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

Abstract

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.     

Derivation of current density distribution by arc pressure measurement in GTA welding

Abstract

Arc pressure is an important factor in understanding physical arc phenomena, especially its effects on the penetration, size and shape of GTA welding. The purpose of the present study is to derive the current density distribution on the anode base metal from the measured arc pressure distribution in GTA welding using the results from previous investigators. Using the measured arc pressure distribution on the anode base metal in GTA welding from the central part to the circumference and the equations of Lin et al. and Maecker, the current density distribution was derived. Applying the derived equation from the present work, the current density distribution was calculated from the low current region to the high current region by means of shield gas mixing ratio. It is compared and discussed with the practical welding current and the derived current by numerical integration of the current density distribution from the central part to the circumference region.     

Arc heating hot wire assisted arc welding technique for low resistance welding wire

Abstract

An assistant arc was used to preheat the wire for hot wire tungsten inert gas (TIG) welding which was independent on the resistance of the welding wires and avoided the drawbacks of the traditional hot wire preheating method. The new method is suitable for low resistance wire such as copper and aluminium. The experimental results showed that the wire temperature varied linearly with preheating current and hyperbolically with wire feeding speed. The temperature of wires achieved 60% of their melting points when LF6, H90, HS201 and H08Mn2Si welding wires were used at a current less than 50 A. With arc assisted hot wire, the welding deposition efficiency of the HS201 wire was increased by 96% compared with the traditional TIG welding method, while its microstructure was similar to that of the cold wire welding.     

Observations of the phenomenon of abnormal arc voltage occurring in pulsed metal inert gas welding of aluminium alloy

Abstract

At the time of arc reignition after short circuiting during electrode positive polarity, cathode spots are newly formed in the centre of the weld pool surface, where oxides scarcely exist. The work function of the cathode surface increases and the cathode spots concentrate because of the lack of sufficient oxides, leading to an increase in the potential gradient across the cathode fall space and the adjoining contraction space. Consequently, the arc voltage becomes abnormally high in spite of the short arc length. Moreover, when electrode polarity is switched from positive to negative immediately after a droplet has detached from the wire tip, cathode spots are newly formed on the surface of the molten metal remaining at the wire tip, where little oxide exists, leading to an abnormal increase in arc voltage as well. Therefore, the change in arc voltage does not necessarily indicate a fluctuation in the arc length.     

Sensing and seam tracking of welding line in backingless V groove welding

Abstract

The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors tried to apply the switch back welding method to butt welding to join discs with flat metal plates without backing plates. During the welding, the gap was detected by processing the weld pool image taken with a CCD camera. Moreover, the tip of the electrode wire is changing owing to a curve of its wire extension in accordance with a rotation of the torch axis. Even though teaching is achieved before the welding, the weaving centre is shifted from the gap centre. The controller of the seam tracking was designed to prevent any steady state error.     

Hybrid laser/arc welding process for controlling bead profile

Abstract

A laser hybrid welding process in which a defocused laser beam is applied beside a gas metal arc weld (GMAW) pool to modify the bead shape was studied. The present paper aims to produce welds with improved toe geometry and better fatigue life than those made with GMAW alone and to apply a numerical simulation to help configure the hybrid process. First, stationary hybrid welds were made to validate weld bead shape predictions and to characterise the spreading of the arc weld deposit to the laser heated spot. Next, the travelling hybrid process was configured with the aid of simulations and fatigue test specimens were welded. Proper application of the laser heat input induced molten metal to spread to the laser heated area, increasing the fillet weld leg length. This produced a larger weld toe angle that decreased the stress concentration and increased the fatigue life of the welds relative to standard mean values.     

Laser stabilisation of arc cathode spots in titanium welding

Abstract

Cathode spot formation is very pronounced during arc welding of titanium and titanium alloys. The dynamic behaviour of these spots was observed to interfere with metal transfer during welding, this interference being a fundamental cause of poor weld quality in these alloys. In the present work, stabilisation of the arc cathode spot with a focused Nd–YAG laser beam during pulsed gas metal arc welding of titanium was investigated. The laser beam was focused near the leading edge of the weld pool and the laser power and focus spot size were varied to determine the values required to confine the cathode spot to the laser focus position. The results showed that, for fixed welding conditions, the laser power required to prevent cathode spot motion varied as a function of focus spot size. The required laser power was minimised at 200 W for a spot size of 0.6 mm. The laser stabilised arcs had lower voltage but approximately the same current density as stabilised arcs. Increased welding speeds required marginally higher laser powers to stabilise the spot, but the minimum power was still attained with a 0.6 mm focus spot diameter. The laser power density required for stabilisation decreased as spot size was increased, varying from almost 10⁶ W cm^?2 at the smallest spot size to approximately 10⁴ W cm^?2 at the largest. Cathode spot stabilisation improved weld quality by reducing spatter generation and weld bead irregularity.     

Influence of welding current shape on expulsion and weld strength of resistance spot welds

Abstract

The present paper presents the influence of welding current shape on weld strength of resistance spot welds of zinc coated mild steel sheets. The influence is analysed at different levels of the electrode wear. Welding currents with different peak values and different RMS (root mean square) values were used in the experiment. The results show that welding current with high peak values implies higher weld strength.     

Analytical thermal model of conduction mode double sided arc welding

Abstract

An analytical thermal model of conduction mode double sided arc welding (DSAW) has been derived and used to predict the weld pool dimensions and shapes and temperatures within 2˙5 and 1˙15 mm thick AA5182 Al alloy sheets as functions of the primary DSAW parameters. Separate Gaussian distributed arc heat sources from a plasma arc welding and gas tungsten arc welding torch were assumed to act on the top and bottom surfaces of the sheets. There was excellent correlation between observed and predicted DSAW weld pool dimensions and shapes provided that suitable values for arc efficiencies and distribution coefficients for the two separate arcs were used in the model. The model is capable of predicting weld pool dimensions and shapes of both full and partial penetration conduction mode DSAW welds made in Al alloy sheet, the welding speed at which there is a transition from full to partial penetration welding and the speed above which no melting occurs.     

Study of single side single pass submerged arc welding using reusable backing strip

Abstract

An attempt has been made to establish a submerged arc welding procedure that will enable the production of welded butt joints in thin steel plate, having proper fusion and reinforcement geometry in a single welding run. In this work, the combined effect of the basic welding parameters, i.e. voltage, current, and speed, along with the effect of thickness, on weld penetration have been studied. Also an attempt has been made to develop a flux filled reusable backing strip. In the present investigation submerged arc welding flux in its powdered form has been used without application of any external pressure and also without any additives. Experiments have been carried out extensively on 6 mm and 8 mm thick C–Mn steel plate specimens. This form of backing strip has shown great promise towards achieving single side single run welding. The welds achieved in this single side single pass welding process are not yet 100% satisfactory, however, the results indicate the feasibility of the process to achieve quality welds to meet relevant quality standards.     

Plasma channel formation for triggering arc welding

Abstract

A method of starting arc welding using a plasma channel formed between electrodes in a tungsten inert gas arc welding system was demonstrated. The plasma channel was generated by gas breakdown in the laser beam path. In a previous study by the present authors, the arc welding could be started using a laser produced plume. Results in the present study indicated that the laser energy required to start the process using the plasma channel was lower than that using the plume.     

Development of low spatter CO2 arc welding process with high frequency pulse current

Abstract

Metal transfer phenomena and spatter generation in CO₂ arc welding with a solid wire were investigated, and a low spatter welding process using a high frequency pulse rectangular current was developed. The optimal conditions of high frequency pulse CO₂ arc welding were determined to be a peak current of 450–550 A and pulse frequency of 450–750 Hz. These high frequency pulse currents influenced the droplet oscillation due to resonance between the applied pulse frequency and the natural frequency of the droplet. A droplet was regularly transferred by 9–11 pulses, and the average interval of metal transfer was ～16 ms, which was half of that in conventional CO₂ arc welding. The average droplet weight is 34 mg, showing a large reduction in comparison with that of the conventional method. As a result, the total spatter weight was reduced by 70% in comparison with the conventional method, and particularly large spatters more than 0·5 mm in diameter were reduced from 25 to 3 mg s^?1.     

Voltage change in the GMAW process due to the influence of a droplet travelling in the arc

Abstract

Experimental measurements have been made to investigate the meaning of the fluctuation or noise of electrical signals for the gas metal arc welding process through globular to spray transfer mode, with particular attention being paid to the so-called 'drop spray' transition mode. The results reveal that the welding arc voltage is significantly affected by the molten droplet travelling in the arc. A sharp drop in arc voltage appears just after the detachment of the droplet, reaching its minimum quickly and then recovering, according to the location of the droplet in the arc. Although the full explanation of the feature requires further study, we believe that an important influence is the geometrical effect. The existence of the droplet in the arc path significantly affects the geometrical shape of the arc and arc attachment at the anode, and hence the overall voltage of the arc.     

Arc power and efficiency in gas tungsten arc welding of aluminium

Abstract

A calorimetric study of gas tungsten arc welding of aluminium is described. The present study comprised experiments in which autogenous welding runs were each made on a block of electrical conductor grade aluminium. The blocks were all approximately cubic in shape which, when combined with the high thermal conductivity of aluminium, ensured that their temperature equalised soon after the completion of a run. Each sample was immersed in insulating material before welding so that heat losses to the surroundings were minimised. Thermocouples were attached to the block in each experiment and the bulk temperature rise was related to the energy input associated with the welding run. The effects of arc polarity, alternating current balance, shielding gas composition, arc length and welding current on the arc power and arc efficiency were investigated. The results obtained with alternating current are compared to those for direct current, and the differences are explained.     

Synergetic effects of hybrid laser/arc welding

Abstract

The present study reports the results of a study examining the synergetic effects of hybrid laser/arc welding. Experiments were carried out with a 500 W Nd:YAG laser in combination with standard gas tungsten arc welding equipment and attention was focused on two aspects: the heat transfer efficiency and the melting efficiency. The heat transfer efficiency was determined by calorimetric measurements, whereas the melting efficiency was obtained from the transverse cross-sections of welds produced under various conditions. In addition, analytic calculations of the melting efficiency were performed on the basis of a modified form of the Rosenthal equation. The results show that the interaction of the laser and the arc does not lead to a noticeable change in the heat transfer efficiency, but results in a significant increase in the melting efficiency. The observed synergic melting effect is caused by addition of the two heat sources (laser and arc) and the contraction of the arc by the laser beam.     

Study on heat efficiency of laser-TIG double-side welding

A series of laser-TIG double-side welding experiments for aluminum alloys were carried out to investigate the heat efficiency of the process. The melting efficiency was introduced to evaluate quantitatively the degree of the mutual effect of the laser and the arc. The results showed that the melting efficiency of laser-TIG double-side welding exceeded the sum of the laser and the arc taken separately. With the increase of heat input, the weld depth and melting efficiency of the laser and the arc were increased signifwantly. This, in fact, implies the strong mutual effect of the laser and the arc as heat sources joined simultaneously in the process. Comparatively, the higher efficiency of the laser constituent of heat sources plays the main role in the increase of the process efficiency. The phenomena of arc column convergence, increased laser absorptivity and the formation of heat accumulation region are the causes of the improvement of heat efficiency.     

3D numerical study of effects of temperature field on sensitisation of Alloy 690 butt welds fabricated by gas tungsten arc welding and laser beam welding

Abstract

This study examines the effects of the temperature field on the sensitisation of Alloy 690 butt welds fabricated using the gas tungsten arc welding (GTAW) method and the laser beam welding (LBW) method respectively. The welding thermal cycles of the two welding methods are simulated using ANSYS software based upon a moving heat source model. The validity of the numerical model is confirmed by comparing the simulation results with the corresponding experimental findings. Agreement is found between the numerical results for the temperature field and the experimental temperature measurements. In addition, it is shown that the LBW weldment experiences a more rapid heating and cooling effect than the GTAW weldment, and therefore has both a smaller heat affected zone and a narrower sensitisation region. Thus, the validity and general applicability of the thermal welding model are confirmed.     

Influence of GMAW-P current waveforms on heat input and weld bead shape

Abstract

Manufacturers use different, proprietary pulse current programming algorithms in their synergic 'one knob control' pulsed gas metal arc welding (GMAW-P) power supplies. Since the different pulse waveforms produced by these supplies can produce somewhat different welding characteristics, inconsistent results can be obtained when transferring welding procedures between different GMAW-P power supplies. A primary objective of this work was to characterise the differences in welding heat input and weld bead shape that could be produced by the pulsed current waveforms from four different commercial supplies. To eliminate the affects of subtle differences in electrical characteristics and to ensure that the exact shape of all waveforms was fully known, the comparison was also done by simulating the waveforms generated by three of the four power supplies on the fourth supply, which was equipped with waveform programming capability. Then, the four waveforms were used to create 'bead on plate' welds over a range of wire feed speed settings, and corresponding heat inputs were calculated from current and voltage samples recorded by a computer data acquisition system during welding. Welds were also done at the same wire feed speed setting using a constant voltage supply. All welds were then cross-sectioned for penetration and dilution measurements. In general, all of the waveforms produced good metal transfer and weld quality. However, the heat input and beads shapes varied noticeably. The heat inputs for the four pulse waveforms and constant voltage welds differed by as much as 150 J mm^?1 (17% of the maximum heat input) at the highest wire feed speed of 212 mm s^?1. The weld penetration differed by 1 mm (22% of the maximum penetration) at a wire feed speed of 169 mm s^?1 and the dilution differed a maximum value of 6.5% dilution (22% of the maximum dilution) at a wire feed speed of 169 mm s^?1 .     

Weld shape variation and electrode protection under Ar–(Ar–O2) double shielded GTA welding

Abstract

Double shielded gas tungsten arc welding (GTA welding or TIG welding) of an SUS304 stainless steel with pure inert argon as the inner layer shielding and the Ar–O₂ active gas as the outer layer shielding is proposed in this study in order to investigate its effect on the tungsten electrode protection and the weld shape variation. The experimental results show that the inner inert argon gas can successfully prevent the outer layer active gas from contacting and oxidising the tungsten electrode during the welding process. The active gas, oxygen, in the outer layer shielding is decomposed in the arc and dissolves in the liquid pool, which effectively adjusts the active element, oxygen, content in the weld metal. When the weld metal oxygen content is over 70 ppm, the surface tension induced Marangoni convection changes from outward into inward, and the weld shape varies from a wide shallow one to a narrow deep one. The effect of the inner layer gas flowrate on the weld bead morphology and the weld shape is investigated systematically. The results showed that when the flowrate of the inner argon shielding gas is too low, the weld bead is easily oxidised and the weld shape is wide and shallow. A heavy continuous oxide layer on the liquid pool is a barrier to the liquid pool movement.     

Microstructures and properties of laser/arc hybrid welds and autogenous laser welds in pipeline steels

Abstract

Lasers are capable of producing welds with deep penetration, low distortion and faster travel speeds, compared to arc welding. More recently, laser/arc hybrid welding processes have also been generating interest for industrial fabrication. In this paper, six carbon–manganese, mainly pipeline, steels were welded using both autogenous Nd:YAG laser welding, and Nd:YAG laser/MAG hybrid welding. The improvements in weld microstructures and weld metal toughness that are possible when using the hybrid process are described and illustrated. Laser/arc hybrid welding is shown to be a process that can generate good quality welds in commercially available pipeline steels. It also has the potential to complete girth welds in these steels with significantly fewer welding passes than are currently required for arc welded pipelines, reducing the joint completion time.