during the course of solidification and followed a specific course. The interface velocity was therefore used as an indicator for the strain rate, which increased in a logarithmic manner with respect to the slope of the laser pulse’s cooling time. The accumulated strain was calculated by measuring the spot weld deformation during solidification. Within the heat-conduction welding regime, hot cracking can be reduced by lowering the interface velocity leading to a reduced strain rate and enhanced permeability of the dendritic microstructure. An over-proportional increase of the accumulated strain was observed for keyhole welding, which led to a high susceptibility to hot cracking regardless of the interface velocity. At low interface velocities, hot cracking was induced by extensive hydrogen diffusion at the solid–liquid interface, which promotes crack initiation.

CFRP-Aluminium Structures Realized by Laser Beam Joining Process

Modern lightweight structures containing hybrid materials allow an improvement of the weight-specific properties. However, to exploit the potential as far as possible novel joint concepts are necessary, enabling an economic structure manufacturing. The DFG-researcher group Schwarz-Silber (FOR 1224) at the University of Bremen aims to explore and develop interface structures for advanced FRP-Al compounds. Considering textile, welding and casting techniques novel joint concepts are under development, in five interdisciplinary projects. Within their work the researcher group focuses on three concepts realizing the transition structures: the usage of wires (titanium), foils (titanium) and fibres (glass fibre) as transition elements between CFRP and aluminium. Typical examples for such hybrid structures can be found in products from the aerospace industry (e.g. hull segments), the car industry (e.g. CFRP roof structures), but also in general mechanical engineering (e.g. rotor blade elements). In this paper, the joint configuration based on titanium wires and a laser beam conduction welding process will be presented. As beam source a lamp pumped Nd:YAG laser (HL4006D) was used. First specimens obtained will be discussed with respect to their properties. It will be shown that the novel approach is in principle suitable to produce load-bearing CFRP-aluminium structures. The wire concept represents a parallel arrangement of miniaturized loop connections. It is characterized by joining a CF-Ti-textile to an aluminium sheet. A carbon fibre loop is threaded through a titanium wire loop by textile technologies on one side. On the side opposite to the CF, the titanium wire loops of the CF-Ti-textile are joined to an aluminium component by welding or casting. A double-sided laser beam heat conduction welding process was applied, for both concepts. During processing, the laser beam was travels along the aluminium edge. The titanium-aluminium structure is welded in two steps. During the first step (i.e. the first weld pass) the aluminium and titanium are heated by the defocused laser beam simultaneously on both sides. An aluminium melt pool is formed, supported by the action of gravity and a certain amount of pre-heating of the titanium-wire or the titanium-foils by the laser beam and by heat conduction through the aluminium melt pool. In the second, immediately subsequent step (i.e. the second weld pass), due to a pre-heating of the materials by the first pass and an increased heat transfer between both materials, a complete wetting of the titanium structures in the joining zone is achieved.

Detection of Seam Offset Based on Molten Pool Characteristics during High-Power Fiber Laser Welding

Laser welding includes the heat conduction welding and the deep penetration welding. Deep penetration welding can not only penetrate the material completely, but also can vaporize the material. An important phenomenon during deep penetration welding is that molten pool in the weldment will appear a keyhole. 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. Small focus wandering off weld seam may result in lack of penetration or unacceptable welds, and largely reduce heating efficiency. In a fiber laser butt-joint welding of Type 304 austenitic stainless steel plate with a high power 6kW continuous wave fiber laser, an infrared sensitive high-speed video camera was used to capture the dynamic images of the molten pools. The configurations of molten pools were analyzed through image processing techniques such as median filtering, partial Otsu threshold segmentation and Canny edge to obtain the edge of keyholes and molten pools. The circular degree and the area of keyholes and the width and average gray of molten pools were defined as characteristic parameters to reflect the seam offset between the laser beam and the weld center. By analyzing the change of characteristic parameters during welding process, it was found that these parameters were related to the seam offset. Welding experimental results and analysis of characteristic parameters confirmed that the seam offset could be monitored and distinguished by molten pools configuration during high-power fiber laser welding.

Analysis of Characteristics of Mg Alloy Welded Joint by Laser Welding

The microstructure, the regular and mechanism of various parameters on formation of weld bead of Mg alloy AMCa403 using a laser welding were investigated. The results show that sound welds without major defects can be produced. Two welding modes of deep penetration welding, heat conduction welding were found, and heat input was found to be the main factor for welding mode and shape. The microstructure of weld metal is significantly finer than the base metal. At the same power, with the increase of welding speed, the microstructure of weld metal is much finer.