Flemming Ove Olsen

Review of laser hybrid welding

In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more efficiently in the future.In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more ...

Hybrid laser-arc welding

Part 1 Characteristics of hybrid laser-arc welding: Advantages and disadvantages of arc and laser welding Fundamentals of hybrid laser-arc welding Heat sources of hybrid laser-arc welding processes Effect of shielding gas on hybrid laser-arc welding Properties of joints produced by hybrid laser-arc welding Quality control and assessing weld quality in hybrid laser-arc welding. Part 2 Applications of hybrid laser-arc welding: Hybrid welding of magnesium alloys Shipbuilding applications of hybrid laser-arc welding Industrial robotic application of laser-GMAW and laser-Tandem hybrid welding Hybrid laser-arc welding of aluminium Hybrid laser-arc welding of dissimilar metals. Part 3 Hybrid laser-arc welding of steel: Hybrid laser-arc welding of steel.

Fundamental mechanisms of cutting front formation in laser cutting

This paper includes a theoretical description of some of the major physical phenomena of thermal cutting processes in general. The work is mainly focussed on the laser cutting process. Qualitatively and some quantitatively descriptions of the melt front propagation and the melt flow is described. The theory, supported by experimental studies shows, that in high quality thermal cutting, the melt flow is in front of the cut kerf with a uniform pressure at the melt surface. Furthermore is shown, that at high cutting rates, evaporation in the lower central part of the cut kerf forces the melt partially around the laser beam, reducing the cut quality. Theoretical estimation of the melt film thickness in thermal cutting is derived. It is shown that there is a maximum cutting speed, where substantial evaporation in the cut front can be neglected. This maximum cutting speed depends upon the thermal properties of the material, the thickness of the material and the pressure from the cutting gas. Furthermore the paper indicates thermal instabilities in the top of the cut front in thermal cutting processes, which is supposed to be responsible for the striation formation.

Cutting Front Formation in Laser Cutting

Summary In order to be able to describe and understand the laser cutting process, the cut front has to be investigated in details, which means that the temperature distribution on the melt surface in the cut front, the melt flow pattern, the temperature gradients through the melt and the melt front propagation through the solid material must be considered. In the paper, theoretical considerations based upon experimental work are reported. These considerations include the thermal energy balance and the balance of momentum of a finite element of the melt surface. Evaluation of the cut front geometry leads to some factors, which can cause the striation formation in laser cutting.

Pulsed Laser Materials Processing, ND-YAG versus CO2 Lasers

Abstract Pulsed laser materials processing is widely applied in fine cutting, welding and hole drilling. In this paper pulsable CO2-lasers and ND-YAG-lasers are compared for pulsed laser cutting, welding and hole drilling. In laser cutting the performance of a superpulsed CO2-laser in Aluminium cutting is described and compared to typical ND-YAG-laser cutting data. In pulsed laser welding, experimental investigations in welding AISI 316 stainless steel, where hot cracking sensitivity has been addressed, applying a ND-YAG-laser and a superpulsed CO2-laser, will be described. Finally metal, polymer and ceramics laser drilling applying different CO2-lasers will be described and compared to ND-YAG-laser performance.

Multibeam fiber laser cutting

The appearance of the high power high brilliance fiber laser has opened for new possibilities in laser materials processing. In laser cutting this laser has demonstrated high cutting performance compared to the dominating cutting laser, the CO2 laser. However, quality problems in fiber-laser cutting have until now limited its application to metal cutting. In this paper the first results of proof-of-principle studies applying a new approach (patent pending) for laser cutting with high brightness and short wavelength lasers will be presented. In the approach, multibeam patterns are applied to control the melt flow out of the cut kerf resulting in improved cut quality in metal cutting. The beam patterns in this study are created by splitting up beams from two single mode fiber lasers and combining these beams into a pattern in the cut kerf. The results are obtained with a total of 550 W of single mode fiber laser power. Burr free cuts in 1 mm steel and aluminum and in 1 and 2 mm AISI 304 stainless steel is d...

Laser welding closed-loop power control

A closed-loop control system is developed to maintain an even seam width on the root side of a laser weld by continually controlling the output laser power of a 1500 W CO2 laser. Quality control is done by a photodiode monitoring the root-side light emission from the process. The control system is successfully demonstrated to work in the bead-on-plate configuration on 2 mm sheets, both at constant speeds and at speeds ranging from 0.32 to more than 0.72 m/min in one welding trial. The power control system is able to deal with sheets of variable thickness in one welding trial, with the following thickness changes made as partly lap welds: 1.25 to 1.25 mm, 0.5 to 2 mm, and 1.25 to 2 mm. Sheets of 0.5 mm lap joined to 2 mm sheets displayed even root seams along the entire seam, except at the immediate edges of thickness change.

Process monitoring during CO2 laser cutting

On-line process control equipment for CO2 laser cutting is not available for industrial applications today. The majority of the industrial cutting machines are regulated off-line by highly-educated staffs. The quality inspection of the samples often is visual, and referred to different quality scales. Due to this lack of automatization, potential laser users hesitate to implement the cutting method and hereby to benefit from the advantages offered by the method. The first step toward an automatization of the process is development of a process monitoring system, and the investigation described in this paper is concentrated in the area of on-line quality detection during CO2 laser cutting. The method is based on detection of the emitted light from the cut front by photo diodes. The detection is made co-axial with the laser beam to assure independence of the chosen processing direction. ZnSe mirrors have been placed in the beam path, reflecting the laser beam but transmitting the visible light emitted from the process. Cut series of 2, 6 and 8 mm mild steel have been performed. Fourier Analyses and statistical analyses of the signals have been undertaken, and from these analyses it is possible to estimate the surface roughness in the cut kerf, dross attachment at the backside of the work piece and the penetration of the laser beam.

Comparison of plasma, metal inactive gas (MIG) and tungsten inactive gas (TIG) processes for laser hybrid welding

In this paper, TIG, plasma, and MIG processes have been individually combined with a 2.6 kW CO2 laser. In a number of systematic laboratory tests, the general benefits and drawbacks of each process have been individually assessed and compared. Aspects such as ease of integration with a CO2 laser source, ignition and running torch stability, weld phase transformation and change in ductility and overall weld quality are described.The results show that all three processes can successfully be integrated with a CO2 laser beam for hybrid welding. Due to the pilot arc in plasma welding, this process enables a more stable ignition and running process than both TIG and MIG hybrid welding. Because of the delivery of extra material from a hot wire, the MIG hybrid process is well suited for bridging gaps of up to 0.6 mm in butt-welding of 2 mm steel. But because of the constant delivery of new material, the MIG process is more difficult to control than laser/plasma and laser/TIG processes.All three types of secondary heat sources enable an increased ductility of the weld as compared to pure laser welding when welding 1.8 mm GA 260 with a TIG torch and 2.13 mm CMn steel with a plasma arc or MIG. For the TIG, plasma, and MIG the reductions in hardness are 19, 27 and 33 %, respectively.In this paper, TIG, plasma, and MIG processes have been individually combined with a 2.6 kW CO2 laser. In a number of systematic laboratory tests, the general benefits and drawbacks of each process have been individually assessed and compared. Aspects such as ease of integration with a CO2 laser source, ignition and running torch stability, weld phase transformation and change in ductility and overall weld quality are described.The results show that all three processes can successfully be integrated with a CO2 laser beam for hybrid welding. Due to the pilot arc in plasma welding, this process enables a more stable ignition and running process than both TIG and MIG hybrid welding. Because of the delivery of extra material from a hot wire, the MIG hybrid process is well suited for bridging gaps of up to 0.6 mm in butt-welding of 2 mm steel. But because of the constant delivery of new material, the MIG process is more difficult to control than laser/plasma and laser/TIG processes.All three types of secondary...

Theoretical Investigations In The Fundamental Mechanisms Of High Intensity Laser Light Reflectivity

In laser processing a major physical phenomenon is the coupling of the laser energy into the workpiece. As high power infrared laser beams are directed to metallic targets, the reflectivity of the target shows a decrease. This paper describe some theoretically investigations in the laser light coupling to metallic targets. The validity of some of the reflectivity theories in relation to high power laser irradiation will be discussed based upon numerical calculations of the theoretical coefficients of reflectivity. Some possible mechanisms responsible for the changes in the optical properties of metal surfaces irradiated by high intensity laser beams, such as multi-photon absorption and effects of inhomogenous target heating will be discussed. One simple model describing the reflectivity as a function of material properties, commonly used in the litterature, is based upon the Hagens-Rubens equations. However, in this paper will be shown, why this model is not valid in the wavelength spectrum of high power lasers.