Category Archives: Laser Arc Welding

Laser Beam Welding of Experimental Trip Steels

Posted on by
Reply

By:  Álvaro Prada

In the last few years, the use of Advanced High Strength Steels in the automotive sector has been increasing steadily for the manufacturing of structural and safety parts. Indeed these steels make possible an improvement of passengers´ safety maintaining a reasonable weight. Five years ago, it was predicted that TRIP steels would be one of the most popular family of AHSS. However nowadays its use is not so significant and generally high strength dual-phase steels or martensitic steels are preferred in spite of the very good forming property of TRIP steels. Probably one of the major reasons is the behaviour of TRIP welded parts, whatever the process. Figure 1 presents comparative forming results between laser welded TRIP and DP joints. In order to understand better the manufacturing process of TRIP steels as well as their behaviour when forming and welding, the Spanish government is funding a project integrated by 4 members, each one leading a specific part of the study: ITMA is designing and manufacturing the steels, CEIT is selecting the best thermo mechanical cycles, CTM is evaluating the forming properties as manufactured and also of the welded joints, and AIMEN is studying the behaviour of experimental steels when welding.

In this project, the main objectives are to design TRIP steels in order to enhance their weldability, to assess the behaviour of experimental TRIP steels versus commercial TRIP steels, to make a comparison between TRIP steels and DP steels behaviour and finally to study the weld behaviour using different processes such as LBW, PAW and RSW.

The consecution of the objectives of the project will make possible:

  • To increase the knowledge about the behaviour of these steels versus welding processes.
    • To develop know-how for the production of TRIP steels and future new steels in Spain.
    • To seek innovative solutions in order to solve the actual limitations of the most common commercial grades.

In the work presented, the main action performed is to study the effect of laser radiation on the microstructure and the mechanical response and weldability of experimental and commercial TRIP steels mainly for two types of joints: the autogenous lap weld employed in reinforcements and in subsets of security and commitment, and the butt weld employed in Tailored Welded Blanks.

The main conclusions obtained are:

  • It was possible to obtain, at laboratory scale, TRIP steels that present similar welding properties to commercial TRIP steels.
  • The butt laser welds present higher tensile strength than the base material, but lower formability.
  • When laser welding, the fusion zones present very high hardness values. No decrease of hardness respect to the base material was observed in the HAZ.
  • Using LBW process, the formability of dissimilar TRIP-XES joint is higher than the DP-XES joint.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Analysis of penetration depth fluctuations in single-mode fiber laser welds

Posted on by
Reply

By: Jung-Ho Cho, Dave F Farson, Matt J Reiter

The Ohio State University, Laboratory for Multiscale Processing & Characterization,

Single-mode fiber lasers produce high power beams with nearly perfect beam quality, meaning that they can be focused to a very small diameter spot with relatively long focal length optics. It is becoming increasingly clear that these laser beam optical qualities are not always ideal for welding. Welds made with these lasers are uniquely sensitive to a defect known as penetration spiking. Such abrupt fluctuations in weld penetration depth have long been a problem in electron beam welding but have not been observed in laser welding before the advent of high power single-mode lasers. In this work, the effect of laser power, travel speed and focus length and spiking severity was studied and techniques for reducing spiking were demonstrated. As an initial step, the frequency response of the weld penetration depth to sinusoidal power modulations was quantified. It was found that the laser weld keyhole responded as a second-order dynamic system for modulation frequencies in the range from 100Hz to 1000Hz. Thus, at upper end of this range, the sinusoidal response of the laser weld penetration to the sinusoidal power modulation was practically undetectable above the background noise of natural “random” spiking fluctations. However, the frequency response tests also showed that  power modulation in the frequency range from 900Hz to 3kHz had the good effects,  significantly decreasing the magnitude of the spiking penetration fluctuations. At some frequencies, the sinusoidal power modulation was able to completely eliminate spiking, but the effect was very sensitive to parameters and hence not very reliable for actual applications.  A second technique for spiking suppression in electron beam welds is “beam stirring”, where the focus spot is scanned in small circles at high frequency as it is scanned along the weld joint at the welding travel speed. This beam stirring technique was investigated for spiking suppression in the single-mode fiber laser welding process using a galvanometer scanner to produce simultaneous circular oscillation and linear travel of the focus spot. This spiking suppression technique was found to be much less sensitive to parameter settings and nearly eliminated spiking over broader ranges of circular oscillation frequency and diameter.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Comparative analysis of GMA, laser, and laser-GMA hybrid welding

Posted on by
Reply

By: Siltanen Jukka

Laser and laser hybrid welding processes have attracted interest during the last few years in engineering industry. The basic reason is the need to increase the productivity in order to be more competitive in global markets. In comparison to traditional arc welding processes, laser welding has several benefits such as low heat input resulting in low amount of distortion, deep weld penetration and high welding speed, high quality of welds and possibility to weld a wide spectrum of materials. However, laser welding means that the groove and processing tolerances become tighter as the focal point diameter of the laser beam used for deep penetration welding is very small. In one view this can be seen as a benefit, because it improves the accuracy of a product or component.

Laser hybrid welding (combination of laser and arc welding) permits looser groove tolerances compared to laser welding. The use of filler metal enables the filling of gaps formed during welding and due to inaccuracies in edge preparation or other earlier manufacturing phases as well as gives the possibility to mix the weld metal such to achieve the desired mechanical properties. Another restriction of laser welding is the high investment cost of the laser source and the welding system with a high accuracy. On the other hand, during the recent years the prices of the laser sources have lowered, but still they are a lot more expensive than the arc welding sources.

In 2006 Rautaruukki Oyj (www.ruukki.com) started a project called Rlaser, the aim of which was to find new ways to use laser technologies and discover new business areas and markets. All three business divisions (Construction, Engineering, and Metals) of Rautaruukki Oyj participate in the project. The project has several independent sub-projects focused on the research areas like mechanical properties of the welds and products, optimizing of the manufacturing chain, economical and technical comparison of the production methods and measuring techniques. The conference paper “Comparative analysis of GMA, laser, and laser-GMA hybrid welding” in ICALEO 2009 is especially focused on the mechanical properties of the joint of mild steel S355. Figure 1 shows the testing arrangement of laser-GMA hybrid welding.

Currently, Rautaruukki Oyj concentrates increasingly on the manufacturing of high and ultra high strength steels (up to 1100 MPa yield strength) and therefore in further studies these steels have an essential role. The use of laser and laser hybrid welding as a joining method is very attractive for such special steels. This is due to the established limits for welding energy and cooling time which are accomplished easily with the laser and laser hybrid welding.

Figure 1. Laser-GMA hybrid welding of mild steel S355

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Numerical Calculation of Laser Beam Path Influenced by High Temperature Gas Above Specimen During Laser Welding

Posted on by
Reply

Beam deflection confronting again?  Take care of media bias during laser material processing.

By: Masami Mizutani and Seiji Katayama

Joining and Welding Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, Japan

It has been generally accepted that the laser beams of about 1 mm in wavelength, such as fiber, disk and YAG lasers have smaller interaction to the media such as plume and plasma between the optics system and the specimen than CO2 laser beam of 10.6 mm.

However, we have recently found that weld bead formation is greatly affected by the state of the gas flow, the atmosphere, the ultrafine particles or the plasma plume above the specimen during fiber laser remote welding. The result showed that a laser beam tended to be deflected and focused on the far position without the positive elimination of the media above the specimen.    Meanwhile, mirage effects and heat haze effects are seen as natural phenomena by varied temperature of the air, which suggest that the air possessing the temperature gradient enable the ray to deflect.

Therefore, in order to understand what extent the temperature distribution of the media between the optics and the specimen affects the laser beam path, a theoretical calculation of the laser beam trajectory in the assumptive media, which possesses refractive index gradients led by temperature gradients, was carried out.  As one of the important results showing in the figure, the beam trajectories propagating through a high-temperature media below, show that the beam diameter at the original focal point (200 mm) is broadened and the concentration area of the beams shifts downward.  This result may interpret why the laser beam focused on the far position without the positive elimination of the media above the specimen during fiber laser remote welding, which has been experienced.

It should be emphasized that the trajectory of the laser beam, the wavelength of which is about 1 mm, is deflected to a certain extent, by the media possessing the refractive index gradient led by the thermal gradient, which is presumably attributed to the thermal and plume-jet-induced updraft, and the reheat of ultrafine particles.  Therefore, it is concluded that the beam profile of the laser, which has propagated through the gaseous matter possessing the thermal gradient, is no longer the same as originally intended.

Computing results of focused beams propagating from entrance to focal point of 200 mm, assumed for original rays without deflection to be focused at, showing differences in beam trajectories between with and without refractive index gradient.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper 1403

Monitoring of Hybrid Laser- Arc Welding of Galvanized Steels in a gap-free lap joint configuration

Posted on by
Reply

By: Shanglu Yang, Rouzbeh Sarrafi, and Radovan Kovacevic

Research Center for Advanced Manufacturing

Southern Methodist University

Dallas, TX USA 75205

With respect to excellent mechanical properties and corrosion-resistant capability, galvanized steels have been used intensively in different industries such as in the automotive industries. However, the successful welding of galvanized steels in a gap-free lap joint configuration is still a great challenge. The highly-pressurized zinc vapor is readily developed at the interface of the two metal sheets due to the lower boiling point of zinc (around 906 ˚C) than the melting point of steel (over 1500˚C). The failure of mitigating the highly pressurized zinc vapor at the interface of the two metal sheets leads to the formation of different weld defects such as the spatters and blowholes, which not only damage the weld surface quality but also deteriorate the mechanical properties of welds. In order to mitigate the effect of the highly pressurized zinc vapor, many welding methods have been developed which includes the removal of zinc coating at the interface of two metal sheets, redesigning the lap joint configuration,  setting of alloy element at the interface, and so on. All of these methods require the pre-processing and post-processing actions and are costly to be used in the practice.

An innovative hybrid welding procedure, which combines the laser welding with the gas tungsten arc welding (GTAW) used as a  preheating source, has been successfully developed to weld the galvanized high strength dual phase steels in a gap-free lap joint configuration. In this new welding procedure, GTAW that leads the laser beam at the specific distance is used to preheat the workpieces. Under the controlled heat input from the GTAW, zinc coating at the top surface is burned and the metal oxides are generated at the top surface of workpience. At the same time, the zinc coating along the weld zone at the interface of two metal sheets is transformed into the zinc oxides, which has the high melting point than that of steel. Furthermore, the thin layer of the generated metal oxides at the top surface of workpiece dramatically increases the coupling of laser beam energy into the welded material. Under these welding conditions, the stable keyhole is produced, which provides the channel for the highly pressurized zinc vapor to be vented out. This new welding procedure offers an efficient and robust way for lap joint of galvanized steels in a gap-free configuration and can be practically used in the industry. The completely defect-free lap joints are achieved. In comparison with the other methods for welding of galvanized steels in a gap-free lap joint configuration, the productivity efficiency can be dramatically increased by the use of this new welding procedure. At the same time, it avoids the high time-consuming and cost of pre-processing and post-processing associated with the traditional ways of welding galvanized steels in a gap-free lap joint configuration.

A machine vision system is also developed to real-time record the images of molten pool during the welding process, which is based on a high speed CCD camera with the frame rate of 4000 fps and one green laser as the illumination source. By the analysis of the recorded images of molten pool, the instability mechanism of laser welding of galvanized steels in a gap-free lap joint configuration is better understood. Furthermore, the developed machine vision system can be used to on-line monitor the various defect formations and provides the calculated geometry of molten pool for controlling the weld quality, thus accomplishing the automation of welding process.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here