Laser welding of structural steels: Influence of the edge roughness level

Laser welding continues to become more extensively used in many industrial applications and in the last 10 years an increasing number of studies have examined ways to increase the efficiency of the process. This study investigates the influence of joint edge surface roughness on weld quality and penetration depth. The characteristics are investigated of welded samples of two low alloyed steels, S355 and St 3, of 20 mm thickness with various joint edge surface roughness levels in butt joint configuration. Welding was performed with different fiber lasers with a wavelength of 1070 nm at power levels from 10 to 15 kW. The absorption characteristics were evaluated at 10 kW power level using a calorimeter. There was a significant positive correlation between edge surface roughness level and the penetration depth. Optimum roughness levels to provide maximum penetration depth are presented. The highest penetration depth at power levels of 14 and 10 kW was achieved at R_a=6.3 μm.     

Laser microspectral analysis of steels

The use of laser microspectral analysis for the quantitative measurement of silicon and chromium, present in concentrations as low as 0.08% in mild steels, is investigated. A comparison is made between the use of Q-switched and normal ruby laser operation, both spark assisted. Time integrated electron temperature measurements of iron plasmas and time resolved spectra of A1 plasmas are presented and used to determine if the ‘local thermodynamic equilibrium’ model applies to these laser initiated plasmas.     

Laser welding of PC boxes

The activities connected with introduction of laser welding into the cycle of PC box manufacturing are briefly described. A laboratory comparison among three Nd:YAG sources with powers of 550W, 1000W and 2000W is reported. The use of lasers introduced original solutions in welding, with important changes in the manufacturing cycle and consequent cost reductions related to painting and handling. Other advantages such as higher production rates, improved quality, reduced maintenance time, ability to generate a just-in-time policy, the possibility of upgrading with no set-up cost, and related consequences on costs, are discussed.     

Laser surface treatment of tool steels

Laser surface treatment is a promising technique for improving the wear and corrosion resistance of materials. In the case of tool steels, laser surface treatment is preferably carried out in the liquid state to allow for complete dissolution of carbides. This paper concerns the application of laser melting to the surface treatment of AISI 420 and 440C martensitic stainless steels and sintered AISI T15 high-speed steel. Usually, laser-melted tool steels contain martensite, retained austenite and carbides. In steels containing large proportions of ferrite-forming alloying elements, -ferrite may also be observed. When applied to sintered steels, laser treatment leads to the elimination of residual porosity. The proportion of retained austenite in laser-melted steels is much higher than in conventionally treated steels. However, the hardness is high because austenite is strengthened by solid solution, dislocations and small grain size. The high volume fraction of retained austenite usually prohibits the application of tool steels in the laser-treated condition. Austenite may be eliminated by multiple tempering treatments at temperatures in the range 550–650°C. During tempering, carbides precipitate within austenite and martensite, and austenite transforms to martensite on cooling or isothermally to ferrite. Strong secondary hardening is often observed and the temperature of the secondary hardening peak of laser-surface-melted steels is higher than after conventional heat treatment.     

Laser welding with filler wire

New applications such as welding of material combinations and the ability to fill opening gaps between the workpieces offer new prospects for laser beam welding processes with filler wire. To guarantee good quality, vertical distance variations between wire tip and weld pool are, above all, not permissible as this causes globular metal transfer and would accordingly result in strongly rippled, unclean welds. A process-internal signal, recorded by a sensor, helps to solve this problem. The automatic tracking of the vertical wire position is possible on-line via a controller. In this way, the running process can be optimized and a consistently good weld quality can be achieved.     

Laser surface alloying of plain carbon steels

Deposition and alloying of metallic powders containing WC on a ferritic-pearlitic plain carbon steel have been investigated. Metallic powder was injected into the melted surface layer of the samples under Argon atmosphere during the laser irradiation. The sample surface was previously graphitizing treated to prevent energy loss by reflection. For comparison, irradiated samples, without powder alloying, were also analysed. The coating layer is mainly composed of alloyed α-Fe; γ-austenite and non-magnetic M₃C carbide are also present. Partial dissolution of WC and the preliminary graphitizing treatment are the causes of the rather high carbon concentration in γ-austenite. As it is to be expected this concentration decreases with increasing distance from the external surface of the coating layer.     

Laser milling of martensitic stainless steels using spiral trajectories

A laser beam with sub-picosecond pulse duration was driven in spiral trajectories to perform micro-milling of martensitic stainless steel. The geometry of the machined micro-grooves channels was investigated by a specifically conceived Scanning Probe Microscopy instrument and linked to laser parameters by using an experimental approach combining the beam energy distribution profile and the absorption phenomena in the material. Preliminary analysis shows that, despite the numerous parameters involved in the process, layer removal obtained by spiral trajectories, varying the radial overlap, allows for a controllable depth of cut combined to a flattening effect of surface roughness. Combining the developed machining strategy to a feed motion of the work stage, could represent a method to obtain three-dimensional structures with a resolution of few microns, with an areal roughness S_a below 100 nm.     

Thermal energy balance in laser welding processes of austenitic stainless steels

Deep penetration weldings with a 2kW CO₂ laser were performed on different austenitic stainless steels in a wide range of thicknesses for two different assistant gases.The energy actually transferred to the samples to perform the welding process was calculated in terms of the LSM model for the different steels examined. Then, the trends of the efficiencies versus the thickness of the samples welded were plotted. These curves have a maximum which corresponds to an execution speed value which is approximately the same for all the different steels and is, moreover, equal to the heat propagation speed inside the material.Furthermore, a semi-empirical rule which takes into account both the thermophysical properties of the steel and the laser power is suggested for evaluating the trend of the weld width with respect to the process speed.     

Laser power stabilization for second-generation gravitational wave detectors

We present results on the power stabilization of a Nd:YAG laser in the frequency band from 1 Hz to 100 kHz. High-power, low-noise photodetectors are used in a dc-coupled control loop to achieve relative power fluctuations down to 5 x 10(-9) Hz(-1/2) at 10 Hz and 3.5 x 10(-9) Hz(-1/2) up to several kHz, which is very close to the shot-noise limit for 80 mA of detected photocurrent on each detector. We investigated and eliminated several noise sources such as ground loops and beam pointing. The achieved stability level is close to the requirements for the Advanced LIGO gravitational wave detector.     

Laser welding of thin polymer films to container substrates for aseptic packaging

Keyhole laser welding of polymers is a subject well covered and researched, but relatively little information exists regarding the welding of thin polymer films, particularly to a heavier substrate. This paper presents the design of a suitable test apparatus for laser welding thin film to a heavier substrate, and shows the results of an investigation into the feasibility of laser welding multi-layer polymer film lids to tubs for the manufacture of aseptic food containers. A consistent weld, free from defects, is the key to process success. Typical welding defects have been synthesised in order to investigate, and consequently remove, their cause. The result is a reliable welding method based on even film clamping. With careful attention to machine design, a seal of high mechanical strength and chemical integrity is possible.     

Laser welding in a high pressure gas environment

A study was undertaken to investigate the effect of a high pressure gas environment on the laser welding mechanism. This was specifically related to high power CO₂ lasers in the power range between 1.2 and 5 kW. A small high pressure chamber rated up to 150 bar was utilized for the trials. Successful laser welding was completed up to a pressure of 50 bar in the pressurized helium environment. The chamber was modified to incorporate a high pressure transmissive zinc selenide window and internal focusing optics. The initial welds exhibited wide and shallow profiles indicating a loss of keyhole penetration welding. By filming the welding action the problem was found to be the formation of a plasma approximately an order of magnitude larger than in normal atmospheric conditions. The solution was to implement a gas jet system and to use a higher power laser. The resulting welds in terms of penetration and quality were significantly improved.     

Laser welding of thin foil nickel-titanium shape memory alloy

In this study, two L27 Taguchi experiments were carried out to study the effect of fibre laser welding parameters and their interactions upon the weld bead aspect ratio of nickel-titanium thin foil. The optimum parameters to produce full penetrated weld with the largest aspect ratio and desirable microstructure were successfully obtained by the Taguchi experimental design. The corrosion property of the optimized NiTi weld in Hank’s solution at 37.5 °C was studied and compared with the as-received NiTi. To improve the corrosion properties of the weld, the effect of post-weld-heat-treatments ranging from 573 to 1173 K was investigated. The corrosion properties, surface morphology, microstructure and Ti/Ni ratio of the heat-treated NiTi weld were analysed. It was found that a post-weld heat treatment at 573 K for 1 h provided the best pitting corrosion resistance at the weld zone.     

Maximum obtainable power of a carnot combined power plant

Maximum obtainable power of a combined endoreversible carnot cycle is analyzed. It is found that there is a bound on the obtainable power of real combined cycles. This bound provides a new theoretical criterion for the evaluation of existing combined power generating systems or for influencing the design of future combined heat engines.     

Laser welding of aluminium lightweight materials: problems,solutions, readiness for application

Using high-power CO₂ and Nd:YAG lasers of high beam quality, high process efficiencies and excellent seam qualities are achieved. A particular method for obtaining almost pore-free weld seams without blowholes is the combining of the beams of two CO₂ lasers. Without the need for filler material, crack-free welds can be produced in sheets of hot-crack susceptible, precipitation hardened alloys up to a welding speed of 5–7m min^-1 for full penetration and up to 3–4m min^-1 for partial penetration. In contrast, AlMg alloys containing more than 2.5wt% Mg and AlSi cast alloys are insensitive to hot-cracking even at high processing speeds. Laser welds possess much better static mechanical properties than gas tungsten arc (GTA) or gas metal arc (GMA) butt welds. For the alloys AlMgSi1 and AlMg5Mn the maximum static strength which can be achieved in laser welding is determined by the alloy type, i.e. the hardening mechanism and the heat-treated condition. Laser butt welded car body sheets without filler material exhibit the same load-bearing capacity under dynamical load as GMA welds with filler material. The latest research work has demonstrated that high-quality tailored blanks with good mechanical properties can also be made out of different aluminium plates.     

Laser welding of low-thickness zinc-coated and uncoated carbon steel sheets

Laser welding is a competitive joining process for low-thickness plates. The speeds achieved with laser welding using a CO₂ laser and the low heat input introduced to the base material are the main advantages of this process. The widespread use of laser welding will increase the productivity and accuracy of welding operations. Nevertheless, there are still important developments to be made.The results of laser welding of thin sheets (below 1.25 mm) of uncoated and zinc-coated carbon steels are presented. A CO₂ laser was used for the welding of sheets of different thicknesses and to make dissimilar welds (carbon steel to zinc-coated steel). The bead dimensions and the distortions of the welded sheets are analysed.     

Laser welding of low carbon steel and thermal stress analysis

Laser welding of mild steel sheets is carried out under nitrogen assisting gas ambient. Temperature and stress fields are computed in the welding region through the finite element method. The residual stress developed in the welding region is measured using the XRD technique and the results are compared with the predictions. Optical microscopy and the SEM are used for the metallurgical examination of the welding sites. It is found that von Mises stress attains high values in the cooling cycle after the solidification of the molten regions. The residual stress predicted agreed well with the XRD results.     

Laser applications in nuclear power plants

This paper reports the state of the art of using a solid-state Nd:YAG laser for material processing applications such as cutting, welding and drilling of several components of operational nuclear reactors in radioactive environment. We have demonstrated several advantages of laser-based material processing over conventional methods, and these are discussed briefly. At NPCIL, we have used laser techniques to cut stainless steel sheets up to 14 mm thickness and stainless steel weld up to a depth of 3 mm. This remotely operable laser system has been engineered for its robustness with proper fixtures and tooling for various material processing operations on industrial scale.     

Pulsed Nd:YAG laser welding of AISI 304 to AISI 420 stainless steels

The technique to weld AISI 304 stainless steel to AISI 420 stainless steel with a pulsed Nd:YAG laser has been investigated. The main objective of this study was to determine the influence of the laser beam position, with respect to the joint, on weld characteristics. Specimens were welded with the laser beam incident on the joint and moved 0.1 and 0.2 mm on either side of the joint. The joints were examined in an optical microscope for cracks, pores and to determine the weld geometry. The microstructure of the weld and the heat affected zones were observed in a scanning electron microscope. An energy dispersive spectrometer, coupled to the scanning electron microscope, was used to determine variations in (weight %) the main chemical elements across the fillet weld. Vickers microhardness testing and tensile testing were carried out to determine the mechanical properties of the weld. The results of the various tests and examinations enabled definition of the best position for the incident laser beam with respect to the joint, for welding together the two stainless steels.